An anaesthetic apparatus and a method for use with an anaesthetic apparatus

Abstract: A anaesthetic machine comprising a control unit (18) and a breathing circuit (2) with a connector (14) for fresh gas, a bellows system (28) and an outlet valve (32). Said anaesthetic machine is devised to operate at least as an open system, the connector (14) then constituting an inspiratory part (6) of the breathing circuit (2) and the bellows system (28) and outlet valve (32) constituting an expiratory part (20) of the breathing circuit (2), are described. In order to reduce the adverse impact of compressible volume in the expiratory section (20) on the anaesthetic machine's function, the anaesthetic machine is devised in a special manner. The control unit (18) is accordingly devised to regulate the bellows system (28) during inspiration in such a way that a counter pressure, largely corresponding to the pressure of fresh gas at the connector (14), is maintained in the expiratory section (20) of the breathing circuit (2).

Description: An anaesthetic apparatus and a method for use with an anaesthetic apparatus The present invention relates to an anaesthetic apparatus according to the preamble to claim 1. The present invention also relates to a method according to the preamble to claim 5. An appartus according to the preamble of claim 1 and a method according to the preamble of claim 5 are known from EP-A-0 697 224. Most anaesthetic machines are devised for operation in different modes, especially with open and closed systems. In an open system, all the gas prepared in the anaesthetic machine and delivered to the patient during inspiration (inhalation) is removed from the anaesthetic machine and carried to an evacuation unit during expiration (exhalation). In a closed system, on the other hand, a larger or smaller part of the gas expired by the patient is returned in the anaesthetic machine to the patient at the next inspiration. Gas is usually returned to the patient via an absorber to remove carbon dioxide from the gas. One problem with anaesthetic machines of this kind is that the compressible volumes in the breathing circuit are much larger than in machines that cannot be switched between open and closed systems. The large compressible volume in the expiratory section of the breathing circuit is a particular problem when the anaesthetic machine operates as an open system. During inspiration, a more than negligible part of the programmed tidal volume may be consumed in compressible volume, since inspiratory pressure is greater than expiratory pressure. One objective of the present invention is to achieve an anaesthetic machine with which the aforementioned problems are eliminated. Another objective of the present invention is to achieve a method that eliminates the aforementioned problems. An anaesthetic machine which reaches this objective is achieved in accordance with the invention in that the anaesthetic machine according to the above is devised as disclosed in the characterising part to claim 1. Advantageous embodiments of the anaesthetic machine are evident from the dependent claims to claim 1. A method is achieved in accordance with the invention by means of the distinguishing features evident from the characterising part of claim 5. The bellows system in the expiratory section of the breathing circuit is normally disconnected in functional respects when the anaesthetic machine operates as an open system. By instead making active use of the bellows system's presence and control the bellows system (and possibly even the outlet valve) by a control unit to supply gas to the expiratory section of the breathing circuit during inspiration, a counterpressure can be build up in this section. None of the fresh gas supplied to the breathing circuit's inspiratory section will then be consumed by compressible volume. One embodiment of the anaesthetic machine and method according to the invention will be described below in greater detail, referring to the figure. The figure shows the most salient features of one embodiment of the anaesthetic machine according to the invention. A breathing circuit 2 is connected to a patient 4 to carry gas to and from the patient 4. Gas lines are depicted with broad lines and electrical lines with lines of normal thickness. The breathing circuit 2 consists of an inspiratory section 6 in which a carbon dioxide absorber 8 is arranged. The absorber 8 is shown mainly to indicate that the breathing circuit 2 is also suitable for closed systems. A first check valve 10 is arranged to keep gas from flowing from the patient 4 into the inspiratory section 6. A fresh gas line 12 is connected to a connector 14 in the inspiratory section 6 to supply fresh breathing gas to the patient 4. A first pressure meter 16 measures pressure in the inspiratory section 6. The value measured for pressure is sent to a control unit 18 which controls the various components and units in the anaesthetic machine (also see below). The breathing circuit 2 also has an expiratory section 20 in which a second check valve 22 is arranged to prevent expired gas from flowing back to the patient 4. A second pressure meter 24 and a flow meter 26 are also arranged in the expiratory section 20. A bellows system 28 which, in a closed system, forces breathing gas through the breathing circuit 2, is connected to the expiratory section 20. The bellows system 28 is operated by a drive unit 30. An outlet valve 32 is arranged at the end of the expiratory section 20 and regulates the discharge of gas into an evacuation unit (not shown). Finally, the figure also shows a switching valve 34 for switching between an open system and a closed system. Compressible volume in the expiratory section 20 is relatively large, i.e. several litres. In a traditional system, this means that 20 to 50 ml of breathing gas are required to build up pressure in the expiratory section 20 during inspiration. Since open systems are primarily used in conjunction with small tidal volumes, i.e. from 100 to 300 ml per breath, it will be realised that a not inconsiderable part of the tidal volume is lost. Compensation for this loss can indeed be made by the addition of extra fresh gas, but this would increase total gas losses considerably and simultaneously impose a delay before the patient 4 receives any gas. An anaesthetic machine according to the invention eliminates the problems in another way. When an inspiration commences through a flow of fresh gas from the fresh gas line 12, the bellows system 28 is simultaneously activated by the control unit. The control unit 18 controls the bellows system 28 according to the gas pressure measured, so the same pressure builds up in the inspiratory section 6 and the expiratory section 20. Minimal compressible volume then persists, and it only causes losses measured in millilitres. The patient 4 therefore receives the entire tidal volume without any needless gas losses in the system. Regulation based on the signal from the flow meter 26 can be used as an alternative to regulation based on pressure values determined by the pressure meters 16, 24. The bellows system 28 is then regulated so flow through the flow meter 26 is zero or close to zero. A zero flow means that pressure in the expiratory section 20 is at least equal to pressure in the inspiratory section 6. However, pressure in the expiratory section 20 should not greatly exceed pressure in the inspiratory section 6. If a small flow is allowed, gas losses would indeed increase somewhat, but pressure in the expiratory section 20 would not exceed pressure in the inspiratory section 6. When the actual volumes in the breathing system 2 and the flow and pressure of supplied fresh gas are known, the control unit 18 can even have the bellows system 18 generate an appropriate counter flow/counter pressure without feedback from the values measured in the system. The outlet valve 32 can also be used during inspiration for achieving rapid compliance with the desired pressure/flow. It should be noted that the bellows system can consist of any suitable pressure-generating unit, e.g. a pump, piston, fan or compressor. However, it would be advantageous if the bellows system 28 could rapidly supply or take up gas, enabling it to respond rapidly to changes in the system.

Claims: An anaesthetic machine comprising a control unit (18) and a breathing circuit (2) with a connector (14) for fresh gas, a bellows system (28) and an outlet valve (32), which anaesthetic machine is devised to operate at least as an open system, the connector (14) then being part of an inspiratory section (6) of the breathing circuit (2) and the bellows system (28) and outlet valve (32) being part of an expiratory section (20) of the breathing circuit (2), characterised in that the control unit (18) is devised to regulate the bellows system (28) during inspiration in such a way that a counter pressure, largely corresponding to the pressure of fresh gas at the connector (14), is maintained in the expiratory section (20) of the breathing circuit (2). The anaesthetic machine according to claim 1, characterised in that a first pressure meter (16) is arranged at the connector (14), and a second pressure meter (24) is arranged in the expiratory section (20) of the breathing circuit (2), the control unit (18) regulating the bellows system (28) according to the pressure measured by the pressure meters (16, 24). The anaesthetic apparatus according to claim 1 or 2, characterised in that a flow meter (26) is arranged in the expiratory section (20) of the breathing circuit (2), the control unit (18) regulating the bellows system (28) according to the flow measured by the flow meter (26). The anaesthetic machine according to any of the above claims, characterised in that the control unit (18) is also devised to regulate the outlet valve (32) during inspiration in order to maintain the counter pressure. A method for reducing the impact of compressible volume on the expiratory section of the anaesthetic machine during inspiration with an open system, characterised by the following method steps: measuring an inspiratory pressure in an inspiratory section of the anaesthetic machine, and generating a counter pressure in the expiratory section corresponding to the inspiratory pressure.

Inventors: EMTELL PAER, KOCK MIKAEL, EMTELL, PAER, KOCK, MIKAEL, Emtell, Pär
Applicants: SIEMENS ELEMA AB, SIEMENS-ELEMA AB
Application year: 1999
Main classification: A61M16
IPC codes: A61M16