For patient ventilation, typically the wind pipe of the patient has special tube placed in it (tracheal tube).

This is conventional management of patient who is not breathing. Lungs are then inflated rhythmically to mimic normal breathing by a mechanical ventilator. The lungs are inflated by gas pushed down tracheal tube by the action of the mechanical ventilator. The ventilator uses energy to do this as it is counteracting the natural recoil of the lungs. To allow the lungs to exhale (breathe out), the ventilator in effect opens the tracheal tube to ambient atmosphere, then the cycle starts again. Typical dose of a delivered breath to an adult would be 800ml and a suitable rate would be 10 of these dosed breaths per minute. Transport of mechanically ventilated patients (for example in an ambulance) may require a large oxygen bottle for long journeys if gas powered ventilator used.

Improved ventilator apparatus has now been devised.

According to a first aspect, the invention provides: a pressurised gas feed arrangement for a driving gas;

a double acting drive piston and cylinder arrangement connected to the pressurised driving gas feed arrangement; a respiration gas pump arrangement operatively driven by the double acting drive piston ; A valve arrangement including switching valve means to switch delivery of the driving gas to the double acting drive piston and cylinder arrangement.

The respiration gas pump preferably comprises a driven piston and cylinder arrangement, the driven piston of the respiration pump being operatively connected to and driven by the drive piston. Beneficially the driven piston of the respiration pump is co-axially aligned with the drive piston.

The swept volume per stroke of the driving piston and cylinder arrangement is preferably significantly smaller than the swept volume displaced by the respiration pump per stroke of the driving piston.

The arrangement provides that energy stored as compression of the driving gas (for example in a bottle or canister of compressed gas) is imparted to a larger volume of gas (in the respiration pump arrangement) in a very efficient manner.

The maximum pressure to be delivered to the lungs should be approximately 60cm of water (approximately. 006 Bar).

Most industrial pneumatic logic systems require a gas pressure of 2-6 Bar to work efficiently. The ventilator apparatus therefore needs the gas pressure in the driving gas cylinder to be reduced using a pressure reduction valve to a working pressure equivalent to the lowest pressure in the pneumatic valve logic circuits (typically 2 Bar). The length of stroke of the driving piston is preferably substantially equal to the length of stroke of the driven piston.

One or more flow regulation valves preferably control the pressure and/or flow rate of the gas fed pressure acting on the inlet side into the drive piston and cylinder arrangement. Flow rate regulation valves are preferably provided, flow wise, between switching valve means and the drive piston/cylinder arrangement. Beneficially, the flow rate regulation valves are included in respect of opposed inlet/outlet drive gas flow lines at either opposed end of the double acting drive cylinder arrangement. Use of flow rate regulation valves (particularly so-called needle valves) to control duration of inspiratory phase and expiratory phase on the inlet side of the drive cylinder is believed to be novel and inventive. Industrial systems commonly use full driving gas pressure to the drive cylinder and arrange the needle control valve in the outlet gas port on the exit side of the drive cylinder. The prior art arrangements give better control of cylinder speed but do not optimise use efficiency of the driving gas. The present invention is designed to maximise the efficiency of use of the available driving gas (typically oxygen gas) and the provision of the control valves operated to control

flow on the inlet side of the gas to the drive cylinder is an important aspect in this respect, as is the lowest working pressure possible for the logic circuits and the smallest diameter as able for the drive cylinder.

Beneficially the switching valve means is arranged to be switched by switching gas pulses acting at predetermined points in the stroke of the driving piston.

The apparatus preferably comprises a switching transducer arrangement including sensor means (preferably magnetic sensor means) arranged to produce a trigger switching output acting at predetermined points in the stroke of the driving piston.

It is preferred that sensors with three gas ports are provided having : a port for constant supply of gas pressure; a port for a switching pulse to be delivered to the switching valve; and a port permitting the sensor to reset itself once the driving piston has been displaced.

Such a three port sensor effectively decompresses internally as it resets and the third port allows a puff of gas (typically oxygen) to be directed to a respiration gas reservoir (where present) to be breathed. An important and inventive aspect of the present arrangement is that the

decompression driving gas from the pistons can be directed to a respiration gas reservoir rather than simply leaking into atmosphere.

It is further preferred that a respiration gas reservoir is connected by a flowpath to the respiration pump, a flowpath also existing from the respiration pump to a patient locatable outlet. The flowpath from the respiration pump to the patient locatable outlet preferably includes a vent flowpath for expired gas. Desirably the vent flowpath is associated with a non-return valve in the flowpath from the respiration pump to the patient locatable outlet, the non- return valve inhibiting gas expired from the patient from returning to the respiration pump or the respiration gas reservoir.

The apparatus preferably operates such that a return stroke of the driving piston causes air to be drawn from the respiration gas reservoir into the respiration pump, the forward stroke of the drive piston effecting displacement of the drawn air from the respiration pump and through the patient locatable outlet.

A non-return valve is preferably positioned in the flowpath from the respiration gas reservoir, thereby inhibiting gas returning toward the respiration gas reservoir. Driving gas preferably directed to combine with gas drawn from the respiration gas reservoir following passing of driving gas from the driving piston cylinder arrangement.

In one embodiment the double acting drive piston and

cylinder arrangement preferably comprises, in parallel, a forward stroke active piston and cylinder arrangement, and a return stroke active piston cylinder arrangement, both arrangements being operatively coupled with the respiration gas pump arrangement.

The return stroke active arrangement is preferably of smaller swept volume than the forward stoke active arrangement, beneficially having a smaller diameter piston/cylinder. This arrangement provides advantages because more force is required to inflate the lung (up to 60cms of water) than is required during exhalation of gas from the lung. During the exhalation the drive piston is not doing a great deal of work (the majority of work being done by contraction of the lungs), the drive piston is merely drawing in air (typically through the open end of the respiration gas reservoir) and overcoming seal friction as the respiration gas pump (typically the driven piston thereof) returns to a"start"position ready for the next inflation of the lungs. A small piston/cylinder is therefore typically coupled in parallel with the larger forward stroke active drive piston cylinder arrangement.

The diameter of the return piston/cylinder can therefore be very small, minimising the volume of drive gas used.

In an alternative embodiment, the drive piston may operatively be coupled with an actuator of the respirator pump arrangement via a lever arrangement in order to impart a degree of mechanical advantage to the system.

In a further alternative embodiment, the respiration gas

pump comprises a driven piston and cylinder arrangement, the piston of the respiration pump being operatively connected to and driven by the drive piston, the apparatus including: a first flowpath from the respiration gas pump to a patient locatable outlet, the first flowpath being active on one of the forward and reverse strokes of the respiration gas pump piston ; and, a second flowpath from the respiration gas pump to the said patient locatable outlet, the second flowpath being active on the other of the forward and reverse strokes of the respiration gas pump piston.

The invention will now be further described in specific embodiments by way of example only and with reference to the accompanying drawings in which: Figure 1 is a schematic view of ventilator apparatus according to the invention; Figure 2 is a schematic view of the ventilator apparatus of Figure 1 at a subsequent point in the operational cycle; Figure 3 is a schematic view of the apparatus of Figures 1 and 2 at a further sequential point in the operational cycle; Figure 4 is a schematic view of the apparatus of Figures 1 to 3 in a further point in the operational cycle; and

Figure 5 is a schematic view of an alternative embodiment of ventilator apparatus according to the invention; Figure 6 is a schematic view of the apparatus of Figure 5 at a different point in the sequence of operation of the cycle ; Figure 7 is a schematic view of the apparatus of Figures 5 and 6 at a still further point in the operational cycle; Figure 8 is a schematic view of the apparatus of Figures 5 to 7 at a still further point in the operational cycle; Figure 9 is a perspective view of an alternative arrangement in accordance with the invention; Figure 10 is a schematic view of an alternative embodiment of apparatus in accordance with the invention; and Figure 11 is a schematic view of the apparatus of Figure 10 at a different point in the operational cycle.

Referring to the drawings and initially to the apparatus of Figures 1 to 4, there is shown ventilator apparatus generally designated 1. The ventilator apparatus 1 includes a delivery flowline 2 extending from an open ended reservoir limb 3 to a connection end 4 for connection to the lungs of a patient. A non-return valve 5 is included in the flowline proximate the patient end 4 to prevent exhaled air returning into the main flowline. Exhaled air is directed to exit through exit port 6. A one way valve

7 is included in the flowline 2 closer to the open ended reservoir limb 3 to maintain pressurisation in the flow line. A branch 8 in the flowline leads to a respiration gas pump arrangement 9 comprising a large diameter driven piston 10 accommodated within a cylinder 11. The piston rod 12 comprises a common piston rod, the other end of which supports a much smaller diameter drive piston 13 which is accommodated within a smaller diameter drive cylinder 14. The drive piston 13 and cylinder 14 arrangement comprises a double acting pneumatic actuator, drive gas being fed into drive cylinder 14 via ports 15,16 at opposed ends of cylinder 14.

; Driving gas flows into or out of drive cylinder 14 via a ports 15 or 16 depending upon whether a forward or reverse stroke of drive piston 13 is required. A shuttle valve 17 switches the driving gas flow to enter/exit via the ports 15,16. Upstream of shuttle valve 17, connected to inlet port 18, is a source of driving gas such as a canister or bottle of pressurised oxygen (not shown). The magnetic sensors 19, 20 sense the position of the piston 13 and trigger a gas pulse (exemplified by arrow A) to cause the shuttle valve 17 to switch supply between the ports 15,16.

A piston sensing arrangement other than magnetic may be utilised. The magnetic arrangement is exemplary only.

Intermediate the shuttle valves 17 and respective ports 15, 16 are respective needle valves 20,21 configured to control the flow rate of the driving gas entering the drive cylinder. This is a departure from typical prior art systems in which the needle valves are typically arranged to control flow of gas exhausting from a drive cylinder.

This is an important feature of the invention because it ensures that the gas flow to drive piston 13 is optimised (minimised). The pneumatic logic circuitry is therefore run at lowest possible gas pressure conserving drive gas.

Interconnecting pipes between components are kept as short as possible and of smallest diameter feasible, for the same reason. Gas exhausts from the drive cylinder 14. The trigger originates from the magnetic sensors. Exhaust gas may be subsequently delivered (for example, via a port 22) to supplement the gas drawn through the reservoir limb 3.

The small diameter drive piston 13 (operating in small diameter drive cylinder 14) directly drives the larger diameter driven piston 10 (operating in cylinder 11). The ratio of cross-sectional areas of the two pistons 10,13 is, for best efficiency, arrived at by consideration of the maximum pressure (typically 60'cm of water) the driven piston 10 requires to achieve inflation of the lungs with the required volume of respiration gas provided by the drive gas pressure delivered to act on the drive piston 13 (typically 2 Bar). The length of stroke of the pistons is tailored to deliver the required breath volume to the patient (typically 800 ml). For example, a 10 cm diameter large driven cylinder has a cross-sectional area of 78.5 cm2 ; to deliver an 800 ml breath to the tracheal tube at outlet 4, the piston 10 of the large cylinder 11 would need to travel a distance of 10.2 cm. With a maximum available gas pressure of, for example, 2 Bar, the cross-sectional area of the small drive cylinder should be approximately 2.36 cm2 giving a diameter of 1.74 cm for the drive piston 13. This assumes 100% mechanical efficiency and does not

include compensation for friction losses.

Starting with the arrangement shown in Figure 1, the left hand end of drive cylinder 14 is made active with gas from the pressurised oxygen store being directed through shuttle valve 17 and needle valve 20 to retract pistons 13 and 10 in the direction of arrow B in Figure 2. As this occurs, air is drawn into the cylinder 11 from the open ended reservoir limb via the one way valve 7. This is the exhalation stoke as far as the patients lungs are concerned and the exhaled gas from the lungs exits through atmosphere via the non-return valve 5. When piston 13 reaches the end of the stoke (as shown in Figure 3) magnetic sensor 20 triggers a gas pulse to re-set shuttle valve 17 such that pressurised oxygen driving gas entering via port 18 is directed to flow through right-hand end port 16 via needle valve 21, thereby initiating the forward stroke of pistons 13 and 10.

As shown in Figure 4, on the forward stroke with the pistons moving in direction of arrow C the air previously drawn into cylinder 11 is forced outwardly through branch 8 and (since it cannot flow backward via one way valve 7) onward along flowline 2 through non-return valve 5 to inflate the patients lungs.

Turning now to the arrangement shown in Figures 5 to 8, the arrangement is generally similar in terms of construction and operation to the embodiments shown in Figures 1 to 4, however, in this embodiment the drive cylinder arrangement comprises a pair of drive pistons 13a, 13b, piston 13a

being of larger diameter than piston 13b. Each respective piston 13a, 13b is received in a separate respective cylinder 14,14b.

The arrangement shown in which pistons 13a, 13b act in parallel provides even more efficient use of the pressurised driving gas entering shuttle valve 17 via port 18. On the forward stroke the gas pressure in drive cylinder 14 acts in a similar way as described in relation to the embodiment of Figures 1 to 4. However, on the return stroke, less volume of pressurised gas is required to return the cylinders to the initial position. This is because in operating on the return stroke less force is required than the force required to inflate the lungs on the forward stroke. During the return stroke the drive piston arrangement is not performing as much work as it is on the forward stroke ; on the return stroke the drive piston arrangement is merely drawing air in through the open end of the reservoir limb 3 and overcoming seal friction.

In other respects operation of the embodiment shown in Figures 5 to 8 is generally similar to operation of the embodiments shown in Figures 1 to 4.

In the arrangement shown in Figure 9, the drive piston 13 is coupled to the driven piston 10 by means of an interposed lever arrangement 25 including a lever arm 26 pivoting around a fixed fulcrum point 27. This permits a degree of mechanical advantage to the applied to the system which may have technical benefits.

In the embodiments shown in Figures 10 and 11, the ventilator apparatus includes a drive piston 113 acting in a similar manner to the drive piston 13 described in preceding embodiments. In this embodiment however the flowpath is more complicated and the driven piston 110 is double acting in cylinder 111. The ratio of the diameters of the two cylinders is calculated along the lines of the preceding embodiments and depends upon maximum pressure that the driven piston 110 required to generate. One way flap valves 107,147 are present to allow the system to draw air from the reservoir in 103 and pump it via lower exit port 104 at near constant flow. As in preceding embodiments, air in the reservoir limb 103 may be enriched with"waste"driving gas (typically oxygen) exiting from the drive cylinder. The system is effectively a gas powered pump for pumping gas at near constant flow, tuned for maximum efficiency of use of the pressurised driving gas. Such a device is useful in certain medical systems such as Continuous Positive Airway Pressure (CPAP) machines which require constant high flow of gas. To use pure oxygen from a compressed bottle would require a very large gas bottle. Pure oxygen can be harmful in babies (for example) and a lower percentage of delivered oxygen may be more appropriate. The system described would permit relatively small flow of oxygen to produce much higher total gas flow.

The systems described herein are particularly useful in medical situations where patients require ventilating. The invention can beneficially provide a self contained ventilator and small oxygen bottle for use by military and

civilian paramedics. Use of a small oxygen bottle improves system portability and enables reasonable endurance of use; this is not possible with other designs of gas powered ventilator.