GB2472214A

GB2472214A - Thermistor apparatus for measuring anaesthetic gas flow

Info

Publication number: GB2472214A
Application number: GB0913113A
Authority: GB
Inventors: Christopher Brown; Ian Glascott; Stephen Neil James Plumridge
Original assignee: MPB IND Ltd
Current assignee: MPB IND Ltd
Priority date: 2009-07-28
Filing date: 2009-07-28
Publication date: 2011-02-02
Anticipated expiration: 2029-07-28
Also published as: GB2472214B; GB0913113D0

Abstract

The invention provides an apparatus for measuring and recording the volume of anaesthetic gas administered to a patient, the apparatus comprising a gas flow sensor (1, see fig 1) for measuring the flow rate of an anaesthetic gas, the gas flow sensor being linked to a data storage and processing device (3, fig 1) for flow data, and means (4, 5, 6, fig 1) for retrieving data from the storage device. The sensor circuit 11 is a thermistor arrangement located in the gas flow across inner sleeve 10, one thermistor (R2, see fig 3) being upstream and exposed, and one R4 being downstream and sheltered from gas flow. A thermal or Wheatstone bridge determines the change in resistance and voltage difference, and thus the gas flow (fig 3). The sensor takes into account the starting gas temperature, and may be calibrated prior to use. The sensor may be fixed to an existing flow tube and fitted to an anaesthetic machine.

Description

APPARATUS FOR MEASURING GAS FLOW

This invention relates to an apparatus for measuring gas flow and more particularly for an apparatus that measures and stores data relating to gas flows, particularly in the context of anaesthetic gases.

Also provided by the invention is a novel sensor for measuring gas flow together with methods for measuring and storing data about gas flow.

Background of the Invention

The amounts of anaesthetic gases being delivered to a patient in an operating theatre are typically measured using flow meters connected in line with the supply of anaesthetic gas. Flow meters in common use comprise a calibrated vertical glass tube with a scale marked along its length. A rotating float moves up and down inside the tube in response to changing gas flows, the height of the rotating float inside the tube indicating the volume of gas passing through the tube. The rotating float, which is typically coloured to aid visibility, provides a visual indicator for the anaesthetist who can see at a glance how much gas is passing through the tube.

In a typical arrangement, a plurality of flow meters is mounted on a trolley which is provided with control valves for controlling the gas flows. The trolley can be wheeled around an operating theatre to the point of need.

Whilst the gas flow meters in common use enable the anaesthetist to monitor current gas flows, they do not provide a permanent record of the amounts of anaesthetic gas used during an operation.

In situations where a surgical procedure goes wrong, it is desirable to be able to indentify the cause of the problem, both from an educational viewpoint and from the viewpoint of the liability of the hospital and surgeon concerned. One factor that should desirably be checked is the level of anaesthesia during the operation and the levels of the different gases used.

Summary of the Invention

The present invention provides a system and apparatus for measuring and recording gas flows through an anaesthesia machine and provides an historical record of the amounts of the various anaesthetic gases supplied to a patient during a surgical procedure. The surgeon and anaesthetist are therefore provided with a means for establishing whether faulty anaesthetic procedures contributed to any problems encountered during an operation. The present invention thus provides an equivalent of a "black box flight recorder" for anaesthetic procedures.

The invention also provides a means of recording information which can later be used for training purposes and for monitoring gas usage for cost control purposes.

Accordingly, in a first aspect, the invention provides an apparatus for measuring and recording the volume of anaesthetic gas administered to a patient, the apparatus comprising a gas flow sensor for measuring the flow rate of an anaesthetic gas, the gas flow sensor being linked to a data storage and processing device for storing and processing flow data, and means for retrieving flow data from the storage device.

The apparatus typically comprises a plurality of sensors for recording the flow rates of a plurality of gases. For example, the apparatus may comprise between two and ten sensors, more typically three to six sensors.

The sensors may be connected to an inlet of a flow meter tube, for example a calibrated glass flow meter tube.

As such, the sensor may comprise a housing having means for connection to the flow meter tube (e.g. glass flow meter tube), the housing having a channel therethrough communicating with an inlet of the flow meter tube (e.g. glass flow meter tube), wherein a sensing element is disposed within the channel to measure the flow rate of the gas passing into the inlet.

The sensor housing may conveniently have a spigot extending therefrom, the spigot being configured to fit into the inlet end of the glass flow meter tube. An annular seal (e.g. 0-ring) may be provided for creating a gas-tight seal between the housing and the glass flow meter tube.

The housing may contain a memory device for storing information relevant to the functioning of the sensor. For example, the memory device may contain data relating to calibration of the sensor, or to the gas type with which the gas flow meter is to be used.

In one embodiment, the sensor measures the flow rate of the gas by making use of a cooling effect of gas passing over a sensing element of the sensor.

Thus, means may be provided for heating a thermally sensitive component of the sensing element, and the cooling effect of the gas flow on the heated thermally sensitive component of the sensing element determined and converted into a flow rate.

In the context of the present invention, the thermally sensitive component is a component of the sensing element which has a thermally dependent property which can be affected by the flow of gas over or past it.

In a particular embodiment, the thermally dependent property can be electrical resistance. For example, the thermally sensitive component may comprise a thermistor. Applying a constant current through the thermistor will lead to heating of the thermistor. As the thermistor is heated, its resistance will change producing a voltage potential across the thermistor. If the thermistor is exposed to the gas flow, this will have a cooling effect leading to a change in the resistance of the thermistor and hence a change in the potential across the thermistor. By comparison with a calibration curve stored in the sensor or the linked data storage and processing device, the change in potential can be converted into a flow rate.

The thermally sensitive component (e.g. thermistor) is preferably located on an upstream surface of the sensing element.

The temperature of the gas will typically influence the magnitude of its cooling effect and, therefore, it is preferred that the apparatus of the invention is provided with means for compensating for the temperature of the gas when determining the flow rate. For example, the sensing element may be provided with a second thermally sensitive component (which may be the same or different type to the first thermally sensitive component) which is substantially shielded from the flow of gas through the sensor, for example by virtue of being located on a downstream surface of the sensing element. The second thermally sensitive component is preferably a thermistor.

In one particularly preferred embodiment, both the first and second thermally sensitive components are thermistors, and the thermistors form part of a thermal bridge (Wheatstone bridge) configuration, e.g. with two further resistance elements. In this arrangement, the effects of the gas temperature on the two first and second thermally sensitive components will cancel each other out. However, as the cooling effect due to the gas flow over the first thermally sensitive component will be greater than the cooing effect over the second thermally sensitive component, the voltage across the thermal bridge will change proportionally to the volume of gas flowing through the sensor.

In another aspect, the invention provides a sensor for use in determining the flow rate of a gas, the sensor comprising a housing having a channel therethrough through which gas may pass; and a sensing element disposed within the channel; wherein the sensing element comprises first and second thermistors; the first thermistor being disposed on an upstream surface of the sensing element so that it is contacted by gas passing through the housing; and the second thermistor being disposed on a downstream surface of the sensing element; and means for calculating the flow rate from a difference in cooling effects exerted by the gas on the first and second thermistors.

The first and second thermistors may advantageously be connected to two further resistance elements in a thermal bridge (e.g. Wheatstone bridge) arrangement.

The sensor preferably contains a memory device as hereinbefore defined for storing information relevant to the functioning of the sensor.

The sensor housing may comprise inner and outer sleeves, wherein an inner wall of the inner sleeve surrounds the channel.

Preferably there is a space (e.g. an annular space) between the inner and outer sleeves within which the memory device is mounted.

An interface circuit for linking the sensor to the data logging unit may also be disposed within the space.

The sensing element may have an anchoring portion which is held within the space between the inner and outer sleeves, and a sensing portion which is disposed within and extends across the channel. For example, the sensing element may be substantially T-shaped, the cross limb of the T-shape serving as the anchoring portion. The sensor is preferably configured to be connectable to an inlet of a flow meter tube, e.g. a calibrated glass flow meter tube. For example, the sensor may have a spigot for insertion into an inlet of a calibrated glass flow meter tube. The spigot is preferably provided with sealing means for creating a gas-tight seal between spigot and tube.

In a further aspect, the invention provides a flow meter tube (e.g. a calibrated glass flow meter tube) having attached thereto (e.g. at an inlet end thereof) a sensor as hereinbefore defined.

In a further aspect, the invention provides an anaesthesia machine comprising a glass flow meter tube having attached thereto (e.g. at an inlet end thereof) a sensor as hereinbefore defined, means for controlling the flow of gas through the machine, and means for connection to a source of anaesthetic gas.

Preferably, the anaesthesia machine has a plurality (e.g. two to ten and more preferably three to six) glass flow meter tubes connected thereto, each flow meter tube having attached thereto a sensor as hereinbefore defined.

In another aspect, the invention provides a method of modifying an anaesthesia machine, the method comprising replacing a flow meter tube (e.g. calibrated glass flow meter tube) of length L with a replacement flow meter tube having connected to the inlet end thereof a sensor as hereinbefore defined, wherein the replacement flow meter tube and sensor have a combined length of L. In a further aspect, the invention provides a method of measuring and recording the volume of anaesthetic gas administered to a patient, the apparatus comprising passing an anaesthetic gas through an apparatus according to the invention and recording and storing data concerning the flow of the gas through the apparatus over a defined period, and thereafter optionally retrieving flow data from the storage device.

In another aspect, the invention provides a method of analysing the quantities of anaesthetic gases administered to a patient over the period of a surgical procedure, which method comprises measuring the flow rates of each anaesthetic during the period of the surgical procedure using an apparatus as hereinbefore defined, storing data concerning the flow rates in digital form and downloading the data for analysis.

Further embodiments and aspects of the invention will be apparent from the description below of a particular embodiment of the invention.

Brief Descri�tion of the Drawings Figure 1 is a schematic view of gas flow measuring and recording apparatus according to one embodiment of the invention; Figure 2A is a side sectional view of the gas flow sensor forming part of the apparatus of Figure 1.

Figure 2B is a schematic plan view of the gas flow sensor of Figure 2A.

Figure 3 is a schematic view of the thermal bridge arrangement used in the sensor of Figures 2A and 2B.

Figure 4 is a schematic view of the downstream side of the gas flow sensor of Figures 2A and 2B.

Figure 5 is a schematic view of the upstream side of the gas flow sensor of Figures 2Aand2B.

Figure 6 is a schematic view showing the internal arrangement of a data logger forming part of the apparatus of Figure 1.

Figure 7 is a plot of sensor voltage versus flow rate for a sensor constructed as shown in Figures 2A, 2B, 5 and 6.

Figure 8 is a plot of flow rate versus sensor voltage for a sensor constructed as shown in Figures 2A, 2B, 5 and 6.

Detailed Description of the Invention

The invention will now be illustrated in greater detail, but not limited, by reference to the specific embodiments illustrated in the accompanying drawings Figures 1 to 8.

An apparatus according to one embodiment of the invention comprises a plurality (in this case five) electronic solid state gas flow sensors 1 which are connected to the inlets of flow meters 2. Each gas flow sensor is connected by means of a multi-core cable 7 to a central data logging unit 3. The data stored in the data logging unit can be accessed and downloaded by one of a variety of means such as a memory card 4 or a computer 5 linked by, for example, a USB cable. Alternatively, the data logging unit 3 can be connected by ethernet connection 6 to a computer network or by wireless link 8 to a remote computer (not shown).

The flow meters 2 are conventional in operation and comprise a calibrated glass tube marked with a flow rate scale on the outer surface and containing a rotating float which moves up and down in the tube in response to variations in the gas flow to provide a visual indication of the flow rate. A gas flow sensor 1 is plugged into the lower end of the flow meter tube. In order to enable the flow meter and gas flow sensor combination to be retro-fitted into existing anaesthetic gas flow machines, the glass tube is made slightly shorter than the normal size for the anaesthetic gas flow machine in question so that the size of the sensor can be accommodated.

The gas flow sensor is illustrated in more detail in Figures 2A, 2B, 3, 4 and 5. As shown in Figures 2A and 2B, the sensor 1 comprises a hollow housing 15 having a spigot 14 extending from its upper end. The spigot 14 is of circular cylindrical form and is configured so that it can be inserted into the lower end of a gas flow tube.

To enable a gas-tight seal to be formed between spigot and tube, the spigot is provided with an annular elastomeric seal (not shown) such as an 0-ring mounted in an annular groove 16 extending around the circumference of the spigot.

The hollow housing 15 comprises an outer sleeve 9 and an inner sleeve 10. A generally T-shaped sensor 11 is mounted in the housing. One limb (1 la) of the sensor extends through a slot in the inner sleeve and across the central gas-flow passageway of the housing. The annular space between the inner and outer sleeves is filled with a potting resin which serves to hold the "cross-limb" (11 b) of the T-shaped sensor in place. The potting resin also prevents gas passing through the slot in the wall of the inner sleeve and coming into contact with the cross limb 11 b of the sensor.

Also mounted within the housing, and connected to the sensor is an interface circuit board 12. A cable 13 connects the interface circuit board 12 to the data logging unit 3. Each sensor is provided with a LED to indicate the status of the sensor. The LED may be set up to indicate status by means of colour coding as follows: Power on -green No gas flow -red flashing Fault -amber flashing.

The power for the sensor is provided through the cable 13 via the interface circuit 12. Contained within each sensor within the interface circuit is a non-volatile digital memory device in which can be stored information relevant to the functioning of the sensor such as the gas type (e.g. 02, C02, N20 etc), calibration curve information, serial number, calibration due date, calibration history and flow parameters (range, etc.). In the event that a flow meter tube for the wrong gas is fitted, the gas type information stored in the memory device enables an alarm to be raised.

The sensor 11 works on a Wheatstone bridge principle, as shown schematically in Figure 3. The limb ha of the sensor comprises a printed circuit board on which are mounted two 0402 size thermistors R2and R4. Thermistor R2 is mounted on the upstream side of limb 11 a, as shown in Figure 5, so that in use it is exposed to the gas flow through the housing. Thermistor R4 is mounted on the downstream side of limb 1 la so that they are substantially shielded from the gas flow. The upper resistor arms of the wheatstone bridge arrangement, Ri and R3, are mounted elsewhere.

Limb 11 a is formed from a material which is sufficiently robust to withstand the gas flows through the flow meter and is sufficiently thick to insulate the gas flow sensing thermistor R2 from the other thermistor R4. FR-4 material of 1.6 mm thickness has been found to be suitable for this purpose. The copper trackwork on the circuit board is kept as thin as possible (1 oz copper clad and trackwork of 8 thousandths of an inch have been found to be suitable) in order to reduce any heat sink effects. In order to provide additional durability, the thermistors are bonded to the circuit board using a suitable adhesive prior to soldering to increase the mechanical strength of the sensor. The downstream side of the circuit board and also the upstream side are coated with a conformal coating to protect the components of the circuit board from any corrosive effects of gases passing through the sensor housing.

As is well known, thermistors are devices whose electrical resistance is proportional to their temperature. If a constant current is applied through a thermistor, then a voltage drop across the thermistor will occur. This will cause heating of the device and the resistance to change, hence a change in voltage drop, following Ohms law, until a state of equilibrium is reached. If a gas flow is introduced across the thermistor, this has a cooling effect, thus changing the resistance and changing the voltage drop. The change in voltage drop can be used as a measure of the flow rate of the gas.

However, the temperature of the gas passing through over the sensor will have an effect and this is compensated for by means of the Wheatstone bridge arrangement. The thermistor R2 used for sensing the gas flow faces upstream, while the other element of the bridge, an identical thermistor for temperature compensation, is shielded from the gas flow. Thus, the temperature sensing thermistor R4 will not be cooled' in the same way as the thermistor directly exposed to the gas flow, thus providing gas temperature compensation.

With the sensing circuit arranged as a bridge with Ri =R3 and R2=R4, as shown in Figure 3, the temperature effects on both R2 and R4 will cancel each other out.

However, as the cooling effect due to the gas flow on R2 will be greater than the effect on R4, the voltage across Vf will change proportionally to the volume of gas flowing.

The response curve of Vf is typically nonlinear with respect to volumetric flow. In order to compensate for the non-linear flow, the sensor must be calibrated prior to despatching to a user (or prior to use) by passing gas through the sensor at known flow rates and constructing a calibration curve. The housing is provided with an embedded microprocessor which stores, amongst other things, the calibration curve for the sensor. Examples of calibration curves generated using the apparatus of the invention are illustrated in Figures 7 and 8 which show sensor voltage-flow rate and flow rate-sensor voltage plots for the sensor.

Flow rate measurements from the sensor are communicated via the interlace circuit and cable 13 to the data logging unit 3. The apparatus illustrated in the drawings allows data from up to five gas flow meters to be logged (although fewer or more channels could be provided). The data logging unit has independent communication ports connected to each flow sensor and provides power to each flow sensor. Figure 6 shows a simplified block diagram of the architecture of the data logging unit.

The data logging unit is mains powered via a medically approved mains power adaptor and incorporates a rechargeable backup battery so that operation of the data logging unit is maintained in the event of a power failure. Any power failure incident will be recorded in the logging unit's internal event log.

The size of the internal non-volatile memory capacity of the data logging unit will vary according to needs but typically will be sufficient to record, for example several thousand hours of data. The data logging unit also includes a real time clock device (RTC) to time and date stamp all measurements taken and events recorded.

The data logging unit may have a display and a keypad so that an anaesthetist can control the start of each operation, set the time and date, and enter an operator code and any other relevant data.

Once a logging process has started, the data logging unit communicates with each sensor in turn to obtain a flow measurement and then stores that frame (a frame being a collection of measurements from all of the sensors) into a non-volatile RAM along with a time and date stamp. The rate of logging will typically be from one frame every five seconds to two frames per second.

The logging unit is capable of recording a variety of events, including sensor disconnection, data download activity, user logon, power failure and battery low alarms. Data obtained from the sensors are stored in the logging unit in a non-volatile memory, but may be transferred to a PC for later analysis. The data are typically provided in a format that can be easily imported into a software application such as a spreadsheet.

The apparatus of the invention provides a means of analysing anaesthetic gas flows after surgery has taken place. For example, any problems arising during surgery can be checked against events and gas flows recorded by the apparatus to establish whether defective or inappropriate anaesthesia has contributed to the problems. The apparatus also allows the total amounts of anaesthetic gases used during an operation to be recorded, information which is useful to administrators in planning for the ordering and replacement of anaesthetic gases.

The sensors used in the apparatus of the invention can be built into new anaesthesia machines or, as illustrated in the embodiment shown in the drawings, can be retrofitted into an existing anaesthesia machine.

Equivalents It will readily be apparent that numerous modifications and alterations may be made to the specific embodiments of the invention described above without departing from the principles underlying the invention. All such modifications and alterations are intended to be embraced by this application.

Claims

CLAIMS1. An apparatus for measuring and recording the volume of anaesthetic gas administered to a patient, the apparatus comprising a gas flow sensor for measuring the flow rate of an anaesthetic gas, the gas flow sensor being linked to a data storage and processing device for storing and processing flow data, and means for retrieving flow data from the storage device.
2. An apparatus according to claim 1 which comprises a plurality of the sensors for recording the flow rates of a plurality of gases.
3. An apparatus according to claim 1 or claim 2 wherein the or each sensor is connected to an inlet of a flow meter tube.
4. An apparatus according to claim 3 wherein the flow meter tube is a calibrated glass flow meter tube.
5. An apparatus according to claim 3 or claim 4 wherein the sensor comprises a housing having means for connection to the flow meter tube, the housing having a channel therethrough communicating with an inlet of the flow meter tube, wherein a sensing element is disposed within the channel to measure the flow rate of the gas passing into the inlet.
6. An apparatus according to claim 5 wherein the sensor housing has a spigot extending therefrom, the spigot being configured to fit into the inlet of the glass flow meter tube.
7. An apparatus according to claim 6 wherein the housing contains a memory device for storing information relevant to the functioning of the sensor.
8. An apparatus according to any one of the preceding claims wherein the sensor measures the flow rate of the gas by making use of a cooling effect of gas passing over a sensing element of the sensor.
9. An apparatus according to claim 8 wherein means are provided for heating a thermally sensitive component of the sensing element, or part thereof, and the cooling effect of the gas flow on the thermally sensitive component of the sensing element is determined and converted into a flow rate.
10. An apparatus according to claim 9 wherein the thermally sensitive component is a component of the sensing element which has a thermally dependent property which can be affected by the flow of gas over or past it, and said thermally dependent property is electrical resistance.
11. An apparatus according to claim 10 wherein the thermally sensitive component comprises thermistor
12. An apparatus according to any one of claims 9 to 11 wherein the thermally sensitive component is located on an upstream surface of the sensing element.
13. An apparatus according to any one of claims 9 to 12 which is provided with means for compensating for the temperature of the gas when determining the flow rate, and said means for compensating for the temperature of the gas comprises a second thermally sensitive component which is substantially shielded from the flow of gas through the sensor.
14. An apparatus according to claim 13 wherein the second thermally sensitive component is located on a downstream surface of the sensing element.
15. An apparatus according to claim 13 or claim 14 wherein the second thermally sensitive component is a thermistor.
16. Apparatus according to any one of claims 13 to 15 wherein both the first and second thermally sensitive components are thermistors, and the thermistors form part of a thermal bridge (Wheatstone bridge) configuration for determining changes in the resistance of the first thermally sensitive component relative to the second thermally sensitive component.
17. A sensor for use in determining the flow rate of a gas, the sensor comprising a housing having a channel therethrough through which gas may pass; and a sensing element disposed within the channel; wherein the sensing element comprises first and second thermistors; the first thermistor being disposed on an upstream surface of the sensing element so that it is contacted by gas passing through the housing; and the second thermistor being disposed on a downstream surface of the sensing element; and means for calculating the flow rate from a difference in cooling effects exerted by the gas on the first and second thermistors.
18. A sensor according to claim 17 wherein the first and second thermistors are connected to two further resistance elements in a thermal bridge arrangement.
19. A sensor according to claim 17 or claim 18 which contains a memory device for storing information relevant to the functioning of the sensor.
20. A sensor according to claim 19 wherein the memory device contains data relating to calibration of the sensor, or to the gas type with which the gas flow meter is to be used.
21. A sensor according to any one of clams 17 to 20 wherein the sensor housing comprises inner and outer sleeves, an inner wall of the inner sleeve surrounding the channel.
22. A sensor according to claim 21 wherein there is a space (e.g. an annular space) between the inner and outer sleeves within which the memory device is mounted.
23. A sensor according to claim 21 or claim 22 wherein there is a space (e.g. an annular space) between the inner and outer sleeves within which an interface circuit for linking the sensor to the data logging unit is disposed.
24. A sensor according to any one of claims 17 to 23 wherein the sensing element has an anchoring portion which is held within the space between the inner and outer sleeves, and a sensing portion which is disposed within and extends across the channel.
25. A sensor according to claim 24 wherein the sensing element is substantially T-shaped, the cross limb of the T-shape serving as the anchoring portion.
26. A sensor according to any one of claims 17 to 25 which is configured to be connectable to an inlet of a flow meter tube, e.g. a calibrated glass flow meter tube.
27. A sensor according to claim, 26 wherein the sensor has a spigot for insertion into an inlet of a calibrated glass flow meter tube.
28. A flow meter tube (e.g. a calibrated glass flow meter tube) having attached thereto (e.g. at an inlet end thereof) a sensor as defined in any one of claims 1 to 27.
29. An anaesthesia machine comprising a glass flow meter tube having attached thereto (e.g. at an inlet end thereof) a sensor as defined in any one of claims 1 to 27, means for controlling the flow of gas through the machine, and means for connection to a source of anaesthetic gas.
30. An anaesthesia machine according to claim 29 having a plurality (e.g. two to ten and more preferably three to six) glass flow meter tubes connected thereto, each flow meter tube having attached thereto a sensor.
31. A method of modifying an anaesthesia machine, the method comprising replacing a flow meter tube (e.g. calibrated glass flow meter tube) of length L with a replacement flow meter tube having connected to the inlet end thereof a sensor as defined in any one of claims 1 to 27, wherein the replacement flow meter tube and sensor have a combined length of L.
32. A method of measuring and recording the volume of anaesthetic gas administered to a patient, the apparatus comprising passing an anaesthetic gas through an apparatus according to any oe of claims 1 to 16 and recording and storing data concerning the flow of the gas through the apparatus over a defined period, and thereafter optionally retrieving flow data from the storage device.
33. A method of analysing the quantities of anaesthetic gases administered to a patient over the period of a surgical procedure, which method comprises measuring the flow rates of each anaesthetic during the period of the surgical procedure using an apparatus as defined in any one of claims 1 to 16, storing data concerning the flow rates in digital form and downloading the data for analysis