US2661734A - Arrangement for recording variations in the electrical resistance of the human body - Google Patents

Description

Dec. 8, 1953 v -W,'HOL'ZER ETAL I 2,661,734

ARRANGEMENT RECORDING VARIATIONS IN'QTHE ELECTRICAL RESISTANCE OF THE'HUMAN zany 'fFile 'd Jan. 17; 1948 =15 C PMJi5/IWER' ACZINE Vomaz' 7 To Osczuoamm INVENTORS. Wall-"GANG HOLZER BY 2? Apozr MARKO AGENT Patented flee. 8, 1953 2,661,734 me cament 170a arcoenm VARIA- Tr'o s m me [ELECTRICAL RESISTANCE OF THE HUMAN BODY Wolfgang Helmet and Adolf Marko, Vienna, Austria, assignors to Hartford National Bank and Trust Company, Hartford, Comm, as trustee A plication January 17, 1948, Serial No. 2,933 In Austria September 6, 1945 Section 1, Public Law 690-, August 8, 1946 Patent expires September- 6, 1965 7 Claims. (Cl. 128-421) The present invention relates to rheocardiographic apparatus adapted to measure the varying resistance of a living body as a function of heart beat. A

Due to heart activity, the electrical resistance of a human or animal organism exhibits a periodic variation. This resistance variation may be recorded by connecting the body resistance in series with a constant resistance to a voltage source and connecting the ends of the constant resistance to the input of an oscillograph. The oscillogram recorded in this manner provides those skilled in the 'art with data which is useful in medical diagnosis and research.

It is conventional to employ a high-frequency source for this technique of investigation. Where the high-frequency source is energized from a rectifying supply connected to an alternating-current power line, it has been found that the amplitude of the high-frequency voltage tends to fluctuate with changes in line voltage. Inasmuch as the body resistance variation to be recorded is very small and only in the order of 0.3%, it is important that the high-frequency voltage have an amplitude which is free from fluctuation. Consequently, it has heretofore been the practice to use a battery supply for the high-frequency oscillator, since an amplitudestabilized oscillator energized by an-alternatingcurrent rectifier would entail expensive components and a complex circuit.

The principal object of the present invention is to provide an improved rheocardiog'raphic apparatus.

More particularly, an object of the invention is to reduce the influence of amplitude fluctuation in the high-frequency rheocardiograph oscillator on the body resistance measuring voltage fed to the oscillograph.

Still another objectof the invention is to permit energizat on of an ordinary high fr'equenc'y oscillator for the rheocar'diogra'ph from a power line by 'means of a conventional rectifier and filter voltage supply.

r In accordance with the invention a first highfrequency current is derived from a highfrequency oscillator which is conducted through a living body, whereby the flrstcurrent is modulated as a function of body resistance. Also taken from the high-frequency o'scillator is a 2 second high-frequency current which is independent of body resistance, the second current being fed into a fi'xed resistance through which also flows the first high-frequency current modulated by the body resistance the second current being applied in a phase relative to the first current at which unwanted amplitude fluctuations tend to cancel out.

For a better understanding of the invention, reference is had to the following detailed description to be read in conjunction with the annexed drawing which is a schematic illustration of a rheocardiograph circuit in accordance with the invention.

The carrier current source for the rheocardiograph is constituted by a conventional highfrequency oscillator including an electron discharge tube l, a condenser 2 connected in parallel with a resistor 3, and a coil 4 connected at one end to grid of tube l and through condenser 2 to cathode thereof. vA direct voltage supply is provided whose positive terminal is connected through the parallel combination of a condenser 5 and a coil '6 to anode of tube the negative terminal being connected to cathode. Thedirect voltage may be obtained from any conventional rectifying and filter circuit energized from an alternating-current power line.

The grid coil 4 and the anode coil '6 are inductively coupled to one another in regenerative relationship by way of a center-tapped output coil 1, whereby a high-frequency wave is sustained in the oscillator.

The high-frequency carrier voltage induced in coil 1 is divided into two components in phase opposition. One componentyielded in the upper half of coil 1 is applied to the body electrodes connected to terminals Ill and H via a coupling capacitor '34 and an output resistor '23,, whereas the other component is applied across output resistor 23 'v'iaa coupling capacitor 35 and a comparison resistor 9. 'For the time being the resistance 36 may be considered to be shorted out of circuit by closing the switch '3"! shunted thereacross. The resistance of resistor '9 is "preferably made equal to the mean value of the body resistance between electrodes l0 and 'H.

Thus a bridge circuit is formed in which a first branch is constituted by the upper half of coil! connected through condenser 34 in "series with the body resistance appearing between terminals l and H, and a second branch constituted by the lower half of coil I connected through condenser 35 in series with comparison resistor 9, the two branches being connected in parallel relation across output resistor 23.

Since the high-frequency voltages applied to the two branches by thetwo halves of coil 1 are, despite any fluctuations therein, of equal amplitude but of opposing phase, should the body resistance at any instant be equal to the fixed value of comparison resistor .9, the resultant branch voltages applied across output resistor 23 would cancel each other and a null voltage would be produced thereacross. Therefore, as the value of the body resistance changesv as. a function of the heart activity, a similarly modulated measuring voltage is developed across resistor 23.

Th measuring voltage developed across resistor 23 is applied to a diode rectifier l3 via .a

condenser i2, a leak resistor l-i being connected across condenser 12 to ensure that variations similar to those in the body resistance occur in the voltage of the condenser. The detected p0 tential across condenser 12 comprises a directcurrenteomponent having superimposed there on a varying component whose variation corresponds to the modulation of the high-frequency carrierivoltage developedv across resistor 23 by reason of the changes "occurring in the body resistance. v

This'varying component develops an alterhating-current drop across resistor it which is connected by, means of multi-contact selector switch 28 in series witlrfone of condensers 25, 23 or 27 across'resistor I5. The voltage developed across resistor l4, however, exhibits a highfrequency component by reason of the highfrequency carrier in the measuring voltage. The voltage set up across resistor 15 is filtered with respect to said high-frequency component by a condenser ll connected serially with resistor i8across resistor iii. The influence of the heartbeat on the electrical resistance of the body is represented by the alternating voltage developed across condenser I! in which the highirequency carrier component is substantially by-passed. This voltage is applied to the input terminals of an oscillograph [9 in which it is impressed on the grid of an amplifier tube 28. The oscillograph which is not otherwise disclosed may beof conventional design, and operates to record the measuring voltage variations subsequent ,to amplification. I g

Since the characteristic curve expressing :the functional relationship between the anode volt:- age and the current of diode i3 is not straight but approximately parabolic, the relationship existing, between the input voltage of;the oscillograph taken from the output of the idiode and the input voltage applied to the diode .is not linear. In order to ensure a linear relationship between the input voltage of the oscillograpli and the varying body resistancefresistor 9 may be given a value differing from the mean resistance of thebody. In this .case the measuring voltage no longer fluctuates about the zero value but has a constant component so that the working point of the diode is shiftedfrom the curved portion of its parabolic characteristic to the substantially straight line portion thereof.

This expedient has an additionaladvantage in that it is not necessary-at each test to bring the value of comparison resistors into correspondence with thebodyresistance, it being pos- 4 sible to operate without changing the value of the comparison resistor. For this purpose comparison resistor 9 is preferably given a value which is smaller than the smallest body resistance that may be expected. Favorable results are obtainable by the use of a comparison resistor whose value is about smaller than the mean resistance of, the body.

The use of the above-described bridge circuit also tends to neutralize the influence of socalled shot effect in the oscillator tube.

Condensers 25, 26 and 21 have different capacities and are selectively inserted into the filtering circuit byymeans of selector switch 28. This adjustable filtering circuit makes it possible to. pass a desired alternating voltage componentand. to reject those of diiferent frequency. 1 This is useful for the purpose of neutralizing, for example, the influence of respiration on the record to be produced, for the volume Y the body-which variations, however, exhibit a frequency, different from that arising from the heart beat.

In order to minimize the ripple eiiect on the record arising from" the alternating-current (usuallygdfi cycles) power; "line serving" to energize the power supply for the high-frequency oscillator, provision is made of a voltage divider 29 connected to the terminals 30 and 35 connected to the A.-C. power line. The movable tap of this divider is connected to the'filter'ing circuit (l8 and 25, 26"o'r2'U via a resistor 32 and a phases hiiter 33." By correctly positio'riin'g the tap, an alternating voltage is'applied to'th'e filtering circuit which compensates for the alternating'current ripple introduced into the system via the high-frequency oscillator. The phase shifter 33, which may be of conventionaidesign, acts to displace the phaseof the alternating voltage derived directly from the line toieffeet cancellation'with respect to the alternating voltage ripple derived from the high-frequency oscillator. J k The condensers 34 and 35 act to block the voltage'generated inith'e body itself by the heart beat (the voltage .used for making the ordinary electrocardiogram)away from the body resistance recording. The capacities of these condensers. must be sufficiently lowto cause the strength of the alternating current generated in the body "to be negligible as'compared to the. intensity of the high-frequency current supplied byv the high-frequency oscillator, but sufficiently high topass'the latter current to a suflicient degree. To facilitate .this discrimination, the operating frequencyiof the high-frequency osci'llator. must substantially exceed the frequency of the heart beat.

It may be desirable, in some cases, to ascertain the phase relationship existing between the rheocardiogram representing the change in body resistance as a function of heart beat and the electrocardiogram representing the change in body generated voltage; as a function ofheart beat. This may be effected by short'circuiting;

the blocking- condensers 34 and 35,. or by substituting-therefor relatively high capacity val ues, whereby the voltage from g the high-fre-' quency oscillator is effectively combined with the body generated voltage and impressed; across body electrodes H] and to produce a composite record. Inthis' case, the so-called R-peak of the electrocardiogram alsomanifests itself in the rheocardiograrn,

No measures need be taken to keep the operating frequency of the oscillator constant. This frequency may vary within wide limits. It will preferably not be made lower than 5 kc. in order that the excitation of the nervous and muscular system may remain below the limit of observation. In addition, the frequency should not be excessively low since otherwise the pass resistance at the electrodes and the resistance of the skin increase to an extent such that the resistance variation becomes excessively low relative to the overall resistance.

On the other hand, itis not desirable to cause the frequency to exceed a given value since otherwise particular precautions must be taken to overcome difficulties incidental to the use of very high frequencies. For this reason it is preferable that the operating frequency lie below 50 kc.

Resistance 36, shunted by switch 37 serves for calibration purposes. Upon opening of switch 31, the resistance of the circuit is increased. This produces a jump in the diagram to be recorded and by measuring the height of the jump calibration may be performed.

It is obvious that many changes and modifications may be made in the above-described embodiment without departing from the spirit and scope of the invention.

What we claim is:

1. In apparatus for indicating the variations in the electrical resistance of a living body due to heart action, the combination which oomprises a high-frequency voltage source, a resistance element, a comparison resistor, means to impress a first voltage derived 'from said source through said body across said element thereby to develop a high-frequency voltage across said element modulated in accordance with the variations in the resistance of said body, and means to impress a second voltage derived from said source and of an amplitude equal to said first voltage through said comparison resistor across said element in phase opposition to said first voltage whereby the resultant modulated voltage established across said element is substantially independent of fluctuations in said source.

2. In apparatus for indicating the variations in the electrical resistance of a living body due to heart action, the combination which comprises a high-frequency voltage source and including a center-tapped output impedance, a comparison resistance, an output resistance, and a bridge circuit including one branch constituted by one half of said output impedance in series with the resistance of said body and another and parallel branch constituted by the other half of said output impedance in series with said comparison resistance, said output resistance being shunted across the parallel branches of said bridge whereby the high-frequency voltage developed thereacross has a modulation component depending on the variations in the resistance of said body and substantially independent of fluctuation in said source.

3. An arrangement, as set forth in claim 2, wherein said comparison resistance has a value which is lower than the lowest value of said body resistance.

4. Apparatus for recording the variations in the electrical resistance of a living body due to heart action comprising a high-frequency voltage source and including a center-tapped output impedance, a comparison resistance, an output resistance, a bridge circuit including one branch constituted by one half of said output impedance in series with the resistance of said body and another and parallel branch constituted by the other half of said output impedance in series with said comparison resistance, said output resistance being shunted across the parallel branches of said bridge whereby the high-frequency voltage developed thereacross has a modulation component depending on the variations in the resistance of said body, an oscillograph, means to detect and filter the voltage developed across said outputresistance to produce a potential whose magnitude depends on said modulation component, and means to apply said potential to said oscillograph.

5. An arrangement, as set forth in claim 4,

wherein said detection and filter means includes 1 a diode rectifier connected in series with a first resistor across said output resistance, whereby developed across said first resistor in a rectified voltage having a direct-current component having superimposed thereon a modulation component, and a condenser connected in series with a second resistor across said first resistor, whereby developed across said second resistor is solely said modulation component.

6. Apparatus for recording the variations in the electrical resistance of a living body due to heart action comprising an oscillator producing a highfrequency voltage and including a center-tapped output coil, a comparison resistance, an output resistance, a bridge circuit including one branch constituted by one half of said output coil in series with the resistance of said body and another and parallel branch constituted by the other half of said output coil in series with said comparison resistance, said output resistance being shunted across the parallel branches of said bridge whereby the high-frequency voltage developed thereacross has a modulation component depending on the variations in the resistance of said body, an oscillograph, means to detect and filter the voltage developed across said output resistance to produce a potential whose magnitude depends on said modulation component, and means to apply said potential as an input to said oscillograph.

7..An arrangement, as set forth in claim 6, wherein said oscillator is energized from a lowfrequency alternating current supply, and further including means to derive a low-frequency voltage from said supply and to apply it to the input of said filter means in such phase 'as to cancel the low-frequency component appearing therein.

WOLFGANG HO-LZER. ADOLF MARKO.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,816,465 Boas et al July 28, 1931 2,298,506 Parker Oct. 13, 1942 2,352,011 Rosa et al June 20, 1944 2,409,749 Foulger et al. Oct. 22, 1946 2,419,682 Guillemin, Jr. Apr. 29, 1947 FOREIGN PATENTS Number Country Date 441,057 Great Britain Jan. 13, 1936 449,686 Great Britain July 1, 1936 472,951 Great Britain Oct. 4, 1937

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