Anaesthetic vaporiser

Abstract: An anaesthetic vaporiser (1) for dosing a liquid anaesthetic (10), comprising a liquid container (8) for the anaesthetic (10), and outlet tube (12) from the liquid container (8) to a dosing point (14), a first regulating means (18) for regulating a flow of anaesthetic (10) from the liquid container (8) towards the dosing point (14) and a control unit (36) for controlling dosing are described. To improve dosing and safety, the anaesthetic vaporiser (1) according to the invention is devised with a return tube (28) connected in parallel across the first regulating means (18) by a first connection point (30), downstream from the first regulating means (18), and a second connection point (32), upstream from the first regulating means, a second regulating means (34; 52) is arranged in the return tube (28) and a third regulating means (22) is arranged in the outlet tube (12), downstream from the first connection point (30) . The control unit (36) controls at least one of the regulating means (18, 34, 22) in dosing the anaesthetic (10).

Description: The present invention relates to an anaesthetic vaporiser according to the preamble to claim 1. An anaesthetic is usually administered to patients who, for various reasons, are about to undergo surgery or some examination. In inhalation anaesthesia, a gas mixture of oxygen and nitrous oxide are usually administered along with an anaesthetic. The anaesthetic is in liquid form and must therefore be vaporised before it reaches the patient. Dosing and vaporisation normally take place in an anaesthetic vaporiser. A number of different types of vaporisers are known. One way to dose and vaporise liquid anaesthetic is to draw some of the gas mixture through the liquid, causing it to pick up anaesthetic vapour. Another way is to draw part of the gas mixture over the surface of the liquid anaesthetic. The surface area of this liquid can be enlarged with the aid of wicks which draws up the liquid. Dosing in these vaporiser types is usually based on the saturation of gas, drawn over or through the liquid, with vaporised anaesthetic. Dosing is affected by factors such as temperature and pressure, in addition to problems in ensuring that saturation really does take place. In another type of vaporiser, the liquid is first boiled or transformed into vapour in some other way. The vapour is then dosed via valves into the gas mixture supplied to the patient. Here, problems may occur in e.g. maintaining a constant pressure for the anaesthetic vapour (variations in pressure affect dosing). A third version is to inject a dosed amount of liquid into the gas flow (directly or through a filter of some kind). Dosing can be regulated by pressurisation of the liquid or by pumping specific quantities of the liquid from a liquid container. Document US-A-5 509 405 discloses a vaporiser of the last type and includes all the features of the preamble of independent claim 1. If a pump is used, it is essential for the pump to be exact and rugged. Pressure also plays an important role here. The pump's ability to pump the correct amount of liquid can be affected by pressure in the liquid container which, in turn, can be affected by the egress of liquid. Liquid held in the tubing between the liquid container and the pump also presents a problem. One objective of the invention is to achieve a rugged and reliable anaesthetic vaporiser that avoids the problems associated with other known anaesthetic vaporisers, as described above. This objective is achieved according to the invention when the anaesthetic vaporiser is devised as is evident from the characterising part of claim 1. Advantageous refinements and embodiments of the anaesthetic vaporiser are evident from the dependent claims of claim 1. Connecting a return tube in parallel across the first regulating means, e.g. a pump, makes it easier to control pressure downstream from the pump. A second regulating means is arranged in the return tube and a third regulating means in the dosing tube. The control unit can dispense anaesthetic in a safe and accurate manner by controlling at least one of the regulating means. In one advantageous embodiment of the anaesthetic vaporiser according to the invention, the second regulating means consists of an additional pump, and the third regulating means consists of a dosing restriction. Pressure before the dosing restriction can be accurately regulated by regulating the pumps, flow through the dosing restriction thereby achieving great accuracy. Regulating one of the pumps is sufficient if the other pumps at a constant rate. Alternately, or as a complement, the dosing restriction can be variable. The control unit can then control the dosing restriction to compensate for any pressure variations occurring when there are rapid changes in dosing. In another advantageous embodiment of the anaesthetic vaporiser according to the invention, the second regulating means consists of a back-flow restriction and the third regulating means of a dosing valve. The dosing valve can be controlled by the control unit according to the pressure, so the correct dosing is achieved. The pump can simultaneously be controlled to maintain the pressure. As an alternative or a complement in this embodiment, the back-flow restriction can be variable. The return tube's second connection point can be arranged in the outlet tube upstream from the pump. The second connection point can alternately be arranged at the liquid container. The liquid container can advantageously be devised with a venting device in order to regulate the negative pressure in the container. It should be devised to minimise the risk of leakage of anaesthetic into atmosphere. Additional control of dosing (and an opportunity to increase accuracy) is achieved by arranging a flow meter in the outlet tube downstream from the first connection point. A pressure stabiliser can be connected to the outlet tube, downstream from the first connection point, in order to enhance the maintenance of pressure downstream from the pump. Arranging an anaesthetic meter to measure the anaesthetic content of the gas mixture increases safety and improves opportunities for refining dosing control. Safety can also be increased by arranging a shut-off valve in the outlet tube downstream from the second regulating means. The anaesthetic vaporiser according to the invention is described below in further detail, referring to the figures in which FIG. 1 shows a first embodiment of the anaesthetic vaporiser, and FIG. 2 shows a second embodiment of the anaesthetic vaporiser. FIG. 1 is a schematic depiction of an anaesthetic vaporiser 2 according to a first embodiment of the invention for dosing anaesthetic into a tube 4 in which a gas mixture flows (according to the arrow 6). The tube 4 can be a part of a fresh gas system or a breathing circuit (re-breathing or non re-breathing). The anaesthetic vaporiser 2 comprises a liquid container 8 holding liquid anaesthetic 10 for dosing. The anaesthetic 10 consists primarily of one of the anaesthetics desflurane, enflurane, halothane, isoflurane or sevoflurane. An outlet tube 12 for the anaesthetic 10 runs from the liquid container 8. The outlet tube 12 leads to a dosing point 14 in the tube 4. The dosing point 14 can be devised in some suitable fashion to facilitate vaporisation of the anaesthetic 10. For example, it can have a nozzle, preferably a whirling nozzle, arranged on the end of the outlet tube 12. The liquid anaesthetic 10 can alternately be carried to a filter or some other porous structure made of a suitable material (sintered metal, mesh, porous plastic, a membrane, activated charcoal etc.). Heat can also be applied to the dosing point 14 to facilitate vaporisation. A number of components are arranged in the outlet tube 12. A filter 16 is arranged immediately downstream from the liquid container 8. The purpose of this filter 16 is simply to filter particles and dirt that could otherwise disrupt dosing, out of the system. A pump 18 is arranged downstream from the filter 16. The pump 18 pumps liquid anaesthetic 10 from the liquid container 8. Operation of the pump 18 in dosing is described in more detail below. A manometer 20, a dosing valve 22, a shut-off valve 24 and a flow meter 26 are arranged downstream from the pump 18. The function and purpose of these components are also described below in more detail. A return tube 28 in parallel across the pump 18 is connected to the outlet tube 12 at a first connection point 30 and a second connection point 32. In this embodiment, the first connection point 30 is arranged between the pump 18 and the dosing valve 22. The second connection point 32 is arranged between the filter 16 and the pump 18. A back-flow restriction 34 is arranged in the return tube 28. The back-flow restriction 34 can be fixed or variable. The figure shows the latter version. The back-flow restriction 34 can alternately be replaced with a pressure regulator. The anaesthetic vaporiser 2 is controlled by a control unit 36. Control can be exercised in any of the following ways (or versions thereof). One way to control the anaesthetic vaporiser 2 to dose the desired amount of liquid anaesthetic 10, is based on controlling the pump 18 so as to maintain a pre-set pressure between the pump 18 and the dosing valve 22. This control can be exercised regardless of the pressure measured by the manometer 20. The flow-through performance of the dosing valve 22 at the prevailing pressure is known, i.e. the amount of liquid flowing through the dosing valve 22 at the prevailing pressure is known. When the dosing valve 22 is regulated, a specific amount of liquid anaesthetic can therefore be dosed. The dosing valve 22 can be e.g. an ON/OFF valve controlled with a duty cycle supplying the correct dosing for the prevailing pressure and the set dose. The pressure drops a little when the dosing valve 22 opens to dose a specific amount. When a fast-acting manometer 20 is used, the pump 18 is able to respond with corresponding swiftness in restoring pressure without affecting flow through the dosing valve 22. Operating pressure between the pump 18 and the dosing valve 22 can be affected by the variable back-flow restriction 34 also. Pressure changes in the system can be performed more rapidly by changing the degree of restriction. Greater operating point dynamics and, accordingly, greater dosing dynamics can therefore be achieved. With the variable restriction 34, the pump 18 can be allowed to run at a constant speed. Pressure can then be maintained by merely controlling the variable restriction 34. An alternative is to allow pressure to drop somewhat during the dosing of liquid anaesthetic. This can be achieved by e.g. having the pump 18 operate with a constant pump output and using a fixed back-flow restriction 34. The measured pressure curve then becomes saw-toothed. The drops in pressure are then proportional to the amount dosed. The amount dosed can therefore be determined from the drops in pressure. The amount dosed can also be checked by measuring the dosed flow with the flow meter 26. The flow meter 26 does not need to be placed next to the dosing point 14 but can be located anywhere in the outlet tube downstream from the first connection point 30. If the amount dosed deviates excessively from the reference amount, supply can be stopped completely by closing the dosing valve 22 or the shut-off valve 24. Shut-off valve 24 could therefore be placed upstream of dosing valve 22 as an alternative arrangement. Even if the figure shows that the measurement signal from the flow meter 26 goes to the control unit 36, this safety feature can be completely separate from the dosing control exercised by the control unit 36. Access to the flow signal also provides scope for more refined regulation of dosing. The different control parameters, i.e. pressure, flow, pump output, back-flow restriction, the dosing valve's opening or duty cycle, can all be given different emphases in order to achieve optimal dosing. For example, a neural network could be gradually taught to control dosing. Irrespective of which of these methods is used, the anaesthetic vaporiser according to the first embodiment of the invention conveys specific advantages. It is, generally speaking, easier to dose liquids than gases since liquids are virtually non-compressible. The two-stage system with the pump in series with the valve, in combination with back-flow via restriction, improves control over operating conditions, such as working pressure etc. Even if the pressure in the liquid container 8 varies (is higher or lower than the operating pressure), this does not, in principle, affect dosing. In those instances in which there is positive pressure in the liquid container 8, the pump 18 can be replaced with a valve, but the back-flow restriction 34 should then be replaced with a pump at the same time in order to ensure that back-flow can occur. However, negative pressure will develop in the liquid container 8 with most anaesthetics as liquid is pumped out. Imposing a limit on this negative pressure may be appropriate. If negative pressure becomes excessive, an increasingly powerful pump 18 will be needed to maintain the working pressure downstream from the pump 18. One way to limit the negative pressure is shown in FIG. 1. An air tube 38 is connected to the liquid container 8 and leads to atmosphere. A check valve 40 opens, at a fixed or pre-set pressure gradient between atmosphere and pressure in the liquid container 8, to admit air. Alternately, a pressure regulator or valve, controlled by the pressure measured in the liquid container 8, can be used. A micro-porous membrane 42 is also arranged in the air tube 38. The membrane 42 passes air but not liquid. The membrane 42 therefore prevents liquid anaesthetic from escaping into atmosphere. If anaesthetic vapour does pass the membrane 42, an adsorption filter 44 is also arranged in the air tube 38. The adsorption filter 44 can be devised to adsorb anaesthetic vapour passing towards atmosphere and desorbs the vapour when the valve 40 opens to the passage of a flow of air from the atmosphere. The anaesthetic vapour is then returned to the liquid container 8. The adsorption filter 44 can suitably contain activated charcoal, e.g. coconut shell charcoal. The order in which the membrane 42, valve 40 and filter 44 are arranged is not critical. Additional safety in both the dosing of liquid anaesthetic and patient safety in general can be achieved by placing an anaesthetic meter 46 downstream from the dosing point 14. In contrast to the flow meter 26, which measures the actual flow of liquid anaesthetic, the anaesthetic meter 46 measures the concentration of anaesthetic in the gas mixture flowing through the tube 4. Two faults may be present when the measured concentration deviates from a reference value for the concentration (apart from faults in the meter(s)). One fault could be erroneous dosing of anaesthetic. The other could be an erroneous flow of the gas mixture in the tube 4. Regardless of which fault is involved, the anaesthetic vaporiser 2 can react immediately to excessive concentrations of anaesthetic by closing the shut-off valve 24. Like the safety thinking behind the flow meter 26, this safety system can be incorporated into the control unit 36 as a completely separate unit. Alternately, the measurement signal from the anaesthetic meter 46 can be used for further refining dosing of the anaesthetic. If the flow of the gas mixture does not deviate too much from the desired flow (and reaches a flow sufficient to assure the patient's safety), dosing can be adjusted with the aid of the measured concentration. At the same time, an alarm should still be sounded or a warning issued to the operator whenever any deviations occur with an impact on the system's performance and/or patient safety. FIG. 1 also shows a pressure stabiliser 48 connected to the outlet tube downstream from the first connection point 30. The purpose of the pressure stabiliser 48 is to facilitate maintenance of the working pressure. The pressure stabiliser 48 can consist of a bellows or some other variable-volume container and be loaded with a spring, piston or some other pressure-exerting component. Pressure loading of the bellows should preferably be adjustable so loading can be set at the working pressure. The volume of the pressure stabiliser 48 can be dimensioned for the volumes to be dosed. It should be noted that only the pump 18, the dosing valve 22 and the back-flow constrictor 34 are essential to the anaesthetic vaporiser's dosing function. A second embodiment of the anaesthetic vaporiser according to the invention is shown in FIG. 2 and is designated 50. All the components and means that can be identical to those in the first embodiment have been assigned the same designations. Thus, the anaesthetic vaporiser 50 comprises a liquid container 8 holding liquid anaesthetic 10. An outlet tube 12 connects the liquid container 8 to a dosing point 14. The liquid container 8 could be a bottle containing the liquid anaesthetic 10. The liquid container 8 could also be a small container to which a bottle containing liquid anaesthetic is coupled, thereby avoiding re-circulation of liquid anaesthetic to the bottle. A filter 16, a pump 18, a shut-off valve 24 and a flow meter 26 can be arranged in the outlet tube 12. A return tube 28 is arranged in parallel across the pump 18 between a first connection point 30 and a second connection point 32. In this embodiment, the second connection point 32 is arranged at the liquid container 8. A specific advantage here (in relation to the first embodiment) is the complete elimination of air bubbles from the part of the system to be filled with liquid. A venting device, consisting of a venting tube 38, a valve 40, a membrane 42 and a filter 44, is connected to the liquid container 8. An anaesthetic meter 46 is connected for measuring the concentration of anaesthetic in the gas mixture. A control unit 36 controls the anaesthetic vaporiser's 50 dosing of liquid anaesthetic into the gas mixture in a tube 4. The main difference compared to the first embodiment is that an additional pump 52 is provided in the return tube 28 instead of a restriction. The second connection point 32 opens into the liquid container 8 instead of the outlet tube 12. The dosing valve has been replaced with a dosing restriction 54. The dosing restriction 54 can be variable. When the dosing restriction 54 is fixed, dosing can be achieved by varying the pressure of the liquid anaesthetic immediately upstream from the dosing restriction 54. The flow characteristics of the dosing restriction 54 at different pressures must naturally be known, but this is easily determined by experimentation. When the dosing restriction 54 is variable, regulation of dosing with greater accuracy becomes possible, even if complexity increases. As the above shows, there are many versions of conceivable regulating methods for the anaesthetic vaporiser according to the invention. They can also be complemented in other ways. For example, more filters can be arrayed to ensure function. Thus, one filter can be located immediately downstream from the first connection point 30. This filter blocks any particles from the pump 18.

Claims: An anaesthetic vaporiser (2; 50) for dosing a liquid anaesthetic (10), comprising a liquid container (8) for the liquid anaesthetic (10), an outlet tube (12) from the liquid container (8) to a dosing point (14), a first regulating means (18) for regulating a flow of liquid anaesthetic (10) from the liquid container (8) towards the dosing point (14) and a control unit (36) for controlling dosing, characterised in that a return tube (28) is connected in parallel across the first regulating means (18) at a first connection point (30), downstream from the first regulating means (18), and a second connection point (32), upstream from the first regulating means, a second regulating means (34; 52) is arranged in the return tube (28) and a third regulating means (22; 54) is arranged in the outlet tube (12), downstream from the first connection point (30), the control unit (36) controlling at least one of the regulating means (18, 34; 52, 22; 54) in dosing the liquid anaesthetic (10). The anaesthetic vaporiser according to claim 1, characterised in that the first regulating means consists of a pump (18), the second regulating means consists of a second pump (52), the third regulating means consists of a dosing restriction (54), and a manometer (20 is arranged between the pump (18) and the dosing restriction (54). The anaesthetic vaporiser according to claim 2, characterised in that the dosing restriction (54) is variable. The anaesthetic vaporiser according to claim 1, characterised in that the first regulating means consist of a pump (18), the second regulating means consists of a back-flow restriction (34), the third regulating means consists of a dosing valve (22) and a manometer (20) is arranged between the pump (18) and the dosing valve (22). The anaesthetic vaporiser according to claim 4, characterised in that the back-flow restriction (34) is variable. The anaesthetic vaporiser according to any of the above claims, characterised in that a venting device (38, 40, 42, 44) is connected to the liquid container (8). The anaesthetic vaporiser according to any of the above claims, characterised in that a flow sensor (26) is arranged in the outlet tube (12), downstream from the first connection point (30). The anaesthetic vaporiser according to any of the above claims, characterised in that a pressure stabiliser (48) is arranged for the outlet tube (12), downstream from the first connection point (30). The anaesthetic vaporiser according to any of the above claims, characterised in that an anaesthetic meter (46) is arranged downstream from the dosing point (14) for measuring the amount of anaesthetic (10) dispensed. The anaesthetic vaporiser according to any of the above claims, characterised in that a shut-off valve (24) is arranged in the outlet tube (12), downstream from the third regulating means (22; 54).

Inventors: EMTELL PAER, PESSALA TOM, EMTELL, PAER, PESSALA, TOM, Emtell, Pär
Applicants: MAQUET CRITICAL CARE AB
Application year: 2000
Main classification: A61M16
IPC codes: A61M16