GB2176610A - Respiration monitor - Google Patents

Abstract

A respiration monitor which is battery operated and suitable to detect the onset of apnoea in infants or post operative patients comprises a permanent magnet mounted resiliently relative to a Hall effect device which detects changes in magnetic field due to movements of the magnet caused by breathing. The sensing device is easily attached to the patient by an elasticated belt, together with associated monitoring electronics in which the sensor signal is compared with a floating reference level in a level detector which is connected through an AND gate 3 to prevent a timer 4 timing out while respiration signals are present. Cessation of respiratory movement for greater than 10 or 20 seconds is indicated by both audible and visual alarms 14 and 15. <IMAGE>

Description

SPECIFICATION Respiratory monitor The present invention is concerned with respiratory monitors.

A respiratory monitor is already known which has a sensor for detecting mechanically the breathing of a patient and an electronic control unit which provides output signals determined by the state of the sensor. The sensor in the known monitor comprises a flattened, hollow, rubber or plastics bulb containing an air bubble and connected by tubing to an air-pressure transducer. The sensor is strapped around a patient's chest or abdomen whereby expansion of the lungs causes the bulb to be squashed and the air pressure therewithin to be increased. The pressure increase results in an electrical signal from the transducer which is detected by the electronic control unit to provide an indication of the incidence of respiration by the patient.

A problem in practice with this known system is that it reacts only to increases in pressure at the transducer. Thus, for example, if the patient should roll over and lie on top of the sensor it could be deformed permanently so that no further increase in pressure can result even if the patient is, in fact, breathing normally. Thus, the known system is liable to false alarms due primarily to the type of sensor used but also to the fact that the electronic control unit only reacts to an increase in pressure in the sensor.

It is an object of the present invention to provide a respiratory monitor in which the disadvantages of the known system are substantially avoided.

In accordance with the present invention, there is provided a respiratory monitor comprising a sensing device having a permanent magnet mounted resiliently relative to a Hall Effect sensor, the sensing device being adapted to respond to respiratory action in a patient to whom the sensing device is attached to provide changes in the level of the output voltage from the sensor and a monitoring circuit which monitors electronically the output of the Hall Effect sensor and responds to change in sensor output to indicate that respiration has occurred.

Preferably, the monitoring circuit includes a timer which is arranged to be reset upon the detection of a change in voltage level from the sensor indicative of respiratory action; if the timer is not so reset within a preselectable time period, then an alarm is caused to be raised.

Advantageously, the alarm is both visual and audible. If, on having raised the alarm, the monitoring circuit then detects a breathing action, the audible alarm may be cancelled but the visual alarm continued for a predetermined period. If respiratory action does not recommence, both audible and visual alarms preferably continue until reset by an investigating person.

Preferably, the monitoring circuit includes a level detector which is adapted to detect changes in the prevailing output level of the sensor, irrespective of what the prevailing output level might be. For this purpose, the level detector comprises a comparator, the reference level of which is provided by a square wave signal supplied by an oscillator.

Preferably, the level detector includes two series connected comparators, the first comparator receiving at its one input the voltage output of the sensor and at its other (a) the integrated output of the sensor and (b) said square wave signal, the output of the first comparator being coupled to the input of the second comparator by way of a diode pumping circuit and a parallel capacitor, the other input of the second comparator being provided with a fixed reference level, whereby when the sensor output remains constant an oscillating output is obtained from the first comparator which charges said parallel capacitor and holds the second comparator in one switching state, but when a change in sensor output occurs a steady signal is temporarily obtained from the first comparator which causes the voltage on the parallel capcitor to decay and the second comparator to change switching states, thereby indicating that respiration has occurred.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, wherein: Fig.1 is a block circuit diagram showing one embodiment of a respiratory monitor in accordance with the present invention; Fig.2 shows a detail of the circuit diagram of Fig.1; and Figs. 3a and 3b are diagrammatic graphs for use in explaining the operation of the portion of the circuit shown in Fig.2.

With reference first to Figure 1, the monitor uses a sensor 1 which is attached to a patient's body to generate electrical signals responsive to respiration of the patient. The sensor (not shown in detail) includes a permanent magnet which is disposed in a flexible mounting in close proximity (for example 1mm) to a Hall Effect sensing device. The magnet can, for example, be suspended over the Hall Effect sensing device by means of a rubber or plastics diaphragm or simply by means of a layer of a resilient rubber foam material. The sensor is strapped by a belt or harness or otherwise mounted on the patient (preferably on the patient's abdomen) such that upon respiration the relative positions of the magnet and Hall Effect device change, whereby to generate an output signal from the sensor.

Normally, with the patient lying on his back and the sensor strapped to his abdomen, res piratory action causes the rubber foam -to compress and the magnet to move towards the sensor, thereby generating an output voltage of specific polarity. However, it will be noted that, depending upon the position of the patient, the actual signal level and signal polarity which is generated during respiration cannot be predicted. For example, the patient may be lying on the sensor so that the rubber is already partially compressed. For this reason, as described hereinafter, the detection circuitry is adapted to respond to any signal change from the sensor above a certain threshold level corresponding to the sensitivity of the circuitry.The detection circuitry is therefore not looking for any particular voltage level but for a change in the prevailing level, whatever the latter level might be.

The sensor 1 which is attached to the patient's body thus detects respiratory movement in the patient and transmits an electrical signal to the monitoring electronics. The latter part of the system can be located at a position remote from the sensor 1.

The sensor signal is applied to a level detector 2 which, as described further hereinafter with reference to Figs. 2 and 3, is provided with a floating reference level by means of an oscillator 10. The output of the level detector forms one input of a NAND gate 3 whose other input is connected to a fixed supply voltage VDD. The output of the NAND gate 3 is connected to the control input of an electronic timer 4 and also to the trigger input of a monostable 8.

The output of the NAND gate 3 is normally low and this is arranged to cause the timer 4 to time out. As explained further hereinafter, each time a respiratory action is sensed by the level detector 2, the output of the NAND gate 3 is caused to go high so resetting the timer and initiating another timing period. Each time the output of NAND gate 3 goes high, the monostable 8 is triggered. This is arranged to produce an audible "click" from an electronic sounder 9, together with a flash of green light from a green LED 12. The purpose of the click and flash is to indicate to a person monitoring the alarm that respiratory action is taking place and that the electronics is functioning.

The timer 4 times out in either ten or twenty seconds selectable by a front panel switch (not shown). If respiratory action fails and the timer does in fact time out, then the alarm is raised by the timer energizing a buzzer 14. At the same time a latch 5 is set which causes a red LED 15 to flash. If then respiratory action should recommence, the audible alarm 14 is stopped as a result of the timer 4 being reset. At the same time, the output of a further NAND gate 6 is arranged to go low to cause a further timer 7 to time out. On timing out, for example after one minute, the output of the timer 7 resets the latch 5 so turning off the red flashing indicator 15.

Thus, even though respiration recommences, the red indicator 15 flashes for a period of one minute to indicate to the person monitoring the alarm that a break in normal respiration has occurred.

The structure and operation of the level detector 2 are now described in more detail with reference to Figs. 2 and 3.

The sensor signal Vfl is applied to the noninverting input (+) of an operational amplifier comparator 21 by way of a resistor R1 and to the inverting input (-0) by way of the series combination of two resistors R2 and R3. The junction of the resistors R2 and R3 is coupled to the Ov line 23 by way of a capacitor C1.

The inverting input of comparator 21 is also connected to the Ov line by way of the series combination of a capacitor C2 and a variable resistor R,1. A square wave derived from the oscillator 10 is injected at the junction of the capacitor C2 and variable resistor RV1 by way of a resistor R4.

The output of the comparator 21 is led to the non-inverting input of a second operational amplifier comparator 22 by way of the series combination of a capacitor Cs and a diode D1.

The junction of the capacitor C3 and the diode D1 is coupled to the Ov line by way of a diode D2. The non-inverting input of the comparator 22 is connected to the Ov line by way of the parallel combination of a capacitor C4 and a resistor Bs. The inverting input of comparator 22 is provided with a d.c. reference level Vre by means of a variable resistor RV, connected across the supply voltage. The output of the comparator 22 provides the output of the level detector coupled to the NAND gate 3, in accordance with Fig. 1.

In operation, in order to prevent the timer 4 timing out, the output of comparator 22 must be arranged to go low each time a breath occurs. For the output of comparator 2 to go low, the voltage V,, at its non-inverting input must drop below V,,. This is achieved as follows: The input voltage V,, applied to the comparator 21 is the amplified output of the Hall sensor 1. This voltage V,,, can be at any value (within the linear range of the sensor output) depending on the degree of compression of the sensing diaphragm. However, a respiratory action will always result in at least a small change in Vm.

It will be appreciated that a conventional comparator arrangement cannot be used to detect such changes since such an arrangement would normally use a fixed reference against which the input voltage Vm would have to be compared. To overcome this problem, the present circuit provides a reference which floats with the prevailing level of Vm, the magnitude of the reference being set by RV,.

The floating reference is achieved by use of the square wave applied across the potential divider formed by R4 and RV,. The capacitor C2 decouples the voltage across RV1 and impresses it upon the voltage appearing at the inverting input of the comparator 21. If V, is constant (i.e. no respiratory action is occurring), the voltage applied to the inverting and non-inverting inputs of comparator 21 would be the same (ignoring for the moment the impressed voltage coupled by C2). Under these conditions, when the impressed voltage is taken into account, the output of comparator 21 switches at the oscillator frequency as the voltage at the inverting terminal swings above and below the voltage at the non-inverting terminal. This situation is illustrated in Fig.3a.

The components C3, D1, D3 act as a socalled diode pump circuit serving to charge capacitor C4. It will be appreciated that, providing the output of comparator 21 continues to oscillate, the voltage V,,, across the capacitor C4 will be maintained. However, if the output of comparator stops oscillating and is either permanently high or low, then the voltage Vout across C4 will disappear (reduce to zero).

Now V,,, changes each time respiratory action takes place. The voltage at the non-inverting terminal of comparator 21 follow V,,, instantly, whereas the voltage at the inverting terminal does not, due to capacitor C1 charging or discharging. The effect is illustrated in Fig.3b for an imagined step change in V,,, Referring to Fig 3b, over the period T, the voltage at the non-inverting terminal always exceeds that at the inverting terminal, so that the output of comparator 21 is permanently high for this period. Since no pumping voltage is applied to the circuit C3, D1, D2, the voltage V,,, decays to zero.As soon as V,,, falls below V,,,, the output of comparator 22 is caused to switch to its low value, signifying that respiratory action has taken place.

It was assumed above that the change in V,,, was a step rise. It will be appreciated that a step fall in V,,, would cause the output of comparator 21 to go low but again V,, would decay to zero, signifying respiratory action.

The sensitivity of the comparator 21 is set by the voltage appearing across BV1. In practice this can be set to a relatively low level e.g. 10 mv peak.

Thus, whatever the direction and magnitude of the change in V,,, (above a threshold magnitude), the circuit will detect such changes as being the result of respiratory action and will react accordingly to prevent the timer 4 from timing out.

The monitoring electronics can be housed in any suitable manner. One convenient arrangement is for the monitoring electronics to be housed in a small hand-held plastics box. The timing out time (ten or twenty seconds-or any other desired periods) is selected by a panel switch on the box. Such a monitor may be powered by batteries, for example four 1.5 volt dry batteries disposed in the box, and is therefore inherently safe. A low voltage indicator can be incorporated to warn when batteries need changing.

Claims (4)

1. A respiration monitor comprising a sensor having a permament magnet mounted resiliently relative to a Hall Effect device, the sensor being able to follow respiratory movement in a patient to whom it is attached. Respiratory movement produces changes in the electrical voltage output of the Hall Device these signals being processed by the monitor to indicate that respiration has occured.

2. A respiration monitor as claimed in claim 1 having a voltage level detector, which can detect changes in the prevailing output level of the sensor (indicative of respiratory movement in any plane) irrespective of what the prevailing output level might be.

3. A level detecting circuit as claimed in claim 2 comprising two series connected comparators. The reference level of the first comparator being provided by a square wave signal supplied by an oscillator. The comparator receiving at its one input the voltage output of the sensor amd at its other(a) the said square wave signal added to the integrated output of the sensor. The output of the first comparator being coupled to the input of the second comparator by the way of a diode pumping circuit and parallel capacitor the other input of the second comparator being provided with a fixed reference level. When the sensors output is constant an oscillating output is obtained from the first comparator, which develops a fixed voltage across the said capacitor, and holds the second comparator in one switching state. When a change in sensor output occurs a steady signal is temporarily obtained from the first comparator which causes the voltage om the parallel capacitor to decay and the second comparator to change switching states, indicating that respiration has occured.

4. A respiration monitor substantially as described herein with reference to Figures 1-2 of the accompanying drawing.