Apparatus for the administration of a respiratory gas and at least one anaesthetic

Abstract: An apparatus (4) for the administration of a respiratory gas and at least one anaesthetic to a living being, in which apparatus (4) the anaesthetic is vaporized in a vaporizing unit (34) by respiratory gas, acquires a safer and more accurate control of the anaeshtetic concentration level in that a regulating valve controls the flow of respiratory gas through the vaporizing unit (34) and in that a regulating device (36) controls the regulating valve with such accuracy that a preselected concentration of vaporized anaesthetic in the respiratory gas is substantially fully maintained. To further increase the control of the apparatus it may also be provided with a feedback system for both the anaesthetic concentration level and the total flow.

Description: The invention relates to an apparatus for the administration of a respiratory gas and at least one anaesthetic to a living being comprising a vaporizing unit which contains liquid anaesthetic, whereby a predetermined concentration of the anaesthetic in the respiratory gas is acquired by vaporizing a defined amount of the liquid anaesthetic in respiratory gas passed through the vaporizing unit, an entrance part via which the respiratory gas, or the gases that form the respiratory gas, is supplied to the apparatus, a regulating valve for controlling the flow of gas through the vaporizing unit and an inlet tube, which conducts the respiratory gas and the vaporized anaesthetic to the living being. An apparatus of this kind is described in US-A-3,794,027. The apparatus, an animal anaesthesia machine, comprises a canister, to which a source of gas is connected via a vaporizer unit. An inlet tube conducts respiratory gas to a patient and an outlet tube conducts the respiratory gas from the patient via a common Y-piece. A second vaporizer unit is connected to the inlet tube. The two vaporizer units may be used separately or together and may contain the same or different anaesthetics. The first vaporizer unit comprises a valve, which can admit the gas from the gas source through the vaporizer unit or divert the gas around the vaporizer unit. When the gas is led through the vaporizer unit, it will bubble through the liquid anaesthetic and vaporize it. The gas containing the anaesthetic is then conducted to the canister and from there to the patient. The second vaporizer unit comprises two hollow sections which communicate via two apertures. The first hollow section forms a part of the inlet line. A substantially triangular vane can be placed in two positions, one of which permits the gas to only pass through the first hollow section. When the vane is brought into its second position, the gas will be led into the second hollow section and pass through a wick which separates the second hollow section into two halves. The wick is partially placed in the liquid anaesthetic and the gas passing through the wick vaporizes the liquid anaesthetic which has been absorbed by the wick. The gas containing anaesthetic then passes through the second aperture and out into the inlet line. The control of the vaporization of the anaesthetic is, for both vaporizer units, performed manually by activating respective switch system (valve and vane) for a defined time when vaporization shall take place. The concentration of anaesthetic depends mainly on two factors: the gas flow through the vaporizer unit, which will determine the amount of anaesthetic that is vaporized for each time unit and the total time during which the gas is led through the vaporizer unit, which will determine the total amount of vaporized anaesthetic and thereby also the concentration. This results in a variation of the concentration during the time anaesthetic is supplied to the patient. In order to avoid concentrations that are too high or too low, an empirically established scheme for the switching on and off of the vaporizer unit should be followed. With this system losses of anaesthetic from the system, through leakage and such, cannot be regarded. Another known apparatus, described in US-A-4,770,168, comprises a chamber with liquid anaesthetic and a positive displacement pump. Via a valve gas can be diverted from a pipe to the chamber in which the gas becomes saturated with anaesthetic. The flow of gas from the chamber back to the pipe is controlled by the pump. By controlling the motor that drives the pump via a feedback system depending on a total flow in the pipe or the anaesthetic concentration in the pipe, the concentration of anaesthetic may be maintained with higher accuracy than the previously described apparatus. The chamber may also comprise a thermistor. The motor may then also be controlled depending on the temperature in the anaesthetic chamber. However, the pump limits the useful range of the flow of anaesthetic saturated gas out into the pipe. This cause problems when a total flow is initially low, as is the case when small children or small animals are to be anaesthetized. Similar problems occur if the total flow is initially high. Furthermore, the required concentration level of anaesthetic may vary depending on the anaesthetic used and on the individual patient. Yet another known apparatus is described in EP-A-166 305. The apparatus comprises a vaporizing unit where liquid anaesthetic is supplied to a gasifier to be vaporized. Through the gasifier flows a respiratory gas and carries the vaporized anaesthetic to a breathing circuit, to which a patient can be connected. The concentration of the anaesthetic is controlled by periodically, with a prescribed frequency and length of period, supplying liquid anaesthetic to the gasifier, i.e. similar to the control in the apparatus of US-A-3,794,027 described above. An object of the invention is to provide an apparatus according to the first paragraph wherein the concentration of anaesthetic may be maintained with extremely high accuracy over a wide range of total respiratory gas flow. This object is achieved in an apparatus constructed in accordance with the present invention in that a regulating device controls the regulating valve so that the gas passing through the vaporizing unit attains a flow which, when saturated with anaesthetic, correlates to a preselected concentration of anaesthetic in the respiratory gas supplied to the living being. A regulating system generally of this type is the object of prior European patent application No. 90120843.9, published as EP-B-483 401. This regulating system can supply a gas flow in a range from approximately 0,6 ml/minute up to 18 litres/minute with an accuracy of ± 1 % of the selected flow. Hereby an exact portion of the total gas flow can be lead to the vaporizing unit, become saturated with anaesthetic and be led back to the main gas flow resulting in a total gas flow with a very accurate concentration level of the anaesthetic. The apparatus is improved in that the vaporizing unit comprises a thermal control device which controls the temperature in the vaporizing unit and maintains it at a preselected level. As the saturation level depends on the temperature, the concentration can be maintained with even higher accuracy if the temperature in the vaporizing unit is kept at a constant temperature. Alternatively this may be obtained in that the vaporizing unit comprises a thermistor for measuring the temperature in the vaporizing unit, in that the thermistor is connected to the regulating device, and in that the regulating device varies the flow of respiratory gas through the vaporizing unit depending on the measured temperature. A further improvement of the apparatus is obtained according to the invention in that a first sensor for measuring the concentration of the anaesthetic in the respiratory gas is located between the vaporizing unit and the living being, in that the first sensor is connected to the regulating device and in that the regulating device controls the flow of respiratory gas depending on the measured concentration of anaesthetic. By placing the first sensor after the vaporizing unit, preferably as close as possible to the living being, feed back the signal corresponding to the actual concentration and control the vaporization of the liquid anaesthetic by means of the regulating device, the system is provided not only with a means of further increasing the accuracy, but also a control or safety system which increases the use of the apparatus. The apparatus maintains an equally high accuracy of the concentration whether the respiratory gas is led to bubble through the liquid anaesthetic, is led through a wick which has absorbed liquid anaesthetic or is led over the surface of the liquid anaesthetic. In accordance with the invention there is a number of advantageous improvements for the apparatus, some of which will be described below. With a flow sensor located in the inlet tube for measuring the total flow of respiratory gas, including the vaporized anaesthetic, the regulating system can be provided with a further parameter for increasing the accuracy. The flow sensor also provides for the possibility of controlling the total respiratory gas flow more accurately because the contribution to the flow of the vaporized anaesthetic can thereby be compensated for. To achieve this, the apparatus comprises a control unit which, depending on a measured flow, controls the entrance part so that the difference between the flow supplied to the living being and a preselected flow is minimized. The safety of the apparatus is increased in that a control signal from the regulating device superimposes a basic signal corresponding to the preselected concentration of the anaesthetic, whereby the superimposed control signal increases the accuracy of the regulating valve. Even if there should be a failure in one or several of the sensors or if the regulating device were to malfunction, or cease operating, the basic signal would still guarantee the maintenance of a concentration of anaesthetic that is harmless for the patient connected to the apparatus. It is an advantage to use a microprocessor as the regulating device. An alarm function for the concentration of anaesthetic is provided for in that a second sensor for measuring the concentration of the anaesthetic in the respiratory gas is located in the inlet tube, in the vicinity of the first sensor, and in that a supervising unit gives an alarm if the concentration of anaesthetic falls outside a predetermined interval around the preselected value. There are basically three systems which can be used for administration of anaesthetic to a patient. First there is the non-rebreathing system in which all gases that are supplied to the patient are evacuated from the system after expiration. Then there is the semi-rebreathing system in which the anaesthetic is continuously recirculated to the patient while all other gases are evacuated. Finally, there is the rebreathing system, in which all gases, except expired carbon dioxide, are continuously recirculated to the patient. In connection with two figures, two embodiments of an apparatus in accordance with the invention are described, whereby Fig. 1 shows a schematic block diagram of a first embodiment with the use of a semi-rebreathing system, and Fig. 2 shows a schematic block diagram of a second embodiment with a non-rebreathing system. Fig. 2 shows a non-rebreathing system apparatus 4 for the administration of a respiratory gas and an anaesthetic to a living being. In the non-rebreathing system neither the respiratory gas nor the anaesthetic is recirculated back to the patient after use, as is the case in a rebreathing or semi-rebreathing system which is described below in connection with figure 1. An entrance part 31 supplies gas, for example oxygen and nitrous oxide, to a mixing unit 32 via a first oxygen tube 3O1 and a nitrous oxide tube 3O2. In the mixing unit 32 the gases are mixed before being conducted into an inlet tube 3O3 leading to a patient's lungs 33. A second oxygen tube 3O4 connects the entrance part 31 with a vaporizing unit 34. The oxygen flow to the vaporizing unit 34 is controlled by a regulating valve in the entrance part 31. In the vaporizing unit 34 liquid anaesthetic is vaporized by the oxygen flow and the oxygen becomes saturated with anaesthetic. Through a connecting tube 3O5 the saturated oxygen is conducted to the inlet tube 3O3 and mixed with the main flow of the respiratory gas before entering the lungs 33 through the patient tube 3O6. The patient tube 3O6 is common for both the inlet tube 3O3 and the outlet tube 3O7. Through the outlet tube 307 expired gas is led back to the entrance unit 31. The expired respiratory gas is then led to an absorber 35, in which anaesthetic is absorbed before the respiratory gas is evacuated. Each tube 3O1, 3O2, 3O4 has a regulating valve (not shown), all of the regulating valves being controlled by a regulating device 36. The regulating device 36 is connected to the entrance part 31 via a control line 3O8 and a signal line 3O9. Via the signal line 3O9 the regulating device 36 is provided with all information that is required in order to control the flow in each tube with highest possible accuracy, for example reference values of the flow, the position of each regulating valve etc. The vaporizing unit 34 comprises a thermal regulation device 37, which maintains a well-defined temperature in the vaporizing unit 34. There are several different ways of saturating the oxygen which flows through the vaporizing unit 34 with anaesthetic. One way is to let the oxygen bubble through the liquid anaesthetic, another is to lead the oxygen through a wick soaked with liquid anaesthetic and a third is to lead the oxygen over the surface of the liquid anaesthetic, thereby continuously exchanging saturated oxygen with unsaturated oxygen, which will become saturated due to evaporation of liquid anaesthetic. As the saturation point is constant at constant temperatures a high accuracy for the concentration of anaesthetic in the total respiratory gas flow is obtained by controlling the flow through the vaporizing unit 34 accurately. It is not necessary to use feedback systems where the concentration level or total respiratory gas flow are measured. The vaporizing unit 34 may, as an alternative to the thermal regulating device 37, comprise a thermistor 38 connected to the regulating device 36, whereby the oxygen flow the vaporizing units 36 is varied depending on fluctuations in the temperature. The apparatus 4 may also be provided with an anaesthetic sensor as an extra safety precaution. If the concentration of anaesthetic, due to any possible reason, should increase or decrease too much, an alarm will be activated. In fig. 1 a semi-rebreathing system apparatus 1 is described, i.e. the expired gas is not recirculated to the patient but conducted away from the patient except for the anaesthetic, which is recirculated. An entrance part 10 supplies, via a nitrous oxide tube 101 and the first oxygen tube 102, a mixing unit 11 with oxygen and nitrous oxide. In the mixing unit 11 the two gases are mixed to form a respiratory gas which during inspiration is conducted to a patient' s lungs 12 through an inlet tube 104. From the lungs 12 the respiratory gas is evacuated via an outlet tube 105 and the entrance part 10. When a patient is to be anaesthetized a vaporizing unit 13 is connected to the system. In the vaporizing unit 13 liquid anaesthetic, e.g. halothane, isoflurane or enflurane, is stored. By leading oxygen through the vaporizing unit 13, liquid anaesthetic is vaporized and the oxygen is saturated with it. The vaporizing unit 13 is supplied with oxygen through a second oxygen tube 103. In the figure it is shown that the second oxygen tube 103 is led around the mixing unit 11, so that the oxygen will not be mixed with the other gases. From the vaporizing unit 13 the oxygen and the vaporized anaesthetic is conducted through a connecting tube 106 to a patient tube 107, which is common for the inlet tube 104 and the outlet tube 105 and in which the gas flow from the mixing unit 11 and the gas flow from the vaporizing unit 13 are mixed. As anaesthetics are expensive and, furthermore, should not be allowed to come into the operating theatre where it may affect a surgeon, the apparatus 1 is provided with a filter 14 in the patient tube 107 which during expiration absorbs the anaesthetic in the expired gas and which during inspiration resorbs the anaesthetic to the respiratory gas. Such a filter 14, also known as a reflector, which will be used as designation henceforth, is described in WO88/07876. An anaesthetic sensor 15, a flow sensor 16 and a second anaesthetic sensor 17 are located in the patient tube 107. The three sensors 15, 16, 17 are respectively connected to a regulating device 18 via an anaesthetic signal line 108, a flow signal line 109 and an alarm signal line 110. The regulating device 18 is connected to the entrance part 10 via a control line 111 and a first reference value line 112 through which a set value of the respiratory flow is transmitted, and to the vaporizing unit 13 via a second reference value line 113, by which a set value for the concentration of anaesthetic is transmitted. The second reference line 113 also connects the entrance part 10 to the vaporizing unit 13. The vaporizing unit 13 may also comprise a thermal control device 37 or a thermistor 38, having the same function as described above. When a patient is to be a anaesthetized a selected gas flow (litres/minute) and a selected relationship between oxygen and nitrous oxide is set on the entrance part 10 and a selected concentration level of anaesthetic is set on the vaporizing unit 13. The set values are transmitted to a microprocessor in the regulating unit 18 through the first reference value line 112 and second reference value line 113. The set concentration level of anaesthetic is also transmitted to a control device in the entrance part 10 via the second reference value line 113. The control device of the entrance part 10 controls the valves which supplies the nitrous oxide tube 101 and the two oxygen tubes 102, 103 with gas. The control device hereby controls the valves so that the set respiratory gas flow is obtained with the selected mixture of oxygen and nitrous oxide. The valve which controls the flow through the second oxygen tube is set to supply an oxygen flow through the vaporizing unit 13, which is correlated to the selected concentration level of anaesthetic. As no anaesthetic has been absorbed by the reflector 14 at this point, i.e. at the beginning of the anaesthetization, a relatively large flow of oxygen is supplied to the vaporizing unit 13 in order to build up the concentration level. Via the anaesthetic sensor 15 and the flow sensor 16 the microprocessor of the regulating device 18 receives actual values of the concentration level and the respiratory flow. By comparing these values with the reference values a control signal is determined and via control line 111 transmitted to the entrance part 10. The determined control signal is superimposed on the control signal from the control device. A very accurate control of the set values is thus acquired at the same time as the apparatus 1 becomes relatively insusceptible to faults in the anaesthetic sensor 15 or in the microprocessor. To compensate for the extra flow that the vaporized anaesthetic causes, the nitrous oxide flow is reduced to maintain the selected total respiratory gas flow. The reason for reducing the nitrous oxide and not the oxygen is of course that the selected oxygen concentration is more important to maintain. As an extra safety precaution the apparatus 1 comprises the second anaesthetic sensor 17, described above. Via the alarm signal line 110 actual values of the concentration level of the anaesthetic is transmitted to the regulating device 18. The regulating device 18 comprises an alarm unit, which is completely separated from the microprocessor and has the function of supervising the concentration level. If the level should fall outside a predetermined range, either lower or higher than the selected level, an alarm will be activated.

Claims: Apparatus (1; 4) for the administration of a respiratory gas and at least one anaesthetic to a living being (12; 33) comprising a vaporizing unit (13; 34) which contains liquid anaesthetic, whereby a predetermined concentration of the anaesthetic in the respiratory gas is acquired by vaporizing a defined amount of the liquid anaesthetic in respiratory gas passed through the vaporizing unit (13; 34), an entrance part (10; 31) via which the respiratory gas, or the gases that form the respiratory gas, is supplied to the apparatus (1; 4), a regulating valve for controlling the flow of gas through the vaporizing unit (13; 34) and an inlet tube (107; 303) which conducts the respiratory gas and the vaporized anaesthetic to the living being (12; 33), characterized in that a regulating device (18; 36) controls the regulating valve so that the gas passing through the vaporizing unit (13; 34) attains a flow which, when saturated with anaesthetic, correlates to a preselected concentration of anaesthetic in the respiratory gas supplied to the living being (12; 33). Apparatus according to claim 1, characterized in that the vaporizing unit (13; 34) comprises a thermal control device (37) which controls the temperature in the vaporizing unit (13; 34) and maintains it at a preselected level. Apparatus according to claim 1, characterized in that the vaporizing unit (13; 34) comprises a thermistor (38) for measuring the temperature in the vaporizing unit (13; 34), in that the thermistor (38) is connected to the regulating device (18; 36), and in that the regulating device (18; 36) varies the flow of respiratory gas through the vaporizing unit (13; 34) depending on the measured temperature. Apparatus according to any of the preceding claims, characterized in that a first sensor (15) for measuring the concentration of the anaesthetic in the respiratory gas is located between the vaporizing unit (13) and the living being, in that the first sensor (15) is connected to the regulating device (18) and in that the regulating device (18) controls the flow of respiratory gas depending on the measured concentration of anaesthetic. Apparatus according to any of the preceding claims, characterized in that only a part of the total respiratory gas flow is led through the vaporizing unit (13, 34) and in that a connecting tube (106, 305) leads the respiratory gas containing vaporized anaesthetic back to the main flow in the inlet tube (107, 303). Apparatus according to any of the preceding claims, characterized in that the vaporizing unit (13, 34) comprises a wick to which liquid anaesthetic is supplied, whereby the liquid anaesthetic is vaporised by the respiratory gas passing through the wick. Apparatus according to any of the preceding claims, characterized in that a flow sensor (16) is located in the inlet tube (107) to measure the total flow of respiratory gas and vaporized anaesthetic and in that the regulating device (18) also controls the vaporization depending on the measured flow. Apparatus according to claim 7, characterized in that a control unit depending on the measured flow controls the entrance part (10) so that the difference between the flow supplied to the living being and a preselected value of the flow is minimized. Apparatus according to any of the preceding claims, characterized in that the control signal from the regulating device (18, 36) superimposes a basic signal corresponding to the preselected concentration of the anaesthetic, whereby the superimposed control signal increases the accuracy of the regulating valve. Apparatus according to any of the preceding claims, characterized in that the regulating device is a microprocessor. Apparatus according to claim 6 in combination with claim 2, characterized in that the filter (14) for the absorption and desorption of the anaesthetic is located in the inlet tube (104, 107). Apparatus according to any of the preceding claims, characterized in that a second sensor (17) for measuring the concentration of the anaesthetic in the respiratory gas is located in the inlet tube (107), in the vicinity of the first sensor (15), and in that a supervising unit (18) gives an alarm if the anaesthetic concentration falls outside a predetermined interval around the preselected value.

Inventors: BERGKVIST RUNE, OLSSON SVEN-GUNNAR, PSAROS GEORGIOS, BERGKVIST, RUNE, OLSSON, SVEN-GUNNAR, PSAROS, GEORGIOS
Applicants: SIEMENS AG, SIEMENS ELEMA AB, SIEMENS AKTIENGESELLSCHAFT, SIEMENS-ELEMA AB
Application year: 1992
Main classification: A61M16
IPC codes: A61M16