IL31730A - Valve arrangement,especially for use in anaesthetic gas supply systems - Google Patents

Description

31730/2 * nonn1? t ¾ npsen naipoa en a'©1? nni' aa , Bines? HTB Valve arrangement, especially for use in anaesthetio gas supply systems.

Ruth Lea Hesse The present invention relates to the valve arrangements of anaesthetic gas supply systems for introducing anaesthetic gas and/or oxygen into the lungs of a person to be treated.

Moresspecifically the invention relates to valve arrangements for controlling a connection between a so-called semi-closed or rebreathing anaesthetic gas supply system and the atmosphere* During narcosis it is in many cases a factor of the greatest importance to accomplish controlled ventilation of the patient, that is to assist the natural breathing of the patient or to administer artificial respiration. The manner in which this may be accomplished is best understood by considering Figure 1 of the accompanying drawings which illustrates a typical anaesthetic system having a semi-closed or rebreathing circuit 1 continuously supplied with gas from a gas source 2 and including an expandable bag or bladder 3 which is filled with the gas source 2 being arranged to supply gas at a rate such as to tend to maintain in the circuit 1 and bag 3 a "normal" pressure that is usually slightly above atmospheric pressure. The system has a connection 4 leading to the respiratory system of the patient, with directional valves such as shown at 5 and 6 causing the gas to flow through the circuit 1 in the correct direction, and an absorber 7 for removing contaminants such as carbon dioxide from the gas in the circuit* A pressure relief or blow-off valve 8 is provided fee venting excess gas from the circuit when required, as will be explained.

With such system as that illustrated, gas may if necessary be forced into the lungs of the patient by compression of the bag or bladder 3, so causing a state of overpressure momentarily to be developed in the circuit 1. When the bladder is subsequently 0 ψ released and the pressure in the circuit drops to its normal value or slightly less, the higher pressure then prevailing in the patient's lungs will force the gas back into the anaesthetic gas supply system circuit 1. As during the course of a complete cycle oxygen and/or anaesthetic gas will have been supplied to the circuit 1 from source 2, and as the amount of gas exhaled is substantially the same as the amount inhaled, the excess gas must be vented from the gas circuit 1 to the atmosphere by the valve 8· The valve arrangements in conventional semi-closed rebreathing systems of the above type usually operate in such a way that the valve disc of the blow-off valve 8 is lifted from its seat whenever the positive pressure in the system exceeds a predetermined or "normal" value of, for example, about 0.5 cm. water, as a result of spontaneous exhalation or excess supply of gas, whereby gas is discharged from the system* ¥hen the pressure prevailing in the system has a value lower than the normal pressure the normal bias, for example a spring or the action of gravity, on the valve disc will hold the latter on its seat to prevent communication between the system and the atmosphere.

During controlled ventilation of the patient in the manner described a considerably higher positive pressure has to be built up in the circuit 1, momentarily, by compression of the bladder, the order of magnitude of this pressure being, for example 30 cm water. In many prior art devices the ability to achieve this high positive pressure is provided by making the gas dischargo capacity of tho blow-off valve 8 such that it is both sufficiently small to allow said high positive pressure to build up in the system momentarily during compression of the bladder and, on the - - ** atmosphere during the course of a cycle without causing an unduly high pressure to be maintained in the system.

A considerable drawback of such valve arrangements may be ascribed to the fact that no direct conclusions can be drawn, from the degree of compression of the bladder as to the amount of gas actually introduced into the lungs of the patient by compression of the baadder. Accordingly, it is common practice to evaluate this factor by considering the expansion of the thorax during the inhalation phase, such evaluation however being of a qualitative rather than a quantitative character* Another drawback of said valve arrangements is that a considerable volume of gas is lost from the system to the atmosphere during each compression of the bladder, this being particularly disadvantageous when the system is being continuously supplied with an expensive anaesthetic gas.

It has been proposed to solve this latter problem by providing for an increase in the resistance to opening of the blow-off valve, i.e. the closing bias of the valve disc to be obtainable by manual operation when required. However, this involves the serious that an increase in bias on the valve disc also means that the valve will open during the exhalation phase of controlled ventilation only at an unduly high overpressure, and the blood circulation of the patient may thereby be adversely influenced because the high average pressure maintained in the lungs opposes the inflow of blood to the heart* These conditions will be aggravated in proportion to the amount of gas supplied to the system.

As breathing and blood circulation are vital functions which when changed during acute situations may involve danger to 31730/2 the life of the patient it would be advantageous or even a necessity to be able so to operate an anaesthetic gas supply system that during controlled ventilation the exhalation phase may take place under normal pressure conditions while, nevertheless, the system provides that any compression of the bag causes an amount of gas substantially corresponding to the amount expelled from the bag to be introduced into the lAngs of the patient.

It is an object of the invention to provide, in such a semi-closed or rebreathing anaesthetic system, a valve arrangement controlling communication between the rebreathing system and the atmosphere in such a way that this communication is held open for discharging excess gas and exhaled air, but is automatically closed when the bag or bladder is compressed to accomplished controlled ventilation and is thereafter reopened when the compression of the bag is discontinued* Such a valve arrangement provides that the pressure in the anaesthetic gas supply system during exhalation, whether natural or after compression of the bag, is limited to a suitably low value that will guarantee the expansion of the bag or bladder due to the continuous supply of gas, whereby the bag's character as a gas supply reservoir will be maintained.

The invention thus provides a rebreathing anaesthetic system characterised by a valve arrangement having a blow-off valve controlling a vent and adapted to open to permit venting of surplus gas when the pressure prevailing in the system exceeds a first predetermined value, said valve arrangement further having a pressure pick-up exposed to the pressure prevailing in the system and adapted to close said blow-off valve when the said pressure attains a second and higher predetermined value.

In some embodiments of the invention, the pressure pickup includes a movable member arranged substantially in axial alignment with the blow-off valve. In such embodiments, the movable member is conveniently arranged to move towards the blow-off valve in response to rising pressure in the system and to engage said valve directly. On the other hand, the movable member may b© coupled with lost motion to the hlow-off valve by mechanical force transferring means, and in the latter event various forms of force-transferring means may be adopted.

Thus, for instance, the force-transferring means, may include a telescopic connection providing the said lost motion, or the force-transferring means may be a flexible cord, chain or like element which is longer than the shortest distance between the movable member and the blow-off valve.

Conveniently, the pressure pick-up is disposed to one side of the blow-off valve and is coupled thereto by a force transfer system including a lever* The pressure pick-up may be of any suitable form, an expansible bellows or a piston and rod arrangement being convenient.

The system may include control means influencing the movement of pressure pick-up and/or of the blow-off valve. Such control means may, for instance, be a counterweight arranged for movement along a lever coupling the pressure pick-up with the blow-off valvo, or the control means may be a magnet-and-armature arrangement, one element of which is displaceable in relation to the other. In the latter, case, preferably the magnet ia disposed near a lever coupling the pressure pick-up to the blow-off valve, the armature being arranged on that arm of said lever which is closer to the magnet* - - The system may further include a release operable to open the blow-off valve against a pressure exceeding said second predetermined value or to prevent closure of the valve i response to such a pressure.

The invention further provides a valvo unit for a re-breathing anaesthetic system, comprising a valve housing having an inlet connection and an outlet connection,, a bladder connection, a blow-off valVe comprising a valve seat and a cooperating valve-closing element, and biasing means adapted to keep the blow-off valve closed whenever the pressure acting on the closing element from the interior of the valve housing is lower than a first predetermined value, a movable member capable of expansion and contraction in response to pressure variations within the valve housing, and a double-armed lever having a force arm adapted to be acted upon by a force produced by expansion of the movable member and a load arm adapted to act on said valve-closing element to close the blow-off valve only when the pressure in the valve housing exceeds a second predetermined pressure value higher than said first predetermined value.

Desirably, the said housing is divided into two separate chambers by a one-way valve which permits gas to flow only from a first of said chambers, with which the blow-off valve communicate to the second of said chambers, into which the bladder connection and the connection to the movable member opens.

A throttling means may be disposed in the valve housing between the said inlet connection and the blow-off valve. Such throttling means may be a partition having an opening of small cross-sectional area* The invention also consists a re-breathing anaesthetic system comprising a re-circulation conduit having a first connection leading to the patient, a second connection for continuously supplying gas to the system, a third connection for a compressible bladder, one-way valves constraining gas to flow in a selected direction in said conduit, a blow-off valve having a valve seat and a cooperating valve-closing element, biasing means adapted to hold said element in a valve-closing position against the valve seat whenever the gas pressure in the system is lower than a first predetermined pressure value appropriate for filling the bladder with gas after compression and subsequent release thereof, an absorber for removing pollutants such as carbon dioxide from gas flowing in said conduit, a movable member having a surface exposed to the pressure prevailing in the conduit and associated with a force transfer means for transferring the force, due to gas pressure acting on said surface, to the valve-closing element of the blow-off valve as to close said valve when the pressure in the conduit exceeds a second predetermined pressure value, due to compression of the bladder, higher than said first predetermined pressure value* The invention will be further explained with reference to the accompanying drawings, in which: FIGURE 1, which has already been described illustrates a conventional semi-closed or re-breathing anaesthetic system ; FIGURES 2a and 2b illustrate a preferred embodiment of valve arrangement for a rebreathing system according to the invention, these Figures respectively showing the disposition of parts of the arrangement during different pressure conditions in the system) FIGURE 4 shows yet another embodiment of valve arrangement} FIGURES 5 and 6 illustrate parts of further embodiments of the valve arrangement; and FIGURE 7 shows still another embodiment of valve arrangement for a rebreathing system according to the invention.

Figures 2a and 2b illustrate, in simplified diagrammatic form* a preferred embodiment of valve arrangement for a rebreathing system according to the invention, Figure 2a illustrating the disposition of. the parts during controlled ventilation by compression of the bladder and Figure 2b illustrating the disposition of the parts during the subsequent exhalation phase upon release of the bladder* Parts of the system corresponding to parts of a conventional system as described vrith reference to Figure 1 have been given the same references in Figures 2a and 2 · The valve arrangement in this embodiment is very compact and uomprises a housing 15 provided with an inlet connection 16, an outlet connection 17, and a bladder connection 18* A gas conduit 19 interconnects the inlet connection 16 and the outlet connection 17 and is continuously supplied with oxygen and/or anaesthetic gas from a gas source 2· The conduit 19 is provided with a branch conduit leading to the patient and an absorber 7 for eliminating contaminants ouch as carbon dioxide from the gas circulating in the conduit 19· The conduit 19 also contains one-way valves such as 5 forcing the gas to flow only in the correct direction in the gas conduit 19* The housing 15 is provided with a relief or blow-off valvo 8 capable of putting the interior of the housing in communication with the atmosphere, said valve preferably being constituted by a valve disc 14 and a corresponding valve seat 20. The valve disc ν by gravity acting on the valve disc. This biasing force is so adjusted that the blow-off valve 8 opens as soon as tho positive pressure in the system attains a relatively low first predetermined value./of', for example, 0.5 cm. water i.e. the pressure required to re-expand the bladder 3 with gas after compression of said bladder.

The housing 15 is also provided with a connection to a movable member 9 which in this embodiment is shaped as a bellows and expands and contracts in accordance with the pressure in the system. The pressure-dependent force developed by the bellows 9 upon expansion thereof is transferred to the valve disc 14 via a force transfer system to exert a closing force on the blow-off valve 8. The force transfer system may be designed in many different ways, a preferred design being shoira in Figures 2a and 2b, wherein the bellows 9 is caused to act upon one end of a lever 13, the other end of which is adapted to come into contact with the valve disc 14 in a direction to close the valve* The lever 13 is double-armed, having a load arm 24 and a force arm 25 and is pivoted to a support structure 26 which is secured to the housing 15. In order to obtain a good force transfer from the bellows 9 to the force arm 25 of the lever, the end of the arm 25 is preferably formed as a thin plate, the surface of which corresponds to the upper surface of the bellows 9» The surface of the movable member 9 exposed to the pressure prevailing in the system, the corresponding surface of the valve disc 14 and the length ration between the force arm 25 and the load arm 24 of the lever 13 are all so related that the pressure- u dependent force transferred from the bellows 9 when acting thro'gh the lever 13 to close the blow-off valve as explained below will always be greater than the pressure-dependent force acting on the valve disc 14 and tending to open the blow-off valve.

The valve arrangement shown in Figures 2a and 2b also includes a preferred mechanism adapted to prevent closure of the blow-off valve during normal exhalation phase, i.e. when relatively low overpressure exists in the system. This mechanism comprises a magnet 21 adapted to attract an armature 22 on the load arm 24 of the lever 13, this armature 22 being positioned in such a way in relation to the magnet 21 that the force of attraction by the magnet 21 counteracts the closing force produced by the movable member 9 to an extent such that the load arm 24 cannot move from the position shown in Figure 2b to close the blow-off valve 8 until the attraction force of the magnet has been the overcome. As/force of attraction between the magnet and the armature decreases rapidly with the distance therebetween there will be a very rapid and effective valve-closing action as soon as the closing force transmitted from the movable member reaches a value sufficient to overcome the said attraction force with the lever 13 in the position shown in Figure 2b. The value of the force of attraction in this position can be controlled by means enabling the magnet 21 to be shifted in relation to the armature 22· A simple way of vafcying the attraction force between the armature 22 and the magnet 21, which is preferaby supported by a carrier 27 secured to the housing 15, is to provide the magnet with a screw thread for engagement with a corresponding thread in the carrier 27, the attraction force then being varied by screwing the magnet more or less into the thread of the carrier} the latter may be provided with a scale cooperating with a mark on the magnet, and indicating for each setting position, for example, In the embodiment shown in Figures 2a and 2b the magnet 21 is disposed above the armature 22, which is disposed on the load arm 24 of the lever 13, but it is, of course, possible 'instead to dispose the magnet below an armature disposed on the force arm 25.

The above described mechanism, comprising the magnet 21 and armature 22 for preventing closure of the blow-off valve 8 at relatively small overpressures operates in a manner such that the force transfer system is not released so as to come into action to operate upon the valve 8 until the overpressure in the system has reached a certain (second) predetermined value . Such an arrangement has the advantage that the blow-off valve is not acted upon by a progressively increasing closing force as the overpressure rises towards said value, but rather by a closing force which upon release of the force transfer system suddenly rises from zero to a relatively high value, producing a rapid closing of the valve 8 when the overpressure reaches the (second) predetermined value. The same result can be achieved in cither ways* Thus if a bellows is used as the movable member as in the arrangement of Figure 2a and 2b,. it may be arranged that the bellhws actuates the force transfer system only fhen a certain degree of extension of the bellows has been reached. If the movable member is instead a piston movable in a cylinder, a corresponding arrangement may consist in causing the piston to release the force transfer system only after a certain movement of the piston in the cylinder has occurred.

The blow-off valve-closing action of the valve arrangement in a system according to the invention can be made more effectivo by providing a non-return valve such as a disc or flap valve in V at which the movable member senses system pressures In this way there may be more reliably obtained, ,on the one hand, effective closing of the blow-off valve in response to overpressure attaining the said second predetermined value as a result of compression of the bladder and, on the other hand, freedom from unwanted closing of the valve i response to pressure variations during the exhalation phase.

Thus in the embodiment of Figures 2a and 2b, the non-return valve 28 is disposed in the housing 15 between the opening communicating with the movable member 9*' and the blow-off valve 8, the non-return valve 28 being orientated to prevent the gas flow generated by compression of the bladder from following the shortest path to the blow-Off valve 8. The movable member 9 is positioned close to the bladder 3, and the blow-off valve 8 closely adjacent to the movable member, so that the pressure surge due to compression of the bladder 3 will actuate the movable member 9 to apply closing force to the valve disc 14 practically without delay, whereas the valve disc 14 of the blow-off valve 8 will experience a corresponding opening force only after propagation of the pressure surge through practically the whole system.

On the other hand, pressure increases in the system due to exhalation will first act on the valve disc 14 and only thereafter on the movable member 9 when the valve 28 has opened. On such occasions, the relatively slow pressure rise in the system, which also will occur if there is an excess supply of oxygen or anaesthetic gas to the conduit 19, the opening of the valve 8 will be effective to prevent the pressure at the member 9 reaching the valve at which the lever 13 is released to apply a closing orce to the valve disc 14.

\ The propagation to the valve 8 of the pressure surge caused by compression of the bladder 3 can be further delayed by introducing a throttling means in the system. A very simple and efficient embodiment of such throttling means is shown in Figures 2a and 2b, wherein the throttling means consists of a disc 29 having a central opening 30 of reduced cross-section. ¾he disc 29 constituting the throttling means in Figures 2a and 2b is disposed between the inlet end of the housing 15 and the blow-off valve 8, but it is also possible to insert the throttling means at some other place in the system downstream of the bladder and upstream of the blow-off valve; Any other convenient throttling means such as a jet or nozzle can be utilized in place of the disc 29 in order to reduce the opening force acting on the valve disc of the blow-off valve during compression of the bladder and during exhalation; Another suitable form of movable member is shown in Figure 3. In this case the movable member is constituted by a piston 10 which is slidable in a cylinder 11 is in communication with the interior of the housing 15 in the region of its connection to the bladder 3j the displacement of the piston 10 being dependent on the pressure in the feousing 15 , which acts on one surface of the piston 10» The piston may be exposed to the action of a spring 12 counteracting the pressure-dependent force to which the piston is exposed and thus permitting transfer of the latter force to the blow-off valve disc 14 only when the pressure-dependent force has become greater than the spring force, i.e. upon the presence of a relatively high over-pressure in the housing caused by compression of the bag or bladder 3. As shown, the piston 10 may be mechanically connected to one end of the lever 13, the other end of which is adapted to come into contact with the valve disc 14 of the blow-off valve , as in the arrangement described with reference to Figures 2a and 2b, or some other force transfer system may be utilized to transmit movement of the piston 10 to the valve disc 14 under the required circumstances. The spring 12 can also be replaced by the magnet arrangement of Figures 2a and 2b.

In another embodiment of the invention which is illustrated in Figure 4, the movable member is shown as a bellows 9 as in the arrangement of Figures 2a and 2b but could also be a piston 10 as in Figure 3. In this embodiment, the force transfer system, again comprising the lever 13, is provided with an adjustable counterweight 31 which is so arranged that the force transferred to the valve disc 14 via the lever 13 will not become large enought to close the blow-off valve 8 until the overpressure in the system has reached a certain value at which the pressure-dependent force exerted on the bellows 9 or the piston 10, as the case may be, is greater than the force exerted by the counter- v wieght 31 acting in the opposite direction* The force exerted by the counterweight 31 and counteracting the force exerted by the movable member 9 is adjusted by shifting the position of the counterweight in relation to the pivot point of the lever 13, the counterweight either being slidable directly on one arm of the lever 13 or upon a bridge 23 on the lever 13 as shown. Also in this caso there may be provided a scale indicating the values required for closure of the blow-off valve overpressure for different positions of the counterweight along the lever 13 or bridge 23* Figures 5 and 6 illustrate arrangements in which the force Λ between the blow-off valve disc 14 and the movable member 9, in which case the forces depending on the pressure prevailing in the system a d acting respectively on the valve disc 14 and on the movable member 9 are oppositely directed substantially along a common axis* This connection may comprise rigid force transfer members 32 with a telescopic connection 34 or some other device providing lost-motion, as shown in figure 5, or pliable members 33 as shown in Figure 6, the latter type of connection being exemplified by cords, belts, chains or flexible straps. The degree of lost-motion provided in all these arrangements prevents such shifting of the movable member 9 as will occur upo small overpressures in the system (excess gas supply, exhalation) from exerting any closing action on the valve disc 14, closing force being applied to the valve disc 14 only when higher over-pressures prevail in the system for example upon compression of the bladder.

The force transfer system may also be such that the movable member acts directly against the valve disc 14 of the blow-off valve 8 or upon a contact surface thereon when valve-closi&g movement is required. Thus Figure 7 shows a bellows 9 which is connected to one limb of a T-piece 35 which is fixed in position opposite to the blow-off valve. The bladder 3 is secured to a second limb of the T-piece 35, the third limb of which communicates with the system via a branch conduit 36· Expansion of the bellows 9 (or shifting movement of a piston which may be used instead of the bellows 9) due to compression of the bladder 3 will cause the bellows 9 or piston, as the case may be, to come into direct contact with the valve disc 14 to clost the blow-off valve 8 whon required. It is important with such an arrangement to prevent - 31730/2 ^ attracting an armature 22 on the bellows 9 or equivalent piston. As in the above described embodiments, bia3sing means (not shown) act on the valve disc 14 to hold it closed until a first predetermined overpressure is attained in the system and it is advantageous to make use of a one-way valve 28 disposed between the blow-off valve 8 and the branch conduit 36 and a throttling means 29, 30 upstream of the blow-off valve.

It will be understood that through faulty operation or adjustment, the pressure in the system may unintentionally rise to the value causing closure of the blow-off valve 8, or when the valve 8 has been closed by compression of the bladder 3, to such a high value that the valve 8 will not reopen upon release of the bladder. For this reason, a manually-operable release such as a push-button which can be locked in either of two positions is provided to enable the blow-off valve to be forcibly opened or prevented from closing when required. In the arrangement shown in Figures 2a and 2b, the release 37 is a push-button 37 which in the position shown in full lines doee not interfere with the force transfer system, but in the second position, indicated by the dotted lines in Figure 2b, the push-button interferes with the force transfer system so as to prevent the force developed by the movable member from being transferred to the blow-off valve. The valve arrangement may also comprise means for making it possible manually to hold or latch the blow-off valve closed, which may be desired during certain types of anaesthesia, in particular when small amounts of gas are supplied not requiring any communication between the system and the surrounding atmosphere.

\ Witi -fche valve arrangements as described above it is thus possible to eliminate the drawbacks of previously luiown semi-clooed or rebreathing anaesthetic systems. An anaesthetic system having such a valve arrangement vents to atmosphere at relatively small overpressures (high enough to overcome the biasing force of the blow-off valve) in the system, thus enabling exhalation arid discharge of excess gas under favourable conditions and yet is shut off from communication with the atmosphere at higher overpressures created in the system by compression of the bladder, whereby control of the amount of gas thus introduced into the respiratory ducts of the patient is facilitated and the loss of gas is minimized. By adjustment of the various adjustable preloading devices used in connection with the invention, such as a shiftable couterweight or a magnet, the system may be readily adjusted for use under varying conditions of continuous gas supply to the system so that the blow-off valve always will open at a suitable selected pressure, for example of an order of magnitude of 0,5 - 1,0 cm. water.

By suitable dimensioning of the movable member and suitably selecting the force transfer system and the preloading thereof, it is possible to ensure that when compressing the bladder the amount of gas which is discharged from the system before the blow-off valve closes will be so small as to be substantially zero* The described valve arrangements may be modified in many ways. For example, the force transfer system may take other mechanical forms from those exemplified and may also comprise hydraulic, pneumatic or electrical force transfer mechanisms* Although it is preferred to arrange the movable member and the blow-off valve adjacent to each other in one compact unit, the invention is hot limited to such an arrangement, it being possible to dispose these

Claims (9)

1. 31730/2 w Claims 1· A rebreathing anaesthetic system characterised by a valve arrangement having a blow-off valve controlling a vent and adapted to open to permit venting of surplus gas when the pressure prevailing in the system exceeds a first predetermined value, said valve arrangement further having a pressure pick-up exposed to the pressure prevailing in the system and adapted to closet .said blow-off valve when the said pressure attains a second and higher predetermined value.

2. An anaesthetic system according to claim 1, in which the pressure pick-up includes a movable member arranged substantially in axial alignment with the blow-off valve.

3. An anaesthetic system according to claim 2, in which the movable member is arranged to move towards the blow-off valve in response to rising pressure in the system and to engage said valve directly 4* An anaesthetic system according to claim 2 , in which the movable member is coupled with lost motion to the blow-off valve by mechanical force transferring means* 5. An anaesthetic system according to claim 4, in which the force transferring means includes a telescopic connection providing the said lost motion* 6. Ah anaesthetic system according to claim 4, in which the force transferring means is a flexible cord, chain or like element which is longer than the shortest distance between the movable member and the blow*off valve* · An anaesthetic system according to claim 1, in which the pressure pick-up is disposed to one side of the blow-off valve and is coupled thereto by a force transfer system including a lever* 31730/2 8. An anaesthetic system according to any one of the preceding claims, in which the pressure pick-up is an expansible bellows. 9· An anaesthetic system according to any one of claims 1 to 7, in which the pressure pick-up is piston-and-rod-arrangement. 10. An anaesthetic system according to any of the preceding claims, including control means influencing the movement of pressure pick-up and/or of the hlow-off valve. 11. An anesthetic system according to claim 10, in which said control means is a counterweight arranged for movement along a lever coupling the pressure pick-up with the blow-off valve. 12. An anesthetic system according to claim 10, in which said control means is a magnet-and-armature-arrangement, one element of which is displaceable in relation to the other. 13. An anesthetic system according to claim 12, in which the magnet is disposed near a lever coupling the pressure pick-up to the blow-off valve, the armature being arranged on that arm of said lever which is closer to the magnet. 1

4. An anesthetic system according to any one of the preceding claims, including a release operable to open the blow-off valve against a pressure exceeding said second predetermined value or to prevent closure of the valve in response to such a pressure. 1

5. A valve unit for a re-breathing anaesthetic system, comprising a valve housing having an inlet connection and an outlet connection, a bladder connection, a blow-off valve comprising a valve seat and a cooperating valve-closing element, and biasing means adapted to keep the blow-off valve closed whenever the pressure acting on the clsoing element from the interior of the valve housing is lower than a first predetermined value, a 31730/2 armed lever having a force arm adapted to be acted upon by a force produced by expansion of the movable member and a load arm adapted to act on said valve-closing element to close the blow-off valve only when the pressure in the valve housing exceeds a second predetermined pressure value higher than - said first predetermined value. 1

6. A valve unit according to claim 15, wherein the valve housing is divided into two separate chambers by a one-way valve which permits gas to flow only from a first of said chambers, with which tho blow-off valve communicates, to the second of said chambers, into which the bladder connection and the connection to the movable member opens. 1

7. A valve unit according to claim 15 or 16, wherein a throttling means is disposed in the valve housing between the said inlet connection and the blow-off valve. 1

8. A valve unit according to claim 17, wherein said throttling means is a partition having an opening of small cross-sectional area. 19· A re-breathing anaesthetic system comprising a re-circulation conduit having a first connection leading to the patient, a second connection for continuously supplying gas to the systenij, a third connection for a compressible bladder, one-way constraining gas to flow in a selected direction in said element in a valve-closing position against the valve seat whenever the gas pressure in the system is lower than a first predetermined pressure value appropriate for filling the bladder with gas after compression and subsequent release thereof, an absorber for removing pollutants such as carbon dioxide from gas flowing in said conduit, a movable member having a surface exposed to 31730/2 -\ force transfer means for transferring the force, due to gas pressure acting on said surface, to the valve-closing element of the blow-off valve as to close said valve when the pressure in the conduit exceeds a second predetermined pressure value, due to compression of the bladder, higher than said first predetermined pressure value. 20. A re-breathing anaesthetic system according to claim 19, wherein the said movable member is disposed downstream of the blow-off valve. 21. A re-breathing anaesthetic system according to claim 20, wherein the bladder is connected to the recirculation conduit at a location downstream of the blow-off valve. 22. A re-brenthing anaesthetic system according to claim 21, wherein a one-way valve is provided in the recirculation conduit at a location downstream of the blow-off valve, but upstream of the movable member and the bladder, said one-way valve being adapted to prevent gas expelled from the bladder from flowing by the shortest route in the conduit to the blow-off valve. 23. A re-breathing anaesthetic system according to claim 22, wherein that bladder and the movable member are disposed near the blow- off valve, whereby gas expelled from the bladder upon compression thereof is caused to flow through almost the Entire recirculation conduit before reaching the blow-off valve. 24. A re-breathing anaesthetic system according to claim 22 or 23, wherein a throttling means is provided in the recirculation conduit between the patient connection and the blow-off valve, said throttling means serving to delay the transfer from the bladder to the blow-off valve of force due to the pressure surge in the conduit caused by compression of the bladder. 31730/2 said throttling means comprises a disc provided with a central opening having small cross-sectional area. 26. A re-breathing anaesthetic system according to claim 25, wherein said throttling means comprises a jet or nozzle. 27. A re-breathing anaesthetic system substantially as described with reference to and as shown in Figures 2a and 2b of the accompanying drawings. 28, A re-breathing anaesthetic system according to claim 27, modified substantially as described with reference to and as shown in Figure 3 of the accompanying drawings. 2

9. A re-breathing anaesthetic system according to claim 27, modified substantially as described with reference to and as shown in Figure 4 of the accompanying drawings. 30. A re-breathing anaesthetic system according to claim 27, modified substantially as described with reference to and as shown in Figures 5 and 6 of the accompanying drawings. 31. A re-breathing anaesthetic system according to claim 27, modified substantially as described with reference and as shown in Figure 7 of the accompanying drawings. For the Applicant Dr. Yitzhak Hess.