WO1999011312A1

WO1999011312A1 - Anaesthetic vaporiser

Info

Publication number: WO1999011312A1
Application number: PCT/GB1998/002593
Authority: WO
Inventors: Eduard Johan Baas; Martinus Pieter Dekker
Original assignee: Medical Industrial Equipment Limited; Medical Insight Limited
Priority date: 1997-08-30
Filing date: 1998-08-28
Publication date: 1999-03-11
Also published as: AU8874898A

Abstract

Flow sensors (4 and 5) monitor the flow rate of a carrier gas through a passage (3 and I) the rate at which a titration pump (18) transfers liquid anaesthetic into a vaporisation chamber (10) is adjusted accordingly. The anaesthetic enters the chamber (10) through a fine needle which is in thermal contact with a heating/cooling surface to ensure complete vaporisation of anaesthetic microdroplets on entry into the chamber (10). A conduit (11) extends from the vaporising chamber (10) to the passage (3) to conduct vaporised anaesthetic agent to the passage, ensuring the delivery of anaesthetic to the patient in an accurately controlled concentration which is independent of pressure in the passage (3). An optical sensor (15) detects the anaesthetic being supplied and the temperature of the vaporisation surface is automatically adjusted to ensure that the anaesthetic is supplied to the patient at the desired concentration.

Description

ANAESTHETIC VAPORISER

TECHNICAL FIELD OF THE INVENTION

This invention relates to anaesthetic vaporisers for use in medical anaesthesia.

BACKGROUND

Most anaesthetic vaporisers are of the bypass type, in which a carrier gas such as air, oxygen and/or nitrous oxide is divided between a first stream which is directed through a vaporising chamber to entrain vapour from a volatile liquid anaesthetic, and a second bypass stream. The two streams are subsequently recombined for delivery to a patient.

US 5 592 934 discloses an anaesthetic vaporiser in which vaporised anaesthetic agent is injected directly into the carrier gas flow at a rate which is determined by the pressure difference between the carrier gas and the anaesthetic vapour. In both forms of anaesthetic vaporiser referred to above, a level of liquid anaesthetic is maintained in the vaporising chamber, which is provided with a heater for vaporising the anaesthetic.

EP 0 761 249-A discloses an anaesthetic vaporiser in which microdroplets of liquid anaesthetic agent are injected into the inspiratory gas flow in a connection piece to which a patient is directly connected. The connection piece can contain a vaporisation element for vaporising the microdroplets. The dispensing of anaesthetic droplets is regulated by a control unit according to the value measured for momentary flow of respiratory gas in the inspiratory line.

The present invention seeks to provide a new and inventive form of anaesthetic vaporiser of the vapour injection type.

SUMMARY OF THE INVENTION

The present invention proposes an anaesthetic vaporiser comprising:

- an inlet for carrier gas;

- an outlet for carrier gas and anaesthetic agent for delivery to a patient;

- a passage which extends between the inlet and the outlet;

- a flow sensor arranged to generate an electrical signal corresponding to the rate at which gas flows through said passage;

- a vaporising chamber for anaesthetic agent, said chamber comprising an anaesthetic inlet through which liquid anaesthetic agent is introduced into said chamber in microdroplet form, and thermal control means disposed adjacent to said inlet such that said microdroplets are vaporised upon entry into said chamber;

- a conduit extending from the vaporising chamber to the passage to conduct vaporised anaesthetic agent to said passage; and

- delivery means for supplying liquid anaesthetic agent to said anaesthetic inlet of the vaporising chamber at a rate which is controlled in accordance with said electrical signal.

With such an arrangement the percentage of anaesthetic agent in the gas supplied to the patient is more accurately controlled than in any of the earlier forms of vaporiser described above. In addition, the concentration of anaesthetic agent at the outlet is independent of pressure.

comprises detection means (15) for detecting the kind of anaesthetic agent being supplied, said detection means being arranged to control the delivery means (18) such as to adjust the rate at which the liquid anaesthetic is supplied to the vaporisation chamber.

The anaesthetic agent is preferably supplied via a connector comprising a fixed part and a removable part and said detection means is arranged to detect anaesthetic identification means associated with the removable part of the connector. The detection means preferably comprises an optical sensor. The operating temperature of the thermal control means is preferably adjusted according to a signal supplied by the detection means.

BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings:

Figure 1 is a general diagrammatic drawing of an anaesthetic vaporiser in accordance with the invention;

Figure 2 is an exploded diagrammatic detail of a fresh gas flow sensor included in the vaporiser;

Figure 3 is an axial vertical section through the connector of anaesthetic bottle for use with the vaporiser;

Figure 4 is an example of an identification plate carried by the connector;

Figure 5 is a sectional view of the vaporisation chamber of the vaporiser, including an inset detail; and

Figure 6 is a functional block diagram of the microcontroller included in the vaporiser.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to Fig. 1 , the vaporiser has an inlet 1 for carrier gas which, in use, may be connected to a gas mixer to deliver the required proportions of fresh air, oxygen and nitrous oxide to the inlet 1. An outlet 2 is also provided for delivery of a controlled mixture of carrier gas and anaesthetic vapour to a patient, e.g. via a ventilator. A passage 3 extends between the inlet 1 and outlet 2, first and second flow sensors 4 and 5 being inserted in the passage 3 adjacent to the input 1. Apart from these flow sensors, no additional valves or flow restrictors are present in the fresh gas flow so that the flow resistance is relatively low and constant. No bypass flows are necessary.

Various kinds of flow sensor could be used, including heated wire and turboprop sensors. In the present example the first sensor 4 uses the Fleisch principle for flow measurement and the second sensor 5 uses a heated wire. As shown in Fig. 2, the sensor 4 comprises a laminar restrictor 6 mounted across the wider input end of a conus 7 whilst a laminar filter 8 is provided at the narrower output end, thereby providing a low and constant restriction over which the pressure drop is measured. The output signals from the sensors 4 and 5 are received by a microcontroller 26 (Fig. 1 ). Apart from being a failsafe precaution, the use of two flow sensors to produce simultaneous flow measurements in the same line has a further advantage. Both sensors will of course monitor the same flow, but a more accurate measurement of the actual flow is achieved, in turn leading to a more accurate percentage of anaesthetic agent in the output gases. The first sensor 4 utilises the specific mass property of the gas flow whilst the second sensor 5 uses the specific heat of the gas flow. Both sensors are calibrated using known flow rates and the resulting readings are stored to be used as a reference. When measuring a specific gas flow there will most likely be a difference between the values obtained from the two sensors, but with the aid of an appropriate algorithm the actual flow can be calculated independently of the nature of the carrier gas or the amount of oxygen in the pathway.

Still referring to Fig. 1 , a closed vaporisation chamber 10 (described in more detail below) is arranged to deliver anaesthetic vapour into the passage 3 via an unrestricted branch conduit 11. Liquid anaesthetic agent is introduced to the vaporiser in a bottle B which may, for example, contain one of the known anaesthetics such as halothane, isoflurane, servoflurane, desflurane or enflurane. The bottle has an agent-specific connector 16 which is sealably engaged with the vaporiser via a mating connector 14. As shown in Fig. 3, the bottle connector 16 includes an axial passageway 51 which leads from the bottom of the bottle via a tube (not shown) and which communicates with a radial passageway 52 in the connector 16. This radial passageway leads to an exit port 53 and incorporates a ball valve member 54 which is normally self-closing by means of a spring 55 which urges the valve member 54 against a seat 56 to prevent loss of anaesthetic from the bottle. The vaporiser connector 14 is provided with a tubular male part 57 so that when the bottle connector 16 is engaged with the connector 14 the male part 57 enters the exit port 53 and urges the valve member 54 away from its seat 56, thus allowing liquid anaesthetic to be removed from the bottle via connector 14. The connector 16 further incorporates a filling port 58 which communicates with the axial passageway 51 and is formed with an agent-specific keyhole configuration so that the bottle can only be refilled with the correct anaesthetic. After filling, the key port 58 is sealed by a closure 59 operated by a lever 60.

The bottle connector 16 also carries an identification plate 40, shown in Fig. 4, having four coded areas 41-44 which may be black or white forming a four-digit code which is specific to the anaesthetic in question. An optical sensor 15 is provided on the connector 14 (Fig. 1 ) to read the code carried by the plate 40 and thus auto-detect the kind of the anaesthetic agent being supplied. The identity of the anaesthetic is signalled to the microcontroller 26, and a liquid level sensor 17 informs the microcontroller when the bottle is almost empty. Liquid anaesthetic is transferred from the bottle B via connector 14 by means of a micro fluid titration pump 18, the pumping rate of which is accurately controlled by the microcontroller 26. The pump may, for example, be capable of delivering from 0.1 uL to 300 uL per second, so that liquid anaesthetic can thus be delivered to the vaporisation chamber 10 at an accurately controlled rate.

Referring to Fig. 5, the liquid enters the vaporisation chamber 10 through an inlet in the form of an injection needle 20 having a chamfered tip 21 (see inset detail). The needle 20 is of fine gauge to dliver anaesthetic agent in the form of microdroplets. The sharp edge of the tip 21 is in contact with a vaporisation surface 22 which is thermally coupled with pettier elements 23 and 24 operated by the microcontroller 26. (One such element would be sufficient but, again, two elements are used as a failsafe.) By reversing the current flowing through the peltier elements the microcontroller can produce heating or cooling of the surface 22, as required (see below). The tip of the needle is thus directly heated from the vaporisation surface 22. Although there could be a small gap between the needle and the surface 22 direct contact is preferred since a small volume of liquid (e.g. about 0.1 uL) would not fall into contact with the surface under gravity. Thus, when liquid anaesthetic enters the chamber 10 the microdroplets come directly into contact with the surface 22 and the anaesthetic agent is immediately vaporised so that the vaporisation chamber never contains any significant quantity of liquid anaesthetic.

The vaporiser includes battery backup 29 (Fig. 1 ) for uninterrupted operation in the event of a power failure. Referring to Fig. 6, the heart of the microcontroller is a microprocessor chip 30 provided with RAM and operating under control of instructions stored in an EPROM. In addition to the in/out pathways already described the microprocessor 30 receives further input from a control panel 31 (also shown in Fig. 1 ) incorporating a two digit display for percentage of anaesthetic agent being delivered in the output gas. Software operation is controlled by a three stage "watch dog" 32, and an alarm condition can be signalled by means of an audio-visual alarm 33. A control unit 34 interfaces with the pump 18 and a thermal control unit 35 interfaces with the peltier elements 23 and 24.

The vaporiser further incorporates four temperature sensors, represented by the temperature input unit 36 in Fig. 6, for monitoring ambient temperature, the temperature of the vaporisation chamber 10, the temperature of the carrier gas at input 1 , and the temperature of the gas flow at output 2. For safety, duplicate sensors may be included for monitoring each parameter.

In order to fully appreciate the advantages of the vaporiser it is necessary to understand the principle of operation. Firstly, if no vaporisation is taking place in the vaporisation chamber the pressures in the passage 3, conduit 11 and chamber 10 will be equalised, but this pressure will be constantly varying due to patient-related factors. If the user sets a desired percentage of anaesthetic agent in the output gas flow by means of the control panel 31 , the microcontroller calculates the required rate of anaesthetic delivery using a software algorithm which takes account of fresh gas flow, as signalled by the sensors 4 and 5, and parameters of the particular anaesthetic being supplied, which is known from the sensor 15, e.g. boiling point etc. The pump 18 supplies anaesthetic to the chamber 10 at the required rate, and as soon as vaporisation commences the pressure in the chamber 10 will rise above that in the passage 3 causing anaesthetic vapour to be injected into the fresh gas flow. This pressure rise is determined by natural constants of the anaesthetic, including the K factor for the difference in volumes before and after vaporisation, so that the amount of anaesthetic injected is independent of the output pressure. As a result, the percentage of anaesthetic in the output gas is considerably more stable and accurately controllable than in known forms of vaporiser, and the vaporiser is independent of external factors such as temperature, pressure, humidity etc.

The temperature of the vaporisation chamber is controlled by the microcontroller to ensure effective vaporisation of the particular anaesthetic agent being supplied. At low anaesthetic delivery rates the chamber is cooled to ensure that vaporisation still takes place at an even rate.

An additional advantage of using the vaporisation chamber described is that, since no liquid anaesthetic remains in the chamber its internal volume can be very low so that there is no bolus during startup and very little lag when the kind of anaesthetic is changed. The vaporiser is easily adapted for use with new anaesthetics simply by changing the bottle/connector system (C and 14) for one which is specific for the new anaesthetic (i.e. labelled and provided with an agent-specific key-filling system) and replacing the EPROM for one which takes account of the parameters of the new agent.

It will be appreciated that the features disclosed herein may be present in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.

Claims

1. An anaesthetic vaporiser comprising:

- an inlet (1 ) for carrier gas;

- an outlet (2) for carrier gas and anaesthetic agent for delivery to a patient;

- a passage (3) which extends between the inlet and the outlet;

- a flow sensor (4) arranged to generate an electrical signal corresponding to the rate at which gas flows through said passage;

- a vaporising chamber (10) for anaesthetic agent, said chamber comprising an anaesthetic inlet (20) through which liquid anaesthetic agent is introduced into said chamber in microdroplet form, and thermal control means (22, 23, 24) disposed adjacent to said inlet such that said microdroplets are vaporised upon entry into said chamber;

- a conduit (11 ) extending from the vaporising chamber (10) to the passage (3) to conduct vaporised anaesthetic agent to said passage; and

- delivery means (18) for supplying liquid anaesthetic agent to said anaesthetic inlet (20) of the vaporising chamber at a rate which is controlled in accordance with said electrical signal.

2. An anaesthetic vaporiser according to Claim 1 , in which said thermal control means comprises a thermal surface (22) which is disposed opposite to said anaesthetic inlet (20).

3. An anaesthetic vaporiser according to Claim2, in which said thermal surface (22) is in thermal contact with said inlet (20).

4. An anaesthetic vaporiser according to Claim 1 , which comprises detection means (15) for detecting the kind of anaesthetic agent being supplied, said detection means being arranged to control the delivery means (18) such as to adjust the rate at which the liquid anaesthetic is supplied to the vaporisation chamber.

5. An anaesthetic vaporiser according to Claim 4, in which the anaesthetic agent is supplied via a connector comprising a fixed part (14) and a removable part (16) and said detection means (15) is arranged to detect anaesthetic identification means (40) associated with the removable part of the connector.

6. An anaesthetic vaporiser according to Claim 4, in which said detection means (15) comprises an optical sensor.

7. An anaesthetic vaporiser according to Claim 4, in which the operating temperature of the thermal control means (22, 23, 24) is adjusted according to a signal supplied by the detection means (15).

8. An anaesthetic vaporiser according to Claim 1 , in which the thermal control means comprises at least one peltier element (23, 24).

9. An anaesthetic vaporiser according to Claim 1 , comprising a second flow sensor (5) arranged to generate an electrical signal corresponding to the rate at which gas flows through said passage.

10. An anaesthetic vaporiser according to Claim 9, in which said flow sensors (4, 5) operate on dissimilar principles.

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