VENTILATOR

• US 3,669,108 A
• Filed: 10/20/1969
• Issued: 06/13/1972
• Est. Priority Date: 10/20/1969
• Status: Expired due to Term

• Current Assignee: Leif J. Sundblom, Louis A. Ollivier
• Original Assignee: Leif J. Sundblom, Louis A. Ollivier
• Inventors: Sundblom, Leif J., Ollivier, Louis A.
• Primary Examiner(s): Rosenbaum, Charles F.
• Assistant Examiner(s): Mitchell, J. B.
• Application Number:
  US04/867,804
• Time in Patent Office: 967 Days
• Field of Search: 128/145.8,145.5-145.7,142-142.4,145
• US Class Current: 128/204.26
• CPC Class Codes: A61M 16/00 Devices for influencing the respiratory system of patients by gas treatment, e.g. mouth-to-mouth respiration; Tracheal tubes stimulating the respiratory movement by mechanical, pneumatic or electrical means, iron lungs combined with gas breathing means A61H31/00
• CPC Class Codes: A61M 16/0012 by Venturi means

Abstract

A ventilator capable of both pressure-cycled and volume-cycled operation. A main valve connects a gas supply conduit to a downstream conduit when the pressure in a command signal conduit reaches a first predetermined pressure level and closes off the downstream conduit from the gas supply conduit when the pressure in the command signal conduit drops below a second predetermined pressure level. In the expiratory cycle, the command signal conduit and the downstream conduit are bled to atmospheric. The inspiratory cycle may be initiated (1) by the patient breathing in and thereby lowering airway pressure, or (2) after a time lapse following the commencement of said expiratory phase, the duration of said time lapse being determined as a set multiple of the time of the preceding inspiratory phase. The expiratory phase may be initiated (1) by the achievement of a predetermined airway pressure or (2) upon the delivery of a predetermined volume of gas.

Claims

1. A ventilator for providing breathing gas to a patient during an inspiratory phase and letting him exhale during an expiratory phase, including in combination:

a gas supply conduit for supplying a breathable gas under pressure, a downstream conduit, a command signal conduit, an airway conduit suitable for connection to a patient, means connecting said downstream conduit to said airway conduit, main valve means connected to said command signal conduit for connecting said gas supply conduit to said downstream conduit during each said inspiratory phase when the pressure in said command signal conduit rises above a first predetermined pressure level, and for closing off said downstream conduit from said gas supply conduit during each said expiratory phase when the pressure in said command signal conduit drops below a second, lower, predetermined pressure level, phase-actuation means connected to said command signal conduit for terminating each said inspiratory phase and commencing a said expiratory phase, by bleeding said command signal conduit to atmosphere, means connected to said command signal conduit for bleeding said downstream conduit to atmosphere when said command signal conduit pressure drops below said second predetermined pressure level at the commencement of said expiratory phase, ratio means connected to said command signal conduit, said downstream conduit and said gas supply conduit for ending the expiratory phase and initiating a new inspiratory phase by increasing the pressure in said command signal conduit above said first predetermined pressure level by supplying gas from said gas supply conduit to said command signal conduit, after a lapse of time following the commencement of said expiratory phase, said ratio means having means for sensing the duration of each inspiratory phase and for establishing a maximum duration of the succeeding expiratory phase as a time ratio of the inspiratory phase to the next succeeding expiratory phase, and means for varying the setting of said time ratio.

2. The ventilator of claim 1 wherein said phase-actuation means is connected to said airway conduit and is actuated by the pressure in said airway conduit reaching a third predetermined pressure level.

3. The ventilator of claim 2 having sensitivity control means connected to said airway conduit for connecting said gas supply conduit to said command signal conduit and thereby to raise the pressure therein above said first predetermined pressure level when the pressure in said airway conduit drops below a fourth predetermined pressure level, due to the commencement by the patient of an inspiratory phase, and to close off said command signal conduit from said gas supply conduit when the pressure in said airway conduit rises above said fourth predetermined pressure level, said sensitivity control means thereby overriding said ratio means whenever said patient initiates an inspiratory phase before the time lapse following commencement of an expiratory phase has not yet reached the time at which said ratio means would have acted to initiate a said inspiratory phase.

4. The ventilator of claim 2 wherein said means for connecting said downstream conduit to said airway conduit comprises means for adjusting the pattern of the flow of gas from an initial peak flow through a gradually decreasing flow to a minimal terminal flow, including means for varying the initial peak flow independently of said minimal terminal flow.

5. The ventilator of claim 4 wherein said means for adjusting the pattern of flow of gas comprises first flow means for providing a minimum constant flow from said downstream conduit to said airway conduit, second flow means for providing an adjustable initial peak flow from said downstream conduit to said airway conduit, and means responsive to the pressure in said airway conduit for continually reducing the flow through said second flow means from said peak flow as the pressure in said aIrway conduit increases.

6. The ventilator of claim 5 wherein said first flow means includes means for adjusting the amount of said constant flow.

7. The ventilator of claim 1 wherein the means for connecting said downstream conduit to said airway conduit comprises means for determining the volume of gas to pass from said downstream conduit into said airway conduit during each inspiratory phase, means for determining the time over which said volume is to be delivered, and means for actuating said phase-actuation means at the conclusion of that said time.

8. The ventilator of claim 7, wherein said means for determining the volume comprises two relatively rotatable conduit means having alignable orifices, a first said conduit means being connected to said downstream conduit and a second said conduit means being connected to said airway conduit, and means for rotating said conduit means relative to each other for varying the area of opening of the orifices that provide fluid connection, said means for determining the time comprises means for displacing said conduit means axially relative to each other to vary the area of opening of the orifices that provide fluid connection and flow valve means connected to said downstream conduit, and sending gas to said means actuating said phase-actuation means.

9. The ventilator of claim 8 wherein said downstream conduit is connected to said first conduit means through a pressure regulator having an orifice regulating the passage of gas from said downstream conduit and pressure-control means regulating the size of said orifice, said pressure control means being connected to said flow valve means downstream thereof.

10. The ventilator of claim 8 wherein said means actuating said phase-actuating means comprises pressure-operated means for bleeding said command signal conduit when the amount of gas that has passed through said flow valve means raises the pressure exerted on said pressure-operated means to a predetermined value.

11. The ventilator of claim 1 having manual valve means for passing gas directly from said gas supply conduit to said command signal conduit, for manual initiation of an inspiratory phase.

12. The ventilator of claim 1 wherein said main valve means comprises a housing divided into chambers by first and second diaphragms, said first diaphragm being larger in effective area than said second diaphragm, said first diaphragm having one side open to the atmosphere, a first chamber between said first and second diaphragms connected to said command signal conduit, a second chamber bounded by said second diaphragm and having an inlet connected to said gas supply conduit and an outlet connected to said downstream conduit, valve means for opening and closing said inlet, said valve means being connected to both said first and second diaphragms for movement with them, and spring means urging said valve means normally to close said inlet, said valve means opening said inlet when the pressure in said first chamber rises to said first predetermined pressure level, said valve means, upon commencement of its opening by the pressure on said first diaphragm being rapidly opened by the pressure of the gas entering said inlet from said gas supply conduit and acting on said second diaphragm, said valve means thereafter preventing closure until the pressure on said first diaphragm drops below said second pressure level, at which time said downstream conduit is being bled by its said bleeding means, so that said valve means rapidly and positively closes against said inlet, due to the lowering of pressure in said second chamber able to act on said second diaphragm.

13. The ventilator of claim 1 wherein said ratio means comprises a reservoir, a restricted orifice connecting said reservoir to said downstream conduit, so that pressure in said reservoir builds up gradually toward a value still somewhat less than that of said downstream conduit during the time that said main valve means connects said gas supply conduit to said downstream conduit, which is during the entire inspiratory phase, check valve means between said downstream conduit and said reservoir, to prevent back flow from said reservoir to said downstream conduit when said downstream conduit is bled to atmosphere, means sensitive to the pressure in said reservoir for opening and closing off a connection between said gas supply conduit and said command signal conduit, said connection being closed off so long as the pressure in said reservoir remains above a predetermined level, and bleed means actuated by the drop in pressure in said downstream conduit to atmospheric for bleeding said reservoir to atmosphere at a predetermined flow rate during said expiratory phase, the time for said bleed means to cause the pressure in said reservoir to drop below the level where said command signal conduit is connected to said gas supply conduit thereby depending on the length of duration of said inspiratory phase, during which the pressure in said reservoir is continually built up.

14. The ventilator of claim 1 having switch means for causing, in a first position, the inspiratory phase to terminate when the pressure in said airway conduit reaches a certain level and for causing, in second position, the inspiratory phase to terminate after a given volume has been delivered from said downstream conduit to said airway conduit.

15. A ventilator for pressure-controlled operation with an inspiratory phase and an expiratory phase, including in combination:

a gas supply conduit for supplying a breathable gas under pressure, a downstream conduit, a command signal conduit, an airway conduit suitable for connection to a patient, means connecting said downstream conduit to said airway conduit, main valve means connected to said command signal conduit for connecting said gas supply conduit to said downstream conduit during each said inspiratory phase when the pressure in said command signal conduit rises above a first predetermined pressure level and for closing off said downstream conduit from said gas supply conduit during each said expiratory phase when the pressure in said command signal conduit drops below a second, lower, predetermined pressure level, sensitivity control means connected to said airway conduit for connecting said gas supply conduit to said command signal conduit and thereby to raise the pressure therein above said first predetermined pressure level when the pressure in said airway conduit drops below a third predetermined pressure level due to the commencement by a patient of an inspiratory phase, and to close off said command signal conduit from said gas supply conduit when the pressure in said airway conduit rises above said third predetermined pressure level, means for bleeding said command signal conduit to atmosphere actuated by the pressure in said airway conduit reaching a fourth predetermined pressure level, as upon commencement of an expiratory phase, means connected to said command signal conduit for bleeding said downstream conduit to atmosphere when said command signal conduit pressure drops below said second predetermined pressure level, as at the beginning of said expiratory phase, and ratio means connected to said command signal conduit, said downstream conduit and said gas supply conduit for ending said expiratory phase and initiating a new inspiratory phase by increasing the pressure in said command signal conduit above said first predetermined pressure level by supplying gas from said gas supply conduit to said command signal conduit, if the patient fails to initiate a new inspiratory phase for himself, after a time lapse following the commencement of a said expiratory phase, said ratio means having means for sensing the duration of each inspiratory phase and for establishing automatically a maximum duration of the succeeding expiratory phase as a time ratio of the inspiratorY phase to the next succeeding expiratory phase.

16. The ventilator of claim 15 having manual valve means for passing gas directly from said gas supply conduit to said command signal conduit, for manual initiation of an inspiratory phase.

17. The ventilator of claim 15 wherein said means for connecting said downstream conduit to said airway conduit comprises a two-position valve having a first position wherein said downstream conduit is connected to said airway conduit through means for adjusting flow, a restricted orifice, and a venturi in series, said venturi having an air inlet from atmosphere and drawing in air therefrom to dilute said gas to a desired dilution, said valve having a second position wherein the connection is through means setting the pattern of the flow of gas from an initial peak flow and a gradually decreasing flow to a minimal terminal flow.

18. The ventilator of claim 15 wherein said means for connecting said downstream conduit to said airway conduit comprises means for adjusting the pattern of the flow of gas from an initial peak flow and a gradually decreasing flow to a minimal terminal flow, including means for varying said initial peak flow independently of said minimal terminal flow.

19. The ventilator of claim 18 wherein said means for adjusting the pattern of flow of gas comprises:

first flow means for providing a minimum constant flow from said downstream conduit to said airway conduit, second flow means for providing an adjustable initial peak flow from said downstream conduit to said airway conduit, and means responsive to the pressure in said airway conduit for continually reducing the flow through said second flow means from said peak flow as the pressure in said airway conduit increases.

20. The ventilator of claim 19 wherein said first flow means includes means for varying the minimum constant flow.

21. The ventilator of claim 18 wherein said means for adjusting the pattern of the flow of gas comprises a housing divided by a diaphragm and a partition into first, second, and third chambers, said first chamber being open to the atmosphere and bounded by said diaphragm, said second chamber being between said diaphragm and said partition and having an inlet connected to said downstream conduit through an adjustable needle valve to deliver said minimal terminal flow, and an outlet connected to said airway conduit, said third chamber having an inlet connected directly to said downstream conduit, said partition having an opening therethrough connecting said second and third chambers, valve means for opening and closing said opening and connected to and controlled by said diaphragm, spring means in said housing for exerting pressure on said diaphragm, and compression means connected to said spring means for compressing said spring means for varying its pressure on said diaphragm, the pressure in said airway conduit and in said second chamber acting on said diaphragm and tending to close said valve means.

22. The ventilator of claim 15 wherein said airway conduit includes an exhalation valve having a housing divided into three chambers by first and second diaphragms, so that there are a first chamber bounded by said first diaphragm and having an inlet, a second chamber between said diaphragms and open to the atmosphere, and a third chamber bounded by said second diaphragm and connected to said airway conduit, first valve means opened and closed by said diaphragms for connecting said third chamber to the atmosphere, spring means normally urging said valve means toward a closed position, and an auxiliary conduit connected to said inlet and connected through an adjustable valve providing a variable-size restricted orifice to said airway conduit.

23. The ventilator of claim 22 provided with a pressure relief valve having a housing divided by a diaphragm into two chambers, one connected to atmosphere and having an inlet connected to said auxiliary Conduit, and the other connected to said downstream conduit, and valve means connected to said diaphragm for opening and closing said inlet, with spring means normally urging said valve means to open said inlet.

24. The ventilator of claim 15 wherein said main valve means comprises a housing divided into chambers by first and second diaphragms, said first diaphragm being larger in effective area than said second diaphragm, said first diaphragm having one side open to the atmosphere, a first chamber between said first and second diaphragms connected to said command signal conduit, a second chamber bounded by said second diaphragm and having an inlet connected to said gas supply conduit and an outlet connected to said downstream conduit, valve means for opening and closing said inlet, said valve means being connected to both said first and second diaphragms for movement with them, and spring means urging said valve means normally to close said inlet, said valve means opening said inlet when the pressure in said first chamber rises to said first predetermined pressure level, said valve means upon commencement of its opening by the pressure on the larger said first diaphragm being rapidly opened by the pressure of the gas entering said inlet from said gas supply conduit and acting on the smaller said second diaphragm, said valve means thereafter preventing closure until the pressure on said first diaphragm drops below said second pressure level, at which time said downstream conduit is being bled to atmosphere by its said bleeding means, so that said valve means rapidly and positively closes against said inlet, due to the lowering of pressure in said second chamber able to act on said second diaphragm.

25. The ventilator of claim 15 wherein said sensitivity control means comprises a housing divided into three chambers by a diaphragm and a partition, a first chamber bounded by said diaphragm and connected to said airway conduit, a second chamber between said diaphragm and said partition and open to the atmosphere, and a third chamber bounded by said partition and having an inlet connected to said gas supply conduit and an outlet connected to said command signal conduit and valve means extending through said partition and sealed thereto and actuated by said diaphragm for opening and closing the connection between said inlet and said outlet and spring means urging said valve means to close said connection between said inlet and said outlet so that when said inlet is opened by creation of a vacuum in said first chamber, gas from said gas supply conduit flows into said command signal conduit.

26. The ventilator of claim 25 having means for adjusting said spring pressure.

27. The ventilator of claim 15 wherein said ratio means comprises a reservoir connected by a restricted orifice to said downstream conduit, so that pressure in said reservoir builds up gradually and continuously during the time when the pressure in said downstream conduit is above atmospheric, that is, during the entire inspiratory phase, check valve means between said reservoir and said downstream conduit preventing flow back from said reservoir to said downstream conduit, valve means sensitive to the pressure in said reservoir for connecting said command signal conduit and said gas supply conduit and for disconnecting them when the pressure in said reservoir is above a predetermined level, and bleed means actuated by the drop in pressure to atmospheric in said downstream conduit, for bleeding said reservoir to atmospheric at a constant flow rate during said expiratory phase.

28. The ventilator of claim 15 wherein said ratio means comprises first housing means divided into three chambers by first and second diaphragms, said first diaphragm being smaller in effective area than said second diaphragm, said first housing means having a first chamber bounded by said first diaphragm and having a first inlet connEcted to said gas supply conduit and a first outlet connected to said command signal conduit, a second chamber between said first and second diaphragms and connected to the atmosphere, a third chamber bounded by said second diaphragm and having a second inlet and a second outlet, first valve means connected to both said first and second diaphragms for normally closing said first outlet, except when the pressure in said third chamber drops below a predetermined level, check valve means connected to said downstream conduit and connected through a restricted orifice to said second inlet, to supply gas from said downstream conduit to said third chamber to gradually build up pressure therein during each inspiratory phase, while preventing flow back from said third chamber to said downstream conduit, a pressure relief valve comprising second housing means divided by a third diaphragm into fourth and fifth chambers, said fourth chamber being connected to said downstream conduit and said fifth chamber having a third inlet connected to the second outlet and connected to atmosphere through an adjustable needle valve, the flow rate of which determines a limiting time ratio between the expiratory phase and the inspiratory phase, third valve means connected to said third diaphragm for opening and closing said third inlet, and spring means urging said third valve means away from closing said third inlet, said third inlet being held closed by the pressure in said downstream conduit until said pressure drops to atmospheric at the end of said inspiratory phase, said third inlet being open during the expiratory phase.

29. A ventilator for pressure-controlled operation with an inspiratory phase and an expiratory phase, including in combination:

a gas supply conduit for supplying a breathable gas under pressure, a downstream conduit, a command signal conduit, an airway conduit suitable for connection to a patient, main valve means connected to said command signal conduit for connecting said gas supply conduit to said downstream conduit during each said inspiratory phase when the pressure in said command signal conduit reaches a first predetermined pressure level and for closing off said downstream conduit from said gas supply conduit during each said expiratory phase when the pressure in said command signal conduit drops below a second, lower pressure level, sensitivity control means connected to said airway conduit for connecting said gas supply conduit to said command signal conduit and thereby to raise the pressure in said command signal conduit to said first predetermined pressure level when the pressure in said airway conduit drops due to the commencement by a patient of an inspiratory phase, to a third predetermined pressure level, and to close off said command signal conduit from said gas supply conduit when the pressure in said airway conduit rises above said third predetermined pressure level, means, actuated by the pressure in said airway conduit rising to a fourth predetermined pressure level, for bleeding said command signal conduit to atmosphere, as upon commencement of said expiratory phase, means connected to said command signal conduit for bleeding said downstream conduit to atmosphere when said command signal conduit pressure drops below said second predetermined pressure level as at the beginning of said expiratory phase, and flow control means connecting said downstream conduit to said airway conduit according to a predetermined flow pattern, with a peak flow rate at the commencement of each said inspiratory phase and a flow rate gradually diminishing to a lower terminal flow rate according to said pattern.

30. The ventilator of claim 29 having means for varying said predetermined flow pattern by changing the peak flow rate without changing the terminal rate.

31. The ventilator of claim 29 wherein said flow control means comprises:

first flow means for providing a minimum constant flow from said downsTream conduit to said airway conduit, second flow means for providing an adjustable initial peak flow from said downstream conduit to said airway conduit, and means responsive to the pressure in said airway conduit for continually reducing the flow through said second flow means from said peak flow as the pressure in said airway conduit increases.

32. The ventilator of claim 31 wherein said first flow means includes means for varying the minimum constant flow.

33. The ventilator of claim 29 wherein said flow control means comprises:

a housing divided by a diaphragm and a partition into first, second, and third chambers, said first chamber being open to the atmosphere and bounded by said diaphragm, said second chamber being between said diaphragm and said partition and having an inlet connected to said downstream conduit through an adjustable needle valve to deliver said minimal terminal flow, and an outlet connected to said airway conduit, said third chamber having an inlet connected directly to said downstream conduit, said partition having an opening therethrough connecting said second and third chambers, valve means for opening and closing said opening and connected to and controlled by said diaphragm, spring means in said housing for exerting pressure on said diaphragm, and compression means connected to said spring means for compressing said spring means for varying its pressure on said diaphragm, the pressure in said airway conduit and in said second chamber acting on said diaphragm and tending to close said valve means.

34. A ventilator for providing breathing gas to a patient during an inspiratory phase and letting him exhale during an expiratory phase, including in combination:

a gas supply conduit for supplying a breathable gas under pressure, a downstream conduit, a command signal conduit, an airway conduit suitable for connection to a patient, means connecting said downstream conduit to said airway conduit, main valve means connected to said command signal conduit for connecting said gas supply conduit to said downstream conduit during each said inspiratory phase when the pressure in said command signal conduit rises above a first predetermined pressure level, and for closing off said downstream conduit from said gas supply conduit during each said expiratory phase when the pressure in said command signal conduit drops below a second, lower, predetermined pressure level, phase-actuation means connected to said command signal conduit for terminating each said inspiratory phase and commencing a said expiratory phase, by bleeding said command signal conduit to atmosphere, means connected to said command signal conduit for bleeding said downstream conduit to atmosphere when said command signal conduit pressure drops below said second predetermined pressure level at the commencement of said expiratory phase, ratio means connected to said command signal conduit, said downstream conduit and said gas supply conduit for ending the expiratory phase and initiating a new inspiratory phase by increasing the pressure in said command signal conduit above said first predetermined pressure level by supplying gas from said gas supply conduit to said command signal conduit, after a lapse of time following the commencement of said expiratory phase, said ratio means having means for sensing the duration of each inspiratory phase and for establishing a maximum duration of the succeeding expiratory phase as a time ratio of the inspiratory phase to the next succeeding expiratory phase, pneumatic timing means connected to said gas supply conduit, and sigh means connected to said phase actuation means and actuated by said pneumatic timing means at intervals longer than the inspiratory-expiratory cycle of the ventilator as controlled without said pneumatic timing means by said phase actuation means and said ratio means, for causing a longer inspiratory phase than usual and a greater volume than usual.

35. The ventilator of claim 34 wherein said pneumatic timing means comprises a needle valve connected to said gas supply conduit, a pressure-actuated valve downstream of said needle valve and having a fixed-capacity chamber wherein the pressure builds up from atmospheric to the pressure for actuating said pressure-actuated valve, and means for bleeding said chamber to atmosphere after actuation of said pressure actuated valve.

36. The ventilator of claim 34 wherein said phase-actuation means is connected to said airway conduit and is actuated by the pressure in said airway conduit reaching a third predetermined pressure level and means actuated by said pneumatic timing means for raising said third predetermined pressure level to a fourth higher predetermined pressure level for a single inspiratory phase.

37. The ventilator of claim 34 wherein the means for connecting said downstream conduit to said airway conduit comprises means for determining the volume of gas to pass from said downstream conduit into said airway conduit during each inspiratory phase, means for determining the time over which said volume is to be delivered, and means for actuating said phase-actuation means at the conclusion of that said time, and means actuated by said pneumatic timing means for continuing to supply said gas to said airway conduit from said downstream conduit for an additional time, thereby increasing the volume of a single inspiratory phase by a predetermined percent.

38. The ventilator of claim 34 wherein the means for connecting said downstream conduit to said airway conduit comprises means for determining the volume of gas to pass from said downstream conduit into said airway conduit during each inspiratory phase, means for determining the time over which said volume is to be delivered, and means for actuating said phase-actuation means at the conclusion of that said time, and means actuated by said pneumatic timing means for increasing the volume of a single inspiratory phase by a predetermined and adjustable percent of the volume set by said means for determining the volume.

39. The ventilator of claim 34 wherein the means for connecting said downstream conduit to said airway conduit comprises means for determining the volume of gas to pass from said downstream conduit into said airway conduit during each inspiratory phase, means for determining the time over which said volume is to be delivered, and means for actuating said phase-actuation means at the conclusion of that said time, and wherein said pneumatic timing means generates a trigger signal when the sigh-frequency time is reached and said ventilator is in a said expiratory phase, means actuated by said trigger signal for resetting the sigh frequency timer to zero, time lengthening means actuated by said trigger signal for lengthening the time set by said means for determining the time so that the connection between said downstream conduit and said airway conduit is maintained during the additional time, for maintaining the trigger signal as the ventilator changes from its expiratory phase to its inspiratory phase, means for returning said trigger signal to a zero value as the ventilator changes from that said inspiratory phase to its next expiratory phase, means actuated when said trigger signal returns to zero for starting the sigh frequency timer on a new cycle and closing off said time lengthening means from said means determining the time.

40. The ventilator of claim 34 wherein said sigh means includes:

sigh volume control means for determining the additional volume delivered during said longer inspiratory phase, a main control valve connected to said pneumatic timing means and initiated thereby, and connected to said gas supply conduit and delivering a limited supply of gas directly therefrom when initiated, a communication control valve actuated when said main control valve delivers said gas from said gas supply conduit upon said initiation, said communication control valve being connected to phase-actuation means and acting when actuated to delay the termination of the next said inspiratory phase, and reset valve means connected to said main control valve and to said communication control valve for resetting them upon completion of said next inspiratory phase to the position and pressure they had prior to said initiation, and connected to said pneumatic timing means for initiating a new timing interval thereby upon completion of said next inspiratory phase.

41. The ventilator of claim 40 wherein said main control valve is connected to said downstream conduit and is biased thereby against initiation so long as the pressure in said downstream conduit is above atmospheric, thereby preventing the master control valve from being initiated during an inspiratory phase.

42. A ventilator for pressure-controlled operation with an inspiratory phase and an expiratory phase, including in combination:

a gas supply conduit for supplying a breathable gas under pressure, a downstream conduit, a command signal conduit, an airway conduit suitable for connection to a patient, means connecting said downstream conduit to said airway conduit, main valve means connected to said command signal conduit for connecting said gas supply conduit to said downstream conduit during each said inspiratory phase when the pressure in said command signal conduit rises above a first predetermined pressure level and for closing off said downstream conduit from said gas supply conduit during each said expiratory phase when the pressure in said command signal conduit drops below a second, lower predetermined pressure level, sensitivity control means connected to said airway conduit for connecting said gas supply conduit to said command signal conduit and thereby to raise the pressure therein above said first predetermined pressure level when the pressure in said airway conduit drops below a third predetermined pressure level due to the commencement by a patient of an inspiratory phase, and to close off said command signal conduit from said gas supply conduit when the pressure in said airway conduit rises above said third predetermined pressure level, means for bleeding said command signal conduit to atmosphere actuated by the pressure in said airway conduit reaching a fourth predetermined pressure level, as upon commencement of an expiratory phase, means connected to said command signal conduit for bleeding said downstream conduit to atmosphere when said command signal conduit pressure drops below said second predetermined pressure level, as at the beginning of said expiratory phase, ratio means connected to said command signal conduit, said downstream conduit and said gas supply conduit for ending said expiratory phase and initiating a new inspiratory phase by increasing the pressure in said command signal conduit above said first predetermined pressure level by supplying gas from said gas supply conduit to said command signal conduit, if the patient fails to initiate a new inspiratory phase for himself, after a time lapse following the commencement of a said expiratory phase, said ratio means having means for sensing the duration of each inspiratory phase and for establishing automatically a maximum duration of the succeeding expiratory phase as a time ratio of the inspiratory phase to the next succeeding expiratory phase, pneumatic timing means connected to said gas supply conduit, and sigh means connected to said means for bleeding said command signal conduit and actuated by said pneumatic timing means at intervals longer than the inspiratory-expiratory cycle of the ventilator as controlled without said pneumatic timing means by said phase actuation means and said ratio means, for raising the actuating pressure level of said means for bleeding said command signal conduit above said fourtH predetermined pressure level, to a higher fifth pressure level.

43. The ventilator of claim 42 wherein said pneumatic timing means comprises a needle valve connected to said gas supply conduit, a pressure-actuated main sigh control valve downstream of said needle valve and having a fixed-capacity chamber wherein the pressure builds up from atmospheric to the pressure for actuating said main sigh control valve, and means for bleeding said chamber to atmosphere after actuation of said main sigh control valve.

44. The ventilator of claim 43 wherein said sigh means comprises a communication control valve connected to said main sigh control valve and actuated thereby when downstream pressure therefrom is above atmospheric for connecting said airway conduit to said means for bleeding said command signal conduit to supply the pressure to raise said pressure from said fourth level to said fifth level, and means, actuated by said means for bleeding said downstream conduit, for de-actuating said communication control valve, disconnecting it from said airway conduit, and for bleeding to atmospheric the connection therefrom to said means for bleeding said command signal conduit.

45. The ventilator of claim 43 wherein said main sigh control valve is connected to and biased by said downstream conduit to prevent its actuation when said downstream conduit is above atmospheric pressure.