Fresh gas system for vaporising a liquid anaesthetic

Abstract: A fresh gas system (2) in an anaesthetic machine, comprising a gas preparation unit (4, 8, 10) for preparing a flow of fresh gas consisting of a gas mixture from at least two sources of gas (6, 12A, 12B), a vaporiser (14) through which the gas mixture is passed to pick up a quantity of anaesthetic related to the viscosity of the gas mixture and a control device (18) for controlling the gas preparation unit (4, 8, 10), is described. When the fresh gas system (2) is devised with a means (4A, 8A, 10A; 20; 22, 24), upstream from the vaporiser (14) and connected to the control device (18), for determining the composition of the gas mixture, the control device (18) can be devised to compensate the fresh gas system for the gas mixture's composition in such a way that a specific amount of anaesthetic is always picked up by the gas mixture, regardless of the mixture's composition.

Description: Fresh gas system for vaporising a liquid anaesthetic The present invention relates to a fresh gas system according to the preamble to claim 1. The user manual "Halothane Vaporizer 950/Enflurane Vaporizer 951/Isoflurane Vaporizer 952", Siemens-Elema AB, October 1992, describes a vaporiser for a fresh gas system in an anaesthetic machine. Two gases, oxygen and air or oxygen and nitrous oxide, are mixed in a gas mixer and passed in a series of pulses, or as a continuous flow, through the vaporiser. A throttle is arranged in the vaporiser. A drop in pressure occurs when the gas mixture passes the throttle. This drop in pressure affects the surface of a liquid anaesthetic in a chamber connected to the flow pathway, upstream from the throttle. The liquid anaesthetic is then forced up through a capillary tube and sprayed into the gas mixture downstream from the throttle where the liquid is vaporised. The throttle is adjustable for the uptake of different amounts of liquid anaesthetic so the fresh gas contains specific end concentrations of the anaesthetic. The throttle is calibrated for a specific gas pressure and for a specific gas mixture. One disadvantage of the described system is that different gas mixtures have different viscosity, and this causes variations in the effect of the drop in pressure across the throttle. The deviation can be up to about 10%. The corresponding problem is also found in other types of vaporisers, especially in vaporiser types in which some of the gas mixture is diverted with the aid of a drop in pressure. The objective of the present invention is to achieve a fresh gas system, which avoids the aforesaid problems. One such fresh gas system is achieved in accordance with the invention in that the fresh gas system is devised as disclosed in the characterising part to claim 1. Advantageous improvements and embodiments are evident from the dependent claims to claim 1. With the aid of a means for identifying the composition of different gas mixtures, the fresh gas system can be compensated for the difference in viscosity for different gas mixtures. One such means can consist of a setting device for the gas mixture's composition, i.e. the setting device the operator uses for selecting the gases, e.g. oxygen and air, or oxygen and nitrous oxide, in the gas mixture and the concentration of the respective gases. A gas analyser, arranged to analyse the gas mixture before it is fed into the vaporiser, is another means for identifying composition. A third means for identifying composition is flow meters for measuring the partial flows of gas from the respective gas source. Compensation can consist of regulation of the amount of liquid anaesthetic introduced into the gas flow or regulation of the flow itself. Compensating for the viscosity of the gas mixture by varying the amplitude of the flow of pulses of gas to the vaporiser is particularly advantageous with vaporisers of the aforesaid type. The duration of pulses in conjunction with the latter form of compensation should be varied in such a way that the volume of gas remains constant in each gas pulse. A fresh gas system according to the invention will be described below in greater detail, referring to the figures in which FIG. 1 shows an advantageous embodiment of the fresh gas system, FIG. 2 is a diagram illustrating compensation according to the gas mixture's viscosity, and FIG. 3 is a diagram showing advantageous embodiments of pulses of the gas mixture. FIG. 1 shows one embodiment of a fresh gas system 2 according to the invention. The fresh gas system 2 comprises a first gas regulator 4, preferably a valve, with a setting means 4A for setting the flow of a first gas from a first gas connector 6. The first gas is suitably oxygen. A second or a third gas can be regulated by means of a second gas regulator 8 with a setting means 8A, depending on the setting of a changeover valve 10. The changeover valve 10 also has a setting means 10A. A second gas connector 12A is connected to the second gas regulator 8 when the changeover valve 10 is in a first position, and a third gas connector 12B is connected to the second gas regulator 8 when the changeover valve 10 is in a second position. The second gas can be air, and the third gas can be nitrous oxide. A gas mixture consisting of oxygen and air, or oxygen and nitrous oxide, is prepared in the anaesthetisation of a patient. The exact composition of the gas mixture is selected by the operator with setting means 4A, 8A, 10A. Alternately, settings can be made on an instrument panel (not shown) for the entire anaesthetic machine. In such a case, the setting means 4A, 8A, 10A correspond to the mechanical control elements that implement the settings in the subsystems. The gases are mixed and then fed into a vaporiser 14 with a setting means 14A. The gas mixture is enriched in the vaporiser with an anaesthetic and then sent to a regulatory system 16 for delivery to a patient circuit in the anaesthetic machine. The regulatory system 16 can comprise a gas reservoir for fresh gas, a regulatory valve, flow meters etc. The vaporiser can advantageously consist of a Halothane Vaporizer 950, an Enflurane Vaporizer 951 or an Isoflurane Vaporizer 952, described in the introduction. A control device 18 controls the fresh gas system 2. The control device 18 is connected to most of the components, receives signals from them and emits control signals to them. In particular, the control device 18 selects appropriate compensation for variations in the viscosity of different gas mixture compositions. In order to compensate for the gas mixture's composition, the control device 18 must receive information on the contents of the mixture. The fresh gas system 2 therefore comprises at least one means for determining the composition of the gas mixture. In principle, a first means has already been described, viz. the setting means 4A, 8A, 10A. When the values for settings are sent to the control device 18, the device can determine an appropriate compensation. Alternately, or as a complement, a gas analyser 20 can be arranged upstream from the vaporiser 14. The gas analyser 20 analyses the composition of the gas mixture and sends the results of the analysis to the control device 18. This solution is appropriate when gas delivery to the vaporiser 14 is largely continuous or when compensation is performed in the vaporiser 14. Another alternative, or complement, is also shown in FIG. 1. A first flow meter 22 and a second flow meter 24 measure the respective gas flows from the various gas connectors 6, 12A/12B. The control device 18 can decide on a compensation from information on flow rates and gas fed to the second gas regulator 8. This solution is also more appropriate when gas delivery to the vaporiser 14 is continuous or when compensation is performed in the vaporiser 14. In the present embodiment of the fresh gas system 2, compensation occurs in the gas regulators 4, 8. Compensation is described below with simultaneous reference to all the figures. The gas regulators 4, 8 send pulsed flows 32, 34, 36 (FIG. 3) to the vaporiser 14. Each flow pulse 32 then serves as a kind of basic pulse for which the vaporiser 14 is calibrated. The flow pulse 34 represents a compensated flow pulse for a gas mixture with a lower viscosity, according to line 28 in FIG. 2. Line 28 depicts linear compensation with an increasing flow rate as the oxygen content in a mixture with air declines. The faster flow is generated to achieve a greater drop in pressure in the vaporiser 14, thereby achieving delivery of the same amount of liquid anaesthetic as with the basic pulse 32. The duration of the flow pulse 34 is reduced so a predetermined volume is maintained. In the corresponding manner, the flow pulse 36 illustrates compensation through a reduction in the flow rate. This takes place according to line 30, which represents a mixture of oxygen and nitrous oxide. Here, duration is instead prolonged so the same volume is achieved as with the basic pulse 32. Compensation eliminates virtually all of the effect of viscosity on the end concentration of the anaesthetic, and the result is more accurate dispensing of anaesthetic. Instead of compensating for the mixture's composition by varying the amplitude of flow, direct regulation can be achieved in the vaporiser, for example, by regulating the throttle in such a way that the drop in pressure across it compensates for the gas mixture's composition.

Claims: A fresh gas system (2) in an anaesthetic machine, comprising a gas preparation unit (4, 8, 10) for preparing a flow of fresh gas consisting of a gas mixture from at least two sources of gas (6, 12A, 12B), a vaporiser (14) calibrated for a gas having a specific viscosity and through which the gas mixture is passed to pick up a quantity of anaesthetic related to the viscosity of the gas mixture, and a control device (18) for controlling the gas preparation unit (4, 8, 10), characterised in that a means (4A, 8A, 10A; 20; 22, 24) for determining the composition of the gas mixture is arranged upstream from the vaporiser (14) and connected to the control device (18), the control device (18) being devised to determine a deviation in viscosity of the gas mixture and the specific viscosity and a compensation for the fresh gas system based on the deviation so that a specific amount of anaesthetic is picked up by the gas mixture, regardless of the mixture's composition. The fresh gas system according to claim 1, characterised in that the means for determining the composition of the gas mixture is a setting means (4A, 8A, 10A) for setting the composition of the gas mixture. The fresh gas system according to claim 1, characterised in that the means for determining the composition of the gas mixture is a gas analyser (20). The fresh gas system according to claim 1, characterised in that flow meters (22, 24), arranged to measure the flow of gas from the respect gas source, are the means for determining the composition of the gas mixture. The fresh gas system according to any of the above claims, the vaporiser (14) in said system releasing anaesthetic according to the drop in pressure across the vaporiser (14), and the gas preparation unit (4, 8, 10) is devised to generate a flow of fresh gas in the form of pulses of fresh gas with a specific flow amplitude, characterised in that the control device (18) causes the gas preparation unit (4, 8, 10) to vary the amplitude of the pulses of fresh gas in order to compensate for the gas mixture's composition. The fresh gas system according to claim 5, characterised in that the control device (18) also causes the gas preparation unit (4, 8, 10) to vary the duration of the pulses of fresh gas so a constant volume of fresh gas is maintained. The fresh gas system according to claim 5 or 6, characterised in that the control device (18) performs linear compensation of the amplitude of the fresh gas flow on the basis of the gas mixture's oxygen content. The fresh gas system according to any of the above claims, characterised in that the gas preparation unit (4, 8, 10) comprises a first valve (4) for regulating a flow of oxygen from a source of oxygen (6) and a second valve (8) for regulating a flow of air or nitrous oxide from a source of air or nitrous oxide (12A, 12B).

Inventors: WALLIN STEN, WALLIN, STEN
Applicants: MAQUET CRITICAL CARE AB
Application year: 1998
Main classification: A61M16
IPC codes: A61M16