WO2000030536A1

WO2000030536A1 - Breathing system

Info

Publication number: WO2000030536A1
Application number: PCT/AU1999/001027
Authority: WO
Inventors: Arthur Barrington Baker; Gyorgy Gusztav Jaros; Martin John Turner
Original assignee: The University Of Sydney
Priority date: 1998-11-20
Filing date: 1999-11-19
Publication date: 2000-06-02
Also published as: AUPP725198A0

Abstract

The invention concerns a breathing system which may be suitable for making measurements on the gases expired by a subject, or the gases to be delivered to a subject for inspiration. The system may also be suitable for delivering gases and volatile agents to a subject, for instance to a patient undergoing anaesthesia. The system includes an expiration chamber (2) to receive expired gas from a subject, a middle chamber (3) and an inspiration chamber (4) from which gas passes to the subject. The three chambers are arranged in series so that air received from the subject passes through each chamber in turn. A gas analyser (21) is arranged to measure samples from the middle chamber (3). There are valves between the chambers and between the middle chamber (3) and the analyser (21), and a valve controller controls the valves during each cycle of expiration and inspiration such that the middle chamber (3) is isolated from the other chambers for at least one period of time in each cycle to allow the analyser (21) to measure samples from the middle chamber (3) while the subject is able to continue to breathe without interruption. In another aspect the invention concerns breathing methods using the system.

Description

Breathing system

Technical Field

This invention concerns a breathing system which may be suitable for making measurements on the gases expired by a subject, or the gases to be delivered to a subject for inspiration. The system may also be suitable for delivering gases and volatile agents to a subject, for instance to a patient undergoing anaesthesia. In another aspect the invention concerns breathing methods for using the system.

Summary of the Invention

In a first aspect the invention is a breathing system including an expiration chamber to receive expired gas from a subject, a middle chamber and an inspiration chamber from which gas passes to the subject. The three chambers are arranged in series so that air received from the subject passes through each chamber in turn. A gas analyser is arranged to measure samples from the middle chamber. There are valves between the chambers and between the middle chamber and the analyser. A valve controller controls the valves during each cycle of expiration and inspiration such that the middle chamber is isolated from the other chambers for at least one period of time in each cycle to allow the analyser to measure samples from the middle chamber while the subject is able to continue to breathe without interruption. This allows the measurements of gases and volatile agents to take place in well-mixed, equilibrated and stationary mixtures with a known volume. This also permits the timely preparation of various gas mixtures well in advance of the inspiratory procedure.

One or more supplementary gases or volatile agents may be fed into the line between the middle and inspiratory chambers from a supplementation source, through an additional controlled valve. In alternative embodiments supplementary gases, volatile agents or liquids may be introduced into other parts of the system

To execute complex measurements of physiological parameters in the subject, the entire system may be totally or partially replicated and two or more systems may be placed in parallel. Each chamber may have compliance and volume that can be changed and measured actively and separately. Between the expiration chamber and the middle chamber, there is at least one variable and controllable gas removal device to remove all or a selective proportion of various gases or volatile agents.

The gas is allowed to flow from the patient or subject to the expiration chamber through an expiratory line controlled by valves. Flow between the inspiration chamber and the patient or subject is allowed through an inspiratory line also controlled by valves.

For mechanical ventilation at least one of the three (or more) chambers has to be powered to effect the movement of the gases in the required direction and with the required patterned time sequence. The powering of the chambers and the opening and closing of the valves to determine the path of gas flow is controlled by a controller. At least one temperature sensor and at least one pressure sensor may measure temperature and pressure respectively of the gas in selected chambers and tubes. A second aspect of the invention is a method of operation of a breathing system including an expiration chamber to receive expired gas from a subject, a middle chamber and an inspiration chamber from which gas passes to the subject. The three chambers are arranged in series so that air received from the subject passes through each chamber in turn. A gas analyser is arranged to measure samples from the middle chamber. The method includes the following steps: isolating the expiration chamber from the middle chamber during early and middle expiration during which time the analyser measures of gas from the middle chamber, and then transferring the contents of the middle chamber to the inspiration chamber; and isolating the contents of the inspiration chamber from the middle chamber during inspiration and transferring the contents of the expiration chamber to the middle chamber.

In this way the middle chamber operates completely independently of the chambers which are connected to the subject, thus allowing the possibe removal of gases and volatile agents, mixing, equilibration and measurement to take place independently while the patient or subject breathes in and out without interruption. In this case the measurements of gases and volatile agents take place in well mixed, equilibrated and stationary mixtures with a known volume. This also permits the timely preparation of various gas mixtures well in advance of the inspiratory procedure. During expiration the patient or subject can breathe into the expiration chamber without influencing mixing, equilibration and measurement in the middle chamber, filling of the inspiratory chamber or adding of gases from the supplementation source. The correct inspiratory mixture is prepared well in advance of inspiration which may occur prematurely in cases of spontaneous breathing or coughing.

During inspiration, transfers between the expiratory chamber and the middle chamber also take place independently from the movement of the gas to the patient. The middle chamber is thus always separated from the subject during normal operation. If. during spontaneous ventilation, the subject attempts to inspire before the inspiratory chamber is filled, or takes a breath larger than the volume available in the inspiratory chamber, then a valve is opened to allow gas of known composition from the middle chamber to be transferred. Using the invention, gas concentration measurements are performed on well-mixed and equilibrated stationary mixtures of which the temperatures and pressures are known. Supplementation may be achieved by mixing known volumes and concentrations of gases, liquids or vapours as opposed to regulating flows, which allows the preparation of an accurately determined well-mixed gas mixture before inspiration. Oxygen utilisation by the patient or subject may be determined from the oxygen supplementation required when the volume of the middle chamber is controlled at a constant level. Carbon dioxide production by the patient or subject may be calculated from the difference between the volumes of the expiration and the middle chambers. The metabolic activity and the respiratory quotient may then be calculated. By rapidly changing the inspired gas concentrations, non- invasive respiratory and cardiovascular measurements for the subject may be made. These considerations are advantageous for low flow closed loop anaesthesia as well for the non-invasive measurement of physiological parameters in anaesthesia, the intensive care unit or laboratory.

Brief Description of the Drawings

An anaesthetic system exemplifying the invention is described below with reference to the accompanying drawings, in which: Figure 1 is a diagram of the main components of the breathing system. Figure 2 is a schematic diagram of the configuration of the system during early expiration and measurement.

Figure 3 is a schematic diagram of the configuration of the system during middle expiration, supplementation and filling of inspiratory chamber.

Figure 4 is a schematic diagram of the configuration of the system during late expiration and anticipation of next inspiration.

Figure 5 is a schematic diagram of the configuration of the system during early inspiration and filling of the middle chamber. Figure 6 is a schematic diagram of the configuration of the system during late inspiration and equilibration.

Figure 7 is a diagram of the general mechanical components of the system.

Detailed Description of Examples of the invention

Referring to Figures 1 and 7. the breathing system consists of three chambers 2. 3 and 4. In use. the system is connected in series with a patient or S ibject 5 (indicated schematically) by means of a respiratory tube 6 or equivalent device (indicated schematically). The respiratory tube 6 is connected to expiratory chamber 2 via line 9 in which a valve 10 can be selectively opened and closed. The expiratory chamber 2 is also connected to middle chamber 3 via line 11 in which a solenoid one way valve 12 can be selectively opened and closed. The volume of expiratory chamber 2 is able to vary and is measured by means of sensor 13. which incorporates temperature measurement.

In line 11 upstream of the entrance to middle chamber 3 there is a device 7 to fully or partially remove one or more selected gases or volatile agents. For instance, soda lime may be used to absorb carbon dioxide, activated charcoal to absorb volatile agents and specific zeolites to absorb a number of gaseous agents. These may be arranged in series or in parallel and some or all may be bypassed by valve(s) 37, if required.

Middle chamber 3 is also connected to inspiratory chamber 4 via line 14 in which a solenoid one way valve 15 is selectively opened and closed. The volume of middle chamber 3 is able to vary and is measured by means of a sensor 16 which also incorporates a temperature measurement. Inspiratory chamber 4 is connected back to the patient tube 6 via line 17 in which a solenoid one way valve 18 can be selectively opened and closed. The volume of inspiratory chamber 4 is able to vary and is measured by means of a sensor 19. The pressure in the respiratory tube 6 may be measured using a sensor

20.

The analyser 21 is connected to the middle chamber 3 via line 26 in which a valve 27 may be selectively opened and closed. A return line 28 runs from the analyser 21 back to middle chamber 3, and valve 29 is selectively opened and closed to control flow in that line, allowing gas from the middle chamber 3 to be conserved.

An additional line 30 interconnects the middle chamber 3 and the patient tube 6 to allow inspiration from the middle chamber 3. A solenoid one way valve 31 in line 30 may control flow in that line to allow abnormal respiratory efforts such as coughing or very deep breathing to be accommodated.

Middle chamber 3 also has an overflow valve 32 to allow the escape of excess gas. and to allow the system to be flushed. This overflow valve can be connected to a scavenging system and / or a volume or flow measuring device.

Supplementary gases or volatile agents may be introduced from a supplementation source 8 using mass flow controllers 35, along line 34 into line 14. These gases may include oxygen, carbon dioxide, nitrous oxide anaesthetic agents or other gaseous and volatile agents, and they are introduced while the inspiratory chamber 4 is being filled. The very high flow in line 14 allows good mixing of the gases.

Finally, bladder pump 36 may be used to control inspiration from chamber 4.

The system 1 is designed so that, in use. expiration chamber 2 receives expiratory gas mixture from the patient or subject 5 following the elastic recoil of the respiratory muscles. From here it is rapidly transferred to the middle chamber 3 during early inspiration, when the expiratory chamber 2 is closed off from the patient.

Expired gases are thoroughly mixed and equilibrated with the gases that remain in the middle chamber 3 from the previous respiratory cycle. Measurement of gas concentrations by the analyser 21 is performed on this equilibrated mixture.

Inspiratory gas is moved rapidly into the inspiratory chamber 4 ahead of the time of inspiration. During inspiration it is either actively pumped or it is allowed to flow passively to the patient or subject during controlled or spontaneous respiration, respectively.

The movement of gases around the circuit formed by the three chambers and the patient or subject is achieved by the timed active changes in the volumes of the chambers and the operation of the five or more solenoid one way valves 10, 12, 15, 18 and 31.

Any one (or more) of the three chambers need to be driven actively, for example, by using linear motors or other means. However, the remaining chambers can be constructed to operate as passive chambers, for instance by incorporation of. say, a bellows or rolling diaphragm. The concentrations of gases and anaesthetic agents are measured during early expiration by a flow-through gas analyser 21 connected to the middle chamber 3. After measurement the mixture is returned to the same chamber. Since these values are used for control purposes they should be measured accurately. Concentrations of gas components in the respiratory gases may be monitored in the respiratory tube 6, and in the expiratory 2 and inspiratory 4 chambers and in any of the lines in the system, either by the same gas analyser 21. or by a separate, analyser.

Mixing in the middle chamber 3 is ensured by the use of mixing jets at the inflows from expiratory chamber 2 as well as from the return inlets from the gas analyser 21. Additional mixing fans may be necessary to achieve complete mixing.

Using solenoid valves the chambers are arranged to form two independent, simultaneously operating circuits namely a patient circuit and a measurement circuit. The configuration of the two circuits changes with the activity of the valves as the expiratory 2 and inspiratory 4 chambers are alternatively allocated to the patient or subject and measurement circuits.

Having an independent measuring circuit allows sufficient time for the mixing, equilibration and measurement of the constituents of the gas mixture before supplementation, and the timely preparation of an inspiratory mixture prior to the onset of inspiratory flow. To achieve the independent operation of the two circuits, the respiratory cycle is divided into five phases:

Early expiration and measurement of concentrations

Middle expiration, supplementation and filling of Inspiratory Chamber Late expiration and anticipation of next inspiration

Early inspiration and filling of Middle Chamber

Late inspiration and equilibration.

The Phases of Operation of the system: The first phase involves early expiration and measurement of concentrations, and is illustrated in Figure 2. During this phase, in the patient circuit, valve 10 is open and the patient 5 exhales passively along line 9 into the expiratory chamber 2. Valves 12 and 15 are closed to separate the three chambers from each other. In the measurement circuit, valves 27 and 29 are open and gas analyser 21 is connected to the middle chamber 3 by lines 26 and 28 to measure the concentration of gases and anaesthetic agents in the middle chamber 3. This phase may have a duration of about half a second.

The second phase involves middle expiration, supplementation and filling of the inspiratory chamber 4, and is illustrated in Figure 3. During this phase, in the patient circuit, the valve 10 is open and the patient or subject 5 continues to expire passively filling expiration chamber 2 along line. Valve 12 is closed, separating the expiratory chamber 2 from the middle 3 and inspiratory 4 chambers. In the measurement circuit, valves 15 and 35 are open, so the inspiratory chamber 4 is filled rapidly from the middle chamber 3 along line 14. and the supplementation inlet line 34. This phase may also have a duration of about half a second.

The third phase involves late expiration and anticipation of the next inspiration, and is illustrated in Figure 4. During this phase, in the patient circuit, valve 10 remains open and the patient or subject 5 continues to expire passively filling expiration chamber 2 along line. The volume of the expired mixture may be measured at the end of this stage. This measurement is used for controlling gas supplementation for closed circuit anaesthesia or for physiological measurement purposes, as well as for calculating the amount of carbon dioxide contained in the expired mixture. Valve 15 is closed to isolate the contents of the inspiratory chamber 4 from the middle chamber 3. and the gas mixture in inspiratory chamber 4 is ready for the next inspiration. At this stage, the gas analyser may be connected to the inspiratory chamber 3 to confirm the desired composition of the prepared mixture. In the case of spontaneous inspiration, the patient or subject is able to draw from this mixture at any time during this phase. The duration of this phase may be varied in controlled ventilation to produce different respiratory rates of up to forty breaths per minute if only three chambers are used. The rate may be increased when two or more systems are used in parallel. In case of spontaneous inspiration, the phase is initiated according to pressure variations in the respiratory tube 6 as measured by a pressure sensor 20. Flow triggering can also be used.

In the measurement circuit, since valves 12. and 15 are closed, there is no movement of gases in and out of the middle chamber 3. The fourth phase involves early inspiration and filling of the middle chamber 3, and is illustrated in Figure 5. During this phase, in the patient circuit valve 18 is open and inspiratory chamber 4 is powered to force the inspiratory mixture to the patient or subject 5 along line 17. Alternatively, in the case of spontaneous breathing, the mixture flows to the patient or subject passively due to the "negative" (subatmospheric) pressure generated by the patient^'s respiratory musculature. The actuating mechanism associated with the inspiratory chamber 4 can be activated to reduce the work of breathing. In the measurement circuit, valve 12 is open and the gas mixture is transferred from expiratory chamber 2 to middle chamber 3 along line 11. Since the patient circuit is isolated from the measurement circuit a pump can be used to increase the flow along line 11 so that it is as fast as the pressure change that the pump can produce. High flow has the advantages of producing a better mixing through the jets and leaving sufficient time for equilibration. The fifth phase involves late inspiration and equilibration, and is illustrated in Figure 6. During this phase, in the patient circuit flow continues from inspiratory chamber 4 to the patient or subject 5 along line 17. In the measurement circuit valves 12. 15. 27 and 29 are closed so there is no flow in the middle chamber 3 during this phase, allowing mixing and equilibration to take place. The maximum respiratory rate is limited by the times required for essential operations, which are estimated as follows:

Phase 1: (measurement): ~ 0.5 second

Phase 2: (filling of inspiratory chamber): — 0.5 second Phase 4: (filling of the middle chamber): — 0.5 second

Total: —1.5 seconds

This allows a maximum respiratory rate of 60/1.5 = 40 breaths per minute, with an expiration time of approximately 1.0 seconds and an inspiration time of approximately 0.5 second. In this case the duration of phases 3 and 5 are approximately zero.

In addition, the system can also cope with spontaneous respiration as well as with abnormal breathing patterns such as that encountered in coughing. In the case when an even faster rate is demanded by the patient, such as during coughing and abnormal breathing, the inspiratory mixture can be channelled directly from the middle chamber 3 to the patient or subject 5 through valve 31. It is also possible, by opening valves 10, 12. 15 and 18 to configure the invention as a conventional anaesthetic circle breathing system, with valves 18 and 10 operating passively as non-return valves. Supplementation of the respiratory gas with oxygen or anaesthetic agents or other gases through valve 35 and monitoring of the respiratory gas composition by the gas analyser 21 is possible in this operating mode.

The breathing system can be used in either volume control or pressure control modes, and with additional well known features such as positive end expiratory pressure (PEEP) or synchronised intermittent mandatory ventilation (SIMV). The measurement of pressure or volume for such modes may be made in the airway or other parts of the breathing system. A hold or pause in the inspiratory phase may be provided to allow profusion of the inspired gases through the arterioles.

Although the invention is described with reference to a particular example, it should be appreciated that it is not limited to that example. For instance, the entire circuit or parts thereof may be replicated and connected in parallel with the first circuit. It should also be appreciated that the valves can be opened and closed in any convenient known manner, such as for example, control signals provided by a computerised controller which monitors operation of the system. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

1. A breathing system, includes: an expiration chamber to receive expired gas from a subject, a middle chamber and an inspiration chamber from which gas passes to the subject, where the three chambers are arranged in series so that air received from the subject passes through each chamber in turn, a gas analyser arranged to measure samples from the middle chamber, valves between the chambers and between the middle chamber and the analyser, and a valve controller to control the valves during each cycle of expiration and inspiration such that the middle chamber is isolated from the other chambers for at least one period of time in each cycle to allow the analyser to measure samples from the middle chamber while the subject is able to continue to breathe without interruption.

2. A breathing system according to claim 1, further including a controlled valve between the middle and inspiratory chambers, to introduce supplementary gases, volatile agents or liquids.

3. A breathing system according to any preceding claim, further including at least one gas removal device between the expiration chamber and the middle chamber, to remove all or a selected proportion of a selected gas or volatile agent.

4. A breathing system according to any preceding claim, further including an expiratory line controlled by valves between the subject and the expiration chamber.

5. A breathing system according to any preceding claim, further including an inspiratory line controlled by valves between the inspiration chamber and the subject.

6. A breathing system according to any preceding claim, where at least one of the chambers is powered to effect the movement of the gases.

7. A breathing system according to claim 6. further including a controller to control the powering of the chambers and the opening and closing of the valves to determine the path of gas flow.

8 A breathing system according to any preceding claim, further including at least one temperature sensor to measure temperature.

9. A breathing system according to any preceding claim, further including at least one pressure sensor to measure pressure.

10. A breathing system according to any preceding claim, further including a valve between the middle chamber and the subject, which opens during spontaneous ventilation if the subject attempts to inspire before the inspiratory chamber is filled, or takes a breath larger than the volume available in the inspiratory chamber, to allow gas of known composition to be transferred from the middle chamber to the subject.

11. A breathing system according to any preceding claim, in combination with another such system arranged in parallel with it.

12. A method of operating a breathing system which includes an expiration chamber to receive expired gas from a subject, a middle chamber and an inspiration chamber from which gas passes to the subject, the three chambers being arranged in series so that exhaled air received from the subject passes through each chamber in turn, and a gas analyser is arranged to measure samples from the middle chamber: the method includes the following steps: isolating the expiration chamber from the middle chamber during early and middle expiration during which time the analyser measures gas from the middle chamber, and then transferring the contents of the middle chamber to the inspiration chamber; and isolating the contents of the inspiration chamber from the middle chamber during inspiration and transferring the contents of the expiration chamber to the middle chamber.

13. A method according to claim 12. including the further step of introducing supplementary gases, volatile agents or liquids to the breathing system.

14. A method according to claim 12. including the further steps of changing and measuring the compliance and volume of each chamber actively and separately.

15. A method according to claim 12. including the further step of determining oxygen utilisation by the subject from the oxygen supplementation required when the volume of the middle chamber is controlled .

16. A method according to claim 12. including the further step of calculating carbon dioxide production by the subject from the difference between the volumes of the expiration and the middle chambers.

17. A method according to claim 12, including the further step of removing a selected proportion of a selected gas or volatile agent from the expired gases.

18. A method according to claim 12, including the further step of calculating the metabolic activity and the respiratory quotient of the subject.

19. A method according to any one of claims 12 to 18, including the further step of powering one or more of the chambers to effect movement of the gases.

20. A method according to any one of claims 12 to 19, including the further step of measuring the temperature of the gas in a chamber.

21. A method according to any one of claims 12 to 20, including the further step of measuring the pressure of the gas in a chamber.

22. A method according to any one of claims 12 to 21, including the further step of making non-invasive respiratory and cardiovascular measurements of the subject, by rapidly changing the inspired gas concentrations.