CA2097502C - Inspiratory airway pressure system - Google Patents

Abstract

An apparatus (10) and method for treating mixed and obstructive sleep apnea by increasing nasal air pressure delivered to the patient just prior to inhalation and by subsequently decreasing the pressure to ease exhalation effortby sensing, tracking, storing, and comparing breathing parameters such as flow, pressure, and sounds against pre-determined values of these parameters and controlling delivered gas pressures thereby. Gas is delivered to the patientby means of a nasal air pillow. An alternate embodiment stimulates breathing by means of a pair of electrodes which electrically stimulate the upper airway muscles. An apparatus for the detection and compensation of leaks and/or pressure drops is also disclosed.

Description

WO 92/11054 " 0 ~3 7 ~ ~ ~ PCI'/US91/04052 ' -1 -INSPIRATORY AIRWAY PRESSURE SYS I t Back~round of the Invention 1. Field of the l~ tiG"
The ,cresenl invention relates to an apparalus and method for facilitating the respiration of a patient and is particularly useful in lledtillg disturbed breathing, snGring, mixed obstructive sleep apnea, and certain cardiovascular sieep condi-tions. More particularly, the present invention is co"cer"ed with an appafalus and method for i",posing a positive pressure on the patient's airways just prior to the onset of inhalation in order to induce and/or permit inhalation, and for subsequent-Iy reducing the pressure on the airways to ease exhalation effort. Another aspect of the invention is concer"ed with ",onrtori,)g sounds associ~led with patient'srespiration and controlling the gas pressure delivered to the patie"l's ~esp..alory p~ss~ges in accordance with the sounds.

2. DescriPtion of the PriorArt Obstructive sleep apnea is a sleep disorder characterized by rel~ tion of the airway including the genioglossus throat muscle tissue during sleep. When this occurs, the relaxed muscle can partially or completely block the patient's airway, a condilion more prevalent in overv~ei~l,l patients. Partial blockage can result in snoring. Complete blockage can result in sleep apnea.
When co",plete blockage occurs, the patient's i"haldlion efforts do not result in the intake of air and the patient bec~",es oxygen deprived. In reactiGn, the patient begins to awaken. Upon reaching a nearly awakened state, the genio-glossus muscle resumes normal tension which clears the airway and allows inhalation to occur. The patient then falls back to a deeper sleep whereupon thegenioglossus muscle again relaxes and the apneic cycle repeats.
Central apnea is when no inspiratory effort occurs or is delayed. Central apnea may be combined with obstructive apnea, known as mixed apnea. Other breathing irregularities such as Cheynes Stockes breathing may have apneic intervals when intake airflow ceases.
In some patients, sleep apnea events can occur dozens of times during the course of a sleep session. In conseq-Jence, the patient never achieves a fully relaxed, deep sleep session because of the repetitive arousal to a nearly 21~9~S~2 -2-awakened state. The patient is also deprived of REM (rapid eye movement) sleep.
People afflicted with sleep apnea are continually tired even after an apparer,lly normal night's sleep.
In order to treat obstructive sleep apnea, the so-called continuous positive airway pressure (CPAP) system has been devised in which a prescribed level of positive airway pressure is continuously imposed on the patient's airways. The presence of such positive pressure on the airways provides a pressure splint to offset the negative inspiratory pressure to maintain tissue pos;tion tension andthereby maintain an open patient airway. The positive airway connection with a patient is typically achieved by way of a nasal pillow such as that ~Jisclosed in U.S.
Patent No. 4,782,832 hereby incorporated by leference in which the nasal pillow seals with the patient's nares and i",poses the positive airway pressure on the nasal pass~ges.
The CPAP system meets with l~bjec~ions from palienls, ho.ve~er, because the patient must exhale against the positive pressure. This increases the work to exhale. Some pali~nls have difficulty getting used to this and as a result, may disco"linue the therapy. Drying of the nose and airway due to continuous circulation of room air is also a complaint. Also, eAi,-'ed carbon dioxide tends to remain in some nasal masks with CPAP therapy.
In prescribing CPAP therapy, it is usually necess~ry for a patient to spend one or two nights in a sleep l.~al",enl laboralory where it is first determined whether the patient has a ~lês,~..dlory d,sorder such as sleep apnea. If so, thepatient is then fitted with a CPAP device whereupon the required gas pressure isdetermined for providing the necess~ry air splint to maintain airway patency.
The required pressure for maintaining patency is usually higher when the patient is sleeping on his or her back than when slaepi"g in a side rest position.
The higher pressure is usually presc,ibed in order to ensure sufficient pressure in all sleeping posilions. The higher pressure is not needed, however, in all circumstances. For example, before the patient has fallen asleep and in the early stages of sleep, the higher pressures are not needed. Adclilionally, the higher pressures are often not needed during deep sleep when the patient is in the siderest position. Furthermore, a given patient may only be subject to sleep apnea under certain conditions such as when the patient is eAl,~r,~tly tired or under the influence of alcohol or sleep- inducing drugs. As a result, the patient is subjected to the discomfort of the high plescription pressures even when not needed.

2 0 9 7 ~ 0 2 Pcr/usg1/040s2 Su.,~ u." of the In~.lt;GII
The il.S~: .alory airway pressure system of the presenl invention solves the prior art problems as outlined above. More particularly, the pre~er,ecJ system hereof i"iliales i"s,~iralory nasal air pressure just prior to inhalalion in order to provide a pressure splint to offset negative inspiratory pressure and retain thenormal posilion of the genioglossus muscle thereby ensuring an open patient airway, and subsequently reduces the pressure for ease of e,d,alaliGn. Airflow during this eAhaldlion is pri"~a,ily the patient's exhalent with desi,Able humidity.
The prefer,ed apparall s is adapled for connectiol- wHh a patient-coupled gas delivery device for pressurizing at least a portion of a patient's resp.. alory pA-ssayes~ such as the nasal passages, with a breathable gas, pr~feraLly ambientair which may be sl~FF!e."er,led with oxygen, at a co"lr~'lable gas pressure. The appar~lus includes means for determining a point in the patient's breathing cycle before the onset of an i"halalion phase and subsequent to a prior inhalation 1~ phase, and further includes gas control means for initiating, at the determined point in the breathing cycle, an increase in the gas pressure toward a s¢lected,and pref~rably prescriL,ed, high pressure level. The gas control means further conl,ols the gas pressure at the higher level during at least a portion of the inha-lation phase and suhse~uently lowers the gas pressure in order to presenl a lower pressure level during at least a portion of the subsequent exl,P'~tion phase.
In preferled forms, the appa,alus tracks the patient's breathing cycle, thereby determines the end of the exhalation phase of the breathing cycle, and iniliales the pressure inc,ease at that point in the breathing cycle. Alternatively, the apparatus determines an interval time as the point in the breathing cycle for increasing the ins~.alory pressure as a function of previous breath rates and inhaldlion and eAhalalion intervals.
The apparalus desi,ably includes a co,ltlc'!z'lle, variable speed blower for supplying ambient air above al",ospheric pressure, a nasal pillow for coupling with the patient's nares, a conduit intercoupling the blower and nasal pillow, and a con-trollable, variably posilionai~le vent valve coupled with the conduit for venting air therefrom The prefer,ed appar~ s also includes a co"lll "er operably coupled with the blower and with the vent valve, and a pressure transducer for se, Isiny the patient's nasal air pressure In operation, the controller maintains a set point pressure by varying the 3~ position of the vent valve to vent greater or lesser amounts of air from the conduit in correspondence wHh patient eAhalalion and inllaldlion. The conl,c"er further tracks the position of the vent valve and thereby tracks the patient's breathingcycle. That is to say, as the patient inhales during the inh~ation cycle, the vent ~ 2 ~ 9 7 ~ ~ ~

valve must close partlally to malntaln the pressure of the amblent air as the patlent inhales. In thls way, the movement of the valve corresponds to the lnhalatlon of the patlent.
Slmllarly, durlng exhalatlon at a preferred lower pressure set polnt, the vent valve must vent greater amounts of amblent air from the condult whlch tracks the patlent's exhalatlon phase.
By such tracklng, even at dlfferent set point pressures, the system hereof is able to increase the set point pressure predictably prior to the onset of inhalatlon, and to subsequently decrease the pressure during the next exhalatlon phase.
In another aspect of the lnventlon, sounds and pressure varlations assoclated with a patlent's resplratory passages are monitored and the set point pressure of the gas dellvered to the patlent's alrways ls varled ln accordance wlth the monltored sounds. Thls aspect of the lnvention takes advantage of the fact that snorlng sounds typlcally precede the onset of obstructlve sleep apnea. That ls to say, sleep apnea and snorlng sounds can be consldered varying degrees of the same phenomenon ln whlch the upper alrway muscles may progresslvely relax resultlng ln vlbratlon of the partlally relaxed alr passage, and then may progress to obstructlon of the alr passage when the upper alrway muscles relax completely. By monltorlng alrway sounds, and ln particular snorlng sounds, the applled pressure can be raised before an apnelc event occurs and thereby prevent the occurrence.
In another embodlment of the lnventlon hereof, an apparatus and method are dlsclosed for determlnlng the alrway C 62948-l7o r 2 ~
-4a-patency of a patlent and for quantifylng that patency. By knowlng the patlent alrway patency, the alrway pressure applied to the patient can be optlmlzed to aid respiration and mlnlmize discomfort associated with excessive pressure. That is to say, by determining patient alrway patency, patlent resplratlon can be better characterlzed ln some clrcumstances than by monltorlng alrway sounds. Other preferred aspects of the present lnventlon hereof are explalned further hereinbelow.
In summary, thls lnventlon seeks to provlde an apparatus for determlnlng the alrway patency of a patlent exhlbltlng a breath cycle havlng lnhalatlon and exhalatlon phases, sald apparatus comprlslng means for supplylng breathable gas from a source thereof under a controllable pressure to at least a portlon of the patlent's alrway;
means for senslng patlent resplratlon flow and pressure and for produclng signals representative of substantially simultaneous flow and pressure; slgnal processlng means for recelvlng sald slgnals and responsive thereto for determlnlng Z0 the patlent's respiratory admittance from said substantially slmultaneous flow and pressure; and means for controlling said pressure ln accordance wlth sald admlttance, sald signal processing means including means for determining said admittance as the dividend of flow divided by pressure.
The invention further seeks to provide a method of determining the airway patency of a patient, the patlent exhlblting breath cycle having inhalation and exhalation phases, sald method comprlslng: uslng senslng means for C

~209750 ~

-4b-repeatedly senslng a plurality of substantially simultaneous patlent resplration flows and pressures durlng a patient inhalation phase and for producing signals representative thereof; uslng slgnal processlng means for recelving said slgnals and respondlng thereto for determlnlng a set of patlent admlttances from sald flows and pressures and storlng admlttance data representatlve of said admlttance set ln a memory device, and uslng said slgnal processlng means for determlning sald admlttances as the divldend of flow dlvlded by pressure; comparlng ln sald slgnal processlng means sald admlttance data wlth predetermlned admlttance templates stored ln sald memory devlce; and determlnlng ln sald slgnal processlng means the closest match of sald admlttance templates wlth sald admlttance data, sald closest match belng representatlve of patlent alrway patency durlng sald phase.
The lnventlon further seeks to provlde an apparatus for determlnlng the alrway patency of a patlent exhlbltlng a breath cycle havlng lnhalatlon and exhalatlon phases, sald apparatus comprlslng: means for supplylng a breathable gas from a source thereof under a controllable pressure to at least a portlon of the patlent's alrwayl sald pressure belng substantlally constant durlng the lnhalatlon phase of a breath cycle; means for senslng patlent resplratlon flow and for produclng flow slgnals representatlve of patlent resplratlon flow; slgnal processlng means for recelvlng sald flow slgnals and for determlnlng patlent alrway patency values from sald flow slgnals; and means for ad~ustlng sald pressure ln accordance wlth sald patlent alrway patency values.

~2~9750 2 The inventlon stlll further seeks to provlde a method for determlnlng the alrway patency of a patlent, the patlent exhlbltlng a breath cycle havlng lnhalatlon and exhalatlon phases, sald method comprlslng: uslng senslng means for repeatedly senslng a plurallty of patlent resplratlon flows being under substantially constant pressure durlng a patlent lnhalatlon phase and for produclng flow signals representatlve thereof; using slgnal processing means for receivlng sald flow slgnals and respondlng thereto for determlnlng a set of patlent alrway patency values from sald flow slgnals and storlng patlent alrway patency data representatlve of sald set of patlent alrway patency values ln a memory devlce, and uslng sald slgnal processlng means for determlnlng sald patlent alrway patency values from sald flow slgnals; comparlng ln sald slgnal processlng means sald patlent alrway patency data wlth predetermlned alrway patency templates stored ln sald memory devlce; and determlnlng ln sald slgnal processlng means the closest match of sald alrway patency templates wlth sald patlent alrway patency data, sald closest match belng representatlve of patlent alrway patency durlng sald phase.

Brlef Descrlptlon of the Drawlng Flgures Flgure 1 ls a plan vlew of the head of a sleeplng patlent shown wearlng the preferred patlent-coupllng head gear for use wlth the present lnventlon;
Flg. 2 ls a slde elevatlonal vlew of the patlent's head and head gear of Flg. 1 shown coupled wlth the preferred 7 ~ ~ ~
-4d-houslng cablnet of the dual condult embodlment of the present lnventlonj 4 2 0 9 7 ~ ~ 2 PCI'/US91/04052 Fig. 3 is a schenldlic represenldtiGn of the single-conduit embodiment of the presenl invention;
Fig. 4 is a scl)emalic represe,)talion of the dual-conduit embodiment of Fig.
2;
Fig. 5 is an elevational view of the pr,_ferled vent valve elen-enl in position over the vent ends of the dual-conduit embodiment of Fig. 4;
Fig. 6 preser,ts ylaph;c-' ilh,~b~tions of a typical breathing cycle including an inhalation phase and an e~halalion phase, of the nasal air pressure imposed on the patient's airway during the breathing cycle, and of the vent valve steps re-quired to maintain the set point pressures;
Fig. 7 is an elect,ical schellldtic illu~t~liGn of the m ~.oconlroller and associated components of the ~resenl invention;
Fig. 8 is an electlical schelllalic of the blower motor control;
Fig. 9 is an ele~tlical schematic of the stepper motor control for the vent 1 5 valve;
Fig. 10 is a schemdtic illusll~lion of a pressure transducer circuit;
Fig. 11 is a computer proyldlll i'~Jchall ilhlSt,dlin9 the START-UP portion of the main routine;
Fig. 12 is a computer proy~a~n i~ cha~l of the MAIN LOOP portion of the main routine;
Fig. 13 is a computer program floJ:cha~l of the VALVE STEP subroutine;
Fig. 14 is a computer proyr~lll flowchart of the ADC interrupt;
Fig. 15 is a computer program flowchart of the CHECK BLOWER SPEED
subroutine;
Fig. 16 is an electrical block dia9~dlll i~ Stldling the spectral sound analysiscircuit;
Fig. 17 is a computer proyldln l'a~chall of the SOUND ANALYSIS
subroutine;
Fig. 18 is a scl)emalic block diagram of another embodiment of the invention for determining patient airway patency;
Fig. 19 is a set of five graphs of the embodiment of Fig. 18 illustrating airway flow, pressure and adlllitldnce, and further illusll~ti~ly two adl,lillance templates;
Fig. 20 is a computer proyram no.~chall for operali"g the microcGnl(c"er of Fig. 18; and Fig. 21 is a computer prog,am f'~wchall of anoll,er program embodiment for operating the microconl,c"er of Fig. 18.

-WO 92/11054 PCI'/US91/04052 ~o9rZSO~ -6-Fig. Z is a block diagra"~ of the pneumatic co",ponent~ of the compensa-tion embodiment of the present invention;
Fig. 23 is a block diayldm of the electronic co,.,ponenls associaled with the compensation embodiment of Fig. 22;
Fig. 24 is a computer prog,dm flowchart of the PRIMARY module for operating the co".pensdlion embodiment;
Fig. 25 is a computer proy,am floJ:cha,l of the INITIALIZE module of the PRIMARY module;
Fig. 26 is a computer program flo. cha.l of the EXHALE module of the PRIMARY module;
Fig. 27 is a computer program flowcha,l of the INHALE module of the PRIMARY module;
Fig. 28 is a computer proglal., ~ cha,l of the CPAP BACKUP module of the PRIMARY module;
Fig. 29 is a computer program flowchart of the BPM CYCLE BACKUP
module of the PRIMARY module;
Fig. 30 is a computer prog.ar.~ noJ~Icha-l of the PATIENT CYCLE BACKUP
module of the PRIMARY module;
Fig. 31A is a computer proyldm flowchart of the first portion of the AJD
INTERRUPT module of the PRIMARY module; and Fig. 31 B is a computer progfa", flowcha,l of the remaining portion of the A/D INTERRUPT module.

Detailed Description of the ~I. f~, .a~l Embodiments With reference to the cJ.~J:;.,g figures Fig. 3 schematically illustrates the single conduit embodiment of the preler~ed inspiratory airway pressure apparatus10 which broadly includes an elongdled, flexible hose or conduit 12 nasal pillow14 connected to one end of conduit 12 vent valve asse"~bly 16 positioned adjacent the opposed open vent end of conduit 12 blower unit 18 fluidically coupled with conduit 12 between pillow 14 and vent valve asse.. ,bly 16 and - conl~cl'er 20 which is adapted for pneumatic conne-1iGn with nasal pillow 14 and elect-ical connection with vent valve assembly 16 and blower unit 18.
In the preler,ed embodiment vent valve asse",bly 16 blower unit 18 and co"ltc' er 20 are housed within cabinet 22 such as that illustrated in Fig. 2 in3~ connection with the dual-conduit embodiment. In this regard conduit 12 presenls an interior portion which is housed within cabinet 22 and exterior portion 26 which extends from the cabinet to nasal piliow 14. Conduit 12 addiliGnally p,esenl~ cou-pling end 28 coupled to nasal pillow 14 inlet end 30 coupled with blower unit 18 75~ ~

for receiving a supply of breathable gas, preferably amblent air therefrom, and vent end 32 positloned ad~acent vent valve assembly 16.
Nasal plllow 14 ls the preferred patlent-coupllng device and ls further lllustrated ln U.S. Patent No.
4,782,832. Head gear 34 holds nasal plllow 14 on the head of a patlent 36 ln order to fluldically couple wlth the resplratory passages of patlent 36, and preferably wlth the patlent's nares. Nasal plllow 14 ls conflgured to present pressure sensor flttlng 38 whlch ls coupled wlth controller 20 by pneumatlc llne 40 whlch ls preferably routed wlthln condult 12 so that llne 40 ls convenlently out of the way and less llkely to be plnched or restrlcted by the patlent durlng use of apparatus 10. Nasal plllow 14 also lncludes vent port 42 defined therethrough whlch contlnuously vents a small amount of pressure from nasal plllow 14 ln order to prevent molsture bulld up and subsequent condensatlon thereln. Port 42 also prevents bulld up of exhaled gases lncludlng carbon dioxlde.
Vent valve assembly 16 lncludes stepper motor 44 and valve element 46 connected to the output shaft thereof. Valve element 46 ls preferably constructed of a flat plate conflgured to present two, opposed, arcuate, cam-llke edges 48a,b as lllustrated ln Flg. 5. Element 46 ls posltloned ad~acent vent end 32 of condult 12 so that as stepper motor 44 rotates valve element 46 ln a clockwlse dlrectlon as vlewed ln Flg. 5, edge 48a progresslvely covers and thereby restrlcts vent end 32. Conversely, as motor 44 rotates element 46 ln a counterclockwlse dlrect lon, edge 48a progresslvely exposes an , -. ~

~ ~ ~ 7 ~ ~ ~

7a lncreaslng area of vent end 32 to vent additlonally gas therefrom.
Flg. 4 lllustrates the dual-condult second embodlment of preferred apparatus 10. Thls embodlment is slmllar to that of Flg. 3 and correspondlng components are numbered the same. Second embodlment 50 addltlonally lncludes exhaust hose 52 presentlng connectlon end 54 fluldlcally coupled to conduit exterlor portlon 26 at ~unctlon 56, and presents exhaust end 58 posltloned ad~acent valve element 46 ln the same openlng/closlng relatlonshlp wlth arcuate edge 48b as vent end 32 presents to arcuate edge 48a. Wlth thls conflguratlon, condult 12 addltlonally presents lnhalatlon hose 60 between ~uncture 56 and blower unlt 18. In the dual hose model, nasal plllow 14 does not lnclude vent hole 42, and the tube between ends 54 and 28 lnclude dlvlder 61 to separate lt lnto two separate passages. Second embodlment 50 may also lnclude lnhalatlon check valve 62 dlsposed wlthln lnhalatlon hose 60 ad~acent ~uncture 56, and exhalatlon check valve 64 disposed wlthin exhaust hose 52 also ad~acent ~uncture 56.
Inhalation check valve 62 prevents passage of patient exhalatlon therethrough toward vent end 32 and thereby requlres that the patlent's exhalatlon exlt the WO 92/11054 PCI'/US91/04052 ~,o9~ 8-system through exhaust end 58. Pneumatic lines 66 and 68 respe.,1i-/ely couple co"l-oller 20 wHh i.,halalion hose 60 and exhaust hose 52.
By way of overview, cGnl-c"er 20 controls appardtus 10 in order to increase the gas pressure presenled to the patient at a time in the patient's breathing cycle just prior to inhalation, and to subse~uently lower the pressure for ease of exhala-tion. The upper graph of Fig. 6 illustrates a typical breath cycle air flow. During inhalalion, the flow rate of gas to the patient gradually increases to a maximumand then decreases. At the end of inhalation, the patient typically eA,.eriences a slight pause before exhalation begins. During exhalation, the exl.a'ed gas flow from the patient gradually increases to a maximum and then decreases again. A
post~l .alalion pause, typically somewhat longer than the post-inhalation pause,follows exhalalion. After the post-exhalation pause, the patient again begins inhaldlion.
The middle graph of Fig. 6 illustrates the nasal airway pressure presenled to patient 36 during operalion of apparalus 10. With patients subject to sleep apnea, it is desirable to increase nasal airway pressure just prior to inhalalion to splint airway pressure in order to positiGn the genioglossus tissue and thereby maintain the airway open. Accordingly, this middle graph illustrates an increasein the nasal airway pressure just prior to inhalaliGIl to a selected presc.iplion pressure level s~ nl to push surrounding tissue aside and open this airway.
After completion of i. ,halalion, the set point pressure presenled to the nasal airway is reduced so that exl,alalion occurs against a low or even zero pressure level relative to ambient. At the end of e,~l ,alalion, the nasal airway pressure is again incleased prior to the next i"haldliGn phase.
To accG",plish these pressure v~,ialions, blower unit 18, in one embodi-ment of the invention, produces a generally con~lanl volume per unit time of brealhable gas which is selectively vented through vent end 32. The vented gas volume is cGr,l.c"e~ by vent valve assembly 16.
The bottom graph of Fig. 6 gndph-cally depicts the various posilions of valve ele",enl 46 in relation to vent end 32 in order to acl.. evc the desired nasal airway pressure profile illustrated in the middle graph. For example, during thepost-exhalation pause, cont,e"er 20 activates stepper motor 44 to rotate valve ele..,enl 46 in a clocl~;se direction (as viewed in Fig. 5) in order to i..crease the nasal airway pressure to the desired set point as sensed by cor,l,cller 20 by way of pneumatic line 40. When the patient begins to inhale, gas output from blower unit 18 is inhaled by the patient. In order to maintain the set point pressure, the conlrcl'er then rotates valve ele."er,l 46 in stepwise fashion further in the clockwise direction to reduce the amount of gas being vented. As inhalalion passes its peak W O 92/11054 2 ~ 9 7 .~ ~ 2 P ~ /US91/04052 flow rate cGn~ ler 20 begins to reverse the position of valve element 46 to ventadditional gas for maintaining the set point pressure.
At the end of inhalalion a lower pressure set point is desired and controller 20 continues in slep.~ise fashion to rotate valve element 46 in the counterclock-wise direction to vent additional amounts of gas for achJev;ng a new lower set point pressure.
At the end of the post-inhalation pause the patient begins to exhale. In order to maintain desired lower set point pressure the additionally exhausted gas from the patient must be vented through vent end 32. Accordingly cGrllrcller 20 causes valve ele."enl 46 to further rotate in a clock~;sc direction to open vent end 32 even further. As the eAl,alalion flow rate dec,eases, cont,c"er 20 rotates valve element 46 in a clockwise direction to dec,ease venting in order to maintain thelower set point pressure. At the end of e,~l ,alalion cor,l,. "er 20 then causes valve ele."enl 46 to rotate further in the clockwise direction to increase the pressure to the higher pressure set point. This induces tension in the genioglossus muscle to open the airway in preparalion for the next inhaldlion phase.
Inspection of the upper and lower graphs reveals a similarity in the profile of the curves. That is to say cGnll~ller 20is able to track a patients breathingcycle by l,acking the stepped posiliGns of valve ele."e"l 46 required to maintain the set point pressures. In this way conl,~"er 20 is able to determine the end of respective inhalation/exhalation phases and to predict exhalation and inhalalioninterval times.
Turning now to cGnl,c"er 20, it provides elect,ical outrutc to control the speed of blower unit 18 and the position of slapper motor 44. Conl,-"er 20 receives ~le 1-ical feedb~ck from blower unit 18 indicative of the speed thereofand a pneumatic input by way of pneumatic line 40 to indicate the pressure at nasal pillow 14 and thereby in the patient's nasal airway p~ssages.
Conlrcl'er 20 includes pressure transducer circuit 700 (Fig. 7) for providing an electrical input indicative of the pressure at nasal pillow 14 to ",ic,oconl,oller circuit 800 (Fig. 8) which in turn provides outputs to blower motor circuit 900 (Fig.
9) and slapper motor circuit 1000 (Fig. 10). Addilionally cor,l,-"er 20 includes a conventional 120 v.a.c. to +5 v.d.c., +12 v.d.c. and +24 v.d.c. power supply (not shown) suitable for digital and analog solid state integraled circuit co",ponenls.
Pressure transducer circuit 700 illustrated in Fig. 7 is typical of the pressuretransducer circuit for both the single and dual conduit embodiments of the present invention. That is to say the single conduit embodiment of Fig. 3 uses only one pressure transducer whereas the embodiment schematically illustrated in Fig. 4 uses two pressure transducers both using a circuit as illustrated in Fig. 7.

wo 92/11054 2 PCr/USgl/040s2 ~og1 ~

The preFer,~d pressure transducer includes SENSYM type SX01 DN having a zero-to 70-cm. water operalional range. The preh~rled transducer includes fourstrain gages a"dnged in a convt7ntional Wl,~at~tone bridge 701 having strain gages X1 X2 X3 and X4 presenting a nominal 4650 ohms each. Bridge 701 presenl:j e~cit~lion terminal 702 conne~1ed to +12 v.d.c. and an opposed excilalion terminal 704 connected to ground as shown. Bridge 701 produces outputs at terminals 706 and 708. Zero adjustment potenlio",eler 710 inlercon-nects terminals 704 and 706.
The output from terminal 708 is connected to the positive input terminal of operalional amplifier 712 (one-haH of Type LT1014). The output of operational amplifier 71 2 provides fee~lback to the negative input terminal thereof, and by way of resistor R1 (1 K ohms) su~ F'ies the positive input terminal of amplifier 714. The output is also connected to ground by way of resistor R2 (750K ohms).
Strain gage bridge output terminal 706 is connectad to the positive input terminal of operalional amplifier 716 (the other half of unit LT1014). The output from amplifier 71 6 provides feedback to the negative input terminal thereof and is connected by way of resistor R3 (1K ohms) to the negative input terminal of amplifier 714.
The output from amplifier 714 provides feedb~ck to the negative input terminal thereof by way of resistor R4 (750K ohms). The output from amplifier 714 is also co"nected by way of resistor R5 to output terminal 718 which by way of the circuitry just described provides output between 0 and +5 v.d.c. co"espond-ing to a pressure of 0 to 25 cm. water.
A similar output is provided at a corresponding terminal 720 if a second pressure transducer is used. In the dual-conduit embodiment two transducers provide additional pressure infor",alion which allows more precise tracking of inhalalion and exl ,alalion gas flows of the patient and thereby more precise breath cycle tracking.
Fig. 8 is an elecl,ical schematic diagral-, of n,.~roconl,~ er circuit 800 whichincludes microconlr. ler 802 (Intel Type 8097BH) p,oyfalnrllable array logic (PAL) (Type PC1 6L8) erasable prog,a",l~,able read-only-memory (EPROM) (Type 27256) acJJless latch 808 (Type 74HC373) ,andG", access memory (RAM) (Type 6264P), inpuVoutput serial data inle"ace (RS232 Type MAX232) p-esc, i,~)lion (RX) switch array 81 4, and input data latch 816.
Microcontl~"er 802 receives power (Vcc) at +5 v.d.c. at terminals VCC
VPD BW RDY VPP, and VREF as shown. Ground is connected to terminals NMI
VSS EA and ANGND. Crystal 802 is coupled betv/e~n terminals XTAL1 and WO 92/11054 2 0 3 7 ~ 0 2 PCr/USgl/04052 XTAL2 as shown and to which respective grounded cAp~citor~ C1 and C2 (33 pF
each) are respecti~/ely coupled for timing signals at 12 MHZ.
MicrocGnlr~"er 802 recei\,es a reset signal at terminal RESET from reset sub-circuit 820. On power up, power is supplied through resistor R5 (1 00K ohms)to grounded capacitor C3 (22 uF) and to the input terminals of SCHMlrr trigger NAND gate 822. Initially, the resultant input voltage to NAND 822 is low, and its output is logic high. This logic high output is s~ ,~pl.ed to output terminal 824 which provides a reset signal to blower motor circuit 900 as ~iscussed further hereinbelo~v. The initially logic high output from NAND 822 is inverted by invertor 826 to provide a logic low signal to microconl(c"er terminal RESET which holds microcontroller 802 in reset until the charge on capacitor C3 builds to the trigger level of NAND 822. This provides time for the system to initialize and for l,a"sie, lls to be suppressed. As the charge on capacitor C3 i,.creases to the trigger level,the reset signal is removed from output terminal 824 and m.~,,ocG"l,cl'er 802. The output from invertor 826 is also connected to one side of pull-up resistor R6 (1 OK
ohms) the other side of which is connectecl to Vcc.
Reset circuit 820 also includes a normally open, reset switch 828 coupled across capacitor C3 which allows manual reset. Diode D1 is coupled access resistor R5 to provide a discharge path for C5 in the event of power off.
MicrocGnl,c"er 802 also r~ceives a pressure transducer input at terminal ACH0 and also at ACH1 if a second transducer is used as in the dual-conduit embodiment. To provide transienl suppression, and to slllovtl, the analog voltage from pressure transducer circuit 700, one side of capacitor C4 (.005 nF) is connected to terminal 718 along with the anode of diode D2 and the cathode of diode D3. The other side of carac.itor C4 and the anode of diode D3 are connect-ed to ground as shown and the cathode of diode D2 is connected to a supply voltage Vcc. An idenlical circuit is provided for terminal 720 using diodes D4, D5 and capAc;lor C5. Microcor,l,e"er802includes internal analog-to-digital converters (ADC) which receive the ,espec1i~/e analog inputs at terminals ACH0 and ACH1 and convert these to digital form for internal use in mic~oconlroller 802.
Microco"l~c"er 802 also lecei\,~es an input at terminal HS1.0 which is a pulse signal from blower motor circuit 900 represe"tati~e of the speed of blowerunit 18, ~iscussed further here;nbelcJ:.
M;e:ocG,-l,oller 802 also uses a cG""~,on address/data bus 830 which inlerconnects microconl,vl'sr 802 for data and addless inf.r"~alion flow with PAL
804, EPROM 806, address latch 808, RAM 81 0, and data latch 816 at the terminalsas shown in Fig. 8. Fig. 8 also illustrates the other con~entional inlerconnection between these components as shown.

WO 92/11054 PCr/US91/04052 ~502 -12-Microcor,l,~"er 802 provides a serial data output from terminal TXD to terminal 11 of i,lte,F~ce 812 and receive~ data from terminal 12 thereof at microcor,l,uller terminal RXD. Inle,~ce terminals 14 and 13 receive RS232 data in and out which enable remote reading and control of "~;e-oconl,."er 802 and thereby apparal.ls 1 0. This feature is particularly useful in a sleep laboralory for example tor adjusting the prescfiplion pressures in order to achieve the optimaltherapy.
Switch array 81 4 includes eight s~lrct~ble swHches for providing input data represenlali~e of the desired prescfi,clion set point pressures for inhalation and exl,alalion. In particular, the top four sw:lcl)es are used to set the prescriplion inhalation pressure and the bottom four switches for presc~iptiGn exhalation pressure. With four switches for each set point sixteen possible s~llings are avail-able ranging beh~Jeen 3 and 16 cm water for inhalalion and 0 and 1 4 cm water for exhalation. Data latch 81 6 is coupled with switch array 814 as shown and latches the plesc,i,(.lion data upon receipt of the latch signal from terminal 1 2 of PAL 804.
The presc,iplion data is transmitted over bus 830.
Mic,oconlr~"er 802 also provides two additional outputs. The first of these is data to stepper motor circuit 1000 by way of six-line output bus 832 from ",i~.ocG"l,c"er terminals P1.0-1.5 to output terminal 834. The second ad~Jitional output is a pu13c wi~l h mod~J!sted signal (PWM) to blower motor circuit 900 by way of line 834 and output terminal 836.
Fig. 9 is an elect,ical schematic diagra,n represenli"g blower motor circuit 900 which receives the pulse width modlJ'~tod signal at terminal 836 from microcGr,l,oller 802 and also rectives an inverted reset signal at terminal 824 from reset circuit 820. Blower motor circuit 900 also provides a pulse output signal at terminal 902 represenlalive of the speed of blower motor 904 to microcontroller 802.
The reset signal recei\f0d at terminal 824 is connected to terminal 10 of motor driver 906 (Type UC3524A). The pulse width modlJl~tçd signal from co"l,~'ler 802 at terminal 836 is provided to terminal 2 of driver 906 by way of low pass filter C6 t1.0 uF) and resister R7 (24.9K ohms).
Driver terminal 7 is connel1ed to ground by way of car~itor C7 (.003 uF) and terminal 6 is connected to ground by way of resistor R8 (49.9K ohms).
Terminal 8 is connected to ground and terminal 15 rec~ives power supply at +12 v.d.c. Driver terminal 12 13 and 16 are conne-1ed to Vcc at +5 v.d.c.
Motor driver 906 converts the input pul_~ ~rJidlll modu!nted signal at 0-5 v.d.c. to a c~r,esponding output at 0 to +12 v.d.c. at terminals 11 and 14 thereof to progldlllmable array logic (PAL) (Type 16L8) terminal 1. These terminals are WO92/110~4 2 0 ~ 7 ~ 0 2 PCI'/US91/04052 also connectad to ground by way of resistor R9 (0.5 ohms). PAL 908 produces respective olJtrUtS at terminals 1 9 and 1 8 as two phases for the stator and rotor of brushless D.C. blower motor 904 (Fasco Corp. Type 70000-S51 7). The PAL 908 outrutC are respe-;ti~/e inputs to level converters 910 and 912 (MC14504) which shiR the voltage level from +5 to +12 v.d.c. The +12 v.d.c. outputs from level converters 910 and 912 are in turn ll dnsrl ,itlad to the respective gates of field effect l,ansistor~ (SENSFET) (1~1 s torola SENSFET Type MTP40N06M) 91 4 and 91 6. The respective drain terminals of SENSFETS 91 4 and 916 are respectively connected to terminals OA and OB of blower motor 904 and provide the respective phase inputs to the stator and rotor thereof.
Power at +12 v.d.c. is additionally provided to level converters 91 0 and 912 and to CGIlllllOn power terminal CP of blower motor 904.
The source terminal of each SENSFET 914, 91 6 is connecte.l to ground as shown.
SENSFETS 914,916 each include an addiliGnal pair Of o.ltrl~C on lines 918 and 920 which provide a sampling of the current flow through the respective SENSFETS. These outputs are courled across resistor R1 0 (1 00 ohms) to provide a current path for the current sample, and thereby a voltage represenl~ /e thereof to terminals 3 and 4 of motor driver 906. Driver 906 is responsive to this inputvoltage represe, Italive of the current flow through blower motor 904 to reduce the duty cycle of the output at terminals 11 and 14 in the event of motor overcurrent.
Blower motor 904 is additionally equipped with Hall effect transducer which is operable to provide a voltage pulse each time a ",agnelic pole of the motor stator passes thereby. These output pulses represer,l the speed of motor 904 andare provided at motor terminal HALL by way of line 922 to output terminal 902, and as feedb~Ack to motor driver 906. The output pulses representative of motor blower speed at terminal 902 are provided to m.~-ocGnl,ol'er 802 at terminal HS1.0 thereof.
The pulses represenlalive of motor blower speed are converted to a represenlalive voltage before input to motor driver terminals 1 and 9. As shown in Fig. 9, line 9Z is connected to one side of cal~Acitor C8 (0.01 uF) the other side of which is connected to one side of resistor R11 (1 OK ohms), and to the anode of diode D6. The other side of resistor R11 is connectecl to ground.
The cathode of diode D6 is connected to one side of grounded carAcitor C9 (0.1 uF), to grounded resistor R12 (1M ohms) and to one side of resistor R13 (1 OOK ohms). The other side of resistor R13 is connecteJ to one side of carAcilor C10 (o.22 uF), to one side of resistor R14 (lOM ohms), and to motor driver 209rt50 -14-- terminal 1 as input thereto. The other side of caracilor C10 and re~lslor R1 4 are conne~1ed to driver terminal 9.
This network of components C8-C1 0 R1-1 -R1 4 and diode D6 convert the frequency pulses on line 922 to a voltage represernalive thereof. That is to saythis network acts as a frequency-to-voltage converter owing to the large capacitance of c~pacitor C9 (0.1 uF) which provides a long time conslanl. The voltage value provided at motor driver terminals 1 and 9 provides lee.Jl,~cl~ to an internal c~."pa,~lor which cGn,pares the voltage to a set point derived from thepulse width modul~ed signal received at terminal 2.
Fig.1 0 illustrates slepper motor circuit 1 000 which activates slepper motor 44 to position valve elemenl 46 in accordance with data rec~ 0d from microcontr-oller 802 at terminal 834 II,ere~-u",. Stepper motor 44 is prefe.ably a VEXTA model available from Oriental Motor Company and is capable of providing one revolutionin 400 "steps" and is also capable of half-slepp.ng if needed. As those skilled in the art will apprecidle motor 44 is operable to shift one step upon the imposition of the next sequential voltage step pattern provided as input at terminal 834 over output bus 832. In particular bus 832 includes six lines which are pattern data for the driver chip.
The step pattern data is provided to step motor driver chip 1002 (Type S'GS' L298N) at terminals A B C and D respectively from terminals P1.0-1.3 of microcG"l-c"er 802. Driver 1002 shifts the input data voltage from +5 v.d.c. to +12 v.d.c. for cGr-e$ponding output at terminals 2 3 13 and 14 which are connected to stepper motor 44 to impose the step pattern thereon at +12 v.d.c.
The anodes of diodes D7 8 9 and 10 are connected to the respective four output lines of driver 1002 and the cdtl,odes thereof are connected to +12 v.d.c. for voltage pull-up. Correspondingly the cathodes of diodes D11 12 13 and 14 are connected respectively to the output lines and the respe.1i~/e diode cathodes connected to ground as shown for voltage pull-down.
As shown in Fig. 10 +5 v.d.c. is provided at driver terminal 9 +12 v.d.c.
at driver terminal 4 and terminals 1 8 and 1 5 are all connected to ground.
Figs. 11-14 are computer prog(a", ~lo.~cha.ls illu~l.dli.l5a the operative progr~n, for microconl,."er 802.
Fig.11 illustrates the START-UP portion of the main routine of the computer progra", for operating microconl.c"er 802. After the logic low reset signal goeslogic high the prog,a,~, enters at step 1102 which pro""~ls conll~"er 20 to shift vent valve assembly 16 to its "home" position. In particular, this step pro",pl~micrGconlrc"er 802 to produce data of sequential pattern outputs by way of line 832 and terminal 834 to stepper motor control circuit 1000. This shifts sle,~per WO 92/11054 PCI'/US91/04052 20s7~02 motor 44 to a mid-range position wherein valve slen,enl 46 blocks conduit ends 32 and 58 about half-way as shown in Fig. 5 or conduit end 32 alone in the single conduit embodiment. Step 1102 also initializes the variables counters interrupt routines and so forth in the prograr".
The proy, dl11 then moves to step 11 04 to read the i"haldlion and exhalation prescription pressure values as set on switch array 814 and read by way of address data bus 830. These values are then stored in PAM. Step 1104 also pr~",pls microco"l,. 'er 802 to set the oper~ting speed ot blower motor 904 in accordal~ce with the preacfiption of pressure set on switch 814. The blower speed should be set at a level fast enough to ensure that sufficient ambient air volume is provided to conduit 12 such that the prescription pressure level can be attained during maximum i"haldlion. Blower motor speed data cGrlesponding to prescfiption settings are stored preferably in a look-up table. Step 1104 also clears any value stored in the internal buffer at m.~:ocontrc ler terminal HS1Ø
The prog, al " then moves to step 11 06 which enables the pl ogra" ,'s timed interrupts to begin timing.
In step 11 08 the progfa", sets the software flag ',~hase" equal to inhalation "I" which initializes the prog, a", from the i, lh~'~tion phase of the patient's breathing cycle. This step also initializes the blower check counter at zero. As disclJssed further hereinbelow the program reads the blower speed after 128 passes through the main loop.
The prog,a", then moves to step 111 0 which starts the internal analog-to-digital converter (ADC) connectad to microconl,~ er input terminals ACH0 and ACH1.
Step 1112 sets the pressure set point for the inhalalion phase according to the inhalalion prescfiplion value set on switch array 814 according to data in a look-up table. This step also defines the start-up mode of the appa,dtus as continuous positive airway pressure (CPAP). That is to say and as explained further hereinbelow the prog,ar" operales apparatus 10 in order to present a continuous positive pressure at the inhalaliGn set point pressure for the first eight breall ,s of a patient. Step 1112 also initializes the breath counter at zero in pre-paration for counting patient breathing cycles.
After completion of step 1112 the progfar" moves to MAIN LOOP 1 200 of the main routine as illustrated in Fig. 12. Step 1202 is the first step of this routine in which the proy,a", c-'e~ tes the average pressure as sensed by pressure transducer 701 over eight ADC converaions. That is to say ",.erocGnl,c'ler 802 includes an internal "ring" buffer which stores the eight most recent pressure readings received at miçroconl,.' er terminal ACH0 (and also ACH1 in the two-W O 92/11054 PC~r/US91/W 052 2 0 9 ~ S ~ ~ 16 conduit embodiment). As discussed further heleinbeloJJ ADC interrupt routine converts the input analog values to. digital form every 22 microsecGnds and continuously stores the most recent digital values in the ring buffer. Step 1020calculates the average value by dividing the cumulative buffer value by eight. Step 1202 also calculates the deviation that is error in the average pressure from the pressure set point.
The prog, a"~ then moves to step 1204 which asks whether the magnitude of the error calculated in step 1202 is greater than allowed maximum error. Thisprovides a so-called "dead band" to prevent the system from "hunting".
If the answer in step 120~ is yes, the progra", moves to step 1206 and calculates the number of steps and direction of slepper motor 44 required to correct the pressure deviation error. That is to say depending upon the volume of air being produced by the blower, the fluid capac;ty of the system, and the leakage II,ere~o"" the number of required steps can be determined approxi",atelyby reference to data previously stored in a look-up table.
The program then moves to step 1208 to execute routine '~ALVE STEP"
illustrated in Fig. 13 and ~iscussed further herei.)below. VALVE STEP routine 1300 sequentially preserils the data patterns required to step the valve for the required number of steps in the direction determined in step 1206.
After execution of sub-routine 1300 or after step 1204 the program returns to step 1210. This step stores the number of valve steps and direction actually implemented in an internal valve slope buffer which continuously stores the previous eight movements of stepper motor 44. With this inIor",aliol) the slope of valve movement can be calculated by dividing the valve slope buffer sum by eight. This represenl~ a slope because the eight values are stored at equal timeintervals and thus the buffer sum divided by eight represenls the first derivative of value movement.
For example and re~er,ing to Fig. 6, after the post~xl~alaliGn pause, and after ac~"ev;ng the desired set point pressure no sig"i~icanl error in pressure versus set point exists. Thus, no change in the value position is required and so the previous eight value steps would equal zero indicating a slope of zero whichis indicated by the flat portion of the valve position curve in Fig. 6. In conlldsl when the patient begins to inhale the valve position must initially and quickly shift toward the closed positiGn to maintain the pressure in conduit 32. With a numberof positive steps executed on stepper motor 44 the values stored in the slope buffer indicate a high positive slope. Conversely near the end of inh-'-tion thevalve must execute a number of steps in the negative direction in order to maintain the pressure in condyit 32 indicating a large negative slope. This slope WO 92/11054 , O 9 7 . Q 2 PCI/US91/04052 information as is ~iscussed further hereinbeloJ~ is used to determine various points in the breathing cycle of a patient.
The proy~ a", then moves to step 1212 which asks w: ,ell ,er the phase flag is set for e,~l,alalio,). The p!og,a"~ was initialized with the phase flag set for inhalalion and so during the first few passes through main loop 1200 the answer -in 1212 is no and the progfa", moves to step 1214 which asks v;: ,~tl,er the phase flag is set for inhalation. Rec~use this flag is initialized as inhalation the answer in step 1214 is yes and the prog,a,n moves to step 1216.
Step 1216 asks whether the variable "timer counter" is greater than the value for variable inhaldlion end time, and whether the slope as calculated in step 1210 is less than or equal to -5. The variable utimer counter" (TMR CNT) is a software counter which was initialized at zero and incle",e"t~ every 13 millisec-onds. The variable inhalalion end time was initialized at a default value represenl-ing inhalalion time equivalent to a predeler"lined average value. As discussed further hereinbelow the variable inhalaliGn end time" is recalculated for each breath cycle after an initial eight passes through main loop 1200. Step 1216 operales to determine v:: ,etl ,er sufficient time has passed for normal inhalation to be complete as addiliG"ally confirmed by the value slope being less than -5 as illustrated by the slope of the value position curve at the end of inh~ -tion in Fig.
6.
During the first few passes through main loop 1200 the answer in step 1216 is no and the progfar" moves to step 1218 which asks whether the blower check counter initialized at zero is equal to 128. Until then the answer in step1218 is no and the program moves to step 1220 to increr"e"l the blower check counter. The program then loops back to step 1202 and repetitively executes steps 1202-1220 until the answer in step 1218 is yes whereupon the program moves to step 1~2? to execute the sub-routine CHECK BLOWER ~l~ttu 1200 as illustrated in Fig.15. As discussed further her~ el~w this step monitors the blower speed to ensure that it is running at the set point speed initially set in step 1104 in accordance with prescfi,ction setlings. The progfarn then returns to step 1224 to reset the blower check counter at zero.
After sufficient time has el~psed to exceed the default time set for the inhalation end time and when the slope of the valve positiGn curve is equal to or less than -5 indicating the end of patient inhalation the answer in step 1216 is yes and the progl ar" moves to step 1218 which asks whether the mode of operation is set for inspiratory nasal air pressure (INAP). This was initialized in the CPAP
mode in step 1112. During the first eight breathing cycle, the answer in step 1226 is no and the pr~y, a", rQoves to step 1228 which asks whether the breath counter WO 92/11054 PCr/US91/04052 ~o9~ ~ ~18-is less than or equal to eight. The breath counter was initialized at zero and during the first pass of the proylalll the answer in step 1220 is yes, and the progfa,nmoves to step 1230 to incre",enl the breath counter.
The progra"~ then moves to step 1232 which sets the variable "cycle time"
equal to the current value e~isti"g on the timer counter. This step is enlered at the end of each inhalation phase and marks the end of one breath cycle and the beginning of another. Thus, the time of one breath cycle, that is, cycle time, equals the time value exisli"g on the timer counter which is reset to zero at the end of each breath cycle, also in step 1232.
Step 1232 also sets a new inhalation interval time equal to the new cycle time divided by three. St~tistically, inhalalion time averages about 40% of a typical breathing cycle. Step 1232, hoJ~ever, sets the inhalation interval equal to 33% of the most recent cycle time in order to ensure that this value clocks out in step121 6 early, that is, before the end of anticirated actual inhalation time.
Step 1232 also sets the variable "i"haldtion start time" equal to the new cycle time divided by two. With the beginning of a cycle marked as the end of aninhalalion phase, the next inhalation start time would normally be expected to occur after 60% of the cycle time has el~pserl Step 1232, ho.vevcr, sets inhalation start time at 50%, that is earlier than the predicted inhalation time in order to ensure an increase in nasal pressure before 9nhalation would be expected to begin.
After main loop 1 200 has cletected eight breath cycles as indicated on the breath counter, the answer in step 1228 is no and the proylalll moves to step 1 234 which sets the operating mode as INAP. The eight cycle delay in setting the INAP mode ensures reliable data in tracking the breath cycle.
With the mode now set as INAP, the answer during the next pass at step 1226 is yes and the prog,d", moves to step 1236 to set the pressure set point equal to the exhaust presc,iplion. That is to say, an i"halalion phase has endedas determined in step 121 6, eight breaths have been tracked as determined in step 1228, the mode is set as INAP which allows a decrease in pressure during e~halalion. With these condilions satisfied, the co"l,~'leG' pressure set point is lowered to the prescribed exhaust presc,iplion set point.
Normally, the exhaust pressure would be prescribed at zero, that is ambient, so that the patient can exhale normally. In some circu",~tances, however, the ll,erapist may desire a slight positive pressure during exl,alalion which is set on the lower four switches of switch array 814 (Fig. 8).
Step 1 236 also sets the phase flag for exhalation.

WO 92/11054 2 0 9 7 ~ G 2 PCI/US91/04052 _19_ During the next pass through main loop 1200, the answer in step 1 212 is now yes, that is, the phase is "eAl,e'~tion", and the prGylarll moves to step 1238 which asks wl ,etl ,er the current value on the timer counter is greater than or equal to the i"haldlion start time as previously set in step 1232. In the alternative, step 1 238 asks wl ,~tl ,er the valve position slope is greater than seven which indepen-dently indicates the end of exl,alation. With l_h3,ence to Fig. 6, at the end ofe~l,alalion, the valve must step in the positive direction rapidly in order to restrict vent end 32 for maintaining the set point pressure. This rapid change indicates a positive slope greater than 70.
If the answer in step 1238 is no, the progra", continues to loop through until the answer is yes at which time the progra", moves to step 1240 to set thephase flag for inhalation, to set the pressure set point at the inhalation presc, iplion value, and to set the value for the variable "inhalation end time" equal to the currently exisling timer count plus the i"halaliGn interval time. The ex;sling value 1 5 of the timer counter cGr,esponds to the time elarsed since the beginning of the current breath cycle, which marked the end of the previous inl ,alation phase. The inhalation phase about to begin should end on or after the current timer count value plus the inhaldlion interval time. Thus, step 1240 provides a new value for inhalation interval time for use in step 1216. Normally, this value is reached before the end of the actual inhalation and is used to ensure that a t,ansi6i,l slope reading does not erroneously mark the end of the inhalation phase. Thus the requirement in step 121 6 for both the expiration of the inhalalion end time and a slope less than or equal to -5.
As those skilled in the art will appreciale, step 1238, in cooperalion with the balance of the operaling pr~yldllll ensures that the inhalalion set point pressure increases before the onset of patient inhaldlion. First, by ,nonilGri"g whether the valve positiGn slope exceeds seven, the end of eAl,dldtion can be detected.
Marking the end of an exhalation phase ensures that this is a point in the breath cycle prior to the beginning of the next inhalation phase. Additionally, an increase in the pressure prior to inhalation is assured by ",onitoring whether the timer counter is greater than or equal to the predicted inhalation start time in step 1238.
Thus, if a sporadic or er,oneous slope reading were determined, an i"cr~ase in nasal pressure would still be ensured prior to inhalation when the timer counterexcess the predicted inl-alalion start time, recalling that the inhaldtion start time was set in step 1232 sG",e/,~:,al shorter than the ex~e~ed start time.
Fig.1 3 illustrates VALVE STEP sub-routine 1 300 which operates to impose sequentially the required step patterns on stepper motor 44 by way of stepper motor circuit 1 000. Sub-routine 1 300 enters at step 1 302 by setting the variable 9~ ~o?, "final valve position" equal to the current valve position plus (or minus) the valve cor,ection required as determined in step 1206 (Fig. 2). Step 1302 also sets thevariable "valve position" equal to the current valve position.
The proy,a,., then moves to step 1304 which asks w:,etl,er the cor,ection direction is greater than zero, that is, in a posit;~,re direction to restrict vent end 32j or in the opposite direction. If the answer in step 1304 is yes, the progra", moves to step 1306 which asks whether the final position as determined in step 1302 exceeds step 160. That is to say, this step determines whether the requested or desired final valve position is beyond the maximum allowed position. If yes, theprogram moves to step 1308 which sets the final valve position equal to 160.
If the answer in step 1306 is no, or after step 1308, the program moves to step 1310 to set the variable "valve position" equal to "valve posilion plus one. In other words, the progfafi, incre",ents slepper motor 44 one step at a time until the final posilion is achieved.
The prog,~", then moves to step 1312 which asks v,ll,etl,er the new valve position is less than or equal to the final valve posilion as determined in step 1302.
If no, which indicates that the desired final valve position has been acl,i~ved, the program returns to main loop step 1210.
If the answer in step 1312 is yes, indicating that the final valve posilion has not yet been achieved the prO9f~1n moves to step 1314 which retrieves the step pattern for the next blower motor step from memory. The prog,am then activates the lines of bus 832 in order to send this step pattern to slepper motor circuit 1000 and thereby to stepper motor 34.
The prog,a", then loops back to step 1310 to continue executing step patterns one at a time in sequence until the final position is obtained.
If the rotational direction for cor,ection requires is negative as delermined in step 1304, the program moves to steps 131 ~1324 as illustrated to execute therequired number of stepping patler"s to shift the valve in the "negative' direction to reduce pressure by venting more air. Step 1316 asks vrhell,er the final posilion determined in step 1302 is less than zero indicating a valve position beyond theallowable limits of travel. If yes, the progral" sets the final position equal to zero in step 1318.
Step 1320 then clecre" ,ents the "valve position" variable and step 1322 asks whether the newly determined "valve position is greater than or equal to the final position desired. If yes, the step moves to prO9f;am 1324 and then loops back tostep 1322. If the answer is step 1322 is no, the proyla", returns to main loop step 1210.

WO 92/11054 PCI~/US91/04052 2097~02 Fig. 14 illustrates ADC interrupt sub-routine 1400 which has its interrupt executed every 1 4 micro-seconds for providing an analog-to-digital conversion for the pressure data receiv0d from pressure transducer circuit 700 and to store this data in memory. Subroutine 1 400 enters at step 1402 which retrieves the currentdata from the ADC reg;sler internal to micfocont, ~" e r 802. This data is then stored in the ADC buffer for use in step 1202 (Fig. 12) of the main loop. This data is stored at location "L" which is one of the eight buffer localions. The progral" then moves to step 1404 to incre",enl localion variable "L" so that the next set of ADC
data is placed in the next buffer localion. The pro~a,d", then moves to step 1406 which asks wl,etl,er "L" is equal to eight which is greater than the number of localions provided in the ADC buffer. H yes the progra", resets "L" at localion zero which is the first localion in the buffer. After step 1408 or if the answer in step 1406 is no, the pfog,an) moves to step 1410 which instructs the ADC to begin another data conversion. The program then returns from the interrupt to the mainloop.
Fig. 15 illustrates CHECK BLOWER SPEED subroutine 1500 which is entered from step 1 ~2 of main loop 1200 and enters at step 1 502 which reads the current blower speed as received at microcont-c"er terminal HS1.0 from the Hall effect transducer in blower motor 94. The program then moves to step 1 504 which retrieves the blower speed set point cor,esponding to the presc,i~lion i"i ,-'~tion pressure and compares the set point to the sensed lower speed. The progfan, then moves to step 1506 which asks w:,elher the blower speed is within a maximum error range of the set point speed. If no the prog,aln adjusts in step1508 the pUI3e Widlll of the pulse width modulated signal produced at micro-cor,l,ol'er terminal PWM and l,ansr"illed to blower motor circuit 900. After step 1 508 or if the answer in step 1506 is yes the progfa", returns to the main loop.

Airway Sounds Embodiment Figs. 16 and 17 illustrate anoli,er aspect of the invention in which patient airway pressure vari~lions and in particular airway sounds are ",onilored and the patient airway pressure conl,~llecl in response. In particular Fig.16 is an electrical block cliag~atn illuslldling sound analysis circuit 1600 which receive3 input from pressure sensor circuit 700 by way of terminal 718 thereof and which delivers OUtplJtC to microcGnl,c"er 802. As those skilled in the art will appreciale sounds are pressure va,idtions and as such, prefe"ed pressure sensor circuit 700 is also operable for sensing pressure vafialions represe"t~tive of airway sounds and in converting these varialio!-s into tepreser,ldli~/e signals at terminal 71 8.

WO 92/11054 ~ PCI/US91/04052 ~ ' -22-The signals from pressure sensor circuit 700 are delivered to preamplifier 1602 which boosts the signal level for delivery to low-pass filter 1604 band-pass filter 1606 band-pass filter 1608 and high pass filter 1610. Low-pass filter 1604 is included to provide output "DC" to miclocont,~"er 802 indicative of low frequency (subaudio) pressure \,arialions and nasal pressure.
Filters 1606-10 split the audio frequency spectrum into three cGI ~ ~ponenls 10-200 Hz. 200-800 Hz. and 800+ Hz. respectively. The outputs from filters 1606-10 pass through respective recti~;ers 1612 1614 and 1616 which in turn provide rectified outputs to low-pass filters 1618 1620 and 1622. Low-pass filters 1618-22 convert the respective rectified inputs to equivalent D.C. voltage outruts "LOW""MED", and "Hl" which represenl the respective audio spectldl co,npol-enls. These three outputs along with output "DC" are provided as inputs to fr,: Crt)CGI ,l,oller 802 which uses internal analog-to-digital conversion to produce digital data represen-tative of the three spectrum co,nponents.
Fig.17 is a computer program flowchart of SOUND ANALYSIS subroutine 1700 which is advanlsgeously included as part of the program for operaLin~ the ".~ocGI~l~cller 802 in connection with the pressure varialion aspect of the invention. Subroutine 1700 enters at step 1702 which initiates analog-to-digitalconversion of the analog inputs "DC" "LO~' "MED" "Hl" recei\fed from circuit 1600. In the preferled embodiment step 1702 is im,cle."ented a number of times (for example ten times) for each inh-'~tion and the conversion values averaged.
The average values of the digital represer,lalions of DC LOW MED and Hl are then used for steps 1706-1716 as discussed further hercinbelo~J.
The program then moves to step 1704 which sets the software variable "old state" (OS) equal to the variable "new state" (NS) determined in the previous passes through the ployldm. This step then sets variable NS equal to zero.
In step 1706 the progfar" asks whether input "DC" is greater than a predeter"lined li,resho'd value. This II,reshc'd value is set at a level sufficient to indicate that detectable airway sounds are occurring. If the answer is no the prog,t.", returns to the main loop. If yes the proyldlll moves to 1708 in which along with subsequent steps conducts a spectral analysis of the airway sounds as determined by circuit 1600. In particular step 1708 asks wheti,er input LOW
is of a predetermined li"esho'.~'. If yes the program moves to step 1710 which incre",enls variable NS by 1.
If the answer in 1710 is no or after step 1710 the prog,d", moves to step 1712 which asks vJhetller input MED is above its associaled ll,reshold. If yes the prog,d", moves to step 1714 which incfefnenl~ variable NS by 2.

2Q9~5Q2 lf the answer in step 1712 is no, or after step 1714, the program moves to step 1716 which asks v:h~tl ,er input Hl is greater than its predelermined ll " eshaI r~, If yes, then step 1718 incre",enls variable NS by 4.
If the answer in step 1716 is no, or after step 1718, the prog,dm moves to step 1720. Step 1720 calculates the variable "ansition" (T) as a function of variables OS and NS as shown in Fig. 17. Variable T provides a spectral quantificalion of the airway sounds for use in determining which action, if any,should be taken concerning the increase or decrease of the gas pressure applied to the respiratory passA9es of the patient. This determination occurs in step 1722 by use of a so-called "action table" which is a look-up table stored in memory using variable T as a pointer. The pref~,r,ed action table is incGr,uoraled as part of the ~lisclosure hereof as Appendix I attached hereto.
Upon determining the proper action including increase, decrease, or maintain pressure from the action table, the proyfalll moves to step 1724 which executes that action. In the prefer-ed embodiment, action~esig.,aled changes in pressure are in incle",ents of 1.0 cm. water pressure.
If the action determined in step 1722 is "none", which indicates that snoring sounds are not occurring, it is prefer.ed in step 1724 that the patient-applied the pressure be decreased by 0.5 cm. water. In this way, the prog.a.)- assures that the pressure is not maintained at a level greater than that necessAry. For example, if the .letectecl airway sounds pro,),pls an i"clease in pressure, and the airway sounds then disarpeA~, it may be that the pressure was increased slightly more than necess~ry. Accordingly, the program will aulo,l,alically dec.ease the pressure over time in small i"cren,enls until airway sounds are again detected.
The aspect of the presenl invention desc.il,ed above in connection with Figs.16 and 17 ",onitor:j airway sounds in the preferred embodiment. It will be appreciated, however, that pressure transducer circuit 700 is sensitive to many types of pressure v~ritllions other than those a-csociz~led wHh airway sounds. For example, circuit 700 could be used to detect inaudible vil,rdlions or pressure varidlions A-qsocia~ed with exl,~ on and inhalation. With this capability, much in~o",)alion can be ga,-,ered about a patient's respiration such as whether the patient's r~ s ~ . alion is rhythmic, erratic, or apneic as well as breath rate, inhaldlion and exhalalion durations, and flow rates. Hence, withthis capability the patient's r~F.alion can be properly charact~ri~ed and aspects of the respiration quantified.
Furthermore, this in~or")alion can be stored in memory for s~ ~bsequent downloading for use by a physician, for example, in diagnosing respiratory af-tions and efficacy of l.~al"~enl. In this way the expense and time consumed WO 92/11054 PCI~/US91/04052 ~,~9~ ~ -24-in sleep lab facilities is avoided or at least minimized. Addilionally, patient cG"If~ l is enhanced bec~use only the minimum required pressure is imposed both during sleep and before the patient falls to sleep. With i"creased cGmfoll the patient is more likely to use the p,esc,il,ed l,~at",enl on a sustained basis and thereby gain the maximum benefit Iller~hG",.
As those skilled in the art will appfec;ale the presenl invention encG",p~sses manyvariations in the prefer,ed embodiments desc,il~ed herein.
For example while the presenl invention is useful in l,eali"g sleep apnea its utility is not so limited but rather the presenl invention is useful in treatingmany conditions in which facilitated respiration is a factor in l,edl"~enl. For example, increased respiratory air pressure beginning just prior to inhalation induces a deeper inl)- otion than might otherwise occur. This may be useful in l,ealing certain cardiovascular conditions where deeper inhalation and thereby greater oxygenalion of the blood is beneficial when accompanied by decleased pressure to ease exhalation. Additionally the presenl invention encompasses the use of any breall,able gas such as anesll,esia or oxygen-supplemented ambient air.
As ~isc-lssed above the nasal pillow is the pref~r,ed means for patient coupling in order to impose the higher bredll,able gas pressure on the res~.. alory passages of the patient. The presenl invention ho.~ever also encomp~ses a nasal mask or a full face mask which may be desired in certain situ~tions such as the a~Fl.~etion of anesthesia as breathable gas as disclJssed above.
In the prele"ed embodiment of the preseril invention the position of the vent valve assel"bly is varied in order to increase or decrease the pressureof the breall,able gas applied to the patient s respiratory pa.ss~ges. As the detailed desc,i~lion reveals ho.vever the apparalus hereof includes the capability of varying the speed of the blower unit which could be used instead to selectively vary the applied pressure. This would eliminate the need for the vent valve and stepper motor and reduce the manufacturing cost which would be advant?geous as another embodiment of the invention.
The preseril invention also encG",p~-cses the va,ialion wherein the bredll,able gas is compressed and stored in a storage bottle for example.
As described above the prefe"ed cont,cl er includes microcGnl,cl er 802 which is operated by a computer program. Other equivalent control wo 92/11054 2 ~ 3 7 5 0 2 PCI/US91/04052 means might include a custom designed chip with all functions implemented in hardv;are without a computer proyr~.".
As ~Jisclosed in Fig. 6 herein and the acco",panying narrative desc, iplion it is prefer,ed to track the patient s breathing cycle by tracking the movement of vent valve asser,lbly 16. Those skilled in the art will appreciate that the breath cycle can be tracked by other means such as monilori,)g chest contractions and eA~.ansion breathing sounds directly sens;ng genioglossus muscle activity or some equivalent pa,a",elar indicative of a breathing cycle.
As a final example some II,erapist~ may prefer that the apparalus start up in a low pressure or zero pressure mode while the breath cycle is inHially tracked. This may provide further patient colll'ull in the use of the invention.
Admittance Embodiment Figs. 18-21 illustrate another embodiment of the presenl invention in which patient airway patency is determined and pr_~rably used as the basis for controlling airway pressure applied to the patient. Turning initially to Fig.
18 apparatus 1800 includes flow sensor 1802 (Hans Rudolph Pneumotach available from the Hans Rudolph Company of Kansas City Missouri) differe"lial pressure (DP) sensor 1804 (SENSYM type SX01DN) pressure sensor 1806 (SENSYM type SX01 DN) operational amplifiers 1808 and 1810, analog signal divider 1812 (Analog Devices model AD539) operably coupled withmicrocolltra er802.
In operation flow sensor 1802 is preferably coupled in exterior portion 26 of conduit 12 so that bre~ll,able gas supplied to the patient passes there-through. Sensor 1802 provides a pair of pneumatic output signals represen-tative of the gas flow to DP sensor 1804 which in turn provides a pair of outputelect,ical analog signals from the internal bridge representdli~/e of the flow to amplifier 1810.
Pressure sensor 1806 is pneumatically coupled with exterior portion 26 prelerably doJIn~t~eam from flow sensor 1802. Pressure sensor 1806 provides a pair of signals from the internal bridge to amplifier 1808 rt:presenlali~e of the gas pressure being suFFI ed to the patient.
Amplifiers 1808 and 1810 provide respective analog output signals represel,l~live of the inslanlaneous gas pressure and flow being supFlied to the patient by way of lines 1814 and 1816 to divider 1812. Divider 1812 pello""s an analog cli;is.on of the flow and pressure signals prese"led on lines 1814 1816 and thereby produces an analog output signal on line 1818 representative of the insl~nlaneous admittance (A) of the patient s airway. That WO 92/11054 2 0 9 7 ~ ~ 2 ~ PCI'/USg1/04052 is to say a-h "itlance is the inverse of in ,pedance, but patient flow can be zero which prevents direct ~'cu'~tion of i",pedance as pressure divided by flow.
I loJ~cver by dividing flow by pressure, and thereby determining adl"illance such problems are avoided.
As explained further her~ bclow in connection with the computer proglah, Flowcha,l of Fig. 21 it may be desirable to eliminate divider 1812 in certain applications and perform the division fur,ctions within microco"l,.ller 802. If such is the case line 181 8 along with divider 1 812 are eliminated and the pressure and flow signals on lines 1814 1816 are provided directly to .nic:oco.lt.c"er 802.
Fig.19 includes graphs 1902 1 904 1 906 1 908, and 191 0 which aid in illuslldlil ,9 how patient airway patency is determined in the presenl invention.
These graphs respectively plot flow F, pressure P, admittance A, te""~lale T1 and te",,~lale T2 versus seven d;~crete times cor,esponding to those times when microcGnt,."er 802 pe,f~r.-,s an~'~g to-digital conversions of the input i"to""alion. The plot of ad~nitldnce in graph 1906 is a function of the flow and pressure data illustrated in graphs 1 902 1 904 respe~ /ely.
Intheoperalionofmicrocont,c"er802inaccorcJancewiththeprogral"s of Figs. 20 and 21 the admmance plot for the inhalation portion of a single breath cycle is col!,pared to adl"itlance templales stored in memory to determine which tel "plalt7 provides the "best fit" with the latest aJI ~ litla~ ~ce plot.
The best fit is determined by using converiliGnal root-mean-square techniques.
The te",plale which fits best is used as a ' ~ e . ,ter" for a look-up table to select action to be taken such as raising or lowering gas pressure delivered to the patient.
Fig. 20 illustrates a computer program flowchart of subroutine 2000 for ope,dling n,.~oconl~"er 802 of the embodiment shown in Fig. 18 using divider 1 812. Routine 2000 enters at step 2002 which activates microco, .l. le, "er 802 to digitize the a.h~illance signal received on line 1818 at the predeter-mined times during patient inhalalion and to store the converted admittance data in data array "A."
After all of the signals for inhalation signals have been digitized step 2004 then normalizes the amplitudes of the amplitude data in array "A." That is to say the peak-to-peak amplitude value of the array data is normalized to a predeler--lined constanl. This is done because in the prefer-ed em-bodiment the shape of the admitldnce data is of interest not the absolute values.

Similarly step 2006 normalizes the time base of the admittance data array "A" so that the time base matches that of the te",plales. This is needed because inhalation times vary from breathe fo breathe.
The prog,an, then moves to step 2008 which computes a root-mean-square (RMS) value for the diff~rences l~et rlocn the cor,esponding data points in array "A" and each te",plate stored in memory according to the formula shown.
The proy~a"~ then moves to step 2010 which determines which lel"pldle presents the lowest RMS value this being the ten,plale that "best fits"
the a.l~"itlance data for that inhalation of the patient. Step 2012then uses the telnplale se ected in step 2010 as a software ',~-c nler to select an ap-pro~l iale action such as increase decrease or maintain pressure from a look-up table such as that illustrated below:
T1 Maintain T2 Increase T3 Increase * *
TN Decrease Fig. 21 is a computer prog,a", flowchart of module 2100 for operating m.e -ocont,c"er 802 in the embodiment of Fig. 18 when divider 1812 and line 1818 are not used and when lines 1814 1816 are conne~1ed directly to microcol,l,c"er 802 for providing the pressure and flow signals. This variation is advanlageous when greater precision is desired because of the non-linear chafa~1erislics of patient airways.
Module 2100 enters at step 2102 which digitizes the flow signals received by ",.-~roconl,c er 802 by way of line 1816 at the predeter"lined interval times. The digitized flow data is then stored in array "F". Step 2104 then normalizes the time base of the data in array "F."
The program then moves to step 2106 which uses Fast Fourier Transform (FFT) to convert the amplitude vs time data in array "F" to amplitude vs frequency data.
Simultaneous with steps 2102-2106 module 2100 executes ane'~gons steps 2108 2110 and 2112forthe pressure in~r",ation ,ecEived by micloconl-roller 802 over line 1814.
After the flow and pressure data have been converted the ~crogrd", moves to step 2114 which computes ad~"-~lance "A" for each cor,esponding flow and pressure data points. In some circumsldnces depending upon the .

WO 92/11054 PCI'/USgl/04052 ~,o9~S~~ ' particular application and the level of accuracy desired, it may be advanta-geous to amplitude normalize the pressure and flow array data or the acJ",itlance data.
Module 2100 then executes 2116 2118 and 2120 which are the same as steps 200~2012 ~iscussed above in conne-1ion with module 2000 and the action table.
It will be appreciated that after determination of the best-fit ter"plale patient airway patency is effectively qua,ltified. That is to say the set of te,nplales stored in memory could represenl a range of palen~ es (in percenlages for example) and the best-fittemplale represenls a cor,esponding patency as a percentage. Additionally the patency te,nplales are preler~bly a set custom-developed for the particular patient being treated. Furthermore it may be advantageous to continuously update the set of te",plales by storing successive admittance array data in memory as a new te",plale. Additionally certain tel"pldles could be designaled as te,nplales characteristic of wakefulness or sleep states. Finally in some circ~""slances the highest level of accuracy is not required a s~""",dlion of the adl"illdnce data of a given inhalation or an average thereof over a number of inhalations could be used itself as a qua"tificalion of airway patency.
Those skilled in the art will appreciate that the present invention e"co,np~-eses many~,arialions in the pr~l~"t:d embodiments described herein.
For example ultrasound techniques could be used to establish airway patency. Additionally when the gas pressure applied to the patient is relativelyconslanl only flow varialions are of interest and are the only variable parameter which may be considered. As a further example a sensitive thermocouple or the""islor could be used as an in~ c~tion of gas flow.

Stimulation Embodiment As described above in connection with Figs. 16 and 17 the spectral sounds embodiment of the present invention analyze the patient airway sounds to determine an appropriate response for preventing an apneic episode. In the spectral sounds embodiment the appropriate action incf~ases decreases or maintains the airway pressure applied to the patient.
In the stimulation embodiment the pfef-_r,ed response is the zFrlic~tion of electrical stimulus externally applied to the neck of the patient in the vicinity of the upper airway muscles although i",planted ele~;t,odes would be used equivalently to stimulate the muscles or the muscle nerves.

2097~02 The preterred appafal-Js includes a flexible, elastic neck collar, a micro-phone carried by the collar, a pair of ele~l,odes also carried by the collar, and control circuitry i,lterconneuli,)g the mic,oplione and the electrodes. The electlodes and ".-~ropl-one could also be affixed by adhesive or other equivalent means instead of the prefer,ed collar. The prefer,ed control circuitry includes the coi"ponenls and prO9f~lll described above in conne~,1ion with the airway sounds embodiment. The primary difference being that instead of increasing air pressure, the action is to activate the stimulating ele~,odes at the beginning of each inhalalion phase of the patient breath cycle.
To use the stimulation embodiment, the patient couples the collar about the neck with the ele~t,odes positioned in front about either side of the neck centerline and just under"eall, the jaw in order to stimulate the upper airway muscles when activated. In operation, the ",:~.ophone detects airway sounds and the control circuitry analyzes these sounds as described above in connection with Figs. 16 and 17. Whenever an action is determined cor,esponding to "increase" pressure (Fig. 17, Step 1724), this is inler~reled as imminence of an apneic event. That is to say, gradual closing of the airway due to lel~J~al;on of the upper airway muscles produces sound patterns indicative thereof which also indicates, that is, predicts that an apneic episode may occur on a subsequent breath. Thus, when it is determined that an increase airway patency is needed, the control circuitry activates the ele_t,odes to stimulate the upper airway muscles. Additionally, it is preferled to vary the sl,enyLl, of the electrical stimulation according to the breath sounds in the same manner that airway pressure is varied in connection with the ad",illance embodiment discussed above in connection with Appendix 1. In the event that inhalation is not dele~1ecl for a predetermined time based upon afixed time or based upon previous breathing p~llt7rns the prefer,ed breathing device activates the electrodes to stimulate the upper airway muscles.
In this way, apneic episodes are prevented while at the same time electrode stimulation is not imposed when not needed. This is in cor,l,asl to the prior art in which stimulation is not provided until an apneic episode has already occurred. This is also in conlfasl to those prior art devices which stimulate on each inhalalion effort such as that set forth in U.S. Pat. No.
4,830,008, hereby incorporated by reference. As those skilled in the art can appreciate, if stimulation is applied with every inh~ ion, the patient effectively gets used to the stimulation and it is no longer as effective. The presenl invention, on the other hand, prevents stimulation when col)clitions are absent indicating, that is, predicting an apneic episode, but yet ensures stimulation WO 92/11054 PCr/US91/04052 2i~9~ ~g~ -30-before an apneic epi~ode. Thus the two main disadvanlages of the prior art stimu!ation techniques are avoided.
As those skilled in the art can apprecidla other means can be used to detect the imminence of an apneic epi;ode. For example by ",on;toring airway adl"illdnce as discussed above in connection with Figs. 19 - 21, an apneic episode can be predicted and stimulation applied when this occurs.
That is to say by ",onitoring ad~"itlance during inhalation a nar,u~,ing of the airway can be detected by monitoring the ad~"illdnce and when ad",itlance decreases to a predeter"~ined level stimulation can be applied. Furthermore the imminence of an apneic episocle could be determined by using airflow sensors such as ll,er",;~lor~ or ll,er",ocouples at the nose or mouth or a static-charge sensitive "bed" or bands for sensing chest or abdomen movement prelerdbly a RESPITF~ACE brand sensor.

C~.. ~cnsation Embodiment The preferled embodiment d;~closed in Figs. 1 - 4 uses pressure sensor 38 mounted adjacent the patient nasal fitting. In some circu",~lances however this may not be practical. Instead for compactness and economy of manufacture it may be desirable to use pressure and flow sensGrs coupled with the patient pneumatic line at the point where this line leaves cabinet 22.
This arrange",enl however may allow inaccuracies in measurement to occur because of downstream pneumatic leaks and pressure drops in the line which vary nonlinearly with flow to the patient. In addition to unir,lended leaks it is prelerable to have a vent at the patient's nasal connection to prevent buildup of carbon dioxide. Thus flow and pressure as measured at the cabinet outlet may not provide accurate data concerning the actual pressure delivered at the patient's nose. The co",pensalion embodiment of the presenl invention measures pressure and flow at the cabinet outlet but still provides accurate measu(e",en( of the pressure presented to the patient by cGI~pensaling for leaks and pressure drops.
Fig. 22 is a sche",alic block cliagfar" illual,dli,)g the pneumatic system 70 which includes some cG",ponents in co"""on with those previously described and are n~""bered the same. System 70 additionally includes inlet air filter 71, exl ,alalion solenoid 72 with exl ,aldlion valve 73 connected thereto bacteria filter 74 flow eleme"l 75 with flow sensor 76 connected in parallel thereto.
Fig. 23 is an elect,ical block diag~a", illusl,a~ing the preler,ed co",ponents of cGnl,eller 20 for controlling and operating pneumatic system WO 92/11054 2 0 9 7 ~ 0 2 PCr/US91/04052 70 of this embodiment. Conlr~"er 20 includes power supply 80 "..~roproces-sor 81 microprocessor memory 82, analog to digital (A~D) conversion circuitry 83 inlei ~ce circuitry 84 serial communication port 85 wHh remote control 86 connected thereto keyboard and display control 87 with keyboard display panel 88 connel1ed thereto.
Figs. 24 - 31B are computer progfar" f~wcha,l:, illu~tldling the operation of the progfa", stored in memory 82 for operating ,r,.~ -oprocessor 81 and thereby for operali,)g cont,oller 20 and pneumatic system 70. Fig. 24 illustrates PRIMARY module 2400 which shows the overall ar,dnge"~enl and operation of the prefe.,ed program. PRIMARY module 2400 enters at step 2402 at power up when power supply 80 is activated. The program then executes INITIALIZE module 2500 (Fig. 25).
Step 2402 then asks whether the control mode is set to exhale or inhale. If set to inhale step 2404 then asks w:,etl,er the control mode has been set. If no the program executes INHALE module 2700 (Fig. 27).
If the control mode has been set to exhale in step 2402, step 2406 then asks vll,etl,er the control mode has been set. If no the proy,a", executes EXHALE module 2600 (Fig. 26).
If the ar,swe ~ in steps 2404 or 2406 are yes or upon return from EXHALE and INHALE modules 2500 and 2600 the program moves to step 2408 which asks which backup mode has been selected. The program then executes the sel~-ctecl backup module illustrated in Figs. 28 - 31 B after whichthe program loops back to step 2402. As Fig. 24 indicates after initialization the program operates alternatively through the exhale and inhale branches to set the respective exhale and inhale pressures and then proceeds to the selected backup module to determine v,~ tl,er backup operation is needed.
Fig. 25 illustrates INITIALIZE module 2500 which enters at step 2502 to set the variables indicated to their initial values as shown. Step 2504 then sets the pressure control mode to inhale and step 2506 clears the control mode flag indicating that the control mode has not been set.
Steps 2508 and 251 0 then set the flow bias (Fbias) variables for inhale and exhale for the amounts cor,esponding to the vent or bleed hole presenl in the pre~e"ed nasal pillow shell used for connection with the patient airways.Step 2512 then reads the presc, ibed pressure setli"gs set on switch 81 4 (Fig.
8).
Next step 2514 sets a software flag indicating that the next analog to digital interrupt will read pressure transducer data (when not set the A/D

WO 92/11054 P~/US91/04052 ~9 -32-interrupt reads flow transducer data). An A/D conversion for pressure is then i"""ediately executed in step 2516.
Blower 18 is then started at a speed sufficient to produce the presc,iplion pressure setting. The progfd", then returns to step 2402 (Fig. 24).Fig. 26 illustrates EXHALE module 2600 which is enlered when the exhale branch of PRIMARY module 2400 detects that the exhale flag has been set. Module 2600 enters at step 2602 which sets the patient pressure at the exhale prescfiption pressure. Step 2604 then opens exl)aldlion valve 73 by activating exl,aldlion solena.d 72.
The phase control flags are then reset in step 2606 and the blanking inteNal counter cleared in step 2608. The program then returns to the PRIMARY module.
Fig. 27 illustrates INHALE module 2700 which is entered at the beginning of inhalation and which is repeatedly executed during patient inhalalion. Module 2700 enters at step 2702 which sets the total breath count to the sum of the exhale and inhale sample counts. As discussed further hereinbelow, each inhalation and exhalation is counted and this step takes the sum of these counts to determine a value which is used as the total breath count.
Step 2704 then asks whether a backup mode has been indicated as discussed further hereinbelow. H no, step 2706 calculates a value for average breath as illustrated. With this step, patient breathing rate is tracked. If theanswer in step 2704 is yes, the average breath rate is set as equal to the previous average breath in step 2708.
After steps 2706 or 2708, step 2710 calculates the average breath volume according to the formula shown. Step 2712 then determines the maximum exl ,aldlion duration and step 2714 determines a value representative of the pneumatic leaks occurring during inhalalion. Steps 2710 - 2714 use values for these calculations which are explained further hereinbelow.
Step 2716 then asks whether the current peak blower inlet valve (BIV) posilion is less than 100. If yes, step 2718 decfe",enls the blower speed. In other words, if the biower is supplying excessive air, the blower speed is decreased. If the answer in step 2716 is no or after step 2718, step 2720 asks whether the current peak BIV position is greater than 130. The difference between 130 in this step and 100 in step 2716 provides a dead zone so that the prog,d", does not continuously hunt for a stable value. If the answer in step 2720 is yes, step 2722 asks v~helher the current blower speed is below WO 92/11054 2 0 9 7 ~ 0 2 PCr/US91/04052 . .

the maximum speed. If yes, step 2724 incremenls the blower speed in order to supply more air.
After step 2724 or if the ans~:er~ in steps 2720 or 2722 are no, step 2726 then sets the pressure control set point to the exhale prescri~lion pressure and step 2728 then opens exhalation valve 73 by activating exhalation s~'encid 72. The phase control flags are then reset in step 2730 and the peak BIV position variable flag is cleared in step 2732. Next, step 2734 clears the blar,hi"g interval counter and the prog,a", returns to step 2408(Fig. 24).
Figs. 28 - 30 illustrate the three s~le~ 1e backup modes which are executed if i,lhalalion is not detened within a time limit based on breath rate.In the CPAP mode (Fig. 28), the pressure is increased to a conslanl value and maintained. In the BPM backup mode (Fig. 29), the patient pressure is increased to a high level and maintained until the earliest occurrence of 1 5 sensed eAh -'~flon or a time cGr,elaled with previous breath rates. The patient backup mode (Fig. 30) results in a high pressure being delivered to the patient for a fixed time not based on previous breath rates, or when e,~l,alalion is sensed, whichever occurs first.
Turning first to Fig. 28, CPAP BACKUP module 2800 enters at step 2802 which asks whether the backup test is true. More particular, this step asks whether the pressure control mode is set for exhale, the backup flag is clear, and the count on the exhale timer is greater than the average of the lastthree exhale periods plus five seconds. If all of these cor,.Jitions are true, then the answer in step 2802 is yes. The progfa", then moves to step 2804 which sets the pressure control mode to inhale and then in step 2806 sets the backup flag as true.
If any of the required conditions for step 2802 is not sali-~';ed, then the answer in step 2802 is no and the proy~dr~l moves to step 2808 which asks whether the backup flag is set. If yes, step 281 0 asks wl ,~ll,er the count on the backup timer is greater than the minimum allowable time which in this step is the average of the last three inhalalion periods (see steps 2706 and 2708).
If the answer in step 281 0 is yes, step 2812 clears the backup flag. After steps 2806 or 2812, or if the ans~.~ers in steps 2808 or 2810 are no, the program returns to step 2408 (Fig. 24).
BPM BACKUP module 2909 (Fig. 29) enters at step 2902 which asks whether the backup flag is clear. If yes, step 2904 asks whether the inhale timer count is greater than or equal to the maximum allowable inhalaliGn time which is 60 divided by the BPM dial setting and this quantity times the fixed WO 92/11054 ~, ~, PCr/US91/04052 ~o91 ~

- inhalalion to exl,alalion ratio (typically 1:1.5). If yes, step 2906 sets the pressure control mode to exhale and step 2908 sets the backup flag as true.
If the answer in step 2902 is no, step 291 0 asks v,~l ,eti ,er the count on the backup timer is greater than or equal to the minimal allowable time which is the same value as that determined in step 2904. If yes, step 291 2 clears thebackup flag.
If the answer in step 2904 is no, step 2914 asks whether the exhale timer count is greater than or equal to the "~aAi",al -"aW~~'Q exl,aldLion time which is 60 divided by the BPM setting quantity divided by the inhalation/exhal-ation ration. If yes, step 291 6 sets the pressure control mode to inhale as step 291 8 sets the backup flag as true. After steps 2908, 291 2 or 291 8, or if the ans~ler~ in steps 291 0 or 2914 are no, the progf~-n returns to step 2408 (Fig.
24).
Fig. 30 illustrates PATIENT BACKUP module 3000 which enters at step 3002. This step asks v:: ,eli)er the backup flag is clear and if yes, the prog, a", moves to step 3004 which asks whether the inhale timer count is greater than or equal to the time duration of the last inhalation. If yes, step 3006 sets thepressure control mode to exhale and step 3008 sets the backup flag as true.
If the answer in step 3002 is no, step 3010 asks whether the count on the backup timer is greater than or equal to the minimal allowable time which is the last inhalalion time as determined by the inhal~lion counter (see step 31 58). If yes, step 3012 clears the backup flag.
If the answer in step 3004 is no, step 3014 asks whether the exhale timer count is greater than or equal to the time duration of the last exhale. Ifyes, step 3016 sets the pressure control mode to inhale and step 301 8 then sets the backup flag as true. After steps 3008, 3012 or 301 8, or if the answersin step 3010 or 3014 are no, the program returns to step 2408 (Fig. 24).
Figs. 31 A - B illustrate A/D INTERRUPT module 3100 which is execlJted every 14 milliseconds. This module enters at step 3102 which asks whether the last conversion was executed for pressure or flow. If pressure, step 31 04 retrieves the A/D value for pressure sensor 38 which value was previously stored during the last conversion for pressure. Step 31 06 then initiates A/D
conversion for flow sensor 76 and the interrupt ends.
If the last conversion was for flow as determined in step 3102, step 31 08 then retrieves the previously stored flow sensor value. This value is thenlinearized according to look-up table values stored in memory which are empirically dcveloped for the particular patient pneumatic hose 26 being used.

2097~02 ln practice, units include slandard hose links so that the look-up table values do not need to be redcvelGped.
Step 3112 then determines the pressure drop in patient hose 26 on the basis of linear flow according to techniques well known to those skilled in the art. The pressure dcvialion from the presc,iption set point is then determined in step 3114. This deviation is the pressure error (Pe) determined by subtractil)y the pressure drop which is the pressure at the patient's nose (Pn) less the pressure drop in the hose at the flow rate (Pdrop) quantity subtracted from the presc,iplion set point.
Step 3116 then asks v::,etl,er the pressure error is greater than 2. If yes, step 3118 opens the blower inlet valve 46 one posilion. If the answer to step 3116 is no, step 3120 asks whether the pressure error is less than -2. If yes, step 3122 closes blower inlet valve 46 one position. The span between +2 in step 3116 and -2 in step 3120 provides a dead zone to prevent hunting for a stable position.
After steps 3118 or 3122, or if the answer in step 3120 is no, the program moves to step 3124 which increments the variable "volume sum" with the current flow value. In this way, the total volume delivered to the patient is determined by adding the sum of periodically stored inslanlaneous flows delivered to the patient. These values are determined at equal time intervals and in this way to total volume delivered equals the sum of the flow values.
Step 3126 then increments the backup timer counter by one. Next, step 3128 inc~e",er,l the sample counter and blanking interval counter each by one. Step 3130 then asks whether the backup flag is set. If yes, step 3132 increments the backup timer by one.
If the answer in step 3130 is no, step 3134 (Fig. 31 B) asks whether the blanking interval counter is greater than its predetermined -"~wable limit (preferably 1.4 seconds which is 100 counts of the interrupt return every 0.014 seconds). If the answer in step 3134 is yes, step 3136 then asks whether the pressure control mode is set to exhale. If yes, step 3138 then asks whether the current flow value is greater than or equal to the exhale flow bias (the predetermined amount of air lost through the vent) plus the amount of leakage occurring during inhaldlion.
If the answer in step 3138 is yes, step 3140 sets the Prx which is the control mode to inhale. Step 3142 then clears the blank interval counter, step 3144 saves the current sample count and step 3146 clears the sample counter.
If the answer in step 3136 is no, step 3148 asks whether the pressure control mode is set to inhale. If yes, step 3150 asks whether the current flow WO 92/11054 ?~, PCr/USgl/04052 ?.,o91 ~Q

is less than or equal to the vent flow bias during inhale plus the Isal~ge during exhale. If this condilion is true, step 3152 sets the Prx control mode flag for exhale. Step 3154 then clears the blank interval counter after which step 3155 clears the volume sum counter and step 3156 saves the current value for volume sum. Step 3156 then resets the variable volume sum to zero, step 3158 saves the current sample count, and step 3160 clears the sample counter.
If the answers are no in steps 3130, 3134 - 3138 or 3148 - 3150, or after steps 3146 or 3160, the proy,d"~ moves to step 3162 which i"ili~les an AID
conversion for the pressure transducer. A/D INTERRUPT module 3100 then ends.

WO 92/11054 PCI~/US91/04052 APPENDIX I
ACTION TABLE
Sounds State Tfansilion Matrix # To: (new) _ 1 _ _ _ - - -Hi 0 0 0 0 From: Med 0 0 1 1 0 0 (old) Low 0 1 0 1 0 1 0 State Desc, i~lion of Co"~" ,ent~
o - No Sound - Smooth Snoring ssnore]
2 - Other (talking) other ]
3 - Turbulent Snoring tsnore]
4 - Start of clearing an obstruction [sclob ]
- Partial obstruction parob ]
6 - Clearing an obstruction clob ]
7 - P~ucous Snoring rsnore]
Transition Action Co"""enls 0 Decrease No sounds 151 Increase Start of ssnore 2 None Start of other 3 Increase Start oftsnore 4 Increase Start of sclob 205 Increase Start of parob 6 Increase Start of clob 7 Increase Start of rsnore 8 None End of ssnore 259 Increase Ssnore continues None End of ssnore 11 I"crease Ssnore totsnore-airway narrowing?
12 Increase Airway nar,o~:;ny?

WO 92/11054 ~o9~ 50~ PC~r/US91/04052 13 Increase Airway narrowing?
14 I"crease Airway narrowing?
Increase Airway ~ ar,GJ.;ng?
16 None End of other 17 Increase Airway opening?
18 None Other 19 Increase Probabletsnore cont.
Increase Clob 21 Increase Clob 22 Increase Clob 23 Increase Rsnore 24 None End of tsnore Increase Tsnore to ssnore 26 Increase Airway na"o~;ng? Airflow decleas;ng?
27 Increase Tsnore 28 Increase Airway narrowing? Airflow incleasiny?
29 Increase Airway na"c /~;ny? Airflow decleasiny?
Increase Airway narrowing? Airflow decreasiny?
31 Increase Airway narrowing? Airflow increasing?
32 None 33 Increase Airway opening post obstruction 34 Increase Airway opening post obstruction Increase Airway opening post obstruction 36 None 37 Increase 38 Increase 39 Increase Airway opening post obstruction None 41 Increase 42 Increase 43 Increase Airway opening post obstruction WO 92/11054 PCI'/US91/04052 2097~02 44 Increase Increase Partially obstructed snore 46 Increase 47 Increase Airway opening post obstruction 48 None 49 I"crease Airway opening post obstruction Increase Airway opening post obstruction 51 I"clease Airway opening Aifilow decreasing 52 Increase Airflow incfeasi"g 53 Increase 54 Increase Increase Airway opening post obstruction 56 None 57 Increase Airway opening 58 Increase Airflow dec~eas;"g 59 Increase Airway opening Increase Airway narrowing? Airflow increasing?
61 Increase 62 Increase 63 Increase

Claims (24)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for determining the airway patency of a patient exhibiting a breath cycle having inhalation and exhalation phases, said apparatus comprising:
means for supplying breathable gas from a source thereof under a controllable pressure to at least a portion of the patient's airway;
means for sensing patient respiration flow and pressure and for producing signals representative of substantially simultaneous flow and pressure;
signal processing means for receiving said signals and responsive thereto for determining the patient's respiratory admittance from said substantially simultaneous flow and pressure; and means for controlling said pressure in accordance with said admittance, said signal processing means including means for determining said admittance as the dividend of flow divided by pressure.

2. The apparatus as set forth in claim 1, said breathable gas including ambient air.

3. The apparatus as set forth in claim 1, said sensing means including flow sensor means for sensing patient respiration flow and for producing signals representative thereof.

4. The apparatus as set forth in claim 3, said flow sensor means including a differential pressure flow sensor.

5. The apparatus as set forth in claim 1, said sensing means including pressure sensing means for sensing a pressure of breathable gas delivered to the patient.

6. The apparatus as set forth in claim 1, said signal processing means including:
means for storing at least one admittance template composed of a plurality of admittances in memory;
means for stoning a set of said patient admittances;
and means for comparing said admittance set with said at least one stored admittance template.

7. The apparatus as set forth in claim 6, said storing means including a plurality of stored templates, said controlling means including means for determining which of said templates presents the closest match with said admittance set, said closest match being representative of patient airway patency.

8. The apparatus as set forth in claim 7, said controlling means including means for controlling said pressure in accordance with said closest match.

9. The apparatus as set forth in claim 7, said templates presenting normalized amplitudes and time bases, said controlling means including means for normalizing the amplitude and time base of said respiratory admittance in accordance with said stored templates.

10. The apparatus as set forth in claim 7, said controlling means including memory means for storing pressure change data in association with each of said templates, said controlling means including means for controlling said breathable gas pressure in accordance with pressure change data associated with said closest match.

11. The apparatus as set forth in claim 1, said admittance being determined as a function of said flow and pressure during patient inhalation.

12. The apparatus as set forth in claim 1, said controlling means including a microprocessor.

13. The apparatus as set forth in claim 7, said controlling means including a microprocessor.

14. A method of determining the airway patency of a patient, the patient exhibiting breath cycle having inhalation and exhalation phases, said method comprising:
using sensing means for repeatedly sensing a plurality of substantially simultaneous patient respiration flows and pressures during a patient inhalation phase and for producing signals representative thereof;
using signal processing means for receiving said signals and responding thereto for determining a set of patient admittances from said flows and pressures and storing admittance data representative of said admittance set in a memory device, and using said signal processing means for determining said admittances as the dividend of flow divided by pressure;
comparing in said signal processing means said admittance data with predetermined admittance templates stored in said memory device; and determining in said signal processing means the closest match of said admittance templates with said admittance data, said closest match being representative of patient airway patency during said phase.

15. The method as set forth in claim 14, said step of determining admittance data including the step of using Fast Fourier Transforms for processing data representative of said flow and pressure.

16. The method as set forth in claim 14, further including the steps of:
supplying the patient with a breathable gas from a source thereof under a controllable pressure to at least a portion of the patient's airway; and controlling said gas pressure in accordance with said patient airway patency.

17. The method as set forth in claim 14, further including the steps of:
storing in said memory device a set of pressure change actions corresponding to said templates; and executing an action corresponding to said template presenting the closest match to said admittance data.

18. An apparatus for determining the airway patency of a patient exhibiting a breath cycle having inhalation and exhalation phases, said apparatus comprising:
means for supplying a breathable gas from a source thereof under a controllable pressure to at least a portion of the patient's airway, said pressure being substantially constant during the inhalation phase of a breath cycle;
means for sensing patient respiration flow and for producing flow signals representative of patient respiration flow;
signal processing means for receiving said flow signals and for determining patient airway patency values from said flow signals; and means for adjusting said pressure in accordance with said patient airway patency values.

19. A method for determining the airway patency of a patient, the patient exhibiting a breath cycle having inhalation and exhalation phases, said method comprising:
using sensing means for repeatedly sensing a plurality of patient respiration flows being under substantially constant pressure during a patient inhalation phase and for producing flow signals representative thereof;
using signal processing means for receiving said flow signals and responding thereto for determining a set of patient airway patency values from said flow signals and storing patient airway patency data representative of said set of patient airway patency values in a memory device, and using said signal processing means for determining said patient airway patency values from said flow signals;
comparing in said signal processing means said patient airway patency data with predetermined airway patency templates stored in said memory device; and determining in said signal processing means the closest match of said airway patency templates with said patient airway patency data, said closest match being representative of patient airway patency during said phase.

20. The method as set forth in claim 19, said step of determining patient airway patency data including the step of using Fast Fourier Transforms for processing data representative of said flow signals.

21. The method as set forth in claim 19, further including the steps of:
supplying the patient with a breathable gas from a source thereof under a controllable pressure to at least a portion of the patient's airway; and controlling said gas pressure in accordance with said patient airway patency.

22. The method as set forth in claim 19, further including the steps of:
storing in a memory device a set of pressure change actions corresponding to said templates; and executing an action corresponding to said template presenting the closest match to said patient airway patency data.

23. A method of determining the airway patency of a patient, the patient exhibiting a breath cycle having an inhalation phase of substantially constant airway pressure, the method comprising:
sensing a plurality of patient respiration flows during a patient inhalation phase;
generating flow signals representative of the patient respiration flows determining patient airway patency values from said flow signals;
comparing the patient airway patency values with predetermined patient airway patency templates; and determining the template having the best fit with the patient airway patency values, wherein the template having the best fit is representative of patient airway patency during the patient inhalation phase.

24. The method as set forth in claim 23, further comprising the steps of:
increasing the airway pressure when the template having the best fit represents patient airway obstruction; and decreasing the airway pressure when the template having the best fit does not represent patient airway obstruction.