GB2046969A

GB2046969A - Respiration monitoring apparatus

Info

Publication number: GB2046969A
Application number: GB8000629A
Authority: GB
Original assignee: FUNK ELEKTRONIK GmbH
Current assignee: FUNK ELEKTRONIK GmbH
Priority date: 1979-01-09
Filing date: 1980-01-09
Publication date: 1980-11-19
Also published as: DE3000254A1

Abstract

A respiration monitoring apparatus has a sensor comprising a temperature- sensitive resistor exposable to the breath flow of a person and connected to an analysis circuit. The analysis circuit comprises a threshold value switch (A4) which is responsive to the output of the sensor, and a timing circuit (OS1) retriggerable during its timing period by the output of the threshold value switch. The sensor is connected to the threshold value switch via a filter circuit (A1, A2) which is adapted to pass only signals which change significantly within a predetermined period of time, so that the effect of any slow variations in ambient temperature are eliminated. This filter circuit comprises an integrator A2, C3 and a feedback loop R4. <IMAGE>

Description

SPECIFICATION Respiration monitoring apparatus The invention relates to a respiration (breathing) monitoring apparatus provided with a sensor exposable to the breath flow of a person and connected to an analysis circuit. The sensor comprises a temperature-sensitive resistor and the analysis circuit comprises a threshold value switch responsive to signals produced by the sensor, and a timing circuit retriggerable during its timing period by the output signals from the threshold value switch.

Such an apparatus is disclosed in West German OLS No. 2107 099.

In this known apparatus it is necessary to fit the sensor into a chamber and direct the breath flow through the chamber. In order to compensate for changes in the resistance of the sensor caused by changes in the ambient temperature, it is necessary to construct the sensor of two thermistors connected in series with each other and to tap the potential at the centre connection to supply the threshold value switch. Therefore, it is necessary to ensure an equal breath flow to both thermistors and to fit the thermistors into a chamber, requiring the use of a probe and making this known respiration monitoring apparatus difficult to use.

It is an object of this invention to provide a respiration monitoring apparatus which is easy to use and in which the sensor does not need to be fitted into a special chamber.

According to the invention, there is provided a respiration monitoring apparatus provided with a sensor exposable to the breath flow of a person and connected to an analysis circuit, the sensor comprising a temperature-sensitive resistor, and the analysis circuit comprising a threshold value switch responsive to signals provided by the sensor and a timing circuit retriggerable during its timing period by output signals from the threshold value switch, the sensor being connected to the threshold value switch via a filter circuit which is adapted to pass only signals which change within a predetermined period of time.

It is possible to use an extremely small temperature-sensitive resistor in apparatus according to the invention, so that its resistance changes rapidly and the sensor in no way influences or impedes the breath flow. Due to the presence of the filter circuit between the threshold value switch and the sensor, slow changes of the signal produced by the sensor, e.g. caused by changes of the ambient temperature, are suppressed, and therefore the temperaturesensitive resistor may be exposed to the free breath flow.

The temperature-sensitive resistor responds to the temperature difference between the inhalation flow and the warmer exhalation flow and changes its resistance according to the respiratory rhythm of the patient to be monitored.

The use of the single temperature-sensitive resistor as a sensor makes it possible to amplify the changes of the resistance caused by inhalation and exhalation, to a high degree in a very simple manner, and a choice between a direct current or an alternating current amplifier is open to the technical designer.

For example it is possible to achieve an amplification of such a high degree that a reliable identification of a temperature difference of only 0.5 C can be achieved using only relatively cheap circuits elements. Such small differences in temperature are overstepped even by the shallow respiration of a baby and therefore the possibility of a false alarm is avoided.

According to a preferred embodiment of the respiration monitoring apparatus, the filter circuit comprises a differential amplifier having one input connected to a reference voltage source and another input connected to the sensor, the output of the differential amplifier being connected to the input of an intergrator and the output of the integrator being connected to the sensor.

Preferably the differential amplifier and the threshold value switch, preferably in the form of a Schmitt-trigger, are supplied by the same reference voltage source, preferably via an operational amplifier connected as an impedance transformer.

Therefore, a closed control loop is formed and only changes of potential corresponding to changes of the impedance of the sensor occuring within a period which is shorter than the time constant of the integrator appear at the output of the differential amplifier. Preferably this time constant is about 10 seconds. In this way, only changes of resistance of the sensor which are caused by respiration are utilized, whereas slower changes of temperature, e.g. changes of the ambinettemperature, are not taken into account.

Further, by means of the integrator, automatic adjustment of the voltage applied to the sensor to equal the value of the reference voltage is achieved.

Moreover, by this arrangement the offset voltage of the differential amplifier is automatically adjusted and therefore no balancing is necessary.

When an operational amplifier is used as the differential amplifier, it is possible to use sensors having nominal resistance values of about 2 k Ohm to 100 K Ohm.

According to a further feature of the invention the threshold value switch activates in one of its two switching conditions an oscillator connected to an audible indicator, preferably of the type which comprises a piezo ceramic disc, as well as a visual indicator, e.g. a light emitting diode. The intensity of the audible and or visual indicator may be controlled by a variable resistor. These features ensure that the respiration of the patient to be monitored activates and deactivates an audible and optionally a visual indicator in sympathy with the changes of resistance of the temperature-sensitive resistor caused by respiration.

According to a preferred embodiment of the invention, the output of the timing circuit is arranged to control a relatively low frequency oscillator, the output of the low frequency oscillator being arranged to control the oscillator connected to the audible indicator when the timing circuit reverts to an inactive state. The frequency of the low frequency oscillator is preferably within the range of 3 to 10 cycles per second. Thus an effctive alarm is achieved by very simple means.

It is advantageous if the volume of the sound produced by the audible indicator is controlled by a variable resistor which is arranged to be shortcircuited when the timing circuit reverts to an inactive state. By this means a great difference is achieved in an easy manner between the normal acoustic signals monitoring inhalation and exhalation and an alarm indicating a respiration stoppage, without the need to provide separate sound generators.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure la is a circuit diagram of the analogue portion of a respiration monitoring apparatus according to the invention, and Figure ib is a circuit diagram of the digital portion of the apparatus.

In the drawings, terminals in Figures 1 a and 1 b which have the same reference characters are connected together.

Referring to Figure 1 a, a pair of resistors R5, R6, together with a capacitor C2, are connected as a potential divider between a supply voltage V, and earth, to derive a reference voltage of about 2 volts.

An operational amplifier A3 is connected as a high to low impedance transformer to buffer the reference voltage.

A sensor, not shown, in the form of a thermistor, is located in the breath flow of a patient whose breathing is to be monitored. The sensor may be fixed for example in a mouthpiece, under an oxygen mask or in a universal connector for use with various artificial respiration systems. The sensor is connected via terminals PR2 and PR4, between the non-inverting input of an operational amplifier Al and earth. Amplifier Al is connected as a noninverting differential amplifier. A capacitor C1 is connected between the output of the amplifier Al and the inverting input to suppress noise spikes.

The output of amplifier Al is connected via a resistor R3 to the inverting input of an operational amplifier A2 which is connected as an integrator. A capacitor C3 is connected between the output of amplifier A2 and its inverting input and the value of this capacitor is chosen such that the integrator has a time constant of about 10 seconds. The output of the integrator is connected via a resistor R4to terminal PR2 so as to apply the output voltage of the integrator across the sensor. The buffered reference voltage available at the output of amplifierA3 is applied via a resistor R1 to the inverting input of amplifier Al, and directly to the non-inverting input of amplifier A2.Amplifiers Al and A2 form a feedback loop with a time constant of about 10 seconds, and this loop acts to adjust the voltage applied to the sensor to equal the value of the reference voltage. Changes in the impedance of the sensor due to temperature changes appear as changes in the output voltage of amplifier Al, but only when these temperature changes occur within a period which is shorter than the time constant of the feedback loop. Temperature changes which take place over a period of more than 10 seconds, such as those due to variations in ambient temperature, are automatically compensated for by the feedback loop. Viewed from another aspect, the feedback loop constitutes a filter which is adapted to pass only signals which change within a predetermined period of time.

The output of the amplifier Al is fed via a resistor R7 to a Schmitt-trigger A4 which constitutes a threshold value switch. The inverting input of the Schmitt-trigger is supplied with buffered reference voltage from the output of amplifier A3. The digital output of the Schmitt-trigger is fed to the digital portion of the circuit, Figure 1 b, and also to a semi-conductor switch S4. The Schmitt-trigger produces an H signal (high level) during exhalation and an L signal (low level) during inhalation.

Referring to Figure 2, terminal A is connected to a pair of differentiating circuits, one consisting of capacitor C7, resistor RIO and diode D3, and the other consisting of capacitor C8, resistor R11 and diode D4. An inverter 14 is connected between terminal A and one of the differentiating circuits. The outputs of both differentiating circuits are connected to a respective input of a NAND gate N3. The output of the NAND gate is fed to the trigger input of a monostable multivibrator OS1 which is triggerable during its transit time and which has a pulse duration of about 6 seconds. The monostable is triggered by both the leading and following edges of the digital pulses from the schmitt-trigger.The monostable is held in its active (astable) state as long as the time between temperature transients, due to a change from exhalation to inhalation or vice versa, is greater than the predetermined pulseduration of the monostable. In the active state, the monostable produces an H-signal at the terminal F and an L-signal at terminal G.

A low frequency oscillator is formed by a NAND gate N4, an inverter 15, resistors R15, R16, and capacitor C10. An inverter 16 is connected to the out put of the oscillator. This oscillator is held in a non-oscillating condition when an L-signal is present at terminal G and under such conditions an H-signal appears at the output of inverter 16.

Referring back to Figure 1 a, the output of the Schmitt-trigger A4 is connected via switch S4 and terminal B to an input of a NAND gate N1 (Figure 1 b).

A second input of this NAND gate receives the output of inverter 16. The output of NAND gate N1 is connected to the input of a second oscillator comprising a NAND gate N2, an inverter Il, resistors R12 R13 and capacitor C9. This oscillator is adapted to generate a signal of about 2.8 KHz. The output of the second oscillator is connected via an inverter 13 to a terminal LS2. A loud-speaker (not shown) of the piezo ceramic disc type is connected between terminals LS1 and LS2. Terminal LS1 is connected via terminals C and POT1 to one connection of a variation resistor, the other connection being connected via terminals POT2 and D to the output of the second oscillator.Inverter 13 serves to increase the amplitude of the 2.8 KHz note emitted by the loud-speaker and the variable resistor acts as a volume control. As long as the monostable OS1 is active resulting in an H-signal at the output of inverter 16, the presence of an L-signal at terminal B (due to inhalation) causes a tone to be emitted by the loud-speaker and the presence of an H-signal (due to exhalation) at that terminal inhibits the generation of a tone.

A light emitting diode (not shown) may be connected between terminals +LED and -LED. The output of NAND gate N1 is fed via an inverter 12 and terminal E to the light emitting diode so that it is illuminated in sympathy with the tone emitted by the speaker, to serve as a visual indicator.

If the monostable OS1 is not triggered during the predetermined 6 second active period, an L-signal is produced at terminal F and H-signal is produced at terminal G. Thus, switch S4 is switched to the non-conducting state so that signals from the Schmitt-trigger no longer reach terminal B. At the same time as S4 is switched, the H-signal at terminal G activates a switch S3 which passes an H-signal to terminal B.

Further, the low frequency oscillator is activated by the H-signal at terminal G, to trigger the second oscillator at a rate of about 3 pulses per second. The H-signal at terminal G also activates a further pair of switch S1 and S2 which are connected in parallel across the terminals PoT1 and PoT2 so as to short-circuit these terminals. Thus, the volume control connected to these terminals is short-circuited and the loud-speaker works at full volume to emit a burst of loud high frequency bleeps as a warning that the patient's breathing has stopped or at least become dangerously slow.

If an acceptable breathing rate is re-established, the monostable is again triggered and the apparatus reverts to the condition last described.

Power is supplied to the apparatus either by a battery, e.g. a 9 volt battery, connected to terminals + + BAT and - BAT or from an a.c. mains driven power supply unit connected to terminals NG1 and NG2. A voltage stabilizer IC3 is used to stabilize the supply voltage to the apparatus when a mains supply is used, the light emitting diode being supplied via diode D2 and the rest of the circuit being supplied via diode Do. when a battery is used the light emitting diode is supplied only when the switch S is in the 'LED ON' position, so that the visual indication can be switched off to conserve power.

A Avariable resistor may be connected in series with the light emitting diode to control the intensity of the light emitted.

Claims

1. A respiration monitoring apparatus provided with a sensor exposable to the breath-flow of a person and connected to an analysis circuit, the sensor comprising a temperature-sensitive resistor, and the analysis circuit comprising a threshold value switch responsive to signals produced by the sensor and a timing circuit retriggerable during its timing period by output signals from the threshold value switch, the sensor being connected to the threshold value switch via a filter circuit which is adapted to pass only signals which change within a predetermined period of time.

2. An apparatus according to claim 1, in which the output of the threshold value switch is connected to an audible indicator.

3. An apparatus according to claim 1, in which the output of the threshold value switch is connected to a visual indicator.

4. An apparatus according to claim 2 or 3, in which the intensity of the audible and/or visual indicator is controllable by a variable resistor.

5. An apparatus according to any preceding claim, in which the filter circuit comprises a differential amplifier having one input connected to a reference voltage source and another input connected to the sensor, the output of the differential amplifier being connected to the input of an integrator and the output of the integrator being connected to the sensor.

6. An apparatus according to claim 5, in which the differential amplifier and the threshold value switch are supplied by the same reference voltage source.

7. An apparatus according to claim 6, in which an operational amplifier is connected as an impedance transformer between the reference voltage source and the differential amplifier threshold value switch.

8. An apparatus according to any preceding claim in which the threshold value switch is a schmitt-trigger.

9. An apparatus according to any preceding claim, in which the threshold value switch activates in one of its two switching conditions an oscillator connected to an audible indicator.

10. An apparatus according to claim 9, in which the output of the timing circuit is arranged to control a relatively low frequency oscillator, the output of the low frequency oscillator being arranged to control the oscillator connected to the audible indicator when the timing circuit reverts to an inactive state.

11. An apparatus according to claim 10, in which the volume of the sound produced by the audible indicator is controlled by a variable resistor which is arranged to be short-circuited when the timing circuit reverts to an inactive state.

12. An apparatus according to claim 10 or 11, in which the low frequency oscillator has a frequency of between 3 and 10 cycles per second.

13. A respiration monitoring apparatus substantially as described with reference to the accompanying drawings.