US3511237A - Plethysmograph - Google Patents

Description

May 12, 197% Filed Dec. 10, 1965 E. JAEGER 3,511,237

PLETHYSMOGRAPH 5 Sheets-Sheet 1 Fig. 1

In ventor:

Ema 721E656 May 12, 1970 E. JAEGER 3, 1, 3

PLETHYSMOGRAPH Filed Dec. 10, 1965 5 Sheets-Sheet 2 /n ven tor fe/ca 72/151452 May 12, 1970 E. JAEGER "3,511,237

PLETHYS MOGRAPH Filed Dec. I 10, 1965 5 Sheets-Sheet 5 In van for .EE/cz/ $95452 May 12, 1970 E. JAEGER 3,

PLETHYSMOGRAPH Filed Dec. 10, 1965 5 Sheets-Sheet 4 In ventor: fK/(W Z3565? May 12, 1970 I E. JAEGER 3,511,237

PLETHYSMOGRAPH Filed Dec. 10, 1965 5 Sheets-Sheet 5 HIIIIII III/III 164 /n v en! 0/": ERIC $95 5 United States Patent 3,511,237 PLETHYSMOGRAPH Erich Jaeger, 5 Rontgenring, Wurzburg, Germany Filed Dec. 10, 1965, Ser. No. 512,894 Claims priority, application Germany, Dec. 11, 1964, J 27,102; Apr. 24, 1965, J 27,991; Oct. 25, 1965,

Int. Cl. A61b 5/08 U.S. Cl. 1282.08 14 Claims ABSTRACT OF THE DISCLOSURE A plethysmograph is understood to mean a medical device for examining the functioning of the lungs, particularly in order to determine the flow resistance of the inner respiratory passages.

Plethysmographic examinations are based on a measurement of the variations in the volume of air displaced by a person to be examined, which occur when the person to be examined subjects the air present in the lungs or in the respiratory passages to excess pressure during breathing out or to reduced pressure during breathing in.

A plethysmograph consists essentially of a cabin, similar to a telephone box, which can be closed in a air-tight manner and in which the person to be examined is placed. The cabin is provided with a measuring instrument, for example a sensitive pressure gauge, which enables the above-mentioned fluctuations in volume to be detected. During the examination, the person to be examined generally breathes into a mouthpiece or a mask to which a so-called breathing tube is connected which contains a flow measuring instrument and or a quick-acting closure which allows closure pressure curves (alveolar pressure curves) to be recorded.

The variations in volume to be detected are small in comparison with the effective internal volume of the cabin and the risk of disturbances as a result of unwanted effects is correspondingly great. The disturbances have essentially two main causes: temperature influences and fluctuations in the ambient pressure. For example, the fact that the air in the cabin is heated by the person to be examined and possibly by electrical equipment giving off heat, which involves a corresponding rise in pressure, is very disturbing. In practice, nobody can be asked to await the adjustment of a temperature equilibrium. Accordingly, an attempt has hitherto been made to carry out the measurements as quickly as possible and the pressure rise occurring during the measurement has been ignored or taken into consideration by appropriate correction of the measured values. The accuracy of the measurement suflers as a result, however.

It is further known that the heating of the inhaled air in the lungs and the cooling of the air which is exhaled again in the cabin causes serious measuring errors. In themselves, these errors could be eliminated if the air in the compartment was maintained substantially at body temperature but this is found very inconvenient. Hitherto, therefore, the person to be examined has been allowed to breathe for a short time in a rubber bag filled "ice with hot water. This presupposes the intelligent and posltive co-operation of the person examined, however, nei ther of which can always be obtained. In addition, the thermal capacity of the water filling and the amount of oxygen in the rubber bag are very limited so that fast working and frequent renewal of the air and water filling are necessary.

With the known plethysmographs, moreover, disturbances very easily occur as a result of fluctuations in the ambient pressure which were caused, for example, by the opening and closing of doors, rapid movement of persons in the vicinity of the equipment etc.

It is the object of the invention to provide a plethysmograph in which the above-mentioned disadvantages are largely avoided.

A plethysmograph for medical lung examinations with a cabin which can be closed in a substantially air-tight manner for the reception of a person to be examined, a measuring device which responds to the variations in the volume of air displaced by the person to be examined, and a breathing tube in which the person to be examined works during the examination, is characterised according to the invention in that connected to the breathing tube is a sealed chamber which is provided inside the cabin and which is shut off at least partially from the rest of the cabin by an easily flexible wall and which contains an electrical heating device as well as an air circulating device. The electrical heating device is preferably controlled by a thermostat in such a manner that the air in the sealed chamber is maintained at such a temperature that substantially no variations in temperature occur during the inhaling and exhaling of air from the sealed chamber or into the sealed chamber. The temperature is preferably adjustable because the temperature of the exhaled air may fluctuate not inconsiderably from person to person.

The sealed chamber is preferably formed by a bottom plate which is connected to the breathing tube and at least one thin-walled plastic bag which is secured tightly round the bottom plate and is supported by clip-like brackets which are secured to the bottom plate. Two plastic bags arranged one inside the other are preferably used, which are each supported by clip-like brackets and are held spaced apart from one another. By this means, heat exchange between the interior of the sealed chamber and the rest of the cabin is largely reduced as a result of which the measurements are not inconsiderably improved.

According to a further important feature of the invention, the measurements of the variations in volume of the breathing patient present inside the cabin are not related to the free ambient air, that is to say the room air, but to a buffer volume which is largely shut off from the ambient air.

The measurement of the fluctuations in volume which are of interest can be effected by means of a sensitive pressure-measuring device, for example a differential pressure gauge, a flow measuring device or a volume measuring device (for example a spirometer bell).

Another important feature of the invention consists in providing, in the cabin wall, an equalising aperture through which pressure differences between the interior of the cabin and the ambient air can become equalised. The cross-section of this equalising aperture is preferably adjustable and the time constant of the equalising process is selected greater than would correspond to the lowest variation in the useful signal still to be detected.

According to a further feature of the invention, air is pumped out' of the cabin in order to compensate for the heating of the air contained in the interior of the cabin. The delivery of the pump should decrease in accordance 3 with a specific function. The delivery may, for example, be proportional to the speed at which the temperature varies in the interior of the cabin.

Examples of preferred embodiments of the invention are explained in more detail below with reference to the drawing, further features of the invention also being discussed.

FIG. 1 shows a side view of a plethysmograph, the cabin of which is shown in section along a line 11 (FIG. 2);

FIG. 2 is a plan view of the cabin in a section plane 2-2 in FIG. 1 and a circuit arrangement for determining the basic metabolism of the person to be examined and for eliminating disturbing thermal influences respectively;

FIG. 3 is a side view, partially in section partially illustrated diagrammatically, of a breathing head such as may beused in the plethysmograph illustrated in FIGS. 1 and 2;

FIG. 4 shows a plan view of the breathing head illus trated in FIG. 3;

FIG. 5 is a diagram to explain the mode of operation of the circuit illustrated in FIG. 2;

FIG. 6 is a view corresponding to FIG. 2 of a somewhat modified form of construction and FIG. 7 is a simplified illustration of the breathing head for the arrangement illustrated in FIG. 6.

The same reference numerals are used for corresponding parts in all the drawings.

The plethysmographs illustrated as examples of embodiments of the invention comprise a cabin 10 to receive a person to be examined, and a measuring and control panel 12.

The cabin 10 has a tightly closing door 14 with a window 16, and its interior is lighted by means of a further window 18 and a light source 20 which is mounted outside the cabin and which is preferably a fluorescent lamp generating little heat. In addition, in the ceiling of the cabin there is also a relatively large aperture 22 with a closure 24, which can be opened from the inside in an emergency. A lamp 25 which is controlled by a floor or seat contact indicates whether the cabin is occupied.

In the interior of the cabin there is a breathing head 28 which is illustrated in more detail in FIG. 3 and which is mounted for adjustment in height on a vertical rod 30 and for swivelling.

During the examination, the patient sits on a kind of chair 32, the seat area and back rest of which are perforated. The under side of the seat area and the back of the back rest are in communication with the interior of the cabin 10. By this means, disturbances are avoided which might otherwise arise as a result of air occlusions between the body of the patient and the chair.

In order to keep the internal volume of the cabin 10 as low as possible, the back wall 34 of the cabin is in the form of a semi-cylinder. In addition, a box 36 which is sealed off from the interior of the cabin, is provided below the chair 32. The interior 38 of this box may be used as a buffer volume as will be explained in more detail below.

The variations in pressure in the cabin 10 can be meas ured in various ways:

(1) By means of a measuring device 40 which is interposed between the interior of the cabin and the ambient air.

(2) By means of a measuring device which is interposed between the interior of the cabin and a buffer volume which is largely or entirely shut off from the ambient air. The interior 38 of the box 36 may, for example, serve as a buffer volume. In this case, a measuring device 42 is interposed between the interior of the cabin and the interior 38 of the box 36. The buffer volume may be outside the cabin and is then connected to the connection 44 of the measuring device 40 remote from the cabin. The external buffer volume may have one Wall in com mon with the cabin 19 or be completely separate therefrom, see FIG. 6. The walls of the buffer container may be provided with reinforcements 37 (FIG. 1).

The measuring devices 40, 42 may be sensitive differ ential pressure gauges or sensitive, flow-responsive flow measuring devices (for example with thermistors) with a following integrating device, or volume measuring devices (for example spirometers).

The interior of the cabin is also connected to the atmosphere through an equalising aperture, the cross-section of which is preferably adjustable, and which is illustrated in FIG. 2 in the form of a valve 46. A rotatable disc may simply be used for this, having apertures of different diameters which can be introduced selectively into a passage connecting the interior of the cabin to the atmosphere, and then determine its narrowest cross-section.

A similar equalising aperture which represents an adjustable flow resistance may also be provided as required between the buifer container and the atmosphere and/or between the buffer container and the interior of the cabin.

The product of volumetric elasticity and volume of the cabin 10 is preferably equal to the product of volumetric elasticity and volume of the buffer container so that pressure fluctuations in the ambient air have no influence on the pressure difference prevailing between the interior of the buifer container and the interior of the cabin.

When the box 36 under the chair 32 is used as a buffer container, the chair 32 is preferably supported against the cabin wall or the cabin floor as shown in FIG. I, but not on the box 36 so that disturbances as a result of deformation of the box 36 on movements of the person to be examined are avoided.

The breathing head 28, illustrated in more detail in FIGS. 3 and 4, comprises a breathing tube 48 with a mouth-piece 50 (or a breathing mask not illustrated), into which the patient breathes. The breathing tube 48 contains a magnetically actuated valve 52 which serves, in known manner, to record closure pressure curves, a correspondingly constructed magnetically actuated valve 54 which is only illustrated diagrammatically and which, in the open condition, allows the person to be examined to breathe air out of the interior of the cabin 10; a flow measuring device 56 which preferably contains a flow resistance 58 in the form of a metal netting which can be heated and a differential pressure gauge 60 which is connected to tube sockets 62 at each side of the flow resistance 58, and a tube socket 64 which is provided in front of the valve 52 and leads to a pressure measuring device 66 which measures the closing pressure (alveolar pressure P with the valve 52 closed. The closing of the valve 52 is preferably controlled by the differential pressure gauge 60 and preferably closes at the end of the breathing out because the closure pressure curve then recorded by means of the pressure meausring device 66 and the measuring device 12 of the plethysmograph enables the functional residual capacity to be determined. For this purpose, the difl'erential pressure gauge 60 is connected to a control device 68 which, as indicated diagrammatically in the rectangle designated by 68, contains a relay with two fixed contacts x, y and a movable contact blade 2, the normal position of which is in the middle. On variations in the differential pressure AP' as shown in the diagram illustrated in FIG. 3, the contact blade z bears alternately against the contact x, y, in accordance with the diagram illustrated below, and also assumes its normal position for a short period of time during which it does not make contact with either of the contacts. Thus the control device 68 has a threshold behaviour. On the changeover from exhalation to inhalation, the control device 68 closes the magnetically actuated valve 52 for a suificient time to allow a closure pressure curve to be recorded which is indicated or recorded in the measuring and control panel 12 together with the pressure variations occurring simultaneously in the cabin 10.

According to the invention, the upper end of the breathing tube 48 leads into a chamber 70 which is sealed 01? from the interior of the cabin and which is bounded by a bottom plate 72. Cylindrical flanges 74, 76, each of which is provided with an annular groove 78, 80, project upwards and downwards respectively from the bottom plate 72. Provided on the upper flange 76, are wire brackets 82 which, at the top, carry a length of tube 84 which can be tightly closed by means of a removable stopper 86. Further wire brackets 88, which are secured at the top to a ring 90, are also provided on the bottom plate 72. The wire brackets 82, 88 serve to support flexible tubes 92 and 94 respectively of thin plastic foil, the upper and lower ends of which are provided with rubber bands 96 which seal off the ends of the flexible tubes in the grooves 78 and 80 respectively or in corresponding grooves at the outside of the length of tube 84. The chamber 70 is therefore sealed oil in an airtight manner from the interior of the cabin in which it is provided; because of the flexibility of the thin plastic foil, of which the flexible tubes 92, 94 consist, no disturbing pressure difference can occur, however, between the chamber 70 and the interior of the cabin 10.

In the chamber 70 there is an air-circulating and temperature-regulating device 98. Through this device 98, the air in the chamber 70 is maintained at a temperature, the value of which is, in practice, just below body temperature and is selected in such a manner that the air exhaled into the chamber 70 by the person being examined is not cooled nor is the air inhaled from the chamber 70 heated. Such cooling or heating would greatly disturb the measurements. The gap between the flexible tubes of plastic foil 92, 94 assured by the wire brackets 82, 88 affords an excellent heat insulation so that no disturbances can occur through surrender of heat from the chamber 70 to the interior of the cabin. Such heat insulation has proved extremely important in practice.

The device 98 contains a tubular housing 100 in which an electric blower 102 is mounted at the top. The blower preferably delivers from the top downwards, the motor is preferably mounted at the top and the fan blades at the bottom. Below the blower is a thermostatically controlled heating device, which comprises heat-exchange plates 104, for example of copper and a heat source which is in satisfactory heat contact therewith. At least one power transistor 106, which is in satisfactory heat-conducting communication with the plates 104, is preferably used as a heat source. The transistor 6 forms a part of a known temperature regulating device which also contains a heat senser 108 which is mounted close beside the transistor and which prevents overheating of the transistor, and a heat senser 110 (FIG. 4) which is mounted at a somewhat greater distance from the transistor and which responds to the temperature of the air flowing past the plates 104. The whole control circuit is preferably mounted on the plates 104 and filled in with a silicone resin as protection against moisture.

The breathing tube 48 may likewise be provided with a heating arrangement 112 which is only illustrated diagrammatically and which may be a thermostatically controlled electric resistance heating device.

A heating blower 114 in the manner of a hair drier may be used for the pre-heating of the breathing head 28 and for the removal of spent air, in which case the stopper 86 is then removed and the device is flushed out with fresh air which may be warm, with the valve 52 open.

When, in the course of an examination, a person is placed in the cabin 10 and the door 14 of the cabin is closed, the temperature and pressure in the cabin begin to rise. This is extremely disturbing for the measurements. A certain remedy is afforded by the equalising aperture 46 (FIG. 2), but a compromise is indicated here: the time constant of the equalisation should not be too short, on

the one hand, because otherwise the fluctuations in pressure to be measured are also falsified, on the other hand, the time constant should not be too great because otherwise the temperature-responsive rise in pressure is not sufliciently attenuated. In order to avoid these difliculties, according to the invention, a pump 116 (FIG. 2), for example a blower is provided which pumps air out of the interior of the cabin 10 at a decreasing rate controlled by a specific program. The relationships are exp ained with reference to the diagram illustrated in FIG. 5 in which the time t is entered in the abscissae and the temperature T of the air contained in the cabin or the pressure P which is substantially proportional to this is entered in the ordinates,

When the cabin containing a person and equipment which has been switched on, is closed, pressure P and temperature T rise in accordance with the curve 118 which consists essentially of the sum of two e-functions 120, 122. The curve 120 corresponds to the surrender of heat by the person to be examined, and the curve 122 to the surrender of heat by the instruments switched on. According to the invention, the pump 116 is controlled in such a manner that its delivery rate decreases substantially according to a curve 124 in such a manner that the interior pressure of the cabin remains largely constant. The curve 124 is likewise the sum of two e-functions 126, 128, which are the reciprocals of the e-functions 120, 122.

Such a pump control can be obtained relatively easily. The drive of the pump 116 is regulated by a regulator into which the desired value is fed in the form of a voltage E. Let it be assumed that a high voltage E corresponds to a low delivery of the pump and vice versa. The voltage E is produced by charging a capacitor 132 through two separate charging circuits. Each charging circuit contains an adjustable series resistor 134 or 136 and an adjustable voltage source 138, 140. The voltage sources may, for example, be potentiometers as illustrated which are connected to a DC voltage U The one charging circuit with the resistor 134 and the potentiometer 13'8 serves to compensate for the heating which is caused by equipment switched on and can therefore be adjusted once and for all when the equipment is produced. The other charging circuit 136, 140 has to be set differently according to the heat output of the person to be examined.

The adjustment of the charging circuit 136, 140 may be effected on the basis of the following measurement: the cabin is closed and the rise in pressure in the cabin is measured, for example by means of the measuring device 42. The charging circuit 136, 140 is interrupted, for example by means of a switch 142, so that only the heating caused by equipment is compensated for. The rise in pressure AP illustrated in the diagram on the left in FIG. 2 must then be attributed exclusively to the heat given off by the person being examined. The pressure signal from the measuring device 42, which rises in accordance with the curve 120 (FIG. 5), is converted by a stage 144, which produces the inverse function, into a signal U which rises linearly and the rising rate of which then represents a measure of the heat output of the person to be examined. The rate of rise may be recorded by a stage 146 or indicated digitally in known manner, this indication representing in the first place a measure of the basic metabolism of the person being examined and at the same time supplying a basis for the adjustment of the charging circuit 136, 140. Instead of compensating for the rise in pressure caused by the equipment in the manner referred to by the pump 116, the pump may be allowed to rest during the measurement of basic metabolism and a voltage U which rises linearly may be derived from the voltage U in a stage 148. The voltage U is then modified by a voltage U representative of the power output of the equipment. In some circumstances, the power output of the equipment can be ignored completely and the arrangement illustrated in FIG. 2 can then be simplified accordingly.

It is particularly appropriate to control the delivery of the pump in proportion to the rate of rise in the temperature of the air in the cabin 10. Such a control is independent of the fluctuations in pressure in the cabin representing the useful signal. A signal representing the rate of rise in temperature dT/dt is produced, in this modification, by a temperature senser 150 and controls the regulator 130 which regulates the speed or delivery of the pump 116. The signal corresponding to the rate of rise in temperature can also be used to determine the basic metabolism in accordance with the above remarks and supplied to the stage 144 for this purpose. No particular correction is necessary for the control of the pump, and the circuit arrangement 132 to 142 can therefore be omitted. The air in the cabin is preferably circulated by means of a blower 152 only illustrated diagrammatically.

FIG. 6 shows a view in section corresponding to FIG. 2 of a modified embodiment of the invention. In FIG. 6, the cabin is double-walled in construction and the gap 160 is filled with a heat-exchange medium, for example water, which is circulated by means of a thermostat device 162. Built on to one side wall of the cabin is a buffer container 38' which is likewise double-walled in construction and which may be incorporated in the thermostat circulation. A heat-exchanger and/or air-circulating device 1'64 is further provided in the interior of the cabin. In addition, the cabin is provided with an intercornmunication system 166 which renders possible easy understanding between the person inside the cabin and the doctor in charge. The door lock is provided with an electric door opener 168 which can be actuated from the interior of the cabin by means of a push button 170 so that the cabin can be opened quickly and easily from the inside.

The inner wall 172 of the cabin preferably has a low thermal capacity so that a state of equilibrium becomes established rapidly.

FIG. 7 shows, in greatly simplified form, a breathing head 28 which is built up in a similar manner to the breathing head 28 (FIG. 3), but also has a connection between the breathing tube 48 and the outside of the cabin 10. This connection is established through two flexible pipes 174 each of which contains a one-way or non-return valve 176 and is connected, through a shutolf valve 178, to the breathing tube 148. Through the arrangement 174 to 178 described, the person to be examined can breathe fresh air from the outside between the actual measurements. The valve 178 is opened in this case, while the valves 52, 54 are closed. The person to be examined then breathes fresh air in through one of the two flexible pipes 174, the one-way valve 176 which opens in the inhaling direction, the open shut-off valve 178, the breathing tube 48 and the mouthpiece 50, while the spent air is ejected during exhalation through the breathing tube 48, the valve 178, that one of the valves 176 which is transmissive in the exhalation direction, and the other flexible pipe 174.

The measuring and control station 12 preferably contains a programme-control device 180, not illustrated in detail, which allows the following operations to take place either individually or in succession as a series of examinations:

(1) Calibration of the device. For this purpose, a master test pump, not illustrated but known per se, is set in operation and produces a periodic, known variation in volume with which the device can be calibrated in known manner.

(2) Determination of the basic metabolism and possible switching on of the compensation pump 116 as explained with reference to FIG. 2. The person to be examined preferably breathes air through the valve 52 out of the sealed chamber, or air out of the interior of the cabin through the open valve 54.

(3) For the recording of a pneumotachogram with the devices 56, 60, the person to be examined breathes air out 8 of the bag 92 through the open valve 52. The valves 54, 178 are closed.

(4) If desired, a pause may now be introduced during which the person to be examined breathes air from the interior of the cabin through the open valve 54 or fresh outside air through the open valve 178. The valve 52 and the valve 178 or 54 are correspondingly closed.

(5) Then a closure pressure measurement is effected. The valves 54, 178 are permanently closed, while the valve '52 is closed briefly, possibly repeatedly at intervals. The control of the valve 52 is preferably effected by means of the devices 60, 68, as explained with reference to FIG. 3.

(6) Then the patient is again brought into communication with the interior of the cabin or the outside air as in FIG. 4.

The sequence given above is not compulsory of course.

The examples of embodiments described can be modified in the most varied ways without departing from the scope of the invention.

What I claim as neW and desire to secure by Letters Patent of the United States of America is:

1. A plethysmograph device for medical lung examination comprising:

a cabin 10 which can be closed in a substantially airtight manner to receive a person to be examined;

a breathing tube 48 located within the cabin and having a first end 50 adapted for breathing thereinto by the person;

a yielding wall 92 forming a sealed chamber 70 connected to the second end of said tube 48 and entirely located within said cabin and separating said chamber from the air in said cabin; pressure sensing means associated with said cabin and said chamber;

and a thermostatically controlled 108, electrical heating device 106, 106' and air circulating means 102 located within said chamber.

2. Device according to claim 1, including a vertical tube 98 located in said chamber, said electrical heating device being mounted in the lower portion of said vertical tube; the air-circulating means including an impeller located above the heating device.

3. Device according to claim 1 wherein said yielding wall is formed of a lastic bag 92; and a second plastic bag 94 is provided surrounding said first plastic bag 92, and spacing means 88 are provided holding said second plastic bag 94 spaced from said first bag 92.

4. Device according to claim 1, including an upright rod 30 mounted in said chamber, said breathing tube 28 being mounted on said rod to be pivotal thereon and height adjustable.

5. Device according to claim 1 wherein said cabin is formed with an entrance means and has an additional opening 46 of controllable size formed therein to provide for communication of the inside of the cabin with the outside thereof independent of said entrance means.

6. Device according to claim 1 wherein said cabin is upright and adapted to receive a person therein in seating position, said cabin being formed with a curved rear wall facing the back of said person.

7. Device according to claim 1 wherein the cabin has a seat 32 located therein; and means 36 forming a buffer air chamber 38 separate from the cabin and located beneath said seat.

8. Device according to claim 1 wherein said cabin has a seat located therein, said seat being covered in the areas in contact with the body of the person with a material which is at least partially perforated.

9. Device according to claim 1 including a pump 116, the inlet of which is connected to the interior of the cabin and the outlet to the ambient air, and pump control means connected to the pump having a time-vs.-pumping delivery characteristic which substantially compensates for rise in pressure caused by a rise in temperature in the interior of the cabin due to the presence of a person therein.

10. Device according to claim 9 including a temperature-measuring sensor 150 mounted in the interior of the cabin and connected to the pump, the control means controlling the speed of the pump so that the pump will have a delivery at least substantially proportional to temperature variation in the interior of the cabin.

11. Device according to claim 10 including a non-linear amplifier 144 connected to the temperature-measuring sensor 150 and delivering an output signal rising at least substantially linearly and at a rate of rise which is proportional to the energy Which is supplied to the internal air in the cabin per unit of time.

12. Device according to claim 9 including a pressure measuring sensor 42 located to be responsive to variations in pressure in the interior of the cabin and a nonlinear amplifier 144 connected to the output of said pressure measuring sensor and delivering an output rising at least substantially linearly with increase in the pressure of the air in the interior of the cabin due to heat supplied at a constant rate of thermal energy per unit of time.

13. Device according to claim 9 wherein said pump control means includes electrical circuit means 132 to 142 to control said pump 116 to have a throughput in accordance with at least one exponential function with a negative exponent.

14. Device according to claim 1 including a quick- References Cited UNITED STATES PATENTS 2,569,849 10/1951 Emerson l282.08 2,970,041 1/1961 Burlis et al. 23256 3,324,848 6/ 1967 Domeier et a1 1282.08

OTHER REFERENCES Journ. of Clinical Investigation, vol. (1956), pp. 321-355.

J oum. of Applied Physiology, vol. 14 (1959), pp. 439- 444.

Journ. of Applied Physiology, vol. 19 (1964), pp. 813- 820.

Journ. of Applied Physiology, vol. 19 (1964), pp. 534- 535.

RICHARD A. GAUDET, Primary Examiner K. L. HOWELL, Assistant Examiner

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