A SUMP FOR LIQUID ANAESTHETIC AGENT

Abstract: A sump for liquid anaesthetic agent for use in an anaesthetic vaporiser, comprises a sum body (1) for containing the liquid anaesthetic agent, which includes an indicator for a minimum level of the agent, and means (15) for measuring the dielectric constant of liquid anaesthetic agent contained within the body, the measuring means being located completely below the indicated minimum level of the agent. The dielectric constant measuring means may be provided by a pair of spaced apart electrically conductive elements. The sump allows anaesthetic agents to be supplied with reduced likelihood of an incorrect agent being supplied inadvertently.

Description: A SUMP FOR LIQUID ANAESTHETIC AGENT The present invention relates to a sump for a liquid anaesthetic agent, for use in an anaesthetic vaporiser*, and to an anaesthetic vaporiser which includes such a sump. An anaesthetic agent is generally is commonly supplied to a patient by means of an anaesthetic vaporiser, in which the agent is taken from a sump, for example by means of a carrier gas, for supply to the patient's breathing circuit. It is of critical importance that the anaesthetic agent supplied from an anaesthetic vaporiser is accurately identified so that the amount of the agent supplied to a patient can be properly measured according to the requirements of the patient, depending on, for example, the nature of the agent, the required depth of anaesthesia, the age, weight of the patient and so on. Containers in which liquid anaesthetic agents are supplied are commonly provided with formations by which the agent in the container can be identified, and which can mate with corresponding formations on the sump of the vaporiser or on a conduit through which the agent is transferred from the container to the sump. The use of formations in this way is disclosed in GB-1193241. Such formations are commonly provided as coded collars located around the necks of the bottles in which the agents are supplied. The use of mateable formations of this type can minimise the chance of an agent being supplied incorrectly to a sump on a vaporiser and, by appropriate mating of a sump with a vaporiser, the risk of an anaesthetic agent being incorrectly identified at the time of supply to a patient in minimised. There remains, however, a requirement for some applications for an anaesthetic agent within a sump on a vaporiser to be positively identified. It has been determined that the dielectric constants of commonly used anaesthetic agents vary across a wide range. The present invention provides for the identification of an anaesthetic agent within the sump of an anaesthetic vaporiser, taking advantage of the range of values of the dielectric constants of such commonly used materials. Accordingly, in a first aspect, the invention provides a sump for liquid anaesthetic agent for use in an anaesthetic vaporiser, which comprises: (a) a sump body for containing the liquid anaesthetic agent, which includes an indicator for a minimum level of the agent, and (b) means for measuring the dielectric constant of liquid anaesthetic agent contained within the body, the measuring means being located completely below the indicated minimum level of the agent. The sump of the invention, has the advantage that it makes possible the accurate identification of an anaesthetic agent contained within it, at all times during use of the vaporiser. The risk of an incorrect anaesthetic agent being supplied to a patient, for example as a result of the formations on a supply container for the agent being tampered with or removed, is therefore reduced significantly. Furthermore, the technique according to the invention by which the agent is identified allows the identity of the agent to be transmitted electrically to an appropriate display for monitoring by the operator of the vaporiser. The operator is then able to ensure that the conditions for supply of the agent to the patient are appropriate for the agent in question. Moreover, the fact the identity of the agent can be transmitted electrically allows the vaporiser itself, without intervention by its operator, to adjust certain of the conditions for supply of the agent to the patient. In this way, the possibility of an incorrect anaesthetic agent or of a mixture of agents (arising from incorrect recharging of the sump) being supplied to a patient can be reduced. The indicator for the level of the anaesthetic agent in the sump body will preferably provide an indication of a maximum level of anaesthetic agent in the vaporiser, as well as a minimum level. More preferably, it will provide a continuous indication of the level of the agent between the minimum and maximum levels. The indicator may simply take the form of a window in the wall of the sump body through which the level of the agent in the body can be monitored visually. However, the indicator might itself provide an electrical signal for transmission to the vaporiser, or otherwise for use by the operator of the vaporiser. For example, the indicator may take the form of a pair of electrically conductive elements by which the dielectric constant of liquid anaesthetic agent between them is measured, the measured value depending on the quantity of such fluid and therefore on the level of the fluid in the sump body. A level indicator of this type is disclosed in GB-A-2177008. Such elements must be positioned so as to extend between the minimum and maximum levels of the anaesthetic agent in the sump in order to operate as required. In order for such a level indicator to operate in this way, it is necessary first for the agent in question to be identified so that the level indicator can be calibrated for that agent. The present invention provides a technique by which the agent can be identified, to allow a level indicator of this type to operate, as well as to allow other aspects of the supply of anaesthetic agent to be conducted more accurately. Preferably, the measuring means comprises a pair of spaced apart electrically conductive elements located completely below the indicated minimum level of the agent. The measuring means is of course positioned below the minimum level of the agent to ensure that the measured value of the dielectric constant does not vary with the level of the anaesthetic agent within the sump. One or both of the elements may be in the form of a tube or a rod. For example, the elements may be provided in the form of a pair of aligned rods, or a pair of concentric tubes which has the advantage of a small apace requirement. More preferably, however, at least one of the elements is in the form of a plate; it is especially preferred that both of the elements are in the form of plates, which are arranged parallel to one another, generally perpendicular to the surface of the anaesthetic agent contained within the sump when in use. One of the elements may be provided by a part of the sump body, when that part at least of the body is formed from an appropriate conductive material. For example, one of the elements may be provided by the base of the body, and the other of the elements may be provided by a plate extending parallel to the base of the body, and located between the base and the indicated minimum level of the anaesthetic agent. One or both of the electrically conductive elements may be coated with a layer of an electrically insulating material. This makes it possible for the sump of the invention to be used in the supply of anaesthetic agents which might otherwise interfere with the measurement of dielectric constant, such as agents which are electrochemically conductive, and agents which are polar. The sump may include means for controlling the temperature of an anaesthetic agent contained within it, for example to minimise fluctuations in vapour pressure. For example, heating or cooling means (or both) may be provided to maintain the temperature of the agent at a predetermined level relative to the boiling point of the agent. This can allow the sump to be used in the supply of an anaesthetic agent whose boiling point is close to the range of temperatures of a vaporiser, commonly about 25 to 35'C. Preferably, the sump includes an electrical connector on the sump body through which connections to the measuring means can be made. More preferably, the connector is positioned so that the connection to the measuring means is made as a result of positioning the measuring means (which might be for example one or more conductive plates or other elements) in the sump body. Preferably, the sump includes an electrical connector on the sump body through which connections between the measuring means and a display can be made. The display may be part of an anaesthetic vaporiser. In another aspect, the invention provides an anaesthetic vaporiser, which includes a sump for liquid anaesthetic agent as discussed above. Preferably, the vaporiser includes a display by which information originating with the said measuring means can be displayed. It is particularly preferred that the vaporiser includes means by which a signal originating with the measuring means and having a value characteristic of an anaesthetic agent in the sump body, is compared with a stored set of signal values so that the agent can be positively identified. Examples of anaesthetic agents which can be supplied from a sump of the type discussed above include 1-bromo-l-chloro- 2,2,2-trifluoroethane (sold under the trade name Fluothane) , 1- chloro-2,2,2-trifluoroethyl difluoromethyl ether (sold under the trade name Isoflurane) , 2-chloro-l,1,2-trifluoromethyl difluoromethyl ether (sold under the trade name Enflurane) , 2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane (sold under the trade name Desflurane) , and fluoromethyl 2,2,2-trifluoro-1- (trifluoromethyl)ethyl ether (sold under the trade name Sevoflurane) . The dielectric constant of the liquid anaesthetic agent may be measured directly. However, in many situations, it can, be preferable to measure the dielectric constant indirectly as a measurement of capacitance. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawing, which is a schematic cross-section through a sump of an anaesthetic vaporiser. Referring to the drawing, a sump for an anaesthetic vaporiser comprises a sump body 1 having an inlet 3 and an outlet 5 for a carrier gas. At least the base portion 9 of the sump body l is formed from an electrically conductive material. The body includes a reservoir portion 7 for liquid anaesthetic agent and a series of tubes 9 through which the carrier gas flows when picking up the anaesthetic agent vapour. A transparent window 11 allows the level of liquid anaesthetic agent in the reservoir to be inspected. Marks 13 are provided to show the minimum and maximum levels of " the liquid for reliable operation of the vaporiser. An electrode 15 formed as a circular disc from an electrically conductive material is provided so as to submerged totally in liquid anaesthetic agent in the reservoir portion of the sump, spaced from and approximately parallel to the base of the sump body. Contacts 17 are provided on the outside of the sump body, to allow electrical connections to be made to the electrode 15 and the electrically conductive base portion 9 of the sump body. A block 15 of an insulating material insulates the contact for the electrode 15 from the sump body. The electrode 15 and the portion of the sump body which are contacted by liquid anaesthetic agent in the reservoir may be coated with a layer of an insulating material to minimise adverse effects on measurements of dielectric constant of a material between them, for example as a result of that material being electrically conductive. Measurements of capacitance of a number of materials using a sump having the construction shown in the accompanying drawing are as follows:

Claims: CLAIMS : 1. A sump for liquid anaesthetic agent for use in an anaesthetic vaporiser, which comprises: (a) a sump body for containing the liquid anaesthetic agent, which includes an indicator for a minimum level of the agent, and (b) means for measuring the dielectric constant of liquid anaesthetic agent contained within the body, the measuring means being located completely below the indicated minimum level of the agent. 2. A sump as claimed in claim 1, in which the measuring means comprises a pair of spaced apart electrically conductive elements located completely below the indicated minimum level of the agent. 3. A sump as claimed in claim 1 or claim 2, in which one of the elements is provided by a part of the sump body. 4. A sump as claimed in claim 2 or claim 3, in which at least one of the elements is in the form of a plate. 5. A sump as claimed in any one of claims 2 to , in which one or each of the elements is coated with a layer of an electrically insulating material. 6. A sump as claimed in any one of claims 1 to 5, which includes an electrical connector on the sump body through which connections to the measuring means can be made. . A sump as claimed in claim 6, in which the connector is positioned so that the connection to the measuring means is made as a result of positioning the measuring means in the sump bodv. 8. A sump as claimed in claim any one of claims l to 7, which includes an " electrical connector on the sump body through which connections between the measuring means and a display can be made. 9. An anaesthetic vaporiser, which includes a sump for liquid anaesthetic agent as claimed in any one of claims 1 to 8. 10. A vaporiser as claimed in claim 9, which includes a display by which information originating with the said measuring means can be displayed. 11. A vaporiser as claimed in claim 10, which includes means by which a signal originating with the measuring means and having a value characteristic of an anaesthetic agent in the sump body, is compared with a stored set of signal values so that the agent can be positively identified.

Inventors: SAMPSON DAVID CHARLES, SAMPSON, DAVID, CHARLES
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1992
Main classification: A61M16
IPC codes: A61M16