Patent Publication Number: US-2005133030-A1

Title: Anaesthetic vaporiser

Description:
The present invention relates to a device for vaporising anaesthetic liquids for administration in a gas stream to a patient. More particularly, but not exclusively, it relates to an anaesthetic vaporiser of the by-pass type with improved consistency of delivery of anaesthetic vapour concentrations and improved anaesthetic reservoir capacity.  
      Many anaesthetic agents used in general anaesthesia are liquids under standard conditions, and are administered as vapours, borne on a flow of carrier gas, such as air, pure oxygen or mixtures of oxygen and gaseous anaesthetics, such as nitrous oxide. Commonly used anaesthetics administered in this fashion include halothane (CHBrCl—CF 3 ), enflurane (CHF 2 —O—CF 2 —CHFCl), isoflurane (CHF 2 —O—CHCl—CF 3 ) and sevoflurane (CH 2 F—O—CH(CF 3 ) 2 ).  
      An anaesthetic vaporiser contains a reservoir of liquid anaesthetic, over or through which is passed a stream of carrier gas, which picks up anaesthetic vapour. The carrier gas stream is delivered to the vaporiser by an anaesthetic machine, and is a blend of oxygen, nitrous oxide, and/or air to produce a mixture with exactly the required composition for administration to the patient through a patient breathing circuit.  
      It is clearly highly desirable for an anaesthetist to be able to rely on a consistent supply of anaesthetic vapour at a known and predeterminable concentration, so that he may concentrate on monitoring the patient and where necessary changing the gas blend at the anaesthetic machine in response to changes in the patient&#39;s condition.  
      A type of vaporiser which has been found to be particularly suitable is the by-pass vaporiser. In this, the carrier gas stream is split into two. A first stream is passed across the liquid anaesthetic in its reservoir to become saturated with anaesthetic vapour, and is then recombined with the second stream which has bypassed the reservoir. In theory, this should produce an output carrier gas stream with a known concentration of vapour.  
      However, the saturated vapour pressure of a liquid depends on the temperature. Not only may ambient temperatures vary, but the latent heat of vaporisation of the liquid leads to it cooling significantly as vapour is formed and borne off in the carrier gas. It is hence customary to control the recombination of the bypass gas stream and the gas stream saturated with vapour by using a temperature sensitive valve mechanism, for example controlled by a bimetallic strip. Thus, when the saturated gas stream has become colder, and so contains a lower saturated vapour concentration, the valve relatively restricts the flow of the bypass gas stream, so that the resulting recombined gas stream remains at the desired vapour concentration. (A separate needle valve arrangement is normally provided, by which an anaesthetist may select a desired vapour concentration).  
      It is hence very important that the temperature sensing means of the valve is responsive to the exact anaesthetic temperature. Vaporisers have been developed in which the valve and temperature sensing means are positioned immediately below a base of the anaesthetic reservoir. A drawback of such an arrangement is the temperature difference between the anaesthetic liquid and gas, which can amount to as much as twenty degrees Celsius, although this depends, for example, on the carrier gas flow rate.  
      An additional problem with this arrangement is that access to the bypass valve for adjustment is very awkward, possibly requiring draining of the reservoir or even dismantling the vaporiser completely.  
      It is hence an object of the present invention to provide a control arrangement for a by-pass anaesthetic vaporiser which is more accurately responsive to vapour temperatures and hence actual saturated vapour concentrations, and which is easier to adjust.  
      Another source of potential variation in supplied vapour concentration is that the carrier gas stream passing through the reservoir may not have a sufficient residence time to become fully saturated with anaesthetic vapour. Various wick arrangements have been proposed with high surface areas to promote rapid transfer of liquid anaesthetic to the vapour phase, as well as indirect gas flow paths to increase residence times and to prevent carrier gas “short cuts” from inlet to outlet of the reservoir. Some proposals have the carrier gas constrained to follow a spiral path, lined with a wick material, from outside to inside, increasing its time in contact with the anaesthetic-soaked wick material. These arrangements and others along similar lines may improve gas/vapour contact, but some problems remain.  
      For example, the anaesthetic vapour is significantly heavier than air. A concentration gradient may thus be set up between a point immediately above the liquid anaesthetic and a point close to a top of the reservoir. A conventional vaporiser with a reservoir capacity of perhaps 120-250 cm 3  will empty in around two to five hours normal use, and thus may require refilling in the course of a prolonged operation. Clearly, the vapour concentration in an upper part of the reservoir can vary greatly and this effect will increase as the liquid level falls in the reservoir. The existing arrangements cannot allow for this, and there is hence a likelihood of a drift or variation in the vapour concentrations produced, particularly at low or high carrier gas flow rates.  
      It is hence a further object of the present invention to provide an apparatus having a carrier gas path and wick arrangement capable of providing improved consistency of vapour saturation, relative to current arrangements.  
      Although an extended carrier gas path length would clearly be beneficial in this respect, there is a potential drawback. An elaborate structure, designed to define a long path length, may result in poor conduction of heat through the structure. Thus, significant temperature differentials and hence differences in saturated vapour pressure may build up across the structure and the path it defines. It would hence be preferable if the carrier gas path could be defined so as to obviate this risk as much as possible.  
      Ideally, arrangements should be made to provide warmth to all parts of the path-defining structure, to compensate for the lost latent heat of vaporisation. Thus the “running temperature” of the vaporiser can be kept relatively high, leading to a relatively high and consistent anaesthetic vapour pressure.  
      A further problem with existing vaporisers results from the use of a mechanical ventilator to deliver the final gas/anaesthetic mixture to the patient. The pumping action of the ventilator can cause a major pulsation in back-pressure, upstream therefrom. This can have an effect as far upstream as the vaporiser, leading to variations in gas flow. Major carrier gas pressure variations can also lead to significant variations in the saturated vapour pressure of the anaesthetic. Conventionally, a large gas volume can be provided above the liquid anaesthetic in the vaporiser, as a “buffer” against back-pressure pulses, but this is not always sufficient, and is in any case a waste of valuable space within the vaporiser.  
      It is hence another object of the present invention to provide an improved means of obviating the effect of back-pressure variations on the operation of the vaporiser.  
      As mentioned, space can be critical within a vaporiser. This is because they are constrained in their external dimensions to fit standardised mountings. Conventionally, a set of one, two or three vaporisers, each possibly containing a different anaesthetic, is mounted side by side in the anaesthetic machine.  
      Thus, a trade-off is usually necessary between the spaces occupied by the various internal components of the vaporiser, which results in a limited reservoir capacity, as mentioned above. The need for the reservoir to be filled periodically during long operations has led to problems. If the anaesthetist is busy monitoring the patient, non-expert personnel may have to fill the reservoir. This has occasionally led to overfilling, and even carry-over of liquid anaesthetic downstream of the vaporiser. There are clear benefits from maximising the reservoir capacity to reduce the need for refilling.  
      According to a first aspect of the present invention, there is provided an anaesthetic vaporiser comprising reservoir means adapted to hold a supply of liquid anaesthetic agent, a plurality of channel means, each extending generally vertically in use, to define a path for carrier gas being passed through the vaporiser, and each being provided with respective wick means extending into said reservoir means or operatively connected thereto so as to permit flow of liquid anaesthetic agent from the reservoir means to each wick means.  
      The anaesthetic agent may thus evaporate from each wick means and enter the carrier gas.  
      Preferably, the channel means are so connected that the carrier gas is directed upwardly along some of said channel means and downwardly along a remainder thereof.  
      Advantageously, the carrier gas is directed alternately upwardly and downwardly along successive channel means.  
      The channel means may be connected serially to form an unbranched path for the carrier gas.  
      Preferably, each second wick means is substantially co-extensive with a respective channel means.  
      Advantageously, each wick means comprises a layer of wick material lining at least part of an inner surface of a respective channel means, optionally a majority thereof.  
      Each channel means may be provided with frame means adapted to hold said layer of wick material to said surface.  
      Each channel means may have an elongate generally cylindrical shape.  
      The frame means may then comprise helical spring means dimensioned to be insertable into a respective channel means with the wick material held between it and the inner surface of the channel means.  
      The wick means may be operatively connected with the reservoir means by a sheet of second wick material extending into said reservoir means and contacting each wick means.  
      The second wick material may line a part of the inner surface of each channel means not lined by the respective wick means.  
      Preferably, the wick means and wick material comprise a non-woven fabric material, such as a felted material.  
      Advantageously, the wick means and wick material comprise a fluorinated plastics material, such as polytetrafluoroethylene.  
      The central block may be provided with upper and lower end caps, each provided with passage means to connect a respective end of each channel means to a corresponding end of another channel means, or to inlet or outlet means for the carrier gas.  
      According to a second aspect of the present invention, there is provided an anaesthetic vaporiser of the by-pass type, as defined hereinabove, comprising reservoir means adapted to hold a supply of liquid anaesthetic agent in a lower, in use, portion thereof, a first carrier gas path extending through an upper portion of the reservoir means and arranged so that carrier gas passing therethrough may become saturated with anaesthetic vapour, a second carrier gas path avoiding contact of carrier gas with anaesthetic vapour, and temperature sensitive valve means adapted to regulate the proportion of carrier gas flowing along each said path, wherein the valve means is so mounted as to sense and respond to the temperature of carrier gas and anaesthetic vapour adjacent the upper portion of the reservoir.  
      Preferably, the first carrier gas path is arranged to pass through said upper portion adjacent the valve means.  
      Advantageously, the valve means comprises a part of the second carrier gas path.  
      The valve means may comprise a variable restriction in the second carrier gas path, controlled by bimetallic strip means.  
      Said restriction may comprise a pair of plate means, between which the carrier gas is constrained to flow, a separation between the plate means being controlled by movement of said bimetallic strip means.  
      According to a third aspect of the present invention, there is provided an anaesthetic vaporiser of the by-pass type as defined hereinabove, comprising a first carrier gas path arranged so that evaporating anaesthetic agent may saturate carrier gas passing therethrough and a third carrier gas path arranged so that carrier gas flows therethrough before flowing through the first carrier gas path, the first and third carrier gas paths extending each adjacent the other such that heat may flow from carrier gas in the third path to replace that lost by evaporation of anaesthetic into carrier gas in the first path.  
      Preferably, each of the first and third carrier gas paths extends through a block of thermally conductive material, for example comprising a metal such as aluminium.  
      Each of the first and third carrier gas paths advantageously comprises a plurality of channel means extending each generally parallelly to the others through the block, optionally connected in series to form a single unbranched path.  
      According to a fourth aspect of the present invention, there is provided an anaesthetic vaporiser provided with buffer means to obviate the effects of variations in back-pressure produced by a mechanical ventilator or the like operatively connected thereto, the buffer means comprising an elongate indirect carrier gas path within the vaporiser.  
      Preferably, said elongate path comprises a plurality of channel means each extending generally parallelly to the others and so connected, optionally in series, that a direction of flow of gas passing therethrough is substantially reversed between each channel means and its neighbours. 
    
    
      An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which:  
       FIG. 1  is a perspective view, partially sectioned, of a wick assembly of an anaesthetic vaporiser embodying the invention;  
       FIG. 2  is a perspective view of components of the wick assembly shown in  FIG. 1 , in separated condition;  
       FIG. 3  is a perspective view of the wick assembly shown in  FIG. 1  in position in a main body of the vaporiser, partially sectioned;  
       FIG. 4  is a perspective view from below of an upper manifold block of the vaporiser;  
       FIG. 5  is a scrap plan view of part of the wick assembly shown in  FIG. 1 , showing details of the wicks thereof;  
       FIG. 6  is a perspective view of the assembly shown in  FIG. 1 , partially sectioned and with the wicks themselves omitted, to show a flow path therethrough;  
       FIG. 7  is a perspective view from behind of the main body, front block and upper manifold block of the vaporiser, separated one from each other to show a flow path through a by-pass valve thereof;  
       FIG. 8  is a frontal elevation of the assembled vaporiser;  
       FIG. 9  is a plan view from above of the assembled vaporiser;  
       FIG. 10  is a cross-sectional view of the vaporiser of  FIGS. 8 and 9 , taken along the line X-X of  FIG. 8 ;  
       FIG. 11  is a cross-sectional view of the vaporiser of  FIGS. 8 and 9 , taken along the line XI-XI of  FIG. 9 ; and  
       FIG. 12  is a cross-sectional view of the vaporiser of  FIGS. 8 and 9 , taken along the line XII-XII of  FIG. 8 .  
    
    
      Referring now to the Figures, and to  FIGS. 1 and 2  in particular, a wick assembly  1  of an anaesthetic vaporiser comprises a wick core block  2  of aluminium alloy, an inner wick  3 , a main wick  4 , an outer wick  5  and a centre wick  6 , each wick  3 - 6  comprising a needle felt wick material made from polytetrafluoroethylene fibres.  
      The wick core block  2  is generally cube-shaped, with its edges extending vertically, in use, being substantially rounded. A plurality of channels  7  (in this embodiment, seventeen) extend vertically through the vertical faces of the core block  2 , separated by a plurality of ribs  8 . Each channel  7  has a generally Ω-shaped profile.  
      The inner wick  3  comprises a sheet of wick material extending around the vertical faces of the core block  2  so as to follow the profile of each channel  7  and rib  8 . A helical spring  9  extends along an interior of each channel  7 , acting as a frame to hold a respective portion of the inner wick  3  in contact with an interior surface of the channel  7 . The inner wick  3  thus lines each channel  7  and is contactable by gas passing therethrough.  
      The main wick  4  extends around the vertical faces of the core block  2  exterior of the channels  7 , contacting the inner wick  3  where it passes over the ribs  8 . The main wick  4  also extends downwardly below the wick core block  2 , so that it may pick up liquid anaesthetic agent from a reservoir (see below).  
      The outer wick  5  extends around an outside of the main wick  4 . It may either also extend downwardly below the core block  2 , as shown in  FIG. 1 , or have a vertical extent generally similar to that of the core block  2  and the inner wick  3 , as shown in  FIG. 2 .  
      The core block  2  is also provided with a substantially cylindrical central aperture  10 , extending vertically therethrough. The centre wick  6  extends around a majority of an inner surface of the central aperture  10 , having the form of a hollow elongate cylinder with a longitudinal slot  11 , and also extends downwardly below the core block  2  so that it may pick up liquid anaesthetic agent from the reservoir.  
      The centre wick  6  is held in contact with the inner surface of the central aperture  10  by a helical central spring  12  of corresponding dimensions.  
      The wick assembly  1  also comprises a base plate  13  (omitted from  FIG. 1  for clarity). This is fastened to an underside of the core block  2 , fitting within the main wick  4 . The base plate has a central aperture  14 , corresponding to the central aperture  10  of the core block  2 , through which the centre wick  6  passes. The base plate  13  is provided on its upper face with an outer set of elongate recesses  15  and an inner set of elongate recesses  16 , whose function is described below.  
      The core block  2  is also provided with a plurality of cylindrical passages  17  (in this embodiment, twelve) extending vertically therethrough, located between the channels  7  and the central aperture  10 .  
      As shown in  FIG. 3 , the wick assembly  1  fits closely inside a reservoir chamber  18 , located within a main body  19  of the vaporiser. The main wick  4  and outer wick  5  are held securely between the core block  2  and corresponding walls of the reservoir chamber  18 . Thus, each channel  7  adopts a generally cylindrical form, entirely lined with wick material. A lower zone  20  of the reservoir chamber  18  is filled, in use, with a supply of a liquid anaesthetic agent. The main wick  4  and centre wick  6  (and optionally the outer wick  5 ) extend into the lower zone  20 . Thus, by capillary action, liquid anaesthetic agent passes upwardly throughout the main wick  4  and centre wick  6 . From the main wick  4 , it passes into the portions of the inner wick  3  over the ribs  8  of the core block  2  and thence to the portions of the inner wick  3  lining the channels  7 . The anaesthetic is then free to evaporate from the wicks  3 ,  4 ,  6  into the respective channels  7  and central aperture  10 .  
      An upper manifold block  21 , as shown in  FIG. 4 , is mounted to an upper surface of the wick assembly  1  and main body  19 . As for the base plate  13  of the wick assembly  1 , the upper manifold block  21  is provided on its surface facing the wick core block  2  with an outer set of elongate recesses  22  and an inner set of elongate recesses  23 .  
      As shown in  FIG. 5 , one channel  7   a  is provided with a groove  24  extending longitudinally thereof, to receive overlapping extremities of the inner wick  3 . The main wick  4  also overlaps with itself at its extremities, as shown, and so the outer wick  5  is sized to leave a gap between its extremities, which receives the overlapping extremities of the main wick  4 .  
      The wick assembly  1  defines a prolonged path in contact with anaesthetic-soaked wick material, along which carrier gas is passed to become fully saturated with anaesthetic vapour.  FIG. 6  shows this gas path, which follows the sequence of arrows  25 .  
      Carrier gas enters the wick assembly  1  from an entry port  26  of the upper manifold block  21  (see  FIG. 4 ), flowing into a passage  17  aligned with the entry port  26 , along arrow  25   a . At a lower end, this passage  17  is aligned with one of the inner set of elongate recesses  16  of the base plate  13 , which connects it to a lower end of an adjacent passage  17 . An upper end of the adjacent passage  17  is aligned with one of the inner set of elongate recesses  23  of the upper manifold block  21 , which in turn connects it to an upper end of a further passage  17 . Thus, the carrier gas flows alternately upwardly and downwardly through each of the passages  17 .  
      This allow heat transfer from the incoming carrier gas to the core block  2  and thence to the wicks  3 ,  4 ,  6  and the anaesthetic evaporating therefrom. Normally, such evaporation leads to a marked localised cooling, reducing the saturated vapour pressure of the anaesthetic in the carrier gas and reducing the rate of evaporation. This heat transfer arrangement thus raises the potential delivery rate of anaesthetic vapour from the wick assembly  1  and reduces its variability.  
      A last elongate recess  27  in the upper manifold block  21  connects a final one of the passages  17  to the central aperture  10  of the core block  2 . Here, carrier gas absorbs, as it flows downwardly, anaesthetic vapour from the centre wick  6 . The slot  11  in the centre wick  6  allows the gas to pass outwardly of the wick  6 , above a point where the wick  6  enters the liquid anaesthetic held in the lower zone  20  of the reservoir chamber  18 .  
      The carrier gas then passes upwardly through a passage port  28  extending through the base plate  13 , and enters a channel  7  aligned therewith. An upper end of the channel  7  is linked via one of the outer set of elongate recesses  22  of the upper manifold block  21  to an upper end of an adjacent channel  7 . The lower end of this adjacent channel  7  is similarly linked by one of the outer set of elongate recesses  15  of the base plate  13  to a lower end of a further channel  7 , and so forth. The carrier gas flows alternately upwardly and downwardly through each of the channels  7 .  
      This prolonged contact with wick material soaked in liquid anaesthetic ensures that the carrier gas becomes thoroughly saturated with anaesthetic vapour. The successive upward and downward flows ensure that there is no appreciable variation in vapour content due to it being heavier than the carrier gas, such as may occur with existing wick arrangements. In any case, there are no “short-cuts” through this assembly, such as may be possible in existing wick arrangements.  
      Thus, carrier gas exiting a final one of the channels  7 , along arrow  25   b , and being led away through an exit port  29  in the upper manifold block  21  aligned therewith, is reliably fully saturated with anaesthetic vapour.  
      A further problem, addressed by the wick assembly  1  shown, is “pumping”. In this, the use of a mechanical ventilator to deliver an ultimate anaesthetic blend to a patient may cause pulsations in back-pressure in the carrier gas stream, as far back in the anaesthetic delivery system as the vaporiser. This can affect both carrier gas flow patterns and the saturated vapour pressure of the anaesthetic vapour in the carrier gas, leading to undesirable variations in the supply of anaesthetic vapour and carrier gas from the vaporiser.  
      Existing solutions include incorporating large gas volumes within a vaporiser, to buffer any pressure variations, or “anti-pumping coils” inserted into the carrier gas path outside the vaporiser. The first solution wastes valuable space within the vaporiser and the latter is an inconvenient extra piece of apparatus. However, in the present wick assembly  1 , the long, relatively restricted carrier gas path extending through each of the passages  17  also acts as an antipumping coil, greatly reducing the effect of back pressure variations from downstream of the vaporiser, without using up valuable space within the vaporiser.  
      The vaporiser shown is a by-pass type vaporiser, in which an incoming carrier gas stream is divided into two, one sub-stream being passed through a wick arrangement to become saturated with anaesthetic vapour and the other sub-stream by-passing the wick arrangement. When the sub-streams are recombined, immediately before leaving the vaporiser, the carrier gas stream produced thus has a known partially-saturated anaesthetic vapour content.  
      However, as mentioned above, any wick arrangement, even the wick assembly  1  of the present invention, will cool to some extent when in use. The saturated vapour pressure of anaesthetic in the carrier gas will thus fall until the vaporiser reaches an equilibrium operating temperature. To achieve a constant net vapour content in the recombined carrier gas stream, more of the carrier gas must be routed through the wick assembly to pick up anaesthetic and less must by-pass it.  
      It is hence customary to incorporate a temperature compensating valve into the path of the carrier gas by-passing the wick. This valve incorporates a bimetallic strip which closes the valve as it cools, restricting flow through the valve and diverting more of the carrier gas through the wick arrangement. In existing vaporisers, this valve is mounted to an underside of the reservoir chamber  18 , so that the bimetallic strip responds to the temperature of the liquid anaesthetic therein. However, the temperatures of the liquid anaesthetic in the reservoir and of the anaesthetic vapour in the wick arrangement can differ significantly.  
      Therefore, the vaporiser of the present invention is provided with a temperature compensating valve  30  mounted adjacent an upper part of the main body  19 , and hence close to the wick assembly  1 , its core block  2 , and the carrier gas and anaesthetic vapour passing therethrough.  
      The temperature compensating valve  30  is mounted to a front block  31  of the vaporiser. It comprises a pair of closely-spaced circular plates  32 , the opposing faces which are machined to a very close tolerance. Carrier gas enters a volume between the plates  32  adjacent their centres and flows radially outwardly. A bimetallic strip  33  is so arranged that, as it cools, it bears on one of the plates  32 , and moves them together. The plates  32  are springloaded so that they move apart again if the bimetallic strip  33  warms and retracts.  
      The carrier gas entering the vaporiser is split into two within the upper manifold block  21 . The sub-stream by-passing the wick assembly  1  flows out of a first by-pass port  35   a  in the upper mounting body (see  FIG. 7 ) and enters the front block  31  through its corresponding first by-pass port  36   a , as shown by arrow  34   a . The sub-stream then passes through the restricted volume between the plates  32 , and leaves the front block  31  through a second by-pass port  36   b , re-entering the upper manifold block  21  through its corresponding second by-pass port  35   b , as shown by arrow  34   b . This sub-stream is then recombined, within the upper manifold block  21 , with the sub-stream that has passed through the wick assembly  1 .  
      This arrangement provides far more responsive compensation for any temperature variations within the wick assembly  1  than do conventional by-pass valve arrangements, and so delivers a significantly more consistent anaesthetic vapour concentration.  
      An additional benefit of this arrangement is that adjustment of the temperature compensating valve  30  is much easier than for a valve buried beneath the reservoir chamber  18  (see  FIGS. 10 and 12  below for details).  
      As the valve  30  is not occupying space within the main body  19  beneath the reservoir chamber  18 , this can be deeper for a given overall height of the vaporiser. Also, with the valve  30  occupying only an upper part of the front block  31 , a lower part of the front block  31  can be hollowed out to form, an extension  37  of the reservoir chamber  18 , further increasing the capacity of the vaporiser without exceeding its maximum possible external dimensions. The embodiment shown has a capacity of 400 cm 3 ; equivalent conventional vaporisers can hold no more than 250 cm 3 .  
       FIGS. 8 and 9  show the assembled vaporiser. A sight glass  38  is provided, allowing direct visualisation of the level of liquid anaesthetic in the reservoir chamber  18  and its extension  37 . The sight glass  38  is provided with indicia showing a maximum fill level and a minimum fill level, below which the reservoir chamber  18  requires urgent replenishment.  
      A pour filler unit  39 , of conventional form, is provided, mounted to the front block  31  and connected to the extension  37 . This allows easy topping-up of the reservoir chamber  18  while the vaporiser is in use. Alternative filling systems can be used in place of the pour filler unit  39  shown.  
      The vaporiser shown has a conventional mounting arrangement  40  incorporated into the upper manifold block  21 , so that it may be mounted to a desired existing mounting rack system (in this case, a Selectatec (Registered Trade Mark) system).  
      The net anaesthetic vapour content provided by the vaporiser is controlled by a conventional control valve arrangement within the upper manifold block  21 , and is set by means of a control knob  41 .  
      FIGS.  10  to  12  show the assembled vaporiser in cross-section. In particular,  FIG. 12  shows how close the bimetallic strip  33  of the temperature compensating valve  30  is to the wick assembly  1  and the channels  7  therethrough, ensuring that it responds reliably to the actual temperature of the carrier gas and anaesthetic vapour therein.  FIG. 12  also shows an adjusting body  32  for fine adjustment of the valve  30 , and  42  for setting the maximum bypass resistance, easily accessible by removing a decorative fascia  43  from the front block  31 .  
      The vaporiser shown provides a supply of anaesthetic vapour, borne in a stream of carrier gas, having a consistent, predetermined anaesthetic content. It undergoes lower temperature variations during operations than do existing vaporisers, and compensates more accurately and rapidly for any variations that do occur. It is easy to keep topped up during prolonged use, and in any case has a significantly greater anaesthetic capacity while having external dimensions compatible with existing mountings. It also has an in-built resistance to “pumping” effects due to mechanical ventilators connected thereto, and the anti-pumping design of the core also provides improved heat transfer.