Movement monitor for cardio-pulmonary and other activity

Abstract: A method and apparatus for monitoring means and for detecting motion in the vicinity of the apparatus, particularly a non-invasive cardio-­ pulmonary activity, is disclosed comprising an electrostatic charge detector (10) for providing an output signal including signals indicative of motion, e.g. cardio-pulmonary activity of a person in the vicinity of the detector. A detection apparatus (Figure 2) is connected to receive the signals and to filter the extraneous noise signals from the signals of interest to provide output signals indicative only of the motion of interest of cardio-pulmonary activity of the person. Breathing signals are applied to a comparator (66) which is tripped if breathing occurs at a rate below a predetermined value. Cardiac signals are applied to a circuit which adjusts the trip level for the breathing rate if the cardiac rate declines to a level suggestive of brady-­ cardia without breathing rate increases. The filtering circuit for the signal supplied by the charge detector incorporates a dual integrator (24) operating as a frequency selective charged amplifier which is effective to substan­ tially eliminate all signals outside of the range of about 0.1 to 15 Hz.

Description: The present invention relates to an apparatus for monitoring cardio-pulmonary activity of a person in the vicinity of the apparatus, and is particularly, but not exclusively concerned with an apparatus for detecting episodes of apnea in infants. Research into the problem of cot deaths or sudden infant death syndrome (SIDS) in which infants have died during sleep have generally indicated that a significant percentage of infants up to the age of about one year may have problems in the central nervous system reflex such that, during sleep, respiration may cease, a condition referred to as apnea. In most instances, the infant will begin breathing again spontaneously. However, if the duration of apnea is excessive, irreversible cerebral damage may be sustained and, in some cases, death occurs. Clinical procedures have been devised for determining which infants are susceptible to onsets of apnea but are relatively inaccurate. When an infant is diagnosed as being susceptible to apnea, it is necessary that some method be provided for monitoring the cardio-pulmonary activity of the infant quickly and in such a manner that assistance may be provided to the infant to restart breathing. Various types of monitoring devices have been developed for detecting apnea. Notable among these is that disclosed in US-A-4,474,185. That patent discloses a motion monitor in which very small variations in dielectric constant are measured and used with appropriate timing circuits to provide an alarm when cessation of motion is noted for a predetermined period. Such motion is generally caused by breathing of the patient being monitored, such breathing being associated with motion of the chest and diaphragm. The monitor disclosed therein includes a pad which may be placed over a cot mattress or the like and upon which the infant or other patient may lie. A related type of arrangement is disclosed in US-A-4,438,771. The apparatus of the latter patent is of the type comprising an electrostatic charge collector having an electrically insulated charge collection layer; amplifying means for producing an amplified signal indicative of variation in electrostatic charge on the charge collection layer at frequencies of normal cardio-pulmonary activity of a person in the vicinity of the charge collection layer; first detecting means for detecting a first characteristic of the amplified signal to produce a signal representing pulmonary activity of the person; indicator means for providing an indication of each pulmonary activity of the person; and alarm means responsive to the pulmonary activity signal for providing an alarm indication when the pulmonary activity is less than a threshold amount. Specifically, in the apparatus of US-A-4,438,771, the detecting means comprises an edge-triggered retriggerable one-shot multi-vibrator, and the alarm means comprises an alarm buzzer. Accordingly, if the amplified signal includes a pulse caused, for example, by pulmonary activity every 20 seconds, for example, the buzzer will not be sounded. However, if more than 20 seconds elapses between the edge of one pulse sufficient to retrigger the multi-vibrator and the corresponding edge of any such succeeding pulse, the buzzer will be sounded. In the apparatus of US-A-4,438,771, there is a single channel signal which is amplified and filtered to pass a signal representing heart beat, breathing movements or both (depending upon the chosen filter components), anti this single signal is detected. The apparatus of the present invention is characterised by: second detecting means for detecting a second characteristic of the amplified signal to produce a signal representing cardiac activity of the person; the alarm means also being responsive to the cardiac activity signal to vary the threshold amount in dependence thereon. If the heart-rate of the person being monitored decreases, the person's breathing may become more rapid since the person may be suffering from hypoxia. However, with these features of the invention, the threshold level of pulmonary activity which trips the alarm can be varied in dependence upon the level of cardiac activity. An embodiment of the invention has the following features each of which is a preferred feature of the invention: a) the second detecting means includes a band pass filler such that the second characteristic represents a component of the amplified signal in a frequency range covering the frequencies of normal cardiac activity of a person. This filter may be a band pass filter having a centre frequency of about 6.8 Hz; b) the second detector means includes means for integrating the second characteristic; c) the first detecting means includes a first low-pass filter such that the first characteristic represents a component of the amplified signal in a frequency range covering the frequencies of normal pulmonary activity of a person; d) the amplifying means has a feedback circuit with a relatively high gain at frequencies below a predetermined value so that the frequency response of the amplifying means falls off rapidly below said predetermined value; e) the charge amplifier and feedback circuit in combination have a pass band from 0.1 to 15 Hz; f) the feedback circuit includes means to increase the large signal gain of the feedback circuit to reduce the large signal gain of the amplifying means; g) the first detecting means comprises means for integrating the first characteristic; h) the electrostatic charge collector includes an electrostatic shield substantially co-extensive with the charge collection layer and spaced therefrom by an insulative layer, the shield being connected to earth ground by a high frequency bypass filter; and i) the apparatus is selectably operable in a first mode in which, as aforesaid, the alarm means is responsive to the cardiac activity signal to vary the threshold amount in dependence thereon, and in a second mode in which the alarm means is not responsive to the cardiac activity signal and in which the threshold amount is fixed. A specific embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of that portion of the present invention for extracting cardio-pulmonary activity signals from a motion monitor; and Figure 2 is a schematic diagram of that portion of the present invention which responds to the signals developed by the circuit of Figure 1 and also adjusts the system alarm level as a function of cardiac rate. The monitoring apparatus and method of the present invention is applicable to many uses for which slight movements of a person are to be detected. However, the invention will be described and explained for use as an apnea monitor and in particular for monitoring the breathing and heart rate, i.e. cardio-pulmonary activity, of an infant while sleeping. Referring now to Figure 1, a motion monitor charge transducer is shown generally at 10 and is preferably constructed in accordance with the teachings of US-A-4,474,185. The transducer 10 contains an upper conductive electrode or layer 12 and a lower conductive electrostatic shield or layer 14. An insulative layer 16 separates the upper and lower conductive layers. The lower conductive layer is connected to an earth ground through a ferrite bead 18 and a high impedance resistor 20. A capacitor 21 is connected in parallel with resistor 20. The ferrite bead provides a high frequency filtering while the resistor 20 provides a discharge path for accumulated charge on the layer 14. Capacitor 21 decouples RF signals to ground. The active electrode or electrical layer 12 is connected through a second ferrite bead 22 to an input terminal of a frequency selective charge amplifier means or dual integrator 24. A capacitor 23 decouples high frequency noise in conjunction with bead 22. The amplifier means 24 includes an operational amplifier 26 having an inverting input terminal to which the signal from layer 12 is connected through high impedance input resistors 28 and 30. A capacitor 32 connected between a junction intermediate resistors 28 and 30 and ground operates in conjunction with the resistors 28 and 30 to provide a high frequency filter. The amplifier 26 includes an AC feedback loop connected between an output terminal and the inverting input terminal. The AC feedback loop includes first and second serially connected capacitors 34 and 36 with a resistor 38 connecting the junction intermediate the two capacitors to ground. A ten picofarad capacitor 40 connected in parallel with the series connection of capacitors 34 and 36 prevents high frequency oscillation and is sometimes referred to as a Miller capacitor. It will be appreciated that the circuit as thus far described provides no DC feedback and will thus tend to drift to either the positive or negative rails over time. In order to prevent such drift and to provide for a very fast low frequency roll off, there is included an active feedback network utilizing a second amplifier 42 connected as an operational amplifier for providing DC feedback. An output terminal of the amplifier 26 is connected through a resistor 44 and resistor 46 to an inverting input terminal of amplifier 42. The resistor 44 is shunted by two reverse parallel connected diodes 48 and 50 which provide a quasilogarithmic amplifier response to reduce large signal saturation of the amplifier 42. The amplifier 42 is also connected as an integrator with a relatively long time constant and as such includes a capacitor 52 connected between its output terminal and inverting input terminal. A resistor 54 connected in series with capacitor 52 sets the AC gain of the amplifier. The non-inverting input terminal of amplifier 42 is connected to earth ground through the parallel combination of a resistor 56 and capacitor 58. An output terminal of amplifier 42 is connected to the non-inverting input terminal of amplifier 26 through a coupling resistor 60. An additional capacitor 62 is connected between the non-inverting terminal of amplifier 26 and ground to provide suppression of thermal noise generated by the large resistance value resistors. The output signal generated by amplifier 42 is a quasi-analog feedback signal. The output of the charge amplifier 24 of Figure 1 at terminal T is primarily the amplified changes in the charge potential on the transducer 10 created by breathing of the person with which the transducer 10 is associated. However, also riding on that signal is a signal of somewhat higher frequency which represents the cardiac rate of the person being monitored. In actual testing, the circuit of Figure 1 has been found to be extremely practical in stripping out the extraneous noise signals created by 60 Hz power lines and radio station signals and other electronic noise generally available in the atmosphere. In constructing the circuit of Figure 1, the following values were used, where R represents resistor and C indicates capacitor: Ohms R₂₈ 10M R₃₀ 1M R₃₈ 1M R₄₄ 10M R₄₆ 4.7M R₅₄ 1K R₅₆ 10M R₆₀ 10M R₂₀ 10M C₂₁ 0.001uf C₂₃ 0.001uf C₃₂ 0.01uf C₃₄ 0.001uf C₃₆ 0.001uf C₄₀ 10pf C₅₂ 10uf C₅₈ 0.01uf C₆₂ 0.01uf D₄₈ 1N914 D₅₀ 1N914 The arrangement described in Figure 1 thus provides amplification of the breathing and cardiac signals developed by a person associated with the transducer 10 and eliminates all of the extraneous signals in which those breathing and cardiac signals are esentially buried. The overall circuit has essentially zero gain for DC and yet has exceedingly high gain between 0.1 Hz and 1.5 Hz (the fifty per cent attenuation points). The roll off rate at the upper frequency is not as quick as the lower end but does drop off at a rate of about 12 db per octave above about 8 Hz. In actual testing, the 60 Hz and 50 Hz power signals are inconsequential at the output developed at terminal T. The ferrite beads 18 and 22 in combination with capacitors 21 and 23 essentially eliminate most of the RF and AM radio station interference. Below 0.01 Hz, the circuit has approximately 100 second time constant so that the DC gain rapidly drops to zero below 0.01 Hz. An important feature of the circuit in Figure 1 is that it is not necessary to decouple the output of the circuit before applying it to a comparator because the circuit automatically zeros itself and long term offset drift of the amplifier is compensated. In prior art circuits of this nature, the filtering network is such that capacitive coupling is required which then requires fairly sophisticated circuitry for low frequency coupling. Thus, the circuit described above provides the unique features and advantages for detecting the relatively low frequency breathing and cardiac rates of an individual and for excluding extraneous noise signals. Referring now to Figure 2, there is shown a comparator circuit for providing an output signal upon occurrence of each breathing cycle of a patient and an alarm circuit for providing an indication of abnormal breathing. There is also illustrated a circuit for varying the alarm signal level as a function of cardiac activity. The terminal T' corresponds to the terminal T in Figure 1. The signal developed by amplifier 24 is coupled through a resistor 64 to an inverting input terminal of a comparator 66. The comparator 66 includes a high frequency shunt capacitor 68 to further filter any high frequency signals on the breathing signal. A bleeder resistor 70 provides a discharge patch for capacitor 68. The non-inverting input terminal comparator 66 is connected to a voltage divider circuit comprised of first and second resistors 72 and 74 serially connected between a voltage reference V cc and earth ground. The values of resistors 72 and 74 are selected so as to allow the comparator to trip upon receipt of a signal indicative of motion or breathing of a patient being monitored. The output signal developed by the comparator 66 is normally at a positive voltage level but switches to a negative voltage level when the input at the inverting input terminal goes positive, that is, when the patient being monitored either breathes or moves. The output terminal of comparator 66 is connected through a coupling resistor 76 to an LED indicating circuit comprising first and second transistors 78 and 80. An LED 82 is serially connected between an emitter terminal of transistor 78 and an emitter terminal of transisitor 80. A collector terminal of transisitor 78 is connected to the positive voltage source V cc and the collector terminal of transistor 80 is connected to a negative of V ss . A capacitor 84 is connected between the voltage V ss and one terminal of the LED 82. When the output signal of comparator 66 is a positive level, the transistor 78 is gated into conduction and current through the transistor charges the capacitor 84. When the signal from comparator 66 goes to a negative level, the transistor 78 is gated off and the transistor 80 is gated into conduction. During that time the charge accumulated on capacitor 84 is discharged through the LED 82 and transistor 80 causing the LED 82 to emit light. Thus, the circuit will provide an indication corresponding to each event associated with moving or breathing of the patient being monitored. The signals developed by the comparator 66 are also coupled to an alarm circuit for energizing a pulsating type of alarm such as the Sonalert (trademark of Mallory) alarm which is a piezoelectric type annunciator. The signal from comparator 66 is coupled through a resistor 86 to an inverting input terminal of a second comparator 88. The non-inverting input terminal of comparator 88 is connected to a reference level established in the block 96 or in the block 90 by first and second resistors 92 and 94 serially connected between the voltage source V cc and earth ground. It will be apparent that the comparator 88 may be operated from the fixed reference source 90 or may be operated from the variable reference source established in the block 96. Either one or the other of the blocks 90 or 96 may be selected but both would not be used simultaneously. The block 90 is used when the comparison is to be a fixed level and not corrected as a function of cardiac rate. For those instances in which it is desirable to adjust the comparison level as a function of cardiac rate, the circuitry shown in the block 96 is used. Thus, the comparator 88 is set to detect an apnea interval and, as such, the pulses developed by the comparator 66 are accumulated in an integrator comprising the resistor 86 and a capacitor 98. The comparator 88 in conjunction with resistor 86 and capacitor 98 essentially forms an interval timer. The time constant for charging the capacitor 98 is set so that the charge on the capacitor 98 will trip the comparator 88 if breathing events do not occur within a predetermined interval such as, for example 20 seconds. When the comparator 88 is tripped, i.e. when the voltage on capactor 98 becomes more positive than the voltage on the non-inverting input terminal, the output of comparator 88 will slew to a negative voltage level allowing current to flow through the alarm 100 and a serially connected light emitting diode 102. The LED 102 will glow thereby providing a visual indication of an alarm condition and the alarm 100 will provide an audible indication of an alarm condition. When the output of comparator 88 is negative, a diode 104 connected between the reference voltage source at the non-inverting terminal comparator 88 and its output terminal will be forward biased and will act as a latch to prevent the alarm from being turned off until a manual reset has been performed by deenergising the unit. This action provides a safety circuit to prevent a reset until someone has been notified of the apnea episode. As was described above, the positive voltage developed by comparator 66 charges the capacitor 98 and when the charge reaches a predetermined value causes the comparator 88 to change states. When a breathing event occurs, the output of comparator 66 goes to a negative value. At this time, the charge on capacitor 98 is removed or discharged. The discharge occurs through resistors 106 and 108 and diode 110. It should be noted that a junction intermediate resistors 106 and 108 is connected through a capacitor 112 to negative voltage source V ss . The capacitor 112 provides an initialising current on a restart, i.e. when power is first applied to the system. Without the capacitor 112, the system would automatically go into alarm condition whenever power was applied. Capacitor 112 forces the comparator 88 inverting input to a negative value to allow it to be initialised with a positive output voltage. Turning now to the circuit indicated in block 96 of Figure 2, it is seen that the input terminal T' is connected to a band pass filter circuit utilizing an operational amplifier 114. The inverting input terminal of amplifier 114 is connected to terminal T' through a coupling resistor 116 and capacitor 118. A capacitor 120 connected between an output amplifier 114 and the junction intermediate resistor 116 and capacitor 118 operates in conjunction with those latter elements to set the band pass for the amplifier. The resistor 122 connected between the output terminal and the inverting input terminal provides the DC feedback while a capacitor 124 connected in parallel with resistor 122 prevents high frequency oscillations. The non-inverting terminal of amplifier 114 is connected through a resistor 126 to earth ground. The band pass filter is designed to have a centre frequency of about 6.8 Hz so as to filter out the pulses caused by breathing of the patient being monitored and to pass only those pulses corresponding to the cardiac activity. As mentioned previously, the cardiac activity pulses appear as essentially a sinusoidal wave modulating the breathing signals. The amplified breathing signals may vary in amplitude between 500 millivolts and 2 volts peak-to-peak depending upon the intensity of the breath and the size and orientation of the patient being monitored. The cardiac pulses are in the range of 100 millivolts in amplitude and are superimposed on the pulmonary activity. The signals representative of cardiac activity are coupled through a decoupling capacitor 128 and rectifying circuit comprising diodes 130 and 132 and then through a coupling resistor 134 to an inverting input terminal of a comparator 136. A filter circuit comprising a resistor 138 and parallel connected to capacitor 140 are connected between an anode terminal of diode 132 and earth ground. The filter circuit acts as an integrator with approximately a one second time constant so that a charge can be developed on the capacitor 140 as the cardiac rate of the patient being monitored increases. If the cardiac rate goes below a set value, the output signal developed by the comparator 136 will slew to a lower level which may be a negative value and will allow the comparison between the voltage on capacitor 98 and the voltage developed by the output of comparator 88 to be based on a much lower value thereby causing the comparator to trip at a reduced breathing time interval. The initial value at which the comparator 136 will trip is set by connecting the non-inverting terminal of comparator 136 to a junction intermediate first and second resistors 142 and 144 which are connected as a voltage divider between negative voltage source V ss and earth ground. The inverting input terminal of comparator 136 is connected through a resistor 146 to positive voltage source V cc whereby the inverting input terminal is normally held at approximately ground potential. A capacitor 148 connected between the output terminal of comparator 136 and its inverting input terminal provides a high frequency shunt. Although other values of components could be selected for use in the circuit of Figure 2, the following is a tabulation of the values used in a preferred embodiment. The operational amplifier and comparator circuits used in the arrangement described in Figure 1 and Figure 2 are of a type well known in the art and available from numerous different manufacturers under different numbers. Although the system has been shown using discreet components, it is apparent that the system could be easily implemented using large scale integrated circuits. Furthermore, although the system has been disclosed as a monitor for cardio-pulmonary activity, it will be apparent that the system will detect motion of any type within the general area of the transducer, including motion associated with inanimate objects. It should also be noted that the transducer uses only a single electrode and that the layer 14 operates only as an electrostatic shield.

Claims: An apparatus for non-invasively monitoring activity of a person, comprising: an electrostatic charge collector (10) having an electrically insulated charge collection layer (12); amplifying means (24 to 62) for producing an amplified signal (T,T') indicative of variation in electrostatic charge on the charge collection layer at frequencies of normal cardio-pulmonary activity of a person in the vicinity of the charge collection layer; first detecting means (64 to 74, 86, 106 to 112) for detecting a first characteristic of the amplified signal to produce a signal representing pulmonary activity of the person; indicator means (76 to 84) for providing an indication of each pulmonary activity of the person; and alarm means (88, 100 to 104) responsive to the pulmonary activity signal for providing an alarm indication when the pulmonary activity is less than a threshold amount; characterised by: second detecting means (96) for detecting a second characteristic of the amplified signal to produce a signal representing cardiac activity of the person; the alarm means also being responsive to the cardiac activity signal to vary the threshold amount in dependence thereon. An apparatus as claimed in claim 1, wherein the second detecting means includes a bandpass filter (116 to 124) such that the second characteristic represents a component of the amplified signal in a frequency range covering the frequencies of normal cardiac activity of a person. An apparatus as claimed in claim 2, wherein the filter is a band pass filter having a centre frequency of about 6.8 Hz. All apparatus as claimed in any preceding claim, wherein the second detecting means includes means (138, 140) for integrating the second characteristic. An apparatus as claimed in any preceding claim, wherein the first detecting means includes a low-pass filter (64,68) such that the first characteristic represents a component of the amplified signal in a frequency range covering the frequencies of normal pulmonary activity of a person. An apparatus as claimed in claim 5, wherein the first detecting means includes means (86,98) for integrating the first characteristic. An apparatus as claimed in claim 6, wherein the integrating means includes a capacitor (98). An apparatus as claimed in any preceding claim, wherein the amplifying means has a feedback circuit (42 to 62) with a relatively high gain at frequencies below a predetermined value so that the frequency response of the amplifying means falls off rapidly below said predetermined value. An apparatus as claimed in claim 8, wherein the charge amplifier and feedback circuit in combination have a pass band from 0.1 to 15 Hz. An apparatus as claimed in claim 8 or 9, wherein the feedback circuit includes means (44,48,50) to increase the large signal gain of the feedback circuit to reduce the large signal gain of the amplifying means. An apparatus as claimed in any preceding claim, wherein the electrostatic charge collector includes an electrostatic shield (14) substantially co-extensive with the charge collection layer and spaced therefrom by an insulative layer (16), the shield being connected to earth ground by a high frequency bypass filter (20,21). An apparatus as claimed in any preceding claim, wherein the apparatus is selectably operable in a first mode in which, as aforesaid, the alarm means is responsive to the cardiac activity signal to vary the threshold amount in dependence thereon, and in a second mode in which the alarm means is not responsive to the cardiac activity signal and in which the threshold amount is fixed.

Inventors: DIAMOND DONALD A, DIAMOND, DONALD A.
Applicants: U T MEDICAL MANUFACTURING INC, U.T. MEDICAL MANUFACTURING, INC.
Application year: 1986
Main classification: A61B5
IPC codes: G01H1, A61B5, G08B21