US3599633A

US3599633A - Volume-limited ventilator with compliance compensator

Info

Publication number: US3599633A
Application number: US813829A
Authority: US
Inventors: Noel F Beasley
Original assignee: Puritan Bennett Corp
Current assignee: Puritan Bennett Corp
Priority date: 1969-04-07
Filing date: 1969-04-07
Publication date: 1971-08-17
Anticipated expiration: 1988-08-17

Abstract

The volume of gas delivered to a patient by a volume-limited ventilator is maintained substantially constant, regardless of change in the delivery pressure which changes the compression of the gas and the size of the delivery tubing, by a compensator which is spring-biased against its delivery stroke and has interconnected pistons, a smaller one delivering a gas volume to be added to that measured in the ventilator and a larger one responsive to the ventilator operating pressure. As a result, the compensator increases the amount of gas delivered to compensate for compression and tube expansion upon increase in delivery pressure.

Description

United States Patent Noel F. Beasley Santa Monica, Calif.

Apr. 7, 1969 Aug. 17, 1971 Puritan-Bennett Corporation Kansas City, Mo.

Inventor Appl. No. Filed Patented Assignee VOLUME-LIMITED VENTILATOR WITH COMPLIANCE COMPENSATOR 19 Claims, 1 Drawing Fig.

References Cited UNITED STATES PATENTS l/l 933 Goodner l28/l45.8

r- V ems/wee Primary Examiner-Richard A. Gaudet Assistant Examiner- Kyle L. Howell Anomey-Fulwider, Patton, Rieber, Lee & Utecht ABSTRACT: The volume of gas delivered to a patient by a volume-limited ventilator is maintained substantially constant, regardless of change in the delivery pressure which changes the compression of the gas and the size of the delivery tubing, by a compensator which is spring-biased against its delivery stroke and has interconnected pistons, a smaller one delivering a gas volume to be added to that measured in the ventilator and a larger one responsive to the ventilator operating pressure. As a result, the compensator increases the amount of gas delivered to compensate for compression and tube expansion upon increase in delivery pressure.

(PAY/5N7) I I Y i VOLUME-LIMITED VENTILATOR WITH COMPLIANCE COMPENSATOR BACKGROUND OF THE INVENTION This invention relates to the field of volume-limited ventilators where a measured volume of a gas is delivered to a patient in administering intermittent positive pressure breathing therapy. I

Respiration apparatus used in administering intermittent positive pressure breathing therapy is known in the art, and it is known to measure at the respirator the volume of air or other gas to be delivered to the patient. However, where the delivery pressure changes, due to a change in the patient's condition, the volume delivered does not remain constant, unless it is adjusted for such pressure change, because of compression or expansion of the gas in the delivery tubing as well as change in the size of the tubing itself. A common value for the tubing compliance of thedelivery system is cc. per centimeter of water change in pressure. Thus, if the ventilator is set to deliver 200 cc. of air or gas to the patient while the pressure in the delivery tubing system is cm. H O, then the patient actually receives 200 cc. less 50 cc. or 150 cc. ofgas, and should the patient's condition change so that cm. H O pressure are required in' the delivery system, then the patient would receive a volume of 200 cc. less 100 cc. or only 100 cc. of gas. The present invention automatically compensates for the compliance of the delivery system in both gas compression and tubing expansion by delivering an increased volume into the delivery system as the delivery pressure increases;

SUMMARY OF THE INVENTION The ventilator of the present invention administers intermittent positive pressure breathing therapy from a delivery chamber having a piston reciprocating therein by the application of pressure to the underside of the piston from acycling source, and measures the volume of the air or gas delivered from the respirator by a volume-measuring device simply illustrated in the drawing as responsive to the stroke of the :piston. The compliance compensator of the present invention employs a pair of interconnected pistons of different area, the larger of which is connected tothe pressure operating the delivery piston of the respirator, and the smaller of which delivers air or gas into the delivery system to the patient. The compensator is normally biased by a springtoward the end of its nondelivery stoke, to prevent movement of the pistons, and. thereby to avoid compensating action, while the operating pressure is below a preselected level.

As a patient's condition changes so as to increase, for example, the pressure in the delivery system, the back pressure on the operated side of the ventilator delivery piston increases a comparable amount and is fed to the larger piston of the compensator. This moves the compensator into a balancing condition where the springforce plus the force from the delivery pressure on the smaller piston of the compensator balances the force on the larger piston of the compensator from the ventilator operating pressure. In the. process ofbalancing, the pistons move in a working or delivery stroke so that gas or air above the smaller piston is moved into the delivery system to add to the volume delivered by the ventilator, with the result of maintaining the tidal volume delivered to the patient substantiallyconstant despite the change in the delivery pressure. By proper selection of the spring rate and the areas of the two DESCRIPTION OF THE PREFERRED EMBODIMENT An exemplification of the present invention has been diagrammatically illustrated in the'drawing in its simplest form, and for more detailed descriptions of the operation of systems of the type under consideration, reference is made to my prior US. Pat. Nos. 3,221,734; 3,368,555 and 3,385,295 which explain the nature of respirator or ventilator operation and the physiological effects thereof at some length, and which are incorporated herein by reference, but are not required for an understanding of the present invention. s

As diagrammatically illustrated, the ventilator 1 comprises a cylinder 2 in which reciprocates a delivery piston 3 sealed to the interior walls of the cylinder 2 byan O-ring or like seal 4. On the delivery side of the piston 3 is a chamber 10 provided with a gas supply pipe 5 adapted to be closed by an outwardly operating check valve 6. As explained in my prior patents, the supply pipe 5 may deliver air, air enriched with oxygen, or such gases with vaporized or nebulized medication. The chamber 10 also connects to a delivery system represented by tubing 7 leading to a receiver 8 establishing communication between the ventilator and the patient's respiratory system by any known device, including, without limitation, face masks and mouthpieces. Reverse flow from the tubing 7 is prevented by a downwardly operating check valve 9.

A chamber 11 beneath the delivery piston 3 is periodically and cyclically connected through tubing 12 to a source of operating pressure and cycling control therefor, indicated generally by the box 20 and of wellknown construction. The chamber 11 is provided with a vent 13 to atmosphere under the control of a bellows valve 14 connected by a tube 15 to the operating pressure supply tubing 12, so that the vent l3 closes upon the application of pressure from the source 20 into the tube 12 and opens when this operating pressure is released.

Any. desired volume-limiting means may be used on the ventilator, and diagrammatically illustrated is a rod 16 moving with the piston 3' to rotate a friction wheel or pinion 17 connected by the dotted line 18 into a volume transducer forming a part of the control 20.

The apparatus so far described is a conventional volumelimited ventilator. The compliance compensator according to v the present invention is shown at 21, and comprises a delivery compensator pistons, the volume added into the delivery system by the compensator can be controlled and made to accord substantially with the compliance of the delivery systemover the expected variation in delivery pressure.

' BRIEF DESCRIPTION OF THE DRAWING cylinder 22 in which reciprocates a piston 23 sealed to the cylinder wall at 24. A delivery chamber 25 above the piston 23 is connected by tubing 26 to the delivery chamber 10 above the piston 3 in the cylinder 2 of the ventilator. The compensator 21 further comprises an operating cylinder 27 larger than the cylinder 22 and having a piston 28 therein with a chamber 29 therebeneath. Tubing 31 connects the chamber 29 with the chamber 11 beneath the piston 3. The piston 28 is sealed to the walls of cylinder 27 by a sealing ring 32.

A compression spring 33 biases the piston 28 downwardly toward the end of its ,nondelivery stroke, the spring 33 abuttingat one end against the upper surface of the piston 28 and its other end beneath the surface of a circularflange 34 rigidly mounted integral with the cylinders 22 and-24. The

pistons

23 and 28 are rigidly connected by a rod 35 for common movement as a unit and the space between the pistons is at all times in communication with atmosphere through the vent 36. Accordingly, the two pistons constitute operators having different areas, and interconnected for common movement to vary the volumes of the chambers in inverse manner.

The operation of the compensated ventilator according to the present invention will now be described.

The inspiration cycle in which gas is supplied to the respiratory system of the patient is initiated while

pistons

3, 23 and 28 are in their lowermost positions at the ends of their nondelivery strokes, at which time chambers 10 and 25 will have filled with the respiratory gas from supply pipe 5. To initiate inspiration, the cycling control at 20 feeds fluid pressure into the chamber llthrough the pressure tubing 12 and expands the bellows valve 14 to close the vent 13 to build up pressure in the chamber 11 and effect an upward delivery stroke of the piston 3. Upon upward movement of the piston 3, valve 6 closes, valve 9 opens, and gas is delivered to the patient through tubing 7 to the receiver mask or other device 8.

The piston 3 will move upwardly a total distance determined by the volume transducer control at 20 as sensed by the wheel or pinion 17 cooperating with the rod 16 and when the set volume is delivered, that is, when the piston has moved through its regulated stroke, the supply of operating pressure from 20 will terminate, and the .vent 13 will open and the piston 3 retract to its rest position at the bottom of the cylinder 2.

During the inspiration cycle, the pressure in chamber will differ from that in chamber 1 l substantially only from the weight of the piston 3 and from frictional resistance. Pressure in chamber 11 is effective over the area of the piston 28, and the pressure in chamber 10 is effective over the area of the piston 23. The

pistons

28 and 23 will be moved through a distance which is a function of their relative areas, the pressures applied thereto, which may be roughly considered equal, and the rate of the spring 33. The relative areas of the

pistons

23 and 28 and the rate of the spring are selected so that at low values of delivery pressures below the preselected value, the spring force will predominate and there will be no movement of the pistons and no delivery of gas from the chamber 25 into the chamber 10 and the delivery system.

On the other hand, should the patient's condition cause an increasein delivery pressure, as for example, by the existence of spasms or other physiological factors, the pressure required in the tubing 7 to deliver the respirator gas to the patient will increase, and because of the compliance of the delivery system, the tidal volume delivered to the patient would or dinarily decrease since the ventilator volume is limited. However, with increase of the pressure in the delivery system, the pressures on the

pistons

28 and 23 are increased and the differential in force will cause movement of the pistons and compression of the spring 33 to compensate and return the compensator to balanced condition. This movement decreases the volume of chamber 25 and delivers gas therefrom through the tubing 26 into 'chamber l0 and the delivery system, with the object of maintaining the tidal volume delivered to the patient substantially constant despite the increase in the delivery pressure. Since the constant of the spring 33 and the areas of

pistons

23 and 28 are fixed, the volume to be added into the delivery system by the piston 23 will be substantially proportional to the value of the delivery pressure over the working range thereof.

While a certain preferred embodiment of this invention has been specifically illustrated and described, it will be understood that the invention is not limited thereto, as many variations will be apparent to those skilled in the art, and the invention is to be given its broadest interpretation within the terms of the following claims.

1 claim:

1. In combination with a volume-limited ventilator for delivering a measured volume of gas into a delivery system adapted to be connected to the respiratory system of a patient; a compensator for maintaining the volume of gas delivered to the patient substantially constant regardless of changes in delivery pressure and the compliance of the delivery system, comprising:

means defining first and second variable volume chambers having first and secondoperators, respectively, of different cross-sectional areas and interconnected for common movement to vary the volumes of said chambers in inverse manner, said first chamber having the operator of smaller area;

first means communicating between said first chamber and the delivery system to carry gas therebetween, whereby said first operator is responsive to the pressure in the system, and gas is delivered to the system upon decrease in the volume of said first chamber;

a source of pressure substantially equal to said delivery pressure but separated from said delivery system,

second means communicating between said source and said second chamber to transmit said substantially equal pressure to said second operator and'means preventing movement of said operators in a direction supplying gas to said delivery system while the pressure in said delivery system is below a selected level.

2. The apparatus defined in claim 1 in which said ventilator comprises: I

a pair of chambers, one of which is connected to the delivery system and the other of which is connected to a cycling operating pressure to cyclically decrease the size of said delivery chamber; v

said first means connecting said first chamber of said compensator to the delivery chamber of the ventilator; and

said second means connecting said second chamber of said compensator to the pressure cycling chamber of said ventilator.

3. The apparatus defined in claim 2 wherein said means for preventing movement of said operators includes: spring means biasing the operators of said toward the end of their nondelivery stoke.

4. The apparatus defined in claim 3 in which said compensator operators are pistons slidable in cylinders, the space between said pistons being vented to the atmosphere;

the chamber on the working face of said second piston being connected to the pressure cycling chamber of the ventilator; and

the chamber on the operating face of said first piston of the compliance compensator being connected to the delivery chamber of the ventilator.

5. The apparatus defined in claim 4 in which said pistons are rigidly interconnected for common movement and the spring means biases said common moving pistons to the end of their nondelivery stroke, the constant of the spring and the areas of the pistons being selected to deliver a volume of gas into the delivery system substantially varying directly with the pressure in the delivery system to compensate for the compliance of the delivery system and deliver a substantially constant volume of air to the patient.

6. The apparatus defined in claim 5 in which the ventilator has a regulated stroke to deliver a constant gas volume from the ventilator, and in which the stroke of the compensator pistons varies in accordance with variation of the delivery pressure to increase the stroke of the compensator and the volume of gas delivered therefrom into the delivery system as the delivery pressure increases.

7. The apparatus defined in claim 1 in which said first compensator chamber is connected to the respiratory gas supply of the ventilator to be filled therefrom as the compensator moves in its nondelivery stroke.

8. The apparatus defined in claim 1 in which:

said ventilator comprises a cylinder having a piston therein defining a delivery chamber on one side thereof and an operating pressure chamber on the other side;

means for cycling operating pressure into said pressure chamber to effect gas delivering strokes of said piston;

means feeding respiratory gas into said delivery chamber in the nondelivery strokes of said piston;

9. The apparatus defined in claim 8 wherein said means for preventing movement of said operators includes spring means biasing the operators of said compensator toward the end of their nondelivery stroke;

said compensator operators being interconnected pistons defining a delivery chamber by said first piston and an operating chamber by said second piston; and

means venting the nonchamber sides of said compensator pistons to atmosphere.

10. In combination with a volume-limited ventilator for delivering ventilating gas to a patient, and including a delivery system, and means for cyclically introducing into said system a measured amount of said gas having a preselected volume when the pressure in said'system'is at a predetermined normal level; the improvement comprising:

compensator means defining a compensator chamber adapted to be filled with said gas;

means communicating between said chamber and said delivery system to carry said gas between the chamber and the delivery system;

means for forcing gas from said chamber into said delivery system through said communicating means, and including an operator movable in said chamber; and

actuating means for said operator for sensing increases in the pressure in said system above said normal level and moving said operator in substantially direct relation to said pressure increases so as to maintain successive measure amounts of said gas substantially at said predetermined volume said actuating means including a means preventing movement of said operator in the direction for forcing gas from said chamber while the pressure in said system is below said normal level.

11. The combination defined in claim in which said introducing means include a delivery chamber, a pressure-actuated piston movable. in said delivery chamber through a regulated stroke to force said measured amount of gas into said system, and a source for supplying gas to said delivery chamber in preparation for said stroke, said actuating means being responsive to an increase in the pressure required to move said piston to actuate said operator.

12. The combination defined in claim 11 in which said actuating means includes a sensing chamber having a sensing piston therein with a working face exposed to the pressure for moving the piston in said delivery chamber, said operator being a compensator piston having a working face in said com pensator chamber of smaller area than the face of said sensing piston, said sensing and compensator pistons beingconnected for movement together so that increases in said moving pressure shift said compensator piston in a direction to force compensating gas into said delivery system.

13. The combination defined in claim 12 wherein said means'for preventing movement of said operator includes a spring acting against said connected pistons and correlated with the area differential thereof to hold the pistons stationary while said moving pressure is below a preselected level, thereby to determine the pressure level above which the compensating action is produced.

14. The improvement defined in claim 10 in which:

said ventilator includes a cylinder having a piston therein with a delivery chamber on one side thereof and an operating pressure chamber on the other side;

means for cycling operating pressure into said pressure chamber to effect gas-delivering strokes of said piston; means feeding respiratory gas into said delivery chamber in the nondelivery strokes of said piston;

means regulating the stroke of said piston to limit the gas volume delivered from the ventilator; and

a sensing chamber constituting part of said actuating means;

means connecting said sensing chamber to said ventilator pressure chamber;

and first and second pistons in said compensator and sensing chambers. respectively, interconnected for common movement, said first piston being smallerthan said second piston whereby movement of said pistons in one direction in response to increases in said pressure chamber forces gas from said compensator chamber into said system.

15. The apparatus defined in claim 14 wherein said means for preventing movement of said operator includes:

spring means biasing said first and second pistons in the other direction, toward the end of their nondelivery stroke;

and means venting the nonchamber sides of said first and second pistons to atmosphere;

said spring means holding said first and second pistons stationary while said operating pressure is below said redetermined level. l In combination with a volume-limited ventilator having a delivery piston movably disposed between drive and delivery chambers to pump gas from said delivery chamber in response to movement of said piston in one direction, means for supplying ventilating gas to said delivery chamber, a delivery system adapted for carrying gas from said delivery chamber to a patient, and means for supplying drive fiuid under pressure cyclically to said drive chamber to reciprocate said piston through strokes of regulated length, thereby to pump said gas to said delivery system during successive strokes of the piston, the improvement comprising:

means defining a pair of variable volume chambers-having operators of difierent areas interconnected for common movement to vary the volumes of said chambers in-inverse manner;

means communicating between the chamber with the operator of smaller area and said delivery chamber so that the smaller operator is responsive to the pressure in the delivery system and delivers gas to said delivery chamber while decreasing the volume of the associated chamber;

means communicating between the chamber with the operator of larger area and said drive chamber so that the larger operator is responsive to the pressure in the drive chamber, whereby said larger operator moves said smaller operator to deliver gas to said delivery system upon an increase in the pressure in said delivery system increasing the back pressure opposing movement of said piston, thereby to compensate for changes in volume of the gas delivered by said regulated strokes when the pressure in said delivery system changes;

and means biasing said operators toward the end of their nondelivery stroke and thereby preventing movement of the operators while the pressure in said drive chamber is below a selected level.

17. The improvement defined in claim 1 in which each of said operators is a piston slidable in a cylinder of comparable area, the space between said cylinders being vented to atmosphere.

18. The improvement defined in claim 17 in which said operator pistons are rigidly interconnected for common movement and said biasing means is a spring urging said operating pistons toward the end of their nondelivery stroke, the constant of the spring and the areas of the pistons being selected to deliver a volume of gas into the delivery system varying substantially directly with the pressure in the delivery system to compensate for the compliance of the delivery system and deliver a substantially constant volume of gas to the patient.

19. The improvement defined in claim 18 further including means for regulating the stroke of the delivery piston to deliver a constant gas volume from the delivery chamber. and for varying the stroke of the compensator pistons in accordance with variation of the delivery pressure to increase the stroke of the compensator pistons and the volume of gas delivered therefrom into the delivery system as the delivery pressure increases.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,599,633 Dated A r 271 Inventor(s) Noel a ley It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

COLUMN 3: line 29, "respirator" should be respiratory-.

COLUMN 4: line 19, "operators" should be --operator-.

line 21, "stoke should be -stroke.

After line 59, please add the following f lines, to complete Claim #8, as follows:

-and means regulating the stroke of said piston to limit the gas volume delivered from the ventilator;

said first means connecting said first compensator chamber to said ventilatorudelivery chamber; and

said second means connecting said second compensator chamber to said ventilator pressure chamber.

line 61, colon (:J should be added after the word, "includes".

COLUMN 5: line 13, "measure" should be -measured.

COLUMN 6 line 44, "1" should be '-l6-.

Signed and sealed this 11th day of April 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Atte sting Officer Commissionerof Patents

Claims

1. In combination with a volume-limited ventilator for delivering a measured volume of gas into a delivery system adapted to be connected to the respiratory system of a patient; a compensator for maintaining the volume of gas delivered to the patient substantially constant regardless of changes in delivery pressure and the compliance of the delivery system, comprising: means defining first and second variable volume chambers having first and second operators, respectively, of different crosssectional areas and interconnected for common movement to vary the volumes of said chambers in inverse manner, said first chamber having the operator of smaller area; first means communicating between said first chamber and the delivery system to carry gas therebetween, whereby said first operator is responsive to the pressure in the system, and gas is delivered to the system upon decrease in the volume of said first chamber; a source of pressure substantially equal to said delivery pressure but separated from said delivery system; second means communicating between said source and said second chamber to transmit said substantially equal pressure to said second operator and means preventing movement of said operators in a direction supplying gas to said delivery system while the pressure in said delivery system is below a selected level.

2. The apparatus defined in claim 1 in which said ventilator comprises: a pair of chambers, one of which is connected to the delivery system and the other of which is connected to a cycling operating pressure to cyclically decrease the size of said delivery chamber; said first means connecting said first chamber of said compensator to the delivery chamber of the ventilator; and said second means connecting said second chamber of said compensator to the pressure cycling chamber of said ventilator.

3. The apparatus defined in claim 2 wherein said means for preventing movement of said operators includes: spring means biasing the operators of said compensator toward the end of their nondelivery stoke.

4. The apparatus defined in claim 3 in which said compensator operators are pistons slidable in cylinders, the space between said pistons being vented to the atmosphere; the chamber on the working face of said second piston being connected to the pressure cycling chamber of the ventilator; and the chamber on the operating face of said first piston of the compliance compensator being connected to the delivery chamber of the ventilator.

8. The apparatus defined in claim 1 in which: said ventilator comprises a cylinder having a piston therein defining a delivery chamber on one side thereof and an operating pressure chamber on the other side; means for cycling operating pressure into said pressure chamber to effect gas delivering strokes of said piston; means feeding respiratory gas into said delivery chamber in the nondelivery strokes of said piston;

9. The apparatus defined in claim 8 wherein said means for preventing movement of said operators includes spring means biasing the operators of said compensator toward the end of their nondelivery stroke; said compensator operators being interconnected pistons defining a delivery chamber by said first piston and an operating chamber by said second piston; and means venting the nonchamber sides of said compensator pistons to atmosphere.

10. In combination with a volume-limited ventilator for delivering ventilating gas to a patient, and including a delivery system, and means for cyclically introducing into said system a measured amount of said gas having a preselected volume when the pressure in said system is at a predetermined normal level, the improvement comprising: means defining a compensator chamber adapted to be filled with said gas; means communicating between said chamber and said delivery system to carry said gas between the chamber and the delivery system; means for forcing gas from said chamber into said delivery system through said communicating means, and including an operator movable in said chamber; and actuating means for said operator for sensing increases in the pressure in said system above said normal level and moving said operator in substantially direct relation to said pressure increases so as to maintain successive measure amounts of said gas substantially at said predetermined volume said actuating means including a means preventing movement of said operator in the direction for forcing gas from said chamber while the pressure in said system is below said normal level.

11. The combination defined in claim 10 in which said introducing means include a delivery chamber, a pressure-actuated piston movable in said delivery chamber through a regulated stroke to force said measured amount of gas into said system, and a source for supplying gas to said delivery chamber in preparation for said stroke, said actuating means being responsive to an increase in the pressure required to move said piston to actuate said operator.

12. The combination defined in claim 11 in which said actuating means includes a sensing chamber having a sensing piston therein with a working face exposed to the pressure for moving the piston in said delivery chamber, said operator being a compensator piston having a working face in said compensator chamber of smaller area than the face of said sensing piston, said sensing and compensator pistons being connected for movement together so that increases in said moving pressure shift said compensator piston in a direction to force compensating gas into said delivery system.

13. The combination defined in claim 12 wherein said means for preventing movement of said operator includes a spring acting against said connected pistons and correlated with the area differential thereof to hold the pistons stationary while said moving pressure is below a preselected level, thereby to determine the pressure level above which the compensating action is produced.

14. The improvement defined in claim 10 in which: said ventilator includes a cylinder having a piston therein with a delivery chamber on one side thereof and an operating pressure chamber on the other side; means for cycling operating pressure into said pressure chamber to effect gas-deliveriNg strokes of said piston; means feeding respiratory gas into said delivery chamber in the nondelivery strokes of said piston; means regulating the stroke of said piston to limit the gas volume delivered from the ventilator; and a sensing chamber constituting part of said actuating means; means connecting said sensing chamber to said ventilator pressure chamber; and first and second pistons in said compensator and sensing chambers, respectively, interconnected for common movement, said first piston being smaller than said second piston whereby movement of said pistons in one direction in response to increases in said pressure chamber forces gas from said compensator chamber into said system.

15. The apparatus defined in claim 14 wherein said means for preventing movement of said operator includes: spring means biasing said first and second pistons in the other direction, toward the end of their nondelivery stroke; and means venting the nonchamber sides of said first and second pistons to atmosphere; said spring means holding said first and second pistons stationary while said operating pressure is below said predetermined level.

16. In combination with a volume-limited ventilator having a delivery piston movably disposed between drive and delivery chambers to pump gas from said delivery chamber in response to movement of said piston in one direction, means for supplying ventilating gas to said delivery chamber, a delivery system adapted for carrying gas from said delivery chamber to a patient, and means for supplying drive fluid under pressure cyclically to said drive chamber to reciprocate said piston through strokes of regulated length, thereby to pump said gas to said delivery system during successive strokes of the piston, the improvement comprising: means defining a pair of variable volume chambers having operators of different areas interconnected for common movement to vary the volumes of said chambers in inverse manner; means communicating between the chamber with the operator of smaller area and said delivery chamber so that the smaller operator is responsive to the pressure in the delivery system and delivers gas to said delivery chamber while decreasing the volume of the associated chamber; means communicating between the chamber with the operator of larger area and said drive chamber so that the larger operator is responsive to the pressure in the drive chamber, whereby said larger operator moves said smaller operator to deliver gas to said delivery system upon an increase in the pressure in said delivery system increasing the back pressure opposing movement of said piston, thereby to compensate for changes in volume of the gas delivered by said regulated strokes when the pressure in said delivery system changes; and means biasing said operators toward the end of their nondelivery stroke and thereby preventing movement of the operators while the pressure in said drive chamber is below a selected level.

19. The improvement defined in claim 18 further including means for regulating the stroke of the delivery piston to deliver a constant gas volume from the delivery chamber, and for varying the stroke of the compensator pistons in accordance with variation of tHe delivery pressure to increase the stroke of the compensator pistons and the volume of gas delivered therefrom into the delivery system as the delivery pressure increases.