SUMP FOR AN ANAESTHETIC VAPORISER

Abstract: A sump for an anaesthetic vaporiser comprises a vaporising chamber (21) for liquid anaesthetic agent, a heater (27) for liquid anaesthetic agent contained within the chamber, and a partition member (31) of which at least a portion extends into the chamber to entrap a portion of liquid anaesthetic agent at a location immediately adjacent the heater, the member having a partition outlet (33) for anaesthetic agent vapour through the partition member at a position above the maximum level of liquid anaesthetic agent within the partition member. The partition member allows a portion only of liquid anaesthetic agent within the vaporising chamber to be heated, so that the temperature required of the liquid agent for operation of the vaporiser can be reached rapidly. The sump finds particular application in connection with the use of low boiling point anaesthetic agents such as 2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane.

Description: SUMP FOR AN ANAESTHETIC VAPORISER This invention relates to a sump for an anaesthetic agent vaporiser, and to a method of vaporising an anaesthetic agent. A known vaporiser for volatile anaesthetic agents includes a vaporising chamber for liquid anaesthetic agent, and an inlet into the vaporising chamber for a carrier gas which entrains anaesthetic agent vapour within the chamber above the surface of the liquid anaesthetic agent and carries the vapour out of the vaporiser for adminstration to a patient. Such a vaporiser is disclosed in GB-1224478. The vapour pressure of many volatile anaesthetic agents is heavily dependent on the ambient temperature at which the vaporiser operates. In order to minimise the dependence of vapour pressure on the ambient temperature in which the vaporiser operates, it is known to provide a heater within the vaporising chamber by which the temperature of anaesthetic agent within the chamber can be controlled. It is to be understood however that the temperature of the anaesthetic agent is maintained significantly below its boiling point; anaesthetic agent vapour is therefore carried out of the vaporising chamber not under its own vapour pressure, but entrained in the carrier gas which is supplied to the chamber. A recently introduced anaesthetic agent, 2-(difluoromethoxy)- 1,1,1,2-tetrafluoroethane has a relatively low boiling point in the region of ambient temperature. Conventional techniques for administration of anaesthetic agents having boiling points well above ambient temperature, using vaporisers such as that disclosed in GB-1224478, are not generally applicable to this new agent. Recently developed vaporisers for low boiling point anaesthetic agents are disclosed in EP-A-438218 and European patent application no. 91306823.5. Subject matter disclosed in those documents is incorporated in this specification by these references to the documents. A vaporiser includes a vaporising chamber for liquid anaesthetic agent, in which the agent is vaporised. The vaporiser further includes a passage which extends from an inlet for carrier gas to an outlet. Anaesthetic agent vapour from the vaporising chamber flows from the chamber under the pressure which is generated as a result of the liquid agent having been heated to cause it to boil, to mix with carrier gas in the passage so that the carrier gas which is discharged from the outlet, for administration to a patient, is a mixture of carrier gas and anaesthetic agent vapour. In vaporisers for low boiling point vaporisers, it is particularly important that the temperature of the liquid anaesthetic agent within the vaporising chamber be controlled carefully, because the pressure of the anaesthetic agent vapour is so heavily dependent on the temperature of the liquid anaesthetic agent relative to its boiling point. It can also be desired in many instances to reach the controlled temperature of the liquid anaesthetic agent as quickly as possible after replenishing the supply of agent to the vaporising chamber or after first turning on the vaporiser. The present invention provides a sump for an anaesthetic vaporiser, which includes a partition member located in the vaporising chamber by which a portion of the liquid anaesthetic agent can be entrapped, for heating by means of an associated heater. Accordingly, the invention provides a sump for an anaesthetic agent vaporiser, which comprises: (a) a vaporising chamber for liquid anaesthetic agent, (b) a heater for liquid anaesthetic agent contained within the chamber, and (c) a partition member of which at least a portion extends into the chamber to entrap a portion of liquid anaesthetic agent at a location immediately adjacent the heater, the member having a partition outlet for anaesthetic agent vapour through the partition member at a position above the maximum level of liquid anaesthetic agent within the partition member. The sump of the invention has the advantage that a quantity of liquid anaesthetic agent can be heated rapidly to a desired temperature, which is above the boiling point of the anaesthetic agent, so that a pressure of anaesthetic agent vapour can be established to enable the vapour to flow from the vaporising chamber for administration to a patient. The provision of a partition member by a portion of the liquid anaesthetic agent in the vaporising chamber can be entrapped, allow the desired temperature in the liquid anaesthetic agent to be reached without having to heat all of the liquid in the vaporising chamber. The significance of this advantage can be appreciated in the context of refilling of the vaporising chamber of a sump, when existing liquid anaesthetic agent which has been heated to cause vaporisation of the agent is supplemented by additional liquid agent which has not been so heated. The resulting reduction in the temperature of the liquid agent in the vaporising chamber will lead to an immediate reduction in the pressure of the anaesthetic agent vapour. This can have significant adverse consequences if prolonged and if the refilling takes place while anaesthetic agent is being administered from the vaporiser to a patient. The sump of the present invention allows the pressure of the anaesthetic agent vapour to be restored to a desired level quickly after such a refilling operation, so that any reduction in the supply of anaesthetic agent to a patient under anaesthesia is minimised. It must further be appreciated that the likelihood of refilling of the vaporising chamber of a vaporiser, while agent is being administered to a patient, is somewhat greater in the case of 2-(difluoromethoxy)-1,1,1,2- tetrafluoroethane and other low boiling point anaesthetic agents than in the case of conventional agents because of the greater volumes of that agent which are administered during anaesthesia. In another aspect, the invention provides a method of vaporising an anaesthetic agent, which comprises providing a sump for an anaesthetic agent vaporiser, comprising: (a) a vaporising chamber for liquid anaesthetic agent, (b) a heater for liquid anaesthetic agent contained within the chamber, and (c) a partition member of which at least a portion extends into the chamber to entrap a portion of liquid anaesthetic agent at a location immediately adjacent the heater, the member having a partition outlet for anaesthetic agent vapour; supplying heat to liquid anaesthetic agent entrapped within the partition member by means of the heater to cause the anaesthetic agent to boil; and allowing anaesthetic agent vapour to flow from within the partition member for supply to a patient, under the pressure of the anaesthetic agent vapour generated as a result of the heat supplied to the liquid anaesthetic agent from the heater. The sump of the invention may be arranged so that anaesthetic agent vapour discharged from within the partition member is supplied directly from the sump into a conduit for administration to a patient, possibly after it has been mixed with a carrier gas. This arrangement has the advantage that * (Fred - any thoughts?) . In another arrangement, the sump may be arranged so that the partition outlet discharges the anaesthetic agent vapour into the vaporising chamber, the vaporising chamber including a chamber outlet for anaesthetic agent vapour from the sump. This arrangement has the advantage that carry over of liquid anaesthetic agent from the vaporising chamber with anaesthetic agent vapour can be minimised, by arranging for the flow of vapour from within the partition member out of the sump to be along a tortuous path, allowing liquid agent associated with anaesthetic agent vapour, for example as a result of agitation on boiling of the liquid, to drop back into the vaporising chamber. For example, the chamber outlet and the partition outlet may be offset from one another so that the two outlets do not overlap with one another. The possibility of minimising liquid carry over has the advantage that the quantity of anaesthetic agent supplied to a patient can be controlled more accurately, since the agent which is being supplied is more likely to be in vapour form which can be quantified, and less likely to include liquid which might vaporise between the vaporising chamber and the patient, or within the patient's breathing circuit, increasing significantly the effective quantity of anaesthetic agent vapour which is received by the patient. When the partition member and the vaporising chamber each have outlets, the path which is followed by anaesthetic agent vapour passing through those outlets may be arranged to be tortuous by arranging the outlets so that vapour flowing through each of the outlets follows respective paths which are non-parallel. For example, one of the outlets may be in a vertical member, and the other of the outlets may be in a horizontal member. In this event, it will be understood that the two outlets will be such that they are offset from one another and do not overlap. When the outlets are in members which are not perpendicular to one another, they are to be considered to be offset from one another and not to overlap when the projection of one of the outlets onto the other outlet does not overlap with that other outlet. The partition member may be provided as a plate which extends across the vaporising chamber and cooperates with the internal wall of the vaporising chamber to define a space within the cha ber for liquid anaesthetic agent to be heated by the heater. The plate may extend for example across the vaporising chamber or around a corner thereof. The plate may include for example one or more openings in it located above the maximum level of the liquid anaesthetic agent in the vaporising chamber, through which anaesthetic vapour can pass from within the partition member, and it may include one or more openings located below the level of the liquid agent in the vaporising chamber, through which liquid agent can pass from outside the partition member, to replace that which has been lost by vaporisation, and so to maintain the levels of liquid agent inside and outside the partition member approximately equal. The provision of a partition member in the form of a plate finds particular application when anaesthetic agent vapour is discharged from it into the vaporising chamber for subsequent administration to a patient through a chamber outlet. The partition member may be provided in the form of a sheath, especially a tubular sheath, extending upwardly through liquid anaesthetic agent located in the vaporising chamber, to entrap a portion of the liquid agent within it. One or more openings may be provided at or towards the base of the sheath to allow liquid agent to pass from outside the sheath, to replace that which has been lost by vaporisation, and so to maintain the levels of liquid agent inside and outside the sheath approximately equal. Anaesthetic agent vapour may be discharged from the sheath into the vaporising chamber for subsequent administration to a patient via a chamber outlet. However, the use of a partition member in the form of a sheath finds particular application when vapour is discharged directly from the sump from the sheath. Preferably, the heater is located within the partition member. It can be preferred particularly that the heater has the form of an elongate member, extending upwardly in liquid anaesthetic agent within the partition member. For example, when the partition member has the form of a tubular sheath, a heater may be provided which has a cross-section corresponding to the internal cross-section of the sheath, extending upwardly within the sheath, defining a region around it between the heater itself and the inner wall of the sheath in which liquid anaesthetic agent to be heated can be located for heating. The partition member may include at least one opening at a point above the maximum level of liquid anaesthetic agent within the vaporising chamber to allow the levels of liquid anaesthetic agent, inside and outside the partition member respectively, to remain approximately equal. When the vaporising chamber includes an outlet for anaesthetic agent vapour received from the partition outlet, the opening and the partition outlet may be provided by a single opening. However, it can be preferred to include an opening in the partition member even when anaesthetic agent vapour is discharged from the sump directly from the partition outlet, to ensure that the levels of the anaesthetic agent inside and outside the partition member remain approximately equal. Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a first embodiment of sump for an anaesthetic vaporiser; and Figure 2 is a schematic representation of a second embodiment of sump. Referring to Figure 1, a sump for an anaesthetic vaporiser includes a vaporising chamber 1 which contains liquid anaesthetic agent for example 2-(difluoromethoxy) -1, 1, 1, 2 tetrafluoroethane. Extending into the vaporising chamber 1 from above (as shown) is a member in the form of a sheath 5. The sheath 5 has a lower (as shown) open end 6 spaced from the base of the vaporising chamber and a branch pipe 7 extends from adjacent its opposite upper (as shown) end outside the vaporising chamber 1. Between the ends of the sheath 5 and within the vaporising chamber at a position above the level of the liquid anaesthetic agent in the vaporising chamber are formed a series of vents 8. Located within the sheath 5 is a heater core 2 which consists of heater 3 and heat sensor 4 which is electrically connected to a control device (not shown) to maintain the core 2 at a steady predetermined temperature. The inside surface of the sheath 5 and the outside surface of the core 2 are so dimensioned that the space between them is occupied by a relatively small liquid volume located adjacent the heater 3. In operation, the vaporising chamber 1 contains liquid anaesthetic agent up to a level below that of the vents 8 in the sheath 5. The core 2 is then energised and reaches a steady operating temperature. The relatively small liquid volume which is located between the heated core 2 and the inner surface of the sheath 5 is heated rapidly to or towards this operating temperature. As vapour is produced the pressure within the vaporising chamber 1 will rise. However, this effect is not instantaneous since some of the vapour produced flows through the vents 8 into the upper part of the vaporising chamber 1. This is necessary to allow more liquid to flow from the vaporising chamber under the open bottom end 6 of the sheath 5 and up into between the inside surface of the sheath 5 and the outer surface the space of the core 2. Initially, with cold anaesthetic agent liquid in the vaporising chamber 1, much of the vapour that flows through the vents 8 condenses on the surface of liquid anaesthetic agent in the vaporising chamber. Nonetheless, the pressure within the vaporising chamber 1 does rise rapidly up to the vapour pressure of the anaesthetic agent at the chosen operating temperature. This is a consequence of the heater arrangement which allows production of large amounts of vapour at a localised "hot spot", namely the small liquid volume between the inner surface of the sheath 5 and the heater core 2 to exist in a state a dynamic equilibrium with a largely separate volume of cold liquid anaesthetic agent. Once the appropriate vapour pressure is reached anaesthetic agent vapour can be drawn from the top of the sheath via the branch pipe 7. Although in the embodiment described above, reference is made to the heating of a liquid anaesthetic agent in the vaporising chamber of an anaesthetic agent vaporiser, the heater arrangement of the invention can be used in other circumstances where it is necessary to "spot heat" a portion of bulk liquid contained within a vessel. Figure 2 shows a sump for an anaesthetic vaporiser which includes a vaporising chamber 21, having an inlet 23 for liquid anaesthetic agent and an outlet 25 for anaesthetic vapour. A cylindrical heater 27 is located within a cylindrical finger 29 for heating liquid anaesthetic agent contained within the vaporising chamber to a temperature which is greater than the boiling point of the agent. A plate 31 is provided between the heater and its associated finger and the outlet 25 from the vaporising chamber. As shown, the plate extends across the chamber and cooperates with the internal wall of the vaporising chamber to define a space within the chamber for liquid anaesthetic agent which is to be heated by the heater. The plate has openings 33 extending through it at its upper end to allow the passage of anaesthetic vapour from within it sheath towards the outlet 25 from the vaporising chamber. The plate also has openings 35 extending through it at its lower end through which liquid anaesthetic agent can pass into the space of the vaporising chamber defined by the plate and the internal wall of the chamber to replace the agent which is lost from that space by vaporisation. The plate controls movement of the liquid anaesthetic agent within it as a result of the application to it of heat which can cause it to become agitated as it boils, so as to minimise the carriage of liquid agent into the outlet 25 from the chamber together with anaesthetic vapour.

Claims: CLAIMS : 1. A sump for an anaesthetic agent vaporiser, which comprises: (a) a vaporising chamber for liquid anaesthetic agent, (b) a heater for liquid anaesthetic agent contained within the chamber, and (c) a partition member of which at least a portion extends into the chamber to entrap a portion of liquid anaesthetic agent at a location immediately adjacent the heater, the member having a partition outlet for anaesthetic agent vapour through the partition member at a position above the maximum level of liquid anaesthetic agent within the partition member. 2. A sump as claimed in claim 1, in which the partition outlet discharges the anaesthetic agent vapour into the vaporising chamber, and in which the vaporising chamber includes a chamber outlet for anaesthetic agent vapour. 3. A sump as claimed in claim 2, in which the chamber outlet and the partition outlet are offset from one another so that the two outlets do not overlap with one another and so that the path followed by anaesthetic agent vapour leaving the sump through the partition and chamber outlets is tortuous. 4. A sump as claimed in any one of claims 1 to 3 , in which the partition member is provided as a plate which extends across the vaporising chamber and cooperates with the internal wall of the vaporising chamber to define a space within the chamber for liquid anaesthetic agent to be heated by the heater. 5. A sump as claimed in claim 4, in which the plate has at least one opening in it through which anaesthetic agent vapour can flow. 6. A sump as claimed in any one of claims 1 to 3, in which the partition member is provided in the form of a tubular sheath. 7. A sump as claimed in any one of claims 1 to 6, in which the heater is located within the partition member. 8. A sump as claimed in claim 7, in which the heater has the form of an elongate member, extending upwardly in liquid anaesthetic agent within the partition member. 9. A sump as claimed in any one of claims 1 to 8, in which the partition member includes at least one opening at a point above the maximum level of liquid anaesthetic agent within the vaporising chamber to allow the pressures within the vaporising chamber, inside and outside the partition member respectively, to equalise. 10. A method of vaporising an anaesthetic agent, which comprises providing a sump for an anaesthetic agent vaporiser, comprising: (a) a vaporising chamber for liquid anaesthetic agent, (b) a heater for liquid anaesthetic agent contained within the chamber, and (c) a partition member of which at least a portion extends into the chamber to entrap a portion of liquid anaesthetic agent at a location immediately adjacent the heater, the member having a partition outlet for anaesthetic agent vapour; supplying heat to liquid anaesthetic agent entrapped within the partition member by means of the heater to cause the anaesthetic agent to boil; and allowing anaesthetic agent vapour to flow from within the partition member for supply to a patient, under the pressure of the anaesthetic agent vapour generated as a result of the heat supplied to the liquid anaesthetic agent from the heater.

Inventors: SAMPSON DAVID CHARLES, THWAITES DAVID JOHN, SAMPSON, DAVID, CHARLES, THWAITES, DAVID, JOHN
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1991
Main classification: A61M16
IPC codes: A61M16