Ventilators for promoting lung function

A ventilator for inducing or assisting lung function in humans is characterized by a control system that provides for semi-automatic operation in which a single complete ventilation cycle in accordance with predetermined ventilation parameters (tidal volume, durations of inhalation and exhalation phases) is initiated by operation of a trigger. The ventilator is particularly useful in one-man resuscitation by CPR. The control system may require a discrete trigger operation to initiate each successive ventilation cycle or it may provide for continuous operation with timed successive cycles, in the manner of an automatic ventilator, by holding or latching the trigger. Various embodiments based on gas-powered ventilators are disclosed but the invention is also applicable to electrically operated or controlled ventilators. In a typical embodiment (FIG. 1) the output line (6) of a pneumatic oscillator is controlled by a diaphragm (22) held closed by a spring (36) loaded by a trigger (31) that when operated relieves the spring load and permits the diaphragm to open the output line. The output line pressure then acts on the diaphragm to hold the output line open until output pressure falls at the termination of the inhalation cycle, whether or not the trigger is released before that instant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns ventilators for inducing or assisting lung function 
in human patients. The invention is especially concerned with ventilators 
for use in resuscitation, rescue breathing and patient transport. 
2. Background Discussion and the Prior Art 
Ventilators for such purposes have been available for many years and two 
main types are in widespread use today: automatic ventilators in which the 
lung ventilation parameters (tidal volume, phase durations) are determined 
by control settings of the ventilator; and manual ventilators in which the 
ventilation parameters are directly controlled by the user. 
Automatic ventilators have the advantage that when once the control 
settings for required parameters have been established--in some cases once 
and for all during manufacture--efficient lung ventilation can be 
consistently maintained through repeated inhalation/exhalation cycles 
without intervention of the user. In manually controlled ventilators, the 
delivered tidal volume of breathable gas and the inhalation and exhalation 
times for each cycle are determined individually by action of the user, 
requiring continual attention and the exercise of judgement by the user. 
Automatic ventilators offer unquestionable advantages for rescue breathing 
and patient transport but can have some disadvantages in one-man 
resuscitation in cases of cardiac arrest, wherein the common resuscitation 
technique (cardiac pulmonary resuscitation, CPR) involves applying a 
sequence of chest compressions interrupted at regular intervals by one or 
more lung ventilations: typically two lung ventilation cycles followed by 
15 chest compressions followed by two further lung ventilation cycles, and 
so on. With an automatic ventilator continuously delivering a train of 
pulses of breathable gas to a face mask at a constant rate, this 
resuscitation technique demands that the face mask be applied to the 
patient for two ventilation cycles and then removed; the required number 
of chest compressions be then applied; and thereafter the face mask be 
reapplied to the patient: all in synchronism with the operation of the 
ventilator so that this delivers a breathable gas pulse immediately the 
mask has been applied or re-applied to the patient. Especially in the case 
of one-man resuscitation, this requires dexterity and skill on the part of 
the rescuer to achieve the required synchronisation of the manual 
operations with the cyclic operation of the ventilator and therefore many 
rescuers opt to use a manual ventilator in such circumstances; the manual 
ventilator allows the rescuer to initiate delivery of a breathable gas 
pulse when required to fit in with the sequence of chest compressions and 
mask application. 
A need can therefore be perceived for a rescue/resuscitation ventilator 
that can be more conveniently used in CPR than existing ventilators, by 
having the ability to deliver instantly on demand a single controlled 
ventilation cycle of predetermined tidal volume and duration, so avoiding 
the synchronisation requirements of the automatic ventilator and also the 
disadvantages inherent in the use of existing manual ventilators. 
There have already been proposals for ventilators that are switchable 
between an automatic mode of operation and a fully manually controlled 
mode. However, in the manually controlled mode, these ventilators exhibit 
the above discussed disadvantages of manually controlled ventilators and, 
thus, cannot offer a satisfactory solution to the problem. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a ventilator is characterised by 
a control system providing a semi-automatic mode of operation in which a 
complete ventilation cycle in accordance with predetermined ventilation 
parameters may be initiated by trigger operation. The control system may 
require a discrete trigger operation to initiate each of a succession of 
ventilation cycles or it may provide for sustained operation of the 
trigger to cause continued operation of the ventilator in the manner of an 
automatic ventilator: latching or functionally equivalent means may 
provide for continuous operation of the ventilator in the manner of an 
automatic ventilator, without user intervention. 
The invention may be implemented in ventilators in which functional control 
of the ventilation cycle is exercised by an electrically operated or 
controlled mechanism. However, because of the many advantages offered by 
gas-powered ventilators, the invention is preferably implemented in a 
gas-powered ventilator in which timing and control functions are provided 
by pneumatic mechanisms. The invention will accordingly be further 
discussed and described in respect of such an implementation. 
Thus, the invention is preferably implemented in a gas-powered ventilator 
the output of which is controlled by a pneumatic oscillator. This may take 
any of a variety of forms and may, for instance, be as disclosed in 
British Patent 1,533,550 or be of the construction in which a piston (or 
its equivalent, e.g. a diaphragm) is reciprocable to open and close a flow 
path between a source of pressurised breathable gas and a gas pulse 
output: examples of such an oscillator are disclosed in French Patent 
1,530,478 and U.S. Pat. No. 3,881,480. A pneumatic oscillator of such 
construction desirably incorporates the inventions disclosed in U.S. Pat. 
No. 4,922,962 and European Patent 0,342,883, and in U.S. Pat. No. 
5,007,457 and European Patent 0,343,818.

DESCRIPTION OF EMBODIMENTS 
Each illustrated embodiment incorporates a pneumatic oscillator that in 
these embodiments comprises a piston 1 reciprocable in a cylinder 2 and 
biased by a spring 3 towards the right as seen in the drawing, to cause an 
annular sealing lip 4 on the piston to engage a resilient seal surrounding 
a port 5 in the end of the cylinder 2. In the arrangement shown, the port 
5 constitutes an outlet port connected to output line 7 by way of an 
outlet branch 6 and a restrictor 8. The outlet branch 6 also connects with 
a feedback line 9 through a restrictor 10, the feedback line 9 connecting 
with the end of the cylinder 2 opposite to that containing the port 5. 
A port 11 at the righthand end of the cylinder 2 and outboard of the port 5 
serves for the admission of pressurised breathable gas, for instance 
compressed air or oxygen, to this end of the cylinder 2 from a source 
shown symbolically at 12. 
In a typical known automatic ventilator incorporating a pneumatic 
oscillator of the form just described, the outlet branch 6 connects 
directly with the port 5 of the cylinder 2 and to understand the following 
description of the operation of the oscillator, such a direct connection 
should be assumed. 
The drawing shows the piston 1 in a flow path-closing position in which the 
sealing lip 4 engages the seal surrounding port 5. With the piston in this 
position, breathable gas from the source 12 entering the port 11 is 
isolated from the port 5. However, source gas pressure acting on the 
annular area of the piston outboard of the sealing lip 5 is sufficient to 
overcome the thrust of spring 3 and to drive the piston to the left as 
seen in the drawings. This movement of the piston takes place, as 
described below, with a snap-action, connecting the port 11 to the port 5 
and thence to the outlet branch 6, so enabling the gas to flow via the 
restrictor 8 to the output line 7 and also, via the restrictor 10, to the 
feedback line 9. As a consequence of the flow of gas through the feedback 
line to the lefthand end of the cylinder 2, at a rate controlled by the 
restrictor 10, pressure builds up in the cylinder to act on the piston 1 
to supplement the force of the spring 3. Eventually the combined effects 
of the gas pressure and spring 3 cause the piston to move to the right as 
seen in the drawing, to return the piston to the illustrated position with 
the sealing lip 4 engaging the seal surrounding the port 5. 
As the piston approaches its illustrated righthand end position, flow to 
the outlet branch 6 is restricted and the pressure therein drops so that 
there is a sudden shift in the balance of forces on the piston 1 and this 
completes its movement to the right with a snap-action to cut off flow to 
the port 5 and the outlet branch 6. 
Pressure in the lefthand end of the cylinder 2 then decays by reverse flow 
through the feedback line and restrictors 10 and 8. When the gas pressure 
in the left hand end of the cylinder 2 has decayed to an appropriate 
extent, source gas pressure acting on the annular area of piston 1 
outboard of the sealing lip 4 once again overcomes the force of the spring 
3 and causes the piston 1 to commence to move towards the left as seen in 
the drawing. As it does so, it re-opens the pathway to the port 5 as above 
described and gas flows into the outlet branch 6, building up pressure 
therein which acts on the central area of the piston 1 to supplement the 
thrust of the source pressure on the outboard annular area of the piston. 
There is in consequence an abrupt change in the balance of forces acting 
on the piston 1 which moves with a snap-action to its lefthand end 
position. The described cycle repeats with a frequency determined by the 
relationship between the annular area outboard of the sealing lip and the 
total cylinder area, the bias force supplied by the spring 3 and the 
characteristics of the restrictors 8 and 10. 
The principles of the operation of this form of pneumatic oscillator may be 
embodied in various arrangements in practical devices. For instance the 
restrictor 10 may be replaced by various restrictor/non return valve 
networks to achieve particular cycling patterns in the output line and to 
provide different operator control possibilities. The biasing of the 
piston may be achieved by means other than a spring: for instance the 
piston may have different areas effective at its opposite ends so that 
when both ends of the piston are exposed to equivalent pressures it 
experiences a net thrust towards the flow path-closing position. Whereas 
in the arrangement shown the port 5 constitutes an outlet port and the 
port 11 constitutes an inlet, the converse arrangement is possible. 
Moreover, the sealing lip 4 may be arranged on the end of the cylinder 
around the port 5 and co-act with a resilient facing on the end of the 
piston. 
In accordance with the invention the oscillator is associated with a 
control system that provides for operation of the oscillator in a 
semi-automatic mode in which it performs individual cycles of operation in 
response to trigger action. 
Thus, in the illustrated embodiments of the invention, the connection 
between the outlet branch 6 and the port 5 in the oscillator configuration 
is modified by inclusion of a valve arrangement 20 comprising a chamber 21 
having a diaphragm 22 forming one wall thereof and movable to engage a 
sealing lip 23 surrounding an inlet port 24 that is connected to the port 
5 of the cylinder 2. Outboard of the sealing lip 23 the chamber 21 has a 
connection 25 to the outlet branch 6. In practical embodiments the chamber 
21 may be formed directly on the righthand end of the cylinder 2 so that 
port 24 is integrated with port 5. Alternatively, the valve arrangement 20 
may be remote from the cylinder 2, its location being determined by the 
need for convenient operation of the trigger mechanism, described below, 
by the user of the ventilator. 
Valve arrangement 20 provides a means for enabling and disabling the 
pneumatic oscillator without affecting any of its connections or the 
settings of the controls--such as restrictors 8 and 10--that determine its 
performance parameters. Thus, when diaphragm 22 is engaged with sealing 
lip 23 to isolate port 24 from port 25 and thus interrupt the connection 
between port 5 and the outlet branch 6, source pressure at port 11 of 
cylinder 2 is effective only to drive the piston 1 back to its lefthand 
end position in cylinder 2, but not to cause a flow of breathable gas to 
output line 7. However, when diaphragm 22 is clear of sealing lip 23, 
outlet branch 6 is effectively connected to port 5 so that oscillator 
action proceeds in the manner described, delivering to output line 7 
breathable gas pulses of predetermined volume at intervals determined by 
the control settings of the oscillator. 
The valve arrangement 20 is controlled by a trigger mechanism 30 comprising 
a trigger lever 31 pivoted at 32 and subject to the thrust of a bias 
spring 33 tending to rotate the lever 31 to engage its end 34 with a stop 
35. A spring 36 acts between lever 31 and diaphragm 22, urging diaphragm 
22 towards contact with sealing lip 23 to interrupt communication between 
port 5 and outlet branch 6, to inhibit operation of the oscillator in the 
manner described. 
The arrangement is such that spring 33 is capable of holding the end 34 of 
lever 31 engaged with stop 35, for any position of the diaphragm 22, 
overpowering spring 36 as required. In this position of the lever 31, 
spring 36 is capable of holding diaphragm 22 in sealing engagement with 
sealing lip 23 against the thrust of source gas pressure acting only on 
the diaphragm area within the sealing lip 23, but not against source gas 
pressure acting on the full area of the diaphragm. 
Thus, with the trigger lever 31 released and engaged with stop 35, and 
diaphragm 22 engaged with sealing lip 23, the oscillator action is held 
arrested with the piston 1 in its lefthand end position. However, rocking 
of trigger lever 31 in a clockwise direction about its pivot 32 as seen in 
the drawing and against the action of spring 33 relaxes the thrust of 
spring 36 upon diaphragm 22 and so enables the latter to lift clear of the 
sealing lip 23. Gas immediately then flows from source 12 via ports 11 and 
5 of cylinder 2 and port 24 into the chamber 21, the pressure of this gas 
then acting on the full area of diaphragm 22 to hold the latter clear of 
sealing lip 23 for so long as source pressure is maintained at port 24 and 
whether or not trigger lever 31 is released and moves under the influence 
of spring 33 to re-engage its end 34 with stop 35. That is to say, the 
source pressure acting on the full area of diaphragm 22 is sufficient to 
overcome the thrust of spring 36 whatever the position of trigger lever 
31. 
It will accordingly be understood that if trigger lever 31 is momentarily 
depressed to relieve the thrust of spring 36 on diaphragm 22, a single 
full oscillator cycle will ensue with output of breathable gas at source 
pressure to outlet branch 6 until piston 1 moves to the right as seen in 
the drawing to interrupt communication between ports 11 and 5 of cylinder 
2 in the manner described. At this point in the oscillator cycle, gas 
pressure in chamber 21 and outlet branch 6 commences to diminish and the 
thrust of spring 36 eventually moves diaphragm 22 into sealing engagement 
with sealing lip 23 to isolate port 5 from outlet branch 6. The oscillator 
cycle terminates when source gas pressure acting on piston 1 has restored 
the latter to its lefthand end position in cylinder 2. 
In the embodiment of FIG. 1, if trigger lever 31 is held depressed so that 
spring 36 is unable to force diaphragm 22 into sealing engagement with 
sealing lip 23, oscillator action will continue, so as to deliver 
breathing gas pulses to output line 7 in the manner of automatic 
ventilator, for so long as trigger lever 31 is held. When lever 31 is 
eventually released, oscillator action terminates with completion of the 
cycle then in progress, and restoration of piston 1 to its lefthand end 
position ready to commence a further cycle or series of cycles whenever 
the trigger lever 31 is again depressed. 
The embodiment of FIG. 1 includes latching means in the form of a switch 
knob 40 having a cam or projection 41 that can be rotated from the 
position shown, in which projection 41 is clear of trigger lever 31, to a 
position in which projection 41 engages and holds trigger lever 31 in its 
depressed position against the action of spring 33, to provide for 
continuous fully automatic operation of the ventilator through continuous 
operation of the oscillator in the manner described, without manual 
intervention by the user. 
It will accordingly be understood that by setting knob 40 in the position 
shown, a single operating cycle of the ventilator may be initiated, with 
instantaneous output of breathing gas, by brief depression of the trigger 
lever 31. The delivered tidal volume and the duration of the ventilation 
cycle will be determined by the ventilation parameters applicable to 
automatic operation: these are unaffected by the duration of depression of 
the trigger lever, provided that such duration is less than the period of 
one ventilation cycle. 
Moreover, for so long as the trigger lever 31 is held depressed, the 
ventilator will continue fully automatic operation: this can result from 
the trigger lever being held depressed manually by the user or by 
appropriate setting of the knob 40. 
The function of the latching means constituted by the knob 40 with its cam 
or projection 41 is to maintain open the connection between port 5 and 
outlet branch 6. This function can of course be achieved by other means, 
for instance a stop valve in a bypass connecting port 5 with branch 6. 
The embodiment illustrated in FIG. 2 differs functionally from that of FIG. 
1 in that depression of the trigger lever 31 is effective to release only 
a single cycle of operation by the oscillator. Thus, in this embodiment, 
the knob 40 is omitted because continuous depression of the lever 31 is 
ineffective to cause continuous automatic operation of the ventilator. 
The embodiment of FIG. 2 is characterised by a diaphragm 51 that acts 
through a spring 52 on the diaphragm 22 to supplement the action of spring 
36. Diaphragm 51 bounds a chamber (not shown) that is connected to outlet 
branch 6 via a non-return valve 53 and a line 54 that is also connected to 
a release orifice 55 that is obturated by a pad 56 on trigger lever 31 
when the latter is depressed. 
Thus, in the embodiment of FIG. 2, when trigger lever 31 is depressed and 
held to relieve the thrust of spring 36 on diaphragm 22, the latter lifts 
clear of the sealing lip 23 to establish communication between the 
cylinder port 5 and the outlet branch 6 in the manner described in 
relation to FIG. 1. As in the arrangement of FIG. 1, while the source gas 
pressure is thus applied to the full area of diaphragm 22 it is effective 
to hold this clear of sealing lip 23 for the duration of the required 
delivery of a gas pulse to output line 7, overpowering spring 36 should 
trigger lever 31 happen to be released during this period. However, in the 
arrangement of FIG. 2, when communication is established as described 
between port 5 and outlet branch 6, source gas pressure is also 
concurrently applied to diaphragm 51 thereby to increase the thrust of 
spring 52 upon diaphragm 22. This increased thrust of spring 52, by itself 
or in conjunction with the thrust of spring 36 with lever 31 released, is 
incapable of forcing diaphragm 22 into sealing engagement with sealing lip 
23 until source gas pressure is cut off by the piston 1 moving to its 
righthand end position in cylinder 2: however, the thrust of spring 52 is 
sufficient to move diaphragm 22 into sealing engagement with the sealing 
lip 23 when source gas pressure is so cut off from the port 24 by action 
of piston 1, and pressure accordingly declines in chamber 21. Thus, should 
the trigger lever 31 be held depressed when piston 1 terminates a gas 
pulse, so that spring 36 is ineffective then to terminate the operation of 
the oscillator, the thrust of spring 52 on diaphragm 22 performs this 
function of arresting operation of the oscillator. The non-return valve 53 
traps source pressure behind diaphragm 51 until such time as trigger lever 
31 is released to enable spring 33 to rock pad 56 clear of the release 
orifice 55. 
A ventilator having its valve arrangement 20 controlled in the manner 
illustrated in FIG. 2 is thus not capable of continued automatic operation 
by holding its trigger lever 31 depressed. Automatic operation, if 
required, must accordingly be obtained by alternative latching means such 
as a stop valve (not shown) in a bypass for valve arrangement 20, or a 
stop valve in the connection between outlet branch 6 and line 54. In the 
case of the latter arrangement, the means for operating the stop valve 
would conveniently include means for holding trigger lever 31 depressed 
for fully automatic operation. 
FIG. 3 illustrates a modification of the arrangement of FIG. 1, in which 
the restrictor 10 is bypassed by a non-return valve 61 and a stop valve 62 
so linked to the knob 40 as to be opened when this is set for manual 
(semi-automatic) operation. By this means the exhalation phase duration is 
reduced during semi-automatic operation of the ventilator under control of 
the trigger lever 31 and ensures that the piston 1 is rapidly returned to 
its lefthand end position in cylinder 2 following termination of the 
delivery of a breathing gas pulse initiated by a depression of the trigger 
lever 31. In this way the oscillator is rapidly "primed" to be able to 
deliver another full-volume breathable gas pulse immediately upon 
depression of the trigger lever 31. This facility can be advantageous in 
enabling another ventilation cycle to be initiated during CPR should the 
chest compression sequence be terminated earlier than the end of a 
"normal" exhalation phase duration. 
The embodiment illustrated in FIG. 4 provides for operation of the 
ventilator in a fully automatic mode at start-up, unless the 
semi-automatic mode is deliberately selected at that time. In this 
embodiment a bypass circuit comprising lines 70 and 71 respectively 
connected to port 5 and outlet branch 6 bridges the valve arrangement 20 
to disable the function thereof to provide fully automatic operation of 
the ventilator when this is required. The bypass includes a valve 73 
comprising a shuttle 74 biassed by a spring 75 towards the position in 
which it opens the bypass to enable fully automatic operation. The shuttle 
74 is also subject to the pressure of source gas supplied from source 12 
over line 76 and the arrangement is such that normal source gas pressure 
produces a thrust on the shuttle that balances the thrust due to spring 
75. 
Shuttle 74 of valve 73 is also mechanically connected to trigger lever 31 
by the connection indicated schematically at 77, so that depression of 
trigger lever 31 to obtain semi-automatic operation of the ventilator is 
effective to displace shuttle 74 to the position in which it interrupts 
the bypass connection between lines 70 and 71, so making valve arrangement 
20 effective to control the oscillator. 
It will be understood that when the supply of breathable gas from source 12 
is interrupted, spring 75 shifts shuttle 74 into the position to open the 
bypass constituted by lines 70 and 71. Accordingly, upon restoration of 
source gas pressure the bypass remains open and the ventilator commences 
operation in its fully automatic mode. However, the first depression of 
trigger lever 31 shifts the shuttle 74 to the position in which the 
connection between lines 70 and 71 is interrupted, the shuttle being held 
in this position thereafter so long as source gas pressure is available 
over line 76 to balance the thrust of spring 75. The valve arrangement 20 
thus becomes effective to control semi-automatic operation of the 
ventilator, under control of trigger lever 31 in the manner described in 
relation to FIG. 1. Means, such as the knob 40 and projection 41 of FIG. 1 
may be provided to hold trigger lever 31 depressed for maintained 
automatic operation after a period of semi-automatic operation. 
A ventilator in accordance with the invention may incorporate all the usual 
facilities of lung ventilators. For instance, the breathable gas output by 
the ventilator may be supplied to any desired patient interface device: 
e.g. face mask or intra-tracheal tube, via an appropriate patient valve if 
required to provide for exhalation without the need for mask or tube 
removal, and the ventilator or the patient interface device may 
incorporate air mix facilities for diluting an oxygen source gas with air 
as and when required. Thus, for instance a gas-powered ventilator in 
accordance with the invention may be arranged to deliver oxygen from a 
suitable source to a patient valve including an entrainment mixer arranged 
in the manner disclosed in British Patent 2,174,609. The ventilator may 
also include a demand valve to permit spontaneous breathing by the patient 
to inhibit ventilator operation, for instance as disclosed in U.S. Pat. 
No. 5,016,626 and European Patent 0,343,824. 
The functional components of a ventilator in accordance with the invention 
can be variously arranged to suit operational requirements. For example, 
the functional components may be arranged in a control unit connected by 
suitable ducting to a patient interface device or some or all of the 
functional components may be integrated with a patient interface device 
such as a face mask for convenient one-man operation. 
As previously noted, the trigger mechanism is desirably arranged to be 
conveniently accessible to the user. This may be accomplished by 
incorporating the trigger mechanism and associated valve arrangement in 
the patient interface device but an alternative possibility is to arrange 
the trigger mechanism in a separate control unit and in a form in which it 
could be operated by the knee, foot or elbow of the user at a distance 
from the patient interface device. 
While the invention has been described in respect of its preferred 
embodiments that involve gas-powered ventilators incorporating a pneumatic 
oscillator, it should be understood, as has been noted above, that the 
concept of the invention may be implemented in ventilators of other types 
and, especially, in ventilators in which functional control is exercised 
by electrical means.