WO1991011139A1

WO1991011139A1 - Breathing monitoring device

Info

Publication number: WO1991011139A1
Application number: PCT/GB1991/000094
Authority: WO
Inventors: Roger Martin Pickard
Original assignee: The Victoria University Of Manchester
Priority date: 1990-01-24
Filing date: 1991-01-22
Publication date: 1991-08-08
Also published as: IE910223A1; GB9001614D0; AU7212791A; PT96570A

Abstract

A breathing monitoring device comprising a temperature sensitive probe (1, 11) for location in an air-way (2, 12) through which a patient breaths in and out. A circuit (7) monitors the output of the temperature sensitive probe to derive an output representative of the patient's breathing. An alarm (9, 21) may be activated if the monitored output indicates a stable temperature for a predetermined period which is long in comparison with an expected breathing cycle period.

Description

BREATHING MONITORING DEVICE

The present invention relates to a breathing monitoring device for use by medical practitioners to monitor breathing in a patient. Breathing monitoring devices may be used for example to detect the cessation of breathing, or to monitor respiratory rate.

Devices are available for monitoring the breathing of patients in, for example, intensive care units. The known devices rely upon monitoring the changes in pressure in an air-way through which the patient breathes. The pressure variations may be used to derive an output representative of respiratory rate for example. Detection of the cessation of breathing may also be achieved, by for example detecting a stable pressure which indicates cessation of breathing and triggering an alarm indicator when a stable pressure is detected. Details of this known breathing monitoring equipment are given in a document issued by Her Majesty's Stationery Office and entitled "Health Equipment Information" No. 169, June 1987 "Evaluation of Ventilator Alarms".

It is an object of the present invention to provide an improved breathing monitoring device.

According to the present invention there is provided a breathing monitoring device comprising a temperature sensitive probe for location in a air-way through which a patient breathes in and out, a circuit for monitoring the output of the temperature sensitive probe, and output means for providing an output representative of the patients breathing.

Preferably, the output means comprises means for activating an alarm if the monitored output indicates a stable temperature for a predetermined period which is long in comparison with an expected breathing cycle period.

The predetermined period may be set to the equivalent in time to for example four cycles of a normal breathing pattern. Means may be provided for disabling the alarm if the output of the probe indicates that the patient has started to breath again. Means also may be provided for disabling the alarm after it is turned on until a predetermined number of breathing cycles has been detected from the probe output.

Means may be provided for manually resetting the alarm.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic illustration of circuitry associated with a first embodiment of the present invention;

Fig. 2 shows two waveforms appearing in the device illustrated in Fig. 1.

Fig. 3 is a diagram illustrating a thermistor and resistor controlled oscillator of a second embodiment of the present invention;

Fig. 4 is a block schematic diagram of the second embodiment; and

Fig. 5 illustrates a connector cable and housing of the second embodiment.

Referring to Fig. 1, a probe in the form of a thermistor 1 is placed in an air-way 2 through which a patient breaths, the probe 1 thus being exposed to air flows in opposite directions as indicated by arrows 3. As a patient breaths in, the air flowing past the probe 1 is at ambient temperature. As a patient breaths out, the air passing probe 1 is at a higher temperature corresponding to the body temperature of the patient.

The thermistor 1 can be in the form of a bead thermistor which can be of small size, for example 5 mm long and 2 mm in diameter. The thermistor is introduced into the air-way through which the patient both inhales and exhales. The thermistor is connected to sensing circuitry by a cable 4, the cable 4 being, for example, a flexible coaxial cable 3 mm in diameter and several metres in length.

The resistance represented by the thermistor 1 may form one arm of a four arm bridge, the other three arms 5 of the bridge each having a nominal resistance value which is the same as the nominal resistance value of the thermistor.

The bridge is energised by a current, for example of 50 microamps, delivered by a battery 6. A resistor (not shown) is connected in series with the battery. This resistor isolates the bridge from direct contact with the battery, limits the current drawn from the battery, and allows the battery to be of a convenient value (normal voltage). The power supply may of course be derived by appropriate circuits through a mains supply or from a conventional or rechargeable battery pack.

The bridge output is delivered to a threshold detector 7. The balance of the bridge is changed as a result of the resistance of the bead thermistor changing as the temperature of the airflow within which it is positioned changes. As shown in the upper waveform of Fig. 2, the output of the bridge will accordingly be amplitude modulated at a frequency corresponding to the frequency of the breathing cycle of the patient. If the patient stops breathing, the air temperature to which the bead thermistor is exposed will stabilise and the output applied to the threshold detector 7 will be a constant voltage. The threshold detector circuit is set to monitor the modulation frequency of the bridge output so that if the thermistor temperature stabilises an alarm is sounded. The threshold as determined by the threshold detector may be manually set or alternatively may be gradually adjusted automatically such that the threshold is always substantially halfway between the peaks and troughs of the bridge output.

The threshold detector supplies a square wave output as illustrated in the lower waveform of Fig. 2 to a timing circuit 8 which is arranged to be reset by each change in amplitude of the square wave delivered to it by the threshold detector 7. If the timer is not reset by the threshold detector 7 within a period corresponding to several cycles of normal breathing, for example fifteen seconds for adults, or five seconds for children, it activates an alarm circuit 9 which can provide an enabling output to for example an alarm siren and/or alarm light.

As an alternative to absolute value threshold detection, the electronic circuitry can be arranged to detect the rate of change of the resistance of the probe, and hence the rate of change of the temperature to which the probe is exposed. The circuitry can be arranged to produce a pulse when the rate of change exceeds a predetermined threshold, indicative of a satisfactory rate of breathing. This pulse is used to reset the timer.

By detecting the rate of change of temperature, rather than the absolute value of the temperature, the absolute value of the temperature becomes unimportant, and setting up or calibration procedures are avoided.

There are alternative circuits which may be used, instead of the bridge configuration, to detect the change in temperature of the gases in the air-way. Some of these circuits may be combined with threshold detecting or rate of change detecting circuits within a single circuit which provides both functions. Alternatively the changes in resistance of the thermistor caused by change in temperature may be incorporated in a repetative timing circuit such that as the temperature changes so the time periods being generated are also changing. In this type of configuration the function of the threshold detector 7 is replaced by circuits which monitor the periods being generated by the repetative timing circuit. If insufficient change is detected between successive timing periods, as occurs on the cessation of breathing, then this circuit does not provide a reset pulse to the timer 8.

A manual reset circuit 10 is also provided to enable the timer to be reset manually to disable the alarm circuit. For example, when the device is to be used in connection with a particular patient, it can be turned on before the thermistor is placed in the patient's air-way.- The alarm should then sound after the preset delay determined by the timing circuit 8 to indicate that the system is working. The timer can then be manually reset and the thermistor bead 1 placed in the patient's air-way. Cessation of breathing by the patient will subsequently reactivate the alarm. Alternatively, the circuitry can be arranged such that it is only activated after it has received an output from the threshold detector 7 indicative of say four normal breathing cycles after the device is first fitted to a particular patient. Some sort of test circuitry would nevertheless have to be provided to give the medical practitioners relying on the device sufficient confidence that it is operating correctly.

Preferably the timing circuit is arranged such that if the patient resumes breathing after a temporary cessation sufficiently long to activate the alarm circuit the alarm circuit is automatically deactivated.

Preferably the "time out" period of the timer may be manually set to adapt the device to different normal breathing rates, for example breathing rates normally found in children as compared with breathing rates normally found in elderly patients.

The various circuit components described above are readily available as discrete circuits which can be interconnected in a conventional manner. The circuitry could however be subminiaturised to form an electronic circuit of a size equivalent to that of a wrist watch. The entire device could then be battery powered and mounted directly on the air¬ way with the bead thermistor in the airstream passing to and coming from the patient. Alternatively, a more sophisticated instrument incorporating a microprocessor programmable to suit particular circumstances could be provided in a small hand¬ held casing. Such a microprocessor-based device is described below with reference to Figs. 3, 4 and 5.

Fig. 3 illustrates the front end circuitry of the second embodiment of the invention. A thermistor 11 is located in an airway indicated by broken lines 12 through which an air flow passes as indicated by arrows 13. Thus assuming that all is in order the temperature to which the thermistor 11 is exposed varies cyclically between the user's body temperature and the ambient temperature. A resistor controlled oscillator 14 in the form of a 74HCT-132 integrated circuit is connected to the thermistor 11. Thus the voltage on output 15 is a square wave the frequency of which corresponds to the frequency of the amplitude modulation of the resistance of the thermistor 11.

The square wave signal on output 15 is applied to a micro-controller system 16 (see Fig. 4). The micro-controller 16 is an Intel 80C31 circuit. The micro-controller 16 is interfaced with a liquid crystal display 17, a power controller and low battery detector 18 powered by a 9 volt battery 19, and a switching unit 20 which turns the device on and off and clears the device ready for use. A piezo sounder alarm device 21 is also connected to the micro-controller. In a conventional manner the micro-controller may also be provided, with an input represented by chain dot line 22 for connection to a built in self test interface and inputs 23 for connection to an intensive care unit control panel interface.

Thus the described system provides to the micro¬ controller an input in the form of a frequency. Providing the input frequency is maintained above a threshold which may be selected by the user or arrived at automatically it can be assumed that breathing is continuing. As soon as this frequency threshold is higher than the measured frequency this can be recognized to generate an alarm signal that causes the sounder 21 to sound.

The frequency of oscillation of the resistor controlled oscillator may correspond to the frequency of breathing or may correspond to a lower frequency determined by the time constant of the circuit and the oscillator trigger threshold. The actual frequency is not important however as the micro¬ controller can be programmed to calibrate the system to allow for varying ambient temperatures and thermistor values. It is particularly important for the system to cope adequately with changes in thermistor values as cheap thermistor have a tolerance often as wide as twenty percent of the nominal value.

Referring now to Fig. 5, this schematically illustrates the connection of a thermistor 22 to a coaxial cable 23 the end of which is terminated by a conventional coaxial plug 24 which is connected to the resistor controlled oscillator. The thermistor 22 is received in a plastic housing 25 and secured therein by a simple grub screw 26. When so secure the sensing portion of the thermistor 22 is located at the centre of apertures 27 in the housing. The housing can then be appropriately positioned so as to be exposed to the flow of air to and from a patient.

Claims

CLAIMS :

1. A breathing monitoring device comprising a temperature sensitive probe for location in a air-way through which a patient breathes in and out, a circuit for monitoring the output of the temperature sensitive probe, and output means for providing an output representative of the patients breathing.

2. A breathing monitoring device according to claim 1, wherein the output means comprises means for activating an alarm if the monitored output indicates a stable temperature for a predetermined period which is long in comparison with an expected breathing cycle period.

3. A breathing monitoring device according to claim 2, wherein said predetermined period is equivalent in time to a predetermined number of cycles of a normal breathing pattern.

4. A breathing monitoring device according to claim 2, or 3, comprising means for disabling the alarm if the output of the probe indicates that the patient has started to breath again.

5. A breathing monitoring device according to claim 2, 3 or 4, comprising means for disabling the alarm after it is turned on until a predetermined number of breathing cycles has been detected from the probe output.

6. A breathing monitoring device according to claim 2, 3, 4 or 5, comprising means for manually resetting the alarm.