Improvements in anaesthetic vaporisers

Abstract: An anaesthetic vaporiser 1 for use particularly with anaesthetic agents having a low boiling point comprises an inlet 2 for carrier gas and an outlet 4 for carrier gas and gaseous anaesthetic agents. A first passage 6 extends between the inlet 2 and the outlet 4. A flow restrictor 8 is located within the passage 6. A second passage 10 extends between the first passage 6 at a location upstream of the flow restrictor 8 and a regulator 14. The regulator 14 controls the pressure of gaseous anaesthetic agent when flowing through a third passage 26 extending from a vaporising chamber 12 containing anaesthetic agent to the first passage 6 at a location downstream of the flow restrictor 8. A flow control valve 28 is located in the third passage 26.

Description: The present invention relates to anaesthetic vaporisers. An anaesthetic vaporiser as defined in the preamble of Claim 1 is disclosed in FR-A-2 008 359. UK Patent No 1 224 478, describes an anaesthetic vaporiser of the by-pass type in which a carrier gas such as oxygen, air or nitrous oxide is initially divided on entry to the vaporiser between a first stream which is directed towards the sump or vaporising chamber of the vaporiser to entrain vapour from a volatile liquid anaesthetic contained therein; and a second by-pass stream, the first and second streams subsequently recombining prior to leaving the vaporiser for delivery to a patient. This known vaporiser has been used successfully over a number of years for delivering anaesthetic agents such as halothane, trichloroethlene and ether derivatives including enflurane, fluoroxene, methoxyflurane and isoflurane. All the aforementioned anaesthetic agents have a boiling point at atmospheric pressure well above 40°C. However, a new anaesthetic agent has been developed namely 2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane which has a boiling point at atmospheric pressure of between 20° and 25°C. This physical characteristic of 2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane renders existing anaesthetic vaporisers unsuitable for delivering said agent to a patient. It is an aim of the present invention to provide an anaesthetic vaporiser which is capable of delivering to a patient a predetermined concentration of an anaesthetic agent having a boiling point at normal atmospheric pressure of approximately 20°C. According to the present invention, the anaesthetic vaporiser comprises the features specified in Claim 1. Embodiments of the invention will now be described, by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which: Figure 1 is a diagrammatic sketch of an anaesthetic vaporiser according to the present invention in which are used ISO/BSI symbols; Figure 2 is a side view partly in cross section of the anaesthetic vaporiser of Figure 1 but with the addition of heaters as illustrated in Figure 4; Figure 3 is a diagrammatic sketch of a modified version of the anaesthetic vaporiser of Figure 1; Figure 4 is a further diagrammatic sketch of a further modified anaesthetic vaporiser as shown in Figure 1; Figure 5 is a diagrammatic sketch of yet a further modified anaesthetic vaporiser as shown in Figure 1; and Figure 6 is a diagrammatic sketch of still a further modification of the anaesthetic vaporiser of Figure 1. Referring first to Figures 1 and 2, an anaesthetic vaporiser 1 has an inlet 2 for carrier gas and an outlet 4 for carrier gas and gaseous anaesthetic agent. Extending between the inlet 2 and outlet 4 is a passage 6 in which is located a laminar flow by-pass restrictor 8. The restrictor 8 exhibits laminar flow characteristics over its operating flow range. Extending from the passage 6 from a location upstream of the restrictor 8 is a second passage 10 communicating with a first chamber 16 of a balance regulator 14. The balance regulator 14 includes a second chamber 18 which is separated from the first chamber 16 by diaphragm 20. Connected to the diaphragm 20 for movement therewith is a valve head 22 which co-operates with a valve seat 24. The valve head 22 and valve seat 24 control the flow of gaseous anaesthetic agent contained in a vaporising chamber 12. Extending from the second chamber 18 is a passage 26 which extends to the passage 6 downstream of the restrictor 8. A laminar flow control valve 28 is located in the passage 26. The vaporiser 1 includes heaters 32 which are controlled by a temperature sensing device (not shown) However, the heaters 32 can, in a modification, be controlled by a device for sensing the pressure of anaesthetic agent in the vaporising chamber 12. In use, energy is supplied to the heaters 32 which converts anaesthetic agent from a liquid to a gaseous state which is contained in the upper (as shown) portion of the vaporising chamber 12. Carrier gas then enters inlet 2 and continues along passage 6 through restrictor 8 towards the outlet 4. The pressure upstream of the restrictor 8 is dependent on the flow rate of carrier gas entering the inlet 2. The pressure in the first chamber 16 of the balance regulator 14 is the same as that upstream of restrictor 8 because of the second passage 10. This causes the diaphragm 20 to move upwards (as shown) taking with it the valve head 22. The valve head 22 will thus separate from the valve seat 24 thereby enabling gaseous anaesthetic agent to leave the vaporising chamber 12 and pass through the second chamber 18 into the passage 26 until the pressure in the passage 26 is the same as that in the chamber 16. The pressure in passage 6 upstream of the restrictor 8 and passage 26 upstream of the control valve 28 are the same. For any position of the control valve 28 the flow rate of gaseous anaesthetic agent will depend on that pressure and hence the carrier gas flow rate at the inlet 2. This ensures that the flow of anaesthetic agent rises when the carrier gas flow rate rises and vice versa and hence the percentage concentration by volume of the anaesthetic agent in the gas delivered to the patient remains constant. The gaseous anaesthetic agent then joins the carrier gas in the passage 6 prior to leaving the vaporiser outlet 4. As is known in the art and as explained above the concentration of gaseous anaesthetic agent in the carrier gas leaving the outlet 4 is controlled by the setting of the laminar control valve 28. The profiles of the valve head 22 and the co-operating valve seat 24 are so shaped that at carrier gas flows of up to 15 litres per minute the pressure of gaseous anaesthetic agent in the passage 26 equals the pressure of the carrier gas entering the inlet 2. Now referring to Figures 3 to 6 where like reference numerals denote like features as those described with reference to Figures 1 and 2. Referring in particular to Figure 3, for very low boiling point anaesthetic agents such as cyclopropane with a boiling point of minus 32°C the pressure of the agent in the vaporising chamber 12 at 22°C can be very high, in the order of 5.17 bar (75 psi). In order to cope with this high pressure a second pressure reducing regulator 30 is located between the vaporising chamber 12 and the balance regulator 14. Referring in particular to Figure 4, when the boiling point of the anaesthetic agent is about ambient temperature then to ensure pressure in the vaporising chamber 12 a heater 32 is provided to raise the temperature of the anaesthetic agent to above its boiling point. In the embodiment illustrated in Figure 4 the heater 32 is positioned within the vaporiser such that, as well as the vaporising chamber 12, the passageways 6, 10, 26 and regulator 14 are also heated to stop the anaesthetic condensing out on the walls of the vaporiser which would otherwise be cooler than the vaporising chamber. The heater 32 is controlled by a temperature sensing device (not shown). In Figure 5 there is illustrated a modification which provides additional safety in case the regulator 14 should fail in that it provides for a second regulator 14′ in series with the regulator 14. As shown, extending from the passage 6 is a passage 10 to the regulator 14 and a separate passage 10′ to the regulator 14′. A further additional safety measure is the provision of a pressure device (not shown) arranged between the passages 6 and 26 which is arranged to monitor the performance of the regulator 14. The pressure device will automatically detect, for example, a situation where the regulator valve head 22 jams open and higher concentrations of anaesthetic agent than those set are being delivered to the outlet 4. Finally, referring to Figure 6, the vaporiser 1 includes a by-pass passage 40 extending from the passage 6 adjacent the inlet 2 to the vaporising chamber 12. A one-way valve 42 is located in the passage 40. Further, a constant back pressure restrictor 44 is located in the passage 6 as shown. When it is necessary to use anaesthetic agents whose boiling points are above ambient, for example, above 20°C within normal operating temperature ranges in the order of 15°C to 35°C then when ambient temperature is above the boiling point, the vaporiser acts as in the embodiment illustrated in Figures 1 and 2 with the one-way valve 42 closed. When the ambient temperature is below the boiling point of the anaesthetic agent the one-way valve 42 opens and the vaporiser acts as a normal vaporiser of the by-pass type with the laminar flow restrictor 8 being temperature controlled. The laminar flow restrictor 8 thus compensates for changing vapour pressure.

Claims: An anaesthetic vaporiser comprising an inlet (2) for carrier gas, an outlet (4) for carrier gas and gaseous anaesthetic agent for delivery to a patient, a first passage (6) extending between the inlet (2) and the outlet (4) in which is located a flow restrictor (8), a second passage (10) extending between the first passage (6) at a location upstream of the restrictor (8) and a regulator (14), a third passage (26) extending from a vaporising chamber (12) containing anaesthetic agent to the first passage (6) at a location downstream of the flow restrictor (8) and a flow control valve (28) located in the third passage (26), characterised in that the regulator (14) controls the pressure of gaseous anaesthetic agent when flowing through the third passage (26) dependent on the pressure of carrier gas in the first passage (6) upstream of the flow restrictor (8). An anaesthetic vaporiser as claimed in claim 1, in which the flow restrictor (8) and the flow control valve (28) are substantially laminar flow devices over their operating flow ranges. An anaesthetic vaporiser as claimed in claim 1 or 2, in which a pressure reducing regulator is located between the vaporising chamber (12) and the regulator (14). An anaesthetic vaporiser as claimed in claim 1, 2 or 3, in which a heater (32) is located within the anaesthetic vaporiser for heating the vaporising chamber (12), the regulator (14) and the control valve (28). An anaesthetic vaporiser as claimed in claim 4, in which the heater is controlled by a temperature sensing device. An anaesthetic vaporiser as claimed in claim 4, in which the heater is controlled by a pressure sensing device which monitors the vaporising chamber pressure. An anaesthetic vaporiser as claimed in any one of claims 1 to 6, in which two regulators are provided in series. An anaesthetic vaporiser as claimed any any one of claims 1 to 7, in which a detection device is provided which monitors if the or each regulator is operating correctly. An anaesthetic vaporiser as claimed in claim 8, in which the detection device is a microswitch which cuts off power to the heater when it senses a regulator failure. An anaesthetic vaporiser as claimed in any one of claims 1 to 9, in which a further passage (40) extends between the first passage (6) at a location adjacent the inlet (2) and the interior of the vaporising chamber (12), a one-way valve (42) being located in said fourth passage to allow the flow of carrier gas in a direction from the first passage towards the vaporising chamber only.

Inventors: MONTGOMERY FREDERICK JOHN, MONTGOMERY, FREDERICK, JOHN
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1991
Main classification: A61M16
IPC codes: A61M16