WO1999002087A1

WO1999002087A1 - Apnoea monitor

Info

Publication number: WO1999002087A1
Application number: PCT/AU1998/000541
Authority: WO
Inventors: Mark Brendon Perkins; Brett Arthur James
Original assignee: Micro Monitoring Systems Pty. Ltd.
Priority date: 1997-07-11
Filing date: 1998-07-10
Publication date: 1999-01-21
Also published as: AUPO785397A0

Abstract

An apnoea monitoring system (1) is disclosed which includes a sensor (3) formed of a movement sensitive transducer (21) arranged to be coupled (4) about a torso of a respiring being (2). The transducer (21) is responsive to respiratory effort (expansion and contraction of the torso) of the being to output a signal (18) interpretable of a respiratory state of the being (2). A processor arrangement (5) is provided to process the signal (18) and comparing the signal against an anticipated respiratory response of the being whereby when the anticipated respiratory response is not detected, an alarm (14) indicative of an abnormal respiratory state of the being is output.

Description

APNOEA MONITOR Field of the Invention

The present invention relates to the monitoring of physiological activity and, in particular, discloses a simple yet effective apnoea monitor. Background Art

An apnoea is a cessation of breathing which may result from occlusions of the airway which are often intermittent and when of short duration, cause few physiological problems, other than restless sleep. When prolonged, apnoea's can cause substantial physiological problems and in extreme cases, even death. With infants, the so-called "sudden infant death syndrome" (SIDS) is of great concern, and it is not certain whether SIDS is caused by physiological abnormalities or merely through the infant accidentally suffocating itself whilst asleep. Irrespective of its cause, many learned persons argue that the detection and/or prevention of apnoea can substantially ameliorate infant mortality rates due to SIDS. Apnoea monitors are generally well known in the medical arts and are typically associated with ventilator equipment used to provide ventilatory assistance to patients being weaned off mandatory ventilatory regimens. Typically, the ventilator includes air /gas flow or pressure sensing apparatus which detects an abrupt change in either flow or pressure indicative of an apnoea occurring and which causes the ventilator to cycle from a synchronised mode, where the ventilatory regime is synchronised to the patient's spontaneous respiratory effort, into a mandatory mode where the ventilator takes charge to provide one or more mandatory breaths to the patient. Such apnoea monitoring systems are complicated and expensive due to their association with ventilator apparatus. Such apparatus is typically far beyond the reach of parents concerned about SIDS and its possible impact upon their newly born child in a domestic environment.

Summary of the Invention It is an object of the present invention to provide a simple and convenient apnoea monitor that can be used with infants in domestic situations. In accordance with one aspect of the present invention there is disclosed an apnoea sensing apparatus comprising a movement sensitive transducer arranged to be coupled to a respiring being via fastening means, said transducer being responsive to respiratory effort of the being to output a signal inteφretable of a respiratory state of the being.

Preferably, the transducer is a piezo-electric transducer arranged between two members, at least one of the members being configured for coupling of the fastening means whereby respiratory movement of the being causes said fastening means to apply a variable force to the transducer which outputs a signal related to the force. Typically, the fastening means comprises a strap wherein on inhalation by the being, a tension on the strap increases thereby increasing the force applied to the transducer.

Generally, one of the members comprises a base plate having two apertures through which the strap is threaded and which is arranged to contact the being, the other of the members being positioned between the threaded strap and the transducer. In accordance with another aspect of the present invention there is disclosed an apnoea monitoring system comprising a transducer associated with the strap configured to be bound about an abdomen of a human, said transducer being responsive to the respiratory movement of the abdomen to output a signal related to the movement, and means for interpreting the signal and comparing the signal against an anticipated respiratory response of the human whereby when the anticipated respiratory response is not detected, an alarm indicative of an abnormal respiratory state of the human is output.

In accordance with another aspect of the present invention there is disclosed a method of detecting an apnoeic event said method including the steps of: physically detecting the respiratory effort of a respiring being; determining a substantially instantaneous respiratory state of said being; monitoring a time related to a change in the respiratory state; and identifying when no change to the respiratory state has occurred over a predetermined period of time, such identification being indicative of an apnoeic event.

In accordance with another aspect of the present invention there is disclosed a method for detecting a respiratory state of a respiring being, said method comprising the steps of: physically detecting the respiratory effort of said being by continually monitoring expansion and contraction of a torso of said being; from said monitoring, determining a substantially instantaneous value of inflation of said torso; from said monitoring, determining an average value of inflation of said torso; and comparing the instantaneous value with said average value whereby when said instantaneous value exceeds said average value, said being is inhaling, and when said average value exceeds said instantaneous value, said being is exhaling. Brief Description of the Drawings

A preferred embodiment of the preferred invention will now the described with reference to the drawings in which:

Fig. 1 is an illustration of an apnoea monitoring system of the preferred embodiment; Fig. 2 is a schematic block diagram representation of the patient processor shown in Fig. 1;

Fig. 3A is a perspective view of the transducer arrangement shown in Fig. 1; Fig. 3B is a side elevation view of the arrangement of Fig. 3A; and Figs. 4A and 4B are flow diagrams indicating the processing arrangement of the preferred embodiment.

Detailed Description Fig. 1 shows a patient monitoring system 1 in which an infant 2 is provided with a patient sensor 3 attached about the abdomen via a strap 4. The sensor 3 connects to a patient processor 5 via an interconnecting shielded cable 18 which, as shown in Fig. 1 , may be fastened to a leg of the patient 2 using a strap 6. In this manner, the sensor 3 can be attached to the infant 2 using the strap 4 prior to sleep and will be relatively unobtrusive to the infant and would minimise entanglement. Referring next to Figs. 3 A and 3B, the patient sensor 3 to includes a rectangular non-conductive base plate 20 including a pair of slots 23 and 24 arranged at ends of the plate 20 and through which the abdomen strap 4 is threaded. A piezoelectric transducer 21 is configured on the surface of the base plate 20 above which is arranged a non-conductive riser 22 configured to be engaged with one face of the strap 4. In this manner, with the strap 4 appropriately tensioned, as the abdomen of the patient 2 expands and contracts during normal respiration, the tension on the strap 4 varies thus varying a compressive force applied via the riser 22 to the transducer 21. With the varying force applied to the transducer 21 , a variable voltage is output therefrom via the shielded cable 18 to the processor 5. The signal amplitude of the transducer 21 during a breathing cycle is typically about 100 mV peak to peak.

Turning now to Fig. 2, the cable 18 connects to an input amplifier 6 which amplifies the signal derived from the piezo transducer 21 to an appropriate instrumentation value in anticipation of digital processing. After being output from the amplifier 6, the signal is applied to a 5 Hz low-pass filter 7 which acts to eliminate noise from the amplified signal such that the output of the low-pass filter 7 is indicative of only the respiratory effort of the infant 2. The output of the low-pass filter 7 is input to a microcontroller/microprocessor 8. In particular, the microprocessor 8 is provided with an analog input configured to receive the low-pass filtered signal and to convert that analog signal into a digital signal for digital signal processing within the microprocessor 8. The microprocessor 8 is preferably implemented by a device incorporating read-only memory (ROM) 70 for storing a controlling program, and random access memory (RAM) 71 for storing program variables during program execution and calculations. The patient processor 5 receives power from a DC voltage source 9 which supplies a battery changeover and charging circuit 10 configured to continually charge a rechargeable battery 11 and supply power to the microprocessor 8 and other circuits in the patient processor 5. Further, the changeover and charging circuit 10 is configured

5 to detect a failure in the DC power input 9 and to then enable operation of the battery 1 1 to power the circuit.

The microprocessor 8 includes a number of system status registers 72, formed in the RAM 71 , that output status signals via a number of status indicators 12 which may be implemented using light emitting diodes (LED's) or an LED display or like ι o structure. Examples of status register values that may be indicated, include mains power, power on, battery charging, breathing cycle, data terminal ready, apnoea alarm, and low battery alarm. A number of push button switches 13 are provided to provide user control of the patient processor 5. The switches 13 typically include power on, power off, status reset and test.

15 The microprocessor 8 outputs an alarm signal to a piezo alarm 14 configured to provide a minimum sound pressure level of 85 dB at a distance of one metre. In this fashion, the patient monitoring system 1 can be configured in the infant's room to monitor the child during times of sleep. When the respiratory pattern of the child deviates significantly from a normal range, the piezo alarm 14 is sounded thereby

20 drawing the attention of others to a possible interruption in the breathing of the infant.

As also seen, the microprocessor 8 outputs via a bus 17 via a isolation module

15 to an RS-232 connection 16. With the connection 16, the patient processor 5 can be interfaced to a personal computer or like apparatus which may be configured to record, over time, the respiratory patterns of the patient. With this configuration, whilst no

25 alarm may sound during a period of sleep, the breathing patterns of the patient can be monitored for any inherent defect or cause of concern. Typically, the RS-232 connection 16 sends a packet of information 10 times per second, with each packet including an identifier byte, the digitised patient sensor signal, the rechargeable battery voltage, and the state of each of the status indicators 12. Such data can then be used for remote monitoring and data logging of the patient's breathing.

Fig. 4A illustrates a main program loop implemented by computer software store in the ROM and executed by the microprocessor 8 and which commences with an initialisation step 30 common in the use of such devices. At step 32, the main program assesses whether or not a 100 millisecond flag is set. The 100 millisecond flag is set via an interrupt service routine shown in Fig. 4B.

Referring to Fig. 4B, the patient processor 5 is configured to generate interrupts 1 ,200 times per second and when an interrupt is received by the microprocessor 8, step 34 services communications devices connected to the microprocessor 8 which is followed by the commencing of two counters, one which effectively causes a division by 3 and another which causes a division by 120. At step 38, the division by 3 counter is checked for being zero. When the division by 3 counter equals zero, which occurs 400 times per second, step 40 services the scanning of the LED display indicators 12 and of the push button switches 13. Next, at step 42, the divide by 120 counter is checked. When equal to zero, which occurs 10 times per second, the 100 millisecond flag is set and the counters are set at step 44. A software timer value is also incremented. The interrupt is then returned at step 46.

Returning to Fig. 4A, after establishing that the 100 millisecond flag is set at step 32, the main program implements step 48 which samples the sensor 3 and also the voltage of the battery 11. At step 50, the sampled sensor and battery voltages are stored. Preferably, the last ten sampled results obtained from the sensor 3 are retained which are used in step 52 for the determination of two filter values. In particular, from the stored samples, a 2 Hz low-pass filtered value and an accumulated average value are determined using traditional signal processing techniques.

At step 54, the 2 Hz low-pass filtered result is compared with the accumulated average value to determine if the abdomen of the infant 2 is expanding. In particular, the 2 Hz low-pass filtered value will vary at a rate faster than the accumulated average value (ie: 500 msec as compared with 1000 msec), and thus is indicative of a substantially instantaneous breathing state. The accumulate average is indicative of a longer term average breathing condition. Accordingly, if the 2 Hz low-pass filtered value exceeds the accumulated average, such will be indicative of the abdomen expanding. If lower, such will be indicative of the abdomen contracting. In this fashion, the system 1 can discriminate between patient inspiration and exhalation.

If the 2 Hz low-pass filtered value is greater than the accumulated average, the software timer, started at step 44 in the interrupt service routine, is reset at step 58. If the low-pass filtered value is not greater than the accumulated average, the value of the timer is checked at step 56. If the timer value equals 20 seconds, an apnoea period status bit is set at step 60. This indicates that, for the last 20 seconds, the abdomen of the patient has not been detected as having expanded, and such is deemed to be indicative of an apnoea occurring.

Steps 58 and 60 are each followed by step 62 which updates the status registers 72 and resets the 100 millisecond flag.

At step 64, the status of the apnoea period bit is checked and if found to be valid, at step 66 the alarm 14 is sounded indicative of the patient not having breathed for the last 20 seconds. If the apnoea period bit is not set, a non-apnoea period is deemed to have been identified and the program returns to await 100 millisecond flag at step 32.

It will be apparent from the foregoing that a comparatively simple microprocessor-based arrangement can be provided that monitors the respiratory effort of a patient and which can generate an alarm where the respiratory cycle is interrupted. Significantly, simplicity is afforded through physically detecting respiratory effort, as compared with monitoring respiratory variables such as patient air flow and/or pressure, as occurs with ventilator equipment which are comparatively invasive of the patient's respiratory system. Further, by being able to be implemented with low cost microprocessor-based apparatus, the system 1 lends itself to domestic application where simplicity and ease of use are fundamental to market acceptance.

The foregoing describes only one embodiment of the present invention and modifications, obvious to those skilled in the art can be made thereto without departing from the scope of the present invention. For example, the time periods and filter values indicated in Figs. 4A and 4B are only indicative of a specific embodiment to applications involving human infants and depending upon the patient and/or hardware being used, alternative periods and values can be used. Further, the 20 second period allowed for detecting an apnoea can be adjusted through reprogramming to suit different circumstances. A further configuration is to provide an adjustable input to the microprocessor to adjust the time period for apnoea detection. Typically, such an input may be set by a physician or other persons skilled with respiratory regimens. Such may be appropriate where the present invention is used human adults or animals.

Further, whilst the preferred embodiment for infant use describes emplacement of the sensor 3 about the abdomen, the sensor 3 may also be placed about the chest or any part of the torso susceptible to reliable repetitive movement caused by respiration.

In an alternative configuration, the sensor 3 may be formed using a device other than the piezo-electric transducer 21. For example, a semi-conductor strain gauge device may be substituted therefor. In such instance, whereas the piezo transducer 21 responds to the compressive forces associated with respiration, the strain gauge can respond to tension forces, for example those being applied to the strap 4.

Claims

CLAIMS:

1. An apnoea sensing apparatus including a movement sensitive transducer arranged to be coupled to a respiring being via fastening means, said transducer being responsive to respiratory effort of the being to output a signal interpretable of a respiratory state of the being.

2. Apparatus according to claim 1 wherein said transducer includes a semiconductor strain gauge device.

3. Apparatus according to claim 1 , wherein the transducer includes a piezoelectric transducer.

4. Apparatus according to claim 2 or 3 wherein said transducer is associated with a support arrangement configured for coupling of said transducer to said fastening means whereby respiratory movement of the being causes said fastening means to apply a variable force to said transducer which outputs a signal related to the force.

5. Apparatus according to claim 4 when dependent on claim 3 further comprising a (first) member configured to transmit compressive forces associated with said fastening means and the respiratory effort of the being to said transducer.

6. Apparatus according to claim 5 wherein said transducer is arranged between said first member and said support arrangement.

7. Apparatus according to any one of claims 1 to 6 wherein the fastening means includes a strap wherein on inhalation by the being, a tension on the strap increases thereby increasing the force applied to the transducer.

8. Apparatus according to claim 7 when dependent on claim 6, wherein said support arrangement comprises a base plate having two apertures through which the strap is threaded and which is arranged to contact the being, said first member being positioned between the threaded strap and the transducer.

9. An apnoea monitoring system including: a movement sensitive transducer arranged to be coupled to a respiring being, said transducer being responsive to respiratory effort of the being to output a signal interpretable of a respiratory state of the being; means for interpreting said signal and comparing the signal against an anticipated respiratory response of the being whereby when the anticipated respiratory response is not detected, an alarm indicative of an abnormal respiratory state of the being is output.

10. A system according to claim 9 wherein said transducer is associated with a support arrangement associated with fastening means for coupling said transducer to the being, whereby respiratory movement of the being causes said fastening means to apply a variable force to said transducer which outputs said signal related to the force.

11. A system according to claim 10, wherein said transducer includes a semiconductor strain gauge device.

12. A system according to claim 10, wherein the transducer includes a piezo- electric transducer.

13. A system according to claim 12, further comprising a (first) member configured to transmit compressive forces associated with said fastening means and the respiratory effort of the being to said transducer.

5 14. A system according to claim 13, wherein said transducer is arranged between said first member and said support arrangement.

15. A system according to any one of claims 10 to 14 wherein the fastening means includes a strap wherein on inhalation by the being, a tension on the strap increases ╬╣ o thereby increasing the force applied to the transducer.

16. A system according to claim 15 when dependent on claim 14, wherein said support arrangement comprises a base plate having two apertures through which the strap is threaded and which is arranged to contact the being, said first member being

15 positioned between the threaded strap and the transducer.

17. A system according to any one of claim 9 to 16 wherein said means for interpreting includes: state means for determining a breathing state of said being; 20 timer means for tracking an anticipated respiratory response of said being;

(first) comparison means for comparing said breathing state with said anticipated respiratory response; and alarm means for producing an alarm signal when a result of said (first) comparison is indicative of a cessation of breathing.

25

18. A system according to claim 17 wherein said state means includes: first means for determining from said signal a (first) indication of a substantially instantaneous breathing condition; second means for determining a (second) indication of a longer term breathing condition relative to the substantially instantaneous breathing condition; and

(second) comparison means for comparing the first indication with the second indication such that if said first indication exceeds said second indication, an inspiratory state of said being is detected.

19. A system according claim 18 wherein said means for interpreting comprises a processor arrangement configured to: sample said signal and to determine a low-pass filtered value of said signal and an accumulated average value of said signal and therefrom determine said breathing state; maintain a timer of a predetermined period indicative of a cessation of breathing; compare a value of said time with said breathing state to identify a cessation of breathing.

20. A system according to claim 19 further comprising means for coupling said processor to a computer arrangement for recording over time the breathing pattern of said being.

21. An apnoea monitoring system including a transducer associated with the strap configured to be bound about an abdomen of a human, said transducer being responsive to the respiratory movement of the abdomen to output a signal related to the movement, and means for interpreting the signal and comparing the signal against an anticipated respiratory response of the human whereby when the anticipated respiratory response is not detected, an alarm indicative of an abnormal respiratory state of the human is output.

22. A method of detecting an apnoeic event said method including the steps of: physically detecting the respiratory effort of a respiring being; determining a substantially instantaneous respiratory state of said being; monitoring a time related to a change in the respiratory state; and identifying when no change to the respiratory state has occurred over a predetermined period of time, such identification being indicative of an apnoeic event.

23. A method for detecting a respiratory state of a respiring being, said method comprising the steps of: physically detecting the respiratory effort of said being by continually monitoring expansion and contraction of a torso of said being; from said monitoring, determining a substantially instantaneous value of inflation of said torso; from said monitoring, determining an average value of inflation of said torso; and comparing the instantaneous value with said average value whereby when said instantaneous value exceeds said average value, said being is inhaling, and when said average value exceeds said instantaneous value, said being is exhaling.

24. A method according to claim 23 wherein said physically detecting is performed using a transducer associated with said torso.

25. Apparatus configured to perform the method of claim 23 or 24.

26. An apnoea monitoring systems substantially as described herein with reference to the drawings.

27. A method of detecting an apnoeic event substantially as described herein with reference to the drawings.