Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B7/00—Instruments for auscultation
  • A61B7/02—Stethoscopes
  • A61B7/04—Electric stethoscopes
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
  • A61B5/024—Detecting, measuring or recording pulse rate or heart rate
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/08—Detecting, measuring or recording devices for evaluating the respiratory organs
  • A61B5/0816—Measuring devices for examining respiratory frequency

Definitions

• the present invention relates to a cardio-respiratory monitor and apparatus for monitoring cardio- respiratory action.
• apnoea in humans, particularly infants, and its detection has become a matter of some importance. Particularly in the case of infants, detection of apnoea may be used to prevent or reduce the occurrence of Sudden Inf nt Death Syndrome (commonly called "Cot Death").
• monitors have been designed for detecting apnoea but they have hitherto relied upon detection of respiratory movements or movements of the body. These have frequently been in the form of pressure detectors which have either been placed under the body to detect movement of the body or attached to the chest to detect movement of the chest, the variation of pressure within a pressure capsule being used to provide an indication signal of respiratory function.
• the present invention provides a method and apparatus for detecting not only respiratory function but also detecting cardiac function-
• the present invention provides, according to one aspect, a cardio-respiratory monitor apparatus comprising a sound detector for detecting sound from a body, means for passing a signal representing the
• OMPI sound detected to an analyser apparatus sai d analyser apparatus including means to distinguish the signal relati ng to the respiratory function and the signal relating to the cardiac function, and means to moni tor thes e separate si gnal s . It has been determined by experiment that, in general terms, both cardiac and respiratory function produce sounds which are of different frequencies which are separate from one another . In practi ce the cardi ac f unction produces sounds of a frequency of approximately 30 to 180 Hz and the respiratory function provides sounds of a general frequency range of 200 to 950 Hz.
• the monitor apparatus therefore includes means to separate these frequencies and to monitor them separately.
• the sound detector may compri se a mi crophone including means to attach the microphone to the chest wall.
• the means to attach the microphone to the chest wall may include means for providing a space between the microphone and the chest wall and in a pref erred arrangement thi s may be in the form adhesive *0" ings .
• the monitor means for monitoring the two sound related signals may include a phase locking means in which, for example, in the case of the cardiac monitor, will analyse the rate of heart beat and will note any missing heart beats.
• the present invention provides a method of monitoring life function comprising detecting sound from a body, analysing a signal derived from the sound detected to provide two output signals, one of which relates to the sound signal relating to respiratory function and the other of which relates to the sound signal relating to the cardiac function, and monitoring these separate signals.
• the present invention also provides a method of determining life function comprising detecting sound from the chest of a human body, determining cardiac function from a first frequency range of detected sound and determining respiratory function from a second frequency range of detected sound.
• FIG. 1 is a diagrammatic perspective view of a cardio-respiratory monitor apparatus .according to the invention.
• Figure 2 is a block diagram showing the process steps in a first stage of the analysis of the respiratory signal.
• Figure 3 is a block diagram showing the process steps in a second stage of the analysis of the cardiac signal,
• Figures 4A to 4E show signal waveforms in the process steps of the analysis of the respiratory signal, r
• Figure 5 is a block diagram of the process steps in a third stage of the analysis of the respiratory signal
• Figure 6 is a block diagram of the process steps in a fourth stage of the analysis of the respiratory signal
• Figure 7 is a block diagram showing the process steps in a first st ge of the analysis of the cardiac signal
• Figure .8 is a block ⁇ diagram showing the process steps in a second stage of the analysis of the cardiac signal.
• the monitor apparatus may be used with respect to a human subject 11 which may be an unsedated infant sleeping in a sound proofed cot in a quiet room.
• the sounds transmitted through the chest wall 10 of the human subject 11 as a result of breathing and cardiac action respectively are detected - by a single sub-miniature sensor 12 consisting of a ' combined condensor microphone and
• OMPI vibration detector applied to the chest wall 10.
• a sub-miniature electret condenser microphone housed in polytetrafluorethylene shells and applied to the chest wall 10 by adhesive "0" rings.
• the sensitivity of the microphone is 10 mV/Pa and the frequency response is uniform between 40 Hz and 3 kHz. In some instances two microphones may be used in which case the gains of each microphone/amplifier combination should be equalised before measurement.
• an absolute sound level calibration of 94 dB re 20 uPa at 1 kHz is also performed.
• the electrical signal from the sensor 12 is passed by cable 13 to a monitor unit 14 containing electronic circuits, control and display panels.
• a monitor unit 14 containing electronic circuits, control and display panels.
• the electrical output from the sensor 12 is amplified by amplifier 16 and divided into each of two high quality precision electronic filters 17, 18.
• the .first filter 17 is set to pass waveforms with frequencies common to the cardiac sounds, that is between .20 and 200 Hz with an attenuation rate at -each .limit of 24 dB/octave.
• the second filter 18 is set to pass .waveforms with frequencies common to the respiratory sounds; thus filter 18 may be a highpass filter with a lower cut off frequency of 180 Hz and an attenuation rate of 48 dB/octave " (Barr & Stroud Variable Filter EF3 UK).
• the output of each filter unit 17, 18 is phase locked and may be finely tuned externally by control 48.
• the filtered sounds may be recorded on a seven channel FM tape recorder at 38 cm/sec. During recording sound quality is monitored through an audio amplifier and headphones.
• the cardiac and respiratory signals on lines 21, 22 are respectively rectified in rectifiers 23, 24 and passed to respective means 26, 27 which pass the signals through preset amplitude and time windows which are used to generate pulses at the start of every first heart sound and towards the end of every inspiratory phase of respiration (ie towards the end of every inward breath).
• the pulses from window means 26 which signal the occurrence of the first heart sound are: 1) counted electronically by counter 28 and displayed on a front panel light-emitting diode display 29, and 2) used to activate a hear rate trend counter 31 with an external output to a visual dipslay unit 32 or plotter 33.
• the pulses from window means 27 which signal the occurrence of respiration are 1) counted electronically and displayed on a front panel light- emitting diode display 37, and 2) used to activate a respiratory rate trend counter 38 vi h an external output to a visual display unit 39 or plotter 40.
• OMPI pass to their respective but interconnecting logic alarm circuitry 41.
• an alarm 45 is activated by the occurrence of a predetermined delay in either the cardiac and/or respiratory signals. 5
• the exact combination of alarm modes can be specified on the front panel controls 42.
• the monitor is intended for medical use principally in
• background noise may also be eliminated by signal averaging techniques during frequency analysis or by subtraction of the simultaneously recorded background noise frequency spectrum.
• a specific microprocessor based extension unit 54 may be attached to an appropriate output socket 53 of -the monitor unit 14. With this addition, it is
• VDU 32,39 and 51 for each separate function and a separate plotter 33, 40, 52 for each separate function it will be understood that these may be combined as a single unit if desired.
• the single VDU then being switchable between modes in which it will show all of the signals simultaneously, or each of the signals separately and similarly, the plotter can be switched between modes in which it will plot all of the signals simultaneously (there being provided sufficient pens for the number of input signals) or a single signal at a time.
• Both inspiratory and expiratory sounds are transients consisting of random noise over a bandwidth 200 to 900 Hz.
• the resonant frequency is above 400 Hz.
• Expiration is less intense a sound than inspiration and may not be detected at all in some breaths.
• mean peak sound pressure level of the inspiratory sound is around 65 dB re 20 _ ⁇ Pa.
• the inspiratory sound normally lasts 300 to 400 msecs and the expiratory sound is shorter. There is a variable break in the sound between the two phases of respiration.- Sound intensity is related
• OMPI exponentially to air flow rate but these are related linearly over the usual operating range. Breaths whose sound level falls below the threshold of detection are usually associated with inadequate ventilation. However, slow deep breathing in quiet (non-REM) sleep may pose a problem to detection in some circumstances even though alveolar ventilation is satisfactory. Instantaneous (breath-to-breath) respiratory rates are highly variable (eg 20 to 120/min). The actual respiratory rate over one minute does not normally exceed 80.
• a single heart beat generates two sounds related to sequential cardiac valve vibrations. These are discrete, non random transients and (unlike the breathing signal) are relatively uniform from beat to beat.
• the resonant frequency is below 100 Hz.
• the "mean peak sound pressure level is 30 to 40 dB greater than that of the respiratory sounds.
• the first heart sound lasts around 70 msec and is usually longer and more intense than the second sound.
• the interval between heart sounds corresponds to the interval between the Q and T waves of an electrocardiogram ie approximately 150 to 200 msecs.
• the upper limit of hear rate is 180/min.
• the microphone 12 is a Knowles CA series insert which uses an electret film and contains an integral FET
• the microphone 12 is centrally in contact with an air space of ⁇ 5mm , sealed by the skin surface of the chest 10. Peripherally, it is in contact with a 1 cm diameter PTFE plate 1mm thick. This plate forms the contact surface of the microphone housing which consists of a polytetrafluorethylene shell containing alternate layers of expoxy resin and either dense latex foam rubber or soft polymetric elastomer.
• the microphone specification is:
• Sensitivity 10 mV/Pa (Breathing signal 0.5 mV approx.)
• the output from the microphone 12 is preamplified to give a signal of 1-4 volts peak to peak. This may increase to 10 V+ when the human subject 11 moves.
• the raw sound signal has to be handled electronically i n a s er i es of i ndi vi dual s teps .
• signal distortion and phase shift are not critical provided that sensitivity and periodicity are maintained. System delays of up to 250 secs are acceptable.
• the signal from the microphonel2 is processed in the manner shown in Figure 2.
• the signal from the microphone 12 is pre-amplified, passed to an LSM highpass filter having a cutt-off frequency of 350 Hz, the signal is then passed to an AC amplifier to amplify it by 50 times, the amplifier signal is passed to an LSM lowpass filter having a cut-off frequency of 650 Hz., and the output from * t ' he lowpass filter is again amplified in an AC amplifier by 10 times.
• the filter 18 therefore comprises both the LSM highpass filter, the LSM .lowpass filter, and the AC amplifier.
• the steps in this part of the process are illustrated in Figure 3.
• the signal from Figure 2 is rectified in rectifier 24 and integrated with a predetermined sampling rate of 6 Hz. '
• the integrator holds the peak value of each 166 msec sample.
• the energy present in each sample during a breath is adequate to provide a satisfactory sound envelope without a significant response to intervening attenuated heart sounds or background noise.
• the signal falls to the value of the noise floor (ie the noise level) between breaths.
• the output is passed from the integrator through a non-linear- gain amplifier.
• An amplitude li mit is set so that a si gnal i s detected but noise is not.
• a retr.iggerable monostable is used for this process such that when the amplitude of the sound envelope exceeds the preset threshold, a pulse is generated.
• Figure 4A shows a signal corresponding to the breath sound
• Figure 4B shows the integrated envelope of that signal
• Figure 4C the antilogged integral of the signal of Figure 4B. If the voltage limit is set to 10% above the noise floor then the signal of Figure 4D is produced and if this is then passed to the retriggerable monostable the pulse output of Figure 4E is produced.
• each pulse represents the beginning of a breath and this point is now used as a trigger.
• the pulse train needs to be "smoothed" still further to prevent inappropriate triggering.
• breaths will, if completely regular, occur no more frequently than once every 750 msecs. So a further constraint can be imposed on the pulse train - that pulses should occur no more frequently than one every 750 msecs (0.75 Hz).
• babies do not breathe regularly and very fast periods may alternate with much slower ones.
• the output from this stage is passed to the logic alarm circuit 41 and also to counter 36.
• LSM lowpass filter is passed to a DC amplifier where it is amplified by 10 times.
• the output signal from this amplifier is passed to an LSM highpass filter having a cut off frequency of 30 Hz and the output signal therefrom is amplified by an AC amplifier by 10 times.
• the amplified output is passed to a clipper .
• the filter 17 of diagrammatic Figure 1 comprises the LSM lowpass filter, DC amplifer and LSM highpass filter and AC amplifier.
• the filtered output .from the previous step is passed to an amplitude clipper module.
• This module effectively extracts the noise band between selectable amplitude limits and rejects it.
• the positive and negative parts of the . signal are then "joined up” again and amplified to a constant level.
• the sound envelope is generated by passing the signal from the previous stage to an integrated circuit in which the RMS (root mean square) value of the signal is calculated and expressed as a DC value.
• the time constant of the system is 120 to 250 msecs. This
• OMP process converts the pair of spike signals from the previous step into an "M" shaped envelope in which the first peak corresponds to the first sound and the next peak to the second heart sound.
• Thi s helps to smooth the heart signal and improves the signal stabi l i ty .
• Thi s s tep can be om i tted but f al s e triggering is more likely.
• the cardiac sound envelope activates a non-retriggerable monostable when the upstroke crosses a preset amplitude limit, set 20% below the peak amplitude level of the envelope. Further smoothing is achieved by specifying the period for which the pulse generator cannot be retriggered using a second monostable module.
• the refractory period selected is 300 msecs, accommodating heart rates approaching an upper limit of 180/min.
• this step can be performed with two variable non-retriggerable monostables, the first of which delivers a pulse of 300 msecs duration triggered on the upstroke of the sound envelope and the second monostable delivers a TTL pulse on the upstroke of the first heart pulse.
• This audio movement signal is normally in the range of 1 to 4 volts RMS after amplification. Signal levels greater than 5 volts RMS indicate additional noise. Signals greater than 7 volts RMS indicate movement or vocalisation.
• the raw sound signal is monitored by an RMS voltage window comparator with a time constant of 2 sees.
• the output from the comparator goes high for signals > 5 V RMS and low for signals ⁇ 1 V RMS.
• the outputs from the comparator are available as DC levels and also operate appropriately coloured lights automati cally .
• the above process has therefore now produced a TTL pulse for each breath and heart beat and also a high DC level which indicates movement.
• the apparatus includes digital circuitry to analyse these rates of pulses and the trend and to cause an alarm to operate if the rates and trends are outside predetermined limits which may be adjustable for age, weight and other factors.
• OMPI may be produced for home use in which expensive components such as the VDU plotter may be deleted, the apparatus si mply produci ng an alarm if predetermined parameters r egardi ng cardi ac and respiratory rate are exceeded. In such a case , the apparatus can be produced more cheaply as the electronic components can be produced in integrated circuit form.

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• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Physics & Mathematics (AREA)
• Biomedical Technology (AREA)
• Medical Informatics (AREA)
• Acoustics & Sound (AREA)
• Veterinary Medicine (AREA)
• Engineering & Computer Science (AREA)
• Cardiology (AREA)
• Heart & Thoracic Surgery (AREA)
• Public Health (AREA)
• Molecular Biology (AREA)
• Surgery (AREA)
• Animal Behavior & Ethology (AREA)
• General Health & Medical Sciences (AREA)
• Physiology (AREA)
• Pathology (AREA)
• Biophysics (AREA)
• Measurement Of The Respiration, Hearing Ability, Form, And Blood Characteristics Of Living Organisms (AREA)

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• 1983
  • 1983-10-31 ZA ZA838100A patent/ZA838100B/xx unknown
  • 1983-10-31 WO PCT/GB1983/000276 patent/WO1984001705A1/en unknown
  • 1983-10-31 EP EP83903553A patent/EP0124568A1/de not_active Withdrawn
  • 1983-10-31 GB GB08329057A patent/GB2129991A/en not_active Withdrawn

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