WO2004004815A1

WO2004004815A1 - Patient ventilating apparatus

Info

Publication number: WO2004004815A1
Application number: PCT/GB2003/002860
Authority: WO
Inventors: Paul Fenton
Original assignee: Paul Fenton
Priority date: 2002-07-03
Filing date: 2003-07-03
Publication date: 2004-01-15
Also published as: AU2003253101A1

Abstract

The present invention provides a patient ventilating apparatus (1) for ventilation of the lungs. The invention comprises a case (2) located on a flexible support (3), the two forming a chamber for containment of respirable gas. The case moves on the support during the respiratory cycle to vary the volume of the chamber. Preferably the case has a means (19) defining a chamber located within it and said means defining a chamber is in fluid communication with the chamber part of the flexible support. The present invention also provides patient ventilating apparatus for ventilation of the lungs comprising a means defining a chamber located at least partially within a case. The region between the case and the means defining a chamber comprises a plenum and said plenum is in fluid communication with a conduit through which gas may be admitted or expelled from said plenum varying the pressure within. The chamber has an outlet such that respirable gas may be expelled in order to supply respirable gas to a patient. The means defining a chamber and the case are arranged such that the volume of said chamber may be adjusted either by: (1) varying pressure to the plenum, or (2) a user applying physical force to the means defining a chamber.

Description

Patient Ventilating Apparatus

Background to the Invention

The present invention relates to an apparatus for the ventilation of a patient's lungs.

Anaesthesia or resuscitation breathing apparatus is used to provide anaesthetic gas or oxygen to a patient during a surgical procedure or during a period of respiratory insufficiency. Anaesthetic gas means a respirable gas containing an inhalation anaesthetic vapour and normally oxygen enrichment as well. In the course of an operation a patient may breathe spontaneously or have respiration assisted manually by an anaesthetist or by a mechanical ventilator. The latter two methods are also called intermittent positive pressure ventilation (I.P.P.N.) to distinguish this mode from the negative pressure created during spontaneous (normal) ventilation. When a mechanical ventilator is incorporated in a breathing system, the patient may breathe spontaneously through the system (ventilator switched off) or receive ventilatory support, which is then called controlled or mechanical ventilation. Controlled ventilation may be required during or after many surgical procedures according to the patient's condition, the type of operation being performed, or during a period of resuscitation.

One type of breathing system for anaesthesia, known as the draw over system, is particularly suited for use in developing countries where bottles of compressed anaesthetic gas may not be available. In the draw-over system, ambient air with or without oxygen enrichment may be drawn through a liquid anaesthetic agent vaporizing device, called a vaporizer, by negative pressure created by the patient's inspiratory effort. The oxygen enrichment may be provided by an oxygen concentrator, an electrical device that extracts oxygen from room air. The resultant anaesthetic gas enters the patient's lungs via a non-rebreathing, inflating valve, the exhaled gas being vented to atmosphere via the same valve. In another configuration, called the circle system, pressurized anaesthetic gas is controlled by a flow device called a rotameter and then passes through a different type of vaporizer into a recirculating breathing system. The patient may breathe spontaneously or receive I.P.PN. from a mechanical ventilator incorporated in the system. Most exhaled gas is re-circulated after absorption of carbon dioxide in a soda lime absorber and is not discharged direct to atmosphere.

Despite being of great utility the draw-over system has several disadvantages: discharge of the expired gas is wasteful of oxygen and anaesthetic agent, the anaesthetic gas is an environmental pollutant and may be toxic to members of the operating theatre team. Scavenging and disposal of waste gas from the non-rebreathing valve is difficult since it is required to be located close to the patient's airway. The valve is relatively large and it may thus be inconvenient, concealed from the anaesthetist by surgical drapes or its weight may cause a tracheal breathing tube to be pulled out. The inflating function of the valve makes it possible to become jammed with consequent pressure damage to the patient's lungs. Few mechanical ventilators are currently available for use with draw-over systems which further limits their use.

The circle system, being less polluting and more economical, is the most widely used anaesthesia system but it depends upon a bottled compressed anaesthetic gas and oxygen supply. Many types of mechanical ventilator are available for circle systems but they all have a requirement for compressed driving gas, and usually electricity, in order to function. Most commonly, the design of ventilator has a transparent rigid dome with an oscillating bellows inside, the movement of which displaces gas from the ventilator and inflates the patients lungs. This type has no facility for manual operation and cannot function in the event of power or gas failure.

It is an object of the present invention to provide an improved anaesthesia or respiratory support system suitable for use with circle, draw-over or other patient breathing system, or at least provide the public with a useful choice. The present invention can be powered by compressed air from the electrically-driven compressor of an oxygen concentrator, the latter configured with a separate air pipeline, or from a separate compressor and can therefore operate independent of bottled compressed gas supplies. The aforementioned dome-and-bellows concept can be retained in the present invention but the anaesthetist may still ventilate the patient by hand in the event of electrical or mechanical failure or for resuscitation or any emergency. In the preferred embodiment, this is done by manually raising a dome, which is mounted on a bellows, to draw room air into the system and depressing it to ventilate the patients lungs.

Summary of the Invention

According to a first aspect, the present invention provides a patient ventilating apparatus for ventilation of the lungs comprising a case located on a flexible support, the two forming a chamber for containment of respirable gas, wherein the case moves on the support during the respiratory cycle to vary the volume of the chamber.

Preferably, a means defining a chamber is located within the case. The means defining a chamber is flexible and in fluid connection with the chamber part of the flexible support. In the preferred embodiment, the means defining a chamber is a second bellows (also referred to herein as the internal bellows), and is mounted on a flange within the case in a gas tight manner such that the patient's respirable gas from the chamber can freely enter the interior of this bellows but cannot enter the space between the bellows and case. The second bellows may be of smaller volume than the flexible support.

According to a second aspect, the present invention provides a patient ventilating apparatus for ventilation of the lungs comprising a means defining a chamber located at least partially within a case, the region between the case and the means defining a chamber comprising a plenum, said plenum in communication with a conduit through which gas may be admitted or expelled from said plenum varying the pressure within, said chamber having an outlet such that respirable gas may be expelled in order to supply respirable gas to a patient, wherein said means defining a chamber and said case are arranged such that the volume of said chamber may be adjusted either by: (1) varying pressure to the plenum, or (2) a user applying physical force to the means defining a chamber.

The conduit for admitting or expelling gas that is in communication with the plenum may take the form of a single conduit, or two separate conduits. Further, the chamber preferably also comprises an inlet as well as an outlet such that respirable gas may pass into the chamber to replace the respirable gas that has been expelled. The inlet and outlet may be separate or combined as one.

Preferably, the case is in the form of a rigid dome attached in a gas tight manner at its base to the flexible support, and the flexible support is a bellows skirt made from a flexible material such as rubber, the dome and skirt forming a single chamber and arranged with a patient breathing system to allow gas from that system to be drawn into and forced out of the chamber by movement of the dome on the bellows. In alternative preferred embodiments, the case may be cylindrical, ovoid or spheroid in shape. However, the case may have any shape that is suitable to contain a volume of air. In other alternative embodiments, the flexible support may be configured as a piston type arrangement instead of a bellows, or with telescoping concentric rings, or indeed any other shape or configuration that permits variation of the chamber volume.

In the preferred embodiment, the lower edge of the skirt bellows has a gas tight attachment to a base plate of a rigid ventilator housing and inlet and outlet valves permit entry and exit of gas to and from the above chamber and direct one way flow in the breaming system. It will be appreciated that these ports may have other configurations, such as a single port with to-and-fro connections to the breathing system, may be located outside the ventilator and may also connect to a draw over, or other type of breathing system. The gas may be oxygen-enriched anaesthetic gas provided by an oxygen concentrator or cylinder, though it will be appreciated that the present invention can be used for patient ventilation with any respirable gas including room air.

Preferably, the case has a gripping means located on its outside, such as a knob, handle or recess, to enable the hand of a user to grasp and lift the case up on the flexible bellows skirt and thus draw gas into the apparatus.

Preferably, the means defining a chamber is biased to maintain a maximum volume. Such biasing can return the means defining a chamber to a maximum volume state after it has been compressed, making it ready for another compression. Preferably the biasing mechanism is a helical spring, more preferably a large diameter light helical spring located inside the means defining a chamber. In the preferred embodiment described below, its lower end rests on the flange inside the case and its upper end urges against a plate fixed to, or integral with, the top of the bellows thus maintaining the latter in an upper position against the underside of the case when the spring is extended. The case may be transparent so that the movement of this bellows is visible.

When the present invention is of an arrangement such that it has a means defining a chamber located within a case, preferably the invention also comprises a means to supply pressure to the plenum located between the means defining a chamber and the case. In the preferred embodiment, a flexible pipe may perforate the rigid base, pass through the chamber and open into the space between the internal bellows and case by perforating the flange. The tube may be configured loosely or have corrugated walls so as to allow unhindered movement of the case on the skirt and be connected to a source of non-respirable driving gas, such as from a cycling pneumatic ventilator control device driven by compressed air from an oxygen concentrator or other compressor, this compressed air being isolated from the respirable gas in the chamber. An increase in pressure from this driving gas source may cause the air to be forced through the flexible pipe into the above space. The bellows is thereby forced down within the case against the spring and causes the volume of the chamber to be reduced. Respirable gas in the chamber may thereby be forced through the outlet port into a patient breathing system to ventilate a patient. Preferably, the case can be held in a predetermined position by a holding means. In a preferred embodiment, the holding means comprises recesses or grooves, typically three or more, provided at the lower outside edge of the case which engage with a corresponding number of spring-loaded ball bearings on the ventilator housing in order to retain the case in the upper position. Negative pressure from within the case or manual pressure on the top of the case may overcome the retaining force of the spring loaded balls and cause the case to descend, to be then supported only by the flexible skirt. Changes in pressure within the chamber, for example caused by the spontaneous breathing of a patient, may cause movement of the case, which effect serves as a monitor of respiration. It will be appreciated that the case may be retained in the upper position by a different means such as by magnets instead of roller bearings.

In the preferred embodiment, during the inspiratory phase of controlled ventilation, the inner bellows is forced down by cycled gas entering the space between the bellows and case. The reactive force on the case resulting from increased positive pressure in the chamber due to lung inflation may cause the dome to be forced into the upper position and be retained there by the roller bearings.

In the preferred embodiment, the ventilator allows three functions during patient respiration as follows. 1. Free flow of respirable gas through the ventilator and unidirectional valves allows the patient to breathe spontaneously. 2. The anaesthetist may administer I.P.P.N. to the patient by elevating and depressing the dome using the knob or handle. 3. Mechanical ventilation may be started by turning on the pneumatic ventilator control. This cycles compressed air into the plenum, i.e. the space between the case and the internal bellows. The internal bellows is forced down displacing the gas in the chamber into the breathing system, thereby administering I.P.P.N. to the patient.

Some of the advantages of the present invention have been mentioned above. Other advantages of the invention include: it can be used without the need for a built in or piped gas supply; it is easy to manufacture and many of the components for certain embodiments already exist in the market, and are low cost; it is easy to use; it is portable compared to some ventilation devices; and it is suitable for both developed and developing nations.

The term "maximum volume" is used herein with reference to the case and to the means defining a chamber. The means defining a chamber may have a compressed state, which is the state of the chamber after air has been expelled, and a non-compressed state which is the state of the chamber immediately before air is to be expelled, and in some embodiments, the case may also have such states. The term "maximum volume" is used to denote the non-compressed state and should not be read as any more limiting.

"Pressure" as used herein refers to gas or air pressure, unless the context otherwise requires.

Detailed Description of the Invention

The preferred embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a sectional view of the preferred embodiment of the present invention, with dome and internal bellows in the upper position.

Figure 2 is another sectional view of the preferred embodiment, but connected with a circle type of patient breathing system with a single to- and fro- breathing pipe to the system. Figure 2a shows a connection with a draw-over type of patient breathing system.

Figure 3 is a schematic plan view of the preferred embodiment, connected to a circle type breathing system. The example shows an oxygen concentrator providing both compressed gas to a pneumatic cycling ventilator control which controls gas flow to the ventilator as well as oxygen via a rotameter and vaporizer into a breathing system using a second pipeline.

Figure 4 is as Figure 1 but with the internal bellows depressed against the spring during the inspiratory phase of mechanical administered I.P.P.N.

Figure 5 is as Figure 1 but with the dome depressed against the bellows skirt during a manually applied positive pressure ventilation with the internal bellows in the upper position.

Figure 6 is a schematic view of the preferred embodiment of the dome viewed from below and Figure 6a shows an enlargement of the configuration of the groove for the spring roller ball, the lower end of the internal bellows, upper end of the skirt bellows, spring support flange and driving gas inlet.

Figure 7 is a perspective view of the embodiment as shown in Figs 2 and 3.

Figure 8 is a sectional view of an alternative embodiment showing a hemispherical dome and internal bellows for a patient ventilating apparatus according to the present invention.

Fig 9 is a sectional view of an alternative embodiment showing a rigid inner dome in place of the internal bellows.

Referring to Figures 1 and 2, the patient ventilating apparatus (1) is arranged with a rigid transparent dome (2) on a flexible bellows skirt (3) forming a single gas tight chamber inside a housing (4) such that free movement of the dome on the skirt will allow the user (anaesthetist) to move the dome and thereby draw gas into or force it out of the chamber via a patient breat ng system (19, 25), with one way valves to direct flow (5, 6, 7 and 8). Such valves are configured according to the type of patient breathing system. A knob (9) allows manual elevation of the dome. The arrangement also allows a patient (P) to breathe spontaneously through the apparatus or to receive manually assisted I.P.P.N. from an attending anaesthetist by the repeated action of elevating and pressing down on the dome.

Inside the dome, an internal bellows (10) with spring (11) inside it is mounted on a flange (12) which is fixed to the dome (2) such that these four components move as one unit on the bellows skirt (3). The top of the spring (11) urges against a plate (13) in the internal bellows (10) and maintains the top of the latter pressed against the underside of the dome (2).

A flexible or corrugated pipe (14) passes through the chamber inside the skirt (3) perforating at its lower end (16) the base of the housing (4) and having its upper end connected in a gas tight manner with a hole (15) in the flange (12) located between the internal bellows (10) and the dome (2). The pipe (14) is coiled loosely so that its rigidity cannot impede free movement of the dome.

A source (17) of compressed gas cycled for inspiration and expiration from a ventilator control unit is connected to the inlet (16) and pipe (14) and thereby increases pressure in the plenum, i.e. the space between the dome (2) and internal bellows (10), during the inspiratory phase of I.P.P.N. This causes the internal bellows to be forced down against the spring (11), reducing the volume of the dome and skirt chamber (3) and thus displacing the respirable gas in the chamber towards the patient (P) resulting in inflation of the lungs. During the expiratory phase, the pressure in the flexible pipe (14) falls to atmospheric and the spring (11) returns the internal bellows (10) to the upper position, at the same time drawing fresh respirable gas into the chamber from the patient breathing system, according to the direction of the one way valves (5,6,7,8). An airway pressure gauge (18) shows the pressure in the chamber.

The preferred embodiment of the present invention which comprises the rigid transparent dome with knob, bellows skirt, internal bellows with spring, perforating flexible pipe and ventilator housing may be connected with a circle type patient breathing system (19) with soda lime carbon dioxide absorber (20) reservoir bag (21) pressure relief valve (22), fresh gas source (23) and balloon type or other type of expiratory valve (24) or may be connected with a draw-over system (25) with patient inflating valve (8) or another type of patient breathing system.

A draw-over type vaporiser (26) open to room air may be connected as a reserve vaporiser to allow anaesthetic vapour and/or air to enter the system in case the fresh gas source (23) fails. Referring to Figure 3, an embodiment of an apparatus (1) for ventilating a patient (P) is arranged with an oxygen concentrator being a source of compressed oxygen and air (27) to supply a rotameter (28) and vaporizer (29) as well as a cycling ventilator control device (30). The pipe (17) supplies compressed gas to the flexible pipe (14) in the ventilator (1). The pipe (31) operates the balloon occluding valve (24).

Referring to Figure 4, the internal bellows (10) is shown forced down against the spring (11) by the cycled compressed gas in the flexible pipe (14) thereby reducing the volume of respirable gas in the dome and skirt chamber (3) and displacing this gas towards the patient, resulting in inflation of the lungs. The consequent rise in airway pressure causes a reactive upward force on the dome (2) pushing it to the upper position, overcoming the roller bearings (32 ) which may retain it in this position.

Referring to Figure 5, the dome (2) is shown in the lower position disengaged from the roller bearings (32 ) at the end of a manually assisted inflation of the lungs with the cycling pneumatic ventilator control switched off. The skirt bellows (3) is compressed with reduced volume in the skirt chamber as in Figure 4, but the internal bellows (10) and spring (11) are in the upper position. The flexible pipe (14) lies coiled in the base of the ventilator housing (4).

Referring to Figure 6, the dome (2) is seen from underneath looking into the internal bellows (10). Figure 6a shows the configuration of the flange (12), lower end of the dome (2) and internal bellows (10), roller bearings (32) and notch for roller bearings, flexible pipe (14) and orifice (15).

Referring to Figure 7, the ventilator may be located on the work surface of an anaesthesia work station with the cycling pneumatic ventilator control unit mounted above, underneath a top shelf or the control unit and ventilator may be used free standing on any bed side trolley.

Referring to Figure 8, a hemispherical dome is shown operating on the same principles.

In an alternative embodiment, the invention comprises a rigid case with a means defining a chamber, for example a rigid dome, located within the case, and a means to manually reduce the volume of the means defining a chamber. A plenum exists between the outer surface of the means defining a chamber and the internal surface of the case and air may be expelled from the chamber either by supplying positive pressure to the plenum to depress the means defining a chamber or by manually reducing the volume of the means defining a chamber through use of the means to manually reduce the volume. More specifically, and with reference to Figure 9, an increase in pressure in the plenum will push down the internal dome (33) and reduce the volume of the chamber defined by the combination of the volume of the bellows skirt (3) and the volume of the internal dome (33) and thereby force gas to a patient. The increase in pressure provided to the plenum is supplied by compressed air passing through flexible pipe (14) which is arranged such that it does not impede movement of the internal dome (33). The internal dome (33) acts against a helical spring (34) which locates on the base of the ventilator. There is provided a ridge (35) which defines the upper position of the inner dome seated against a flange at the base of the outer dome (2) and an air tight sealing ring (36) to ensure the compressing gas passing through the orifice (15) is separate from the respirable gas in the chamber. The internal dome (33) will return to the upper position when the pressure supplied by the compressed air is reduced. Thus, by varying the pressure within the plenum, the internal dome (33) will move up and down can provide ventilation to a patient. As for other embodiments described, ventilation may also be provided manually by a user physically moving the outer dome (2) up and down.

In another alternative embodiment, the invention comprises a flexible case and a means defining a chamber, such as a bellows, located within the case. The means defining a chamber is flexible and, again, the space between the outer surface of the means defining a chamber and the inner surface of the case defines a plenum. The case is formed of a flexible material such that when a user applies manual pressure to the case, such as placing hands on the case and pressing down, the volume of the case and the, for example, bellows is reduced and respirable gas is expelled from the bellows. The case, while being made of a flexible material, is formed such that when in the maximum volume (or non-compressed) state, it will not expand significantly when pressure is supplied to the plenum. With such an arrangement gas can also be expelled from the bellows via increasing pressure within the plenum. Again, the bellows may be biased to return to a maximum volume state.

In a further alternative embodiment, the invention comprises a rigid case with a means defining a chamber, for example a bellows, located within the case, and a means to manually reduce the volume of the means defining a chamber. The means defining a chamber is flexible and again the space between the outer surface of the means defining a chamber and the inner surface of the case defines a plenum. In this embodiment air may be expelled from the bellows either by supplying pressure to the plenum, as for above embodiments, or by manually reducing the volume of the bellows through use of the means to manually reduce the volume. The means to manually reduce the volume may take the form of a shaft protruding through the case and having a broad flat surface in contact with the bellows, such that the volume of the bellows may be reduced by applying pressure to the shaft. The shaft may be attached to the bellows such that withdrawing the shaft increases the volume of the bellows, or, again, the bellows may be biased to return to a maximum volume state.

The apparatus of the present invention can be produced at a relatively low cost. Its uses therefore include the ventilation of patients in hospitals and medical clinics in both developed and developing countries. Because a patient can be ventilated manually with the apparatus of the invention, it is particularly useful in situations where a steady source of electricity can not be guaranteed, especially for emergency relief clinics or military clinics. The apparatus of the invention can also be used for both medical or dental patients, as well as for veterinary applications.

The skilled person will be appreciate that the ventilator of the present invention may be used in the course of anaesthesia or in other circumstances where respiratory support is needed for a patient, such as post-operatively or in resuscitation.

Although the invention has been particularly described, it should be appreciated that the invention is not limited to the particular embodiments described and illustrated, but includes all modifications and variations falling within the scope of the invention and as defined in the appended claims.

Claims

Claims:

1. A patient ventilating apparatus for ventilation of the lungs comprising a case located on a flexible support, the two forming a chamber for containment of respirable gas, wherein the case moves on the support during the respiratory cycle to vary the volume of the chamber.

2. An apparatus as claimed in claim 1 wherein the flexible support is a bellows.

3. An apparatus as claimed in claim 1 wherein the flexible support is a piston or telescoping concentric rings.

4. An apparatus as claimed in claimed in claim 3 wherein the piston or concentric rings are provided with a gas tight liner to prevent gas leakage.

5. An apparatus as claimed in any preceding claim wherein the case is adapted to move back and forth such that the internal volume of the flexible support increases and decreases and thereby provide intermittent positive pressure to the patient's lungs, when the apparatus is connected to a patient breathing system.

6. An apparatus as claimed in any preceding claim wherein said case has a means defining a chamber located within it and wherein said means defining a chamber is in fluid communication with the chamber part of the flexible support.

7. An apparatus as claimed in any preceding claim wherein said means defining a chamber is a bellows.

8. An apparatus as claimed in either of claims 6 or 7 wherein said means defining a chamber is biased to a maximum volume.

9. An apparatus as claimed in claim 8 wherein said bias is provided by a helical spring.

10. An apparatus as claimed in any one of claims 6 to 9 wherein a plenum is located between the outer surface of the means defining a chamber and the inner surface of the case such that said means defining a chamber is compressed when positive pressure is provided to said plenum.

11. An apparatus as claimed in any one of claims 1 to 10 further comprising a holding means to detachably hold said case in a predetermined position.

12. An apparatus as claimed in claim 11 wherein said holding means comprises a support and either spring loaded bearings biased into a recess acting between the support and the case or a magnetic force acting between the support and the case.

13. An apparatus as claimed in either of claims 11 or 12 wherein said predetermined position is a position that allows the flexible support to be at a maximum volume.

14. An apparatus as claimed any one of claims 1 to 13 wherein the flexible support is free to expand and contract with fluctuating pressure and wherein said fluctuating pressure can be provided by a patient breathing spontaneously, such that said expansion and contraction reflects the patient's lungs expanding and contracting.

15. A patient ventilating apparatus for ventilation of the lungs comprising a means defining a chamber located at least partially within a case, the region between the case and the means defining a chamber comprising a plenum, said plenum in communication with a conduit through which gas may be admitted or expelled from said plenum varying the pressure within, said chamber having an outlet such that respirable gas may be expelled in order to supply respirable gas to a patient, wherein said means defining a chamber and said case are arranged such that the volume of said chamber may be adjusted either by: (1) varying pressure to the plenum, or (2) a user applying physical force to the means defining a chamber.

16. An apparatus as claimed in claim 15 wherein said plenum is suitable to receive fluctuating pressure such that the volume of said chamber located within the case will vary when fluctuating pressure is applied.

17. An apparatus as claimed in either of claims 15 or 16 wherein said means defining a chamber is located partially within the case and partially outside the case, and said case is formed of a rigid material.

18. An apparatus as claimed in claim 17 wherein said case is adapted to move under application of physical force said movement causes the volume of the portion of said means defining a chamber that is outside the case to be altered.

19. An apparatus as claimed in either of claims 17 or 18 wherein said case is located above the portion of said means defining a chamber that is outside the case and force applied to the case along a vertical axis will move the case and alter the volume of the portion of said means defining a chamber that is outside the case.

20. An apparatus as claimed in any one of claims 15 to 19 wherein said, or the portion of said, means defining a chamber located within the case is biased such that it maintains a maximum volume.

21. An apparatus as claimed in claim 20 wherein the bias is provided by a helical spring.

22. An apparatus as claimed in any preceding claim wherein the case is transparent.

23. An apparatus as claimed in any one of claims 17 to 22 further comprising a holding means to detachably hold said case in a predetermined position.

24. An apparatus as claimed in claim 23 wherein said holding means comprises a support and either spring loaded bearings biased into a recess acting between the support and the case or a magnetic force acting between the support and the case.

25. An apparatus as claimed in either of claims 23 or 24 wherein said predetermined position is a position that allows the portion of said means defining a chamber that is outside the case to be at a maximum volume.

26. An apparatus as claimed any one of claims 17 to 25 wherein the portion of said means defining a chamber that is outside the case is free to expand and contract with fluctuating pressure and wherein said fluctuating pressure can be provided by a patient breathing spontaneously, such that said expansion and contraction reflects the patient's lungs expanding and contracting.

27. An apparatus as claimed in any one of claims 17 to 26 wherein the portion of said means defining a chamber that is outside the case is biased to maintain maximum volume.

28. An apparatus as claimed in any one of claims 17 to 27 wherein either positive or negative air pressure can be supplied to the plenum via a conduit that passes through the inside of the portion of the means defining a chamber that is outside the case.

29. An apparatus as claimed in any one of claims 1 to 14 or 17 to 28 wherein the case comprises a gripping means to enable a user to grip it to assist in moving the case back and forth for the purpose of drawing air into and expelling it from the apparatus.

30. An apparatus as claimed in claim 29 wherein the gripping means is a knob, handle or recess on or set into the surface of the case.

31. An apparatus as claimed in any preceding claim adapted to expel respirable gas from a chamber and to a patient.

32. An apparatus as claimed in claim 31 wherein said respirable gas is oxygen enriched.

33. An apparatus as claimed in either of claims 31 or 32 wherein said respirable gas is an anaesthetic gas.

34. An apparatus as defined in claim 5 wherein the case is adapted to be moved manually.

35. An apparatus as defined in any one of claims 6 to 34 wherein said means defining a chamber is formed of a rigid material.

36. An apparatus as defined in any one of claims 6 to 34 wherein said means defining a chamber is formed of a flexible material.

37. A patient ventilating apparatus for mechanical ventilation of the lungs substantially as herein described with reference to any example thereof and/or the accompanying drawings.