CA1131524A

CA1131524A - Nested hollow fiber humidifier

Info

Publication number: CA1131524A
Application number: CA331,617A
Authority: CA
Inventors: Richard R. Jackson
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-07-12
Filing date: 1979-07-11
Publication date: 1982-09-14
Also published as: DE2964203D1; BR7904407A; US4381267A; EP0009543A1; EP0009543B1

Abstract

ABSTRACT
A medical humidifier is disclosed for a respiratory flow path connected directly to the air way of a patient, through which life-supporting air can be channeled, comprising a compact nest of discrete, elongated air-transmitting hollow relatively large bore, relatively thin wall fibers con-nected in parallel by inlet and outlet connections and having walls of sub-stance permeable to water vapor and impermeable under operating conditions to liquid water. A relatively negative-pressure water-filled chamber surrounds the effective length of the fibers. The fibers are adapted to divide dry inhalation air for a patient which enters said inlet connection into a series of air-flow filaments for humidification by water vapor that permeates the wall of the fibers from the surrounding relatively negative-pressure water. The fibers are provided in size and number sufficient to freely pass under normal air-flow conditions an aggregate air-flow volume correspond-ing to the life-supporting need of the patient, and to provide effective fiber surface area sized to pass water vapor sufficient to humidify air at normal flow rates substantially to saturation from said relatively negative-pressure water heated to about body temperature. The outlet connection is arranged to collect the thus humidified filaments of air and direct the unified flow into the patient, whereby the medical humidifier can provide full humidification of air directed into a patient in a practical, fail-safe, low flow resistance compact unit that can be disposable.

Description

113~5Z~ `

This invention relates to a medical humidifier for humidifying and delivering gases such as air or oxygen directly to a patient. It constitutes an improvement over my prior patents United States 3,616,796;
3,871,373; 3,912,795 and to later efforts of Dobritz, Unites States 4,010,748 and 4,086,305.
According to my prior patents, a new type of medical humidifier is provided which operates according to the diffusion principle in which water vapor from a water supply permeates a wall or membrane and enters a stream of breathing air while the water supply is maintained separate from the air stream by the wall. Such a diffusion humidifier offers a number of potential advantages over other methods of humidification, but no satis-factory form for its manufacture has been found. To be suitable for pro-duction and wide use I have recognized that numerous difficult criteria should be met. As the humidifier is to be connected directly to the airway of a patient and life-supporting air is to be channeled through it, i~
should have low air-flow resistance, preferably so low that the patient can breathe through the humidifier without assistance (e.g. with a flow resistance comparable to a standard endotracheal tube). It should be capable of fail-safe operation to aviod any substantial risk that the patient might inhale large volumes of gross (i.e. liquid) water. It should be of small size to avoid introducing a large compressible air volume in the path and to enable placement close to the mouth, for example as a tubular component of a breathing circuit. It should have a long shelf life. And it should be of simple and rugged construction and of such low cost as to be disposable.
Hollow fibers with walls of water-permeable substance have been used for many years in filters and the like, see e.g. ~nited States patents 3,228,877, 3,339,341 and 3,342,729.
In the context of a heat exchanger, United Statespatent 4,098,852 d~

has suggested to condition or humidify air by use of fibers. But the structure employed fails to meet many of the essential criteria.
According to the invention there is provided a humidifier for a respiratory flow path, especially a medical humidifier, comprising a water filled chamber including a bundle of discrete elongated air transmitting hollow fibres forming a compact nest and connected in parallel by inlet and outlet connections and having walls of a substance permeable to water vapor, whereby the water filled chamber surrounds the effective length of the fibres, characterized in that thP fibres incorporate a wetting agent, the fibres have a bore greater than ~.27 mm and means are provided to expose the exterior of the fibres to water under relatively negative pressure.
Preferably, a pressure relief valve is connected to the water side of the unit, constructed to vent to the atmosphere in the event of any condition tending to produce a positive pressure in the water that surrounds the fibers; the substance of the fibers has dry stability and a transmission

-2-'i 113~524 characterlstic of the order of 4.8 kilograms of ~ater per meter squared of fiber wall area per 0.127 mm wall thickness, per hour, when subjected to test with water at 37.5C, at 103 torr negative pressure disposal on the exterior of fibers, and with a flow of anhydrous oxygen at a rate of 200 liters per minute introduced to flow in parallel through a nest of fibers of about 127 mm length;
the aggregate air humidifying capacity of the fibers is of the order of 50 liters per minute flow rate or higher; the fibers have an aggregate air-flow capacity between about 180 and 450 liters per minute with an air pressure drop of 5 cm water from end to end of the hollow fibers; the fibers define an aggregate effective water vapor transmitting surface area of the order of about 465 square cms; the hollow fibers have wall thickness that is 10% or less of the internal diameter of the fibers; the fibers have an internal diameter greater than 1.27 mm and less than 1.78 mm; the substance of the fibers is selected from the group consisting of pol~sulfones and acrylic co-polymers; the substance of the fibers incorporates a wetting agent, preferably glycerine or sodium sulfo succinate; and water inlet and outlet conduits that provide a flow of heated water over the exterior of the fibers are rigid to resist collapse when vacuum is applied, the inlet conduit is a suction line adapted for connection to a body of heated water and the outlet conduit is adapted to be connected to the inlet of a discharge pump, whereby due to water being sucked through the humidifer by the pump the water surrounding the ai~-transmitting fibers applies a negative pressure to the exterior of the fibers.
In preferred embodiments the air-flow resistance value R is in the range between 52 and 134, (corresponding to the resistance respectively of 9 and 7 mm standard endotracheal tubes), the presently preferred standard being R = 79 ~the resistance of an 8 mm standard endotracheal tube). For fibers of constant diameter, R = L x ~, where L is the air-transmitting length

- 3 -A ' 6f~ QS ~ t~ t~QS
in inchos, D is the internal diameter of the fibers in inchcs and N is the number of fibers.
In the accompanying drawings:-Figure l is a longitudinal cross-sectional view of a preferred embodiment of the invention;
Figure 2 is a transverse cross-sectional view taken on line 2-2 (omitting the fibers) and showing a closed mushroom valve in cross-section, while Figure 2a is a view of the mushroom valve when open;
Figure 3 is a partial transverse cross-sectional view taken on line 3-3 of Figure 1, on a magnified scale;
Figure 4 is a longitudinal cross-sectional view on magnified scale taken on line 4-4 of Figure 3;
Figure 5 is a perspective view showing the embodiment in use with a patient; and Figure 6 is a diagrammatic view of the flow arrangement for air to be humidified and the negative pressure water.
Referring to Figures 1-4 the medical humidifier 50 comprises a bundle 10 of straight, elongated fibers, made of a substance ~glycerinized polysulfone~ which is permeable to water vapor and impermeable to water.
The fiber bundle is loosely arranged in its major mid-portion 12 to provide water flow passages 13 (Figures 3 and 4) over the exterior of the fibers, while the end portions 14 and 16 of the bundle are potted (bonded together) in water-impermeable bonding material 1~ ~epoxy). Similarly the sides of the potted end portions 14, 16 are joined to end portions of chamber 20 defined by a rigid cylindrical wall (high density polystyrene). Between these ends the housing defines a water chamber capable of withstanding nega-tive pressure. End caps 22 and 24 are provided at the respective ends of the il315;~4 housing, terminating in tapered external members 28. These serve as standard male breathing circuit connectors for insertion into mating female connectors or hose.
The humidifierunitis connected inany suitable respiratory flow path. In figure S the humidifier is shown in the inhalation leg from the respirator to a patient, with a respirator controlled exhalation valve 31 at the end of a Y connector exhausting to ambient.
As shown in Figure 1, the end caps 22 and 24 each provide open spaces 30 and 32 beyond the ends of the fiber bundle, to define inlet and outlet air plenums. Air from the respirator 36 in figure 5 enters the male connector 28 at figure 1 at the right hand side, fills inlet air plenum 30 and is there distributed across the end face of the fiber bundle, where~it splits, to enter the numerous hollow fibers as filaments of air flow. These proceed through the fibers under the pressure of the respirator, exiting into outlet air plenum 32 on the left hand side of figure 1 where they rejoin.
The unified flow proceeds through the connector to the patient.
The water chamber 20 has water inlet and outlet connectors 21, 23 (of different configuration to avoid mix-up~ at the right and left hand sides of figure 1 respectively. These connectors and the conduits, 21a, 23a are also of sufficiently rigid material to withstand negative pressure. Inlet conduit 21a extends to a water reservoir 40 (Figure 6) where the end is sub-merged in water. A heater element 42 maintains the water at the desired temperature under the control of probe 41 andthermostatic controlswitch 43.
The water outlet conduit 23a is connected to the inlet of discharge pump P
which discharges excess water into the reservoir 40. Water is drawn by the pump through the inlet 21a and through the space between the loosely nested fibers~ thus filling the entire free volume of chamber 20 with water under , 0,~
negative pressure, e.g. - 2 p~. The water is drawn through discharge con-113~52~

duit 23a under the pull of discharge pump P.
A positive pressure relief valve 60 (Figure 2) is incorporated in thewall of the housing. As shown in Figures 2 and 2a, this valve is of the so-called, well-known mushroom type. It comprises a leaflet 62 overlying relief holes 64 which extend from the interior of the chamber 20 to the atmosphere. A
stem 66 is integrally joined to leaflet 62 and ends in enlarged inward end 58 which is larger than the passage through which the stem 62 extends and therefore holds the leaflet in place. Under negative water pressure conditions as shown in Figure 2 the leaflet seals the passages 64. However, referring to the magnified view of Figure 2a, in the event that positive pressure should accidently be applied to the water volume of chamber 20, this positive pressure acts through the passage 64 to deflect the leaflet 62 to allow escape of liquid until the positive pressure is relieved.
The fibers have internal diameter of l.S2 mm and wall thickness 0.127 mm They are glycerinized to make them hydrophyllic by being immersed in glycerine during spinning of the fibers, in a stage while the polysulfone is still soft tas produced by Amicon Corporation).
The fibers are 7.6 cm in length with 9.5 to 12.7 mm of each end sealed in the potting material, resulting in an effective vapor-transmitting length 2Q between the ends in the range of 5.1 and 5.7 cm.
In this embodiment 178 of the fibers are nested together and provide an aggregate vapor transmitting area in the range of 432 to 542 cm2, depending upon the length of the potted end regions, and a capacity to humidify ambient air to saturation at 37.5C at air-flow rates between about 85 and 95 liters/
min.
The flow resistance of this nested fiber module is represented by the figure of merit value R = L x ~ = 79 where L is the air transmitting 11315;~

(overall) length of the fibers in centimeters, D is the internal diameter of the fibers in centimeters and N is the number of fibers. Using this module an air flow of 300 liters/min. is achievable ~ith an air pressure drop of 5 cm. H20 from end to end of the hollow fibers.
This provides an air-flow resistance that is equivalent to the resistance of a standard 8 millimeter endotracheal tube. 8y observing this standard, the humidifier can be placed in any of the variety of respirator circuits and flow paths and indeed the patient can spontaneously inhale through it with no assistance of a respirator.
Suitable air flow characteristics in a practical, compact module are preferably achieved with fibers ranging in internal diameter upwards from 1.27 mm to about 1.78 mm, the length of the fibers being dependent upon their diameter and upon the number N of the fibers to be employed. The number N is also dependent upon the specific vapor transmitting characteristic of the fiber material and wall thickness selected. In some instances long modules may be convenient and in that case even larger bore fibers may be employed.
The glycerinized fibers have the advantage of high vapor rate transmission and longdr~ shelf life and when used with the positive pressure relief valve, offer a fail-safe operation. Where even further safety is desired it is possible to use fibers which are more immune to transmission of liquid water even in the event of accidental application of positive pressure to the water chamber. In this case preferably the fibers are produced to the hardened stage as hydrophobic polysulfone fibers and are subsequently treated with a wetting agent such as sodium sulfo succinate (stool softener marketed by Mead Johnson) and/or with glycerine. In this case, because of the added protection offered by the fi~ers it is in principle possible to omit the positive water pressure relief valve, but the valve is still preferred to _j ~ - 7 -11315Z~

maximize the safety of the device, particularly in view of the relatively larger number of fibers which are used when using material of the somewhat less vapor transmitting capability. The same is true if the fibers are formed of acrylic co-polymers such as the XM formulation manufactured by Amicon Corporation.
The water chamber 20 can advantageously be formed by injection molding of any suitable rigid plastic used in medical appliances, for instance the high impact polystyrene, mentioned above. The end caps, including the connectors, and defining the air plenums, can be of the 10 ~ same or similar material and can for instance be solvent-bonded to the exterior of the housing 20 in the manner shown. In a typical construction the assembly of fibers, having a length longer than the housing, is inserted loosely in the housing, and the potting material, epoxy or others such as silicone rubber is introduced in the end regions, through capillary action along the fibers, sometimes assisted by centrifugal force. After the potting material is set, the extreme ends of the matrix of potting material and hollow fibers can be sliced, as with a knife, flush with the ends of the housing 20, to provide a smooth end face with the fiber ends open to transmit air. The resulting bundle of fibers can readily provide a diffusion wall a Y6~' ~q~ c,~i /p~ep~
20 A surface area of the order of ~ ee~, effective to humidify an aggregate air flow volume of the order of 50 to 75 liters per minute at the instant of peak flow during the respirator cycle.
Referring to the figures the gross stream of inlet air A, Figure 1, from the respirator 36 (Figure 6) is divided into air stream filaments Bl, B2, etc. in Figures 3 and 4 by the many fibers of the nest. The heated water, as indicated diagrammatically in Figures 3 and 4, flows over the exterior of these fibers while water vapor produced by this water supply permeates the thin walls of the fibers, in opposition to the pressure differential, 1~315Z~

and humidifies the dry air filaments. The air stream filaments Bl, B2, etc.
after transmitting the length of the fibers 12 are humidified to saturation at body temperature. The air filaments are then restored to a unified air flow C in the discharge plenum 32, which proceeds into the patient.
In a typical operation the respirator 36 of Figure 5 operates as an open cycle system in which exhaled air is discharged to the atmosphere through exhalation valve 31. During the inspiration phase of the cycle the respirator gradually increases the pressure on conduit 54, supplying air through the humidifier 50 to the end of endotracheal tube 52 inserted into lQ the airway of the patient. The value 31 in the discharge leg 56 of the Y
fitting is closed during this phase by a control line from the respirator, hence all air-flow from the respirator is channeled through the humidifier 50 and into the patient.
D~ring the expiration phase the patient exhales spontaneously.
At this time valve 31 is released by the control line from the respirator to rel~vethe exhaled air to the atmosphere. A check valve, not `shown, dur-ing the expiration phase prevents back-flow of exhaled air through the humidi-fier.
The hollow fibers being subjected to a decreasing pressure gradient from inside to outside take advan~age of the substantial tensile strength of the wall of the fibers to ensure that they do not collapse. With the water under vacuum, little liquid water will enter the airway in the event of accidental rupture of a fiber wall. Even if a reverse pressure differential is encountered as by accidental misconnection of the pump, it is important to realize that the hollow fibers, due to their small size, demonstrate a sufficient degree of structural rigidity to resist crushing that would block the air-flow to the patient.
By use of the particular fibers described, a sufficient vapor-transmitting surface area to volume ratio is obtained while still obtaining 1131SZ~

sufficient air-transmitting capacity to enable a small number of fibers, in the preferred range of 100 to 200, to be employed. The feature permits a large diffusion surface to be obtained in a small geometric volume. Such small size permits the humidifier to be mounted close to the patient, and permits it to be an inexpensive disposable component, to be replaced period-ically, for instance once a day, at the same time that the hoses are ordinarily changed The construction leads to the possibility of extending the time between sterilizations of the inlet hose, from the respirator to the humidifier, owing to the fact that it now does not contain moist warm air and therefore is not a place where bacteria multiply rapidly.
Certain features of the invention are useful without other features of the invention. For instance a humidifier using static water at atmospheric pressure as by use of a collapsible outer wall can in certain instances be used to good effect with the specified fiber construction.

Claims

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

1. A humidifier for a respiratory flow path, especially a medical humidifier, comprising a water filled chamber including a bundle of discrete elongated air transmitting hollow fibres forming a compact nest and connected in parallel by inlet and outlet connections and having walls of a substance permeable to water vapor, whereby the water filled chamber surrounds the effective length of the fibres, characterized in that the fibres incorporate a wetting agent, the fibres have a bore greater than 1.27 mm and means are provided to expose the exterior of the fibres to water under relatively negative pressure.

2. The humidifier of claim 1 characterized in that said hollow fibres have a wall thickness that is 10% or less of the bore of said fibres.

3. The humidifier of claim 1 characterized in that said hollow fibres have a bore less than 1.78 mm.

4. The humidifier of claim 3, wherein the substance of said fibres has a transmission characteristic of 4.8 kg water per m2 per hour of fibres wall area per 0.127 mm wall thickness, with water at 37.5°C and 103 Torr negative pressure on the exterior of said fibres and with a flow of anhydrous oxygen at an aggregate rate of 200 litres per minute flowing in parallel through a bundle of fibres of 127 mm length.

5. The humidifier of claim 1 characterized in that the substance of said fibres is selected from the group consisting of polysulfones and acrylic copolymers.

6. The humidifier of claim 1 or 5 characterized in that said wetting agent is glycerine.

7. The humidifier of claim 1 characterized in that said wetting agent is dioctyl sodium sulfo succinate.

8. The humidifier of claim 1 characterized in that said bundle of fibres has an aggregate air flow figure of merit resistance value R between 52 and 134 where L and D are length and internal diameter of fibres in cm, and N is the number of fibres.

9. The humidifier of claim 1 or 8 characterized in that the number of the fibres is in the range of 100 to 200.

10. The humidifier of claim 1, 2 or 3 characterized in that the effective length of the fibres is 12.7 cm.

11. The humidifier of claim 1 characterized in that said water chamber has a positive pressure relief valve which opens from the interior of the water chamber to the atmosphere.

12. The humidifier of claim 1, 2 or 3 characterized in that the walls of said fibres are impermeable to liquid water under operating conditions in which said walls are 0.127 mm or less in thickness, the pressure of the air within the fibres is essentially atmospheric pressure and the pressure of the water on the exterior of the fibres is 103 torr negative pressure.

13. The humidifier of claim 1 characterized in that the nest of fibres has a characteristic gas pressure drop of 5 centimeters of water across the length of the fibres when gas flows through the nest of fibres at the rate between 180 and 450 litres per minute.

14. The humidifier of claim 2 characterized in that the nest of fibres has a characteristic gas pressure drop of 5 centimeters of water across the length of the fibres when gas flows through the nest of fibres at the rate between 180 and 450 litres per minute.

15. The humidifier of claim 3 characterized in that the nest of fibres has a characteristic gas pressure drop of 5 centimeters of water across the length of the fibres when gas flows through the nest of fibres at the rate between 180 and 450 litres per minute.

16. The humidifier of claim 13, 14 or 15 constructed for the average adult, characterized in having an actual characteristic flow rate of 50 litres per minute or higher.

17. The humidifier of claim 1, 2 or 3 constructed for the average adult, ~ 13 characterized in that the fibres define an aggregate water-vapor transmitting surface of 0.045 m2.