GB2188731A - Respiratory failure alarm - Google Patents

Abstract

An alarm system, principally intended for detecting respiratory failure in infants comprises a variable capacitance 10 which has its capacitance continuously varied by the infant's breathing and is connected in an oscillator circuit 11 to provide a continuously varying oscillation frequency. Circuits are provided to activate an alarm 17 should the oscillation frequency remain constant for a predetermined period. These comprise an amplifier 12 and rectifier 13 which produce a D.C. signal proportional to the oscillation frequency, a comparator 14 which produces pulses when the DC signal exceeds a reference level, and a missing pulse detector 15 which produces an output when the time between pulses exceeds a preset limit. The variable capacitance element may comprise a mat or blanket on which the infant may be lain, the blanket including two flexible conductive layers separated by a resilient compressible insulating layer and enclosed in an impervious cover. <IMAGE>

Description

SPECIFICATION Improvements in or relating to alarm systems This invention relates to alarm systems and is particularly concerned with a system operative to activate an alarm when a baby stops breathing.

Various systems are known for monitoring the respiratory action of infants, but in general these systems are complicated and expensive, and frequently require trained personnel to operate them. Consequently they are normally only used in hospitals or similar places where trained staff are available.

It is therefore an object of the present invention to provide an alarm system which is relatively cheap and simple to operate and can therefore be used, for example, in the infant's own home, or even when the infant is in a pram or the like outside the home.

One of the known systems for monitoring the respiratory action of an infant includes an air bag which is secured to the infant's abdomen. The air bag is connected by means of a flexible tube to a pressure-responsive device which produces an electrical signal in response to pressure in the tube generated by the respiratory movement of the infant. These signals are normally applied to a visual display unit which shows the infant's breathing pattern. A further device is usually provided to sound an alarm if the pattern indicates that breathing has ceased.

From one aspect the present invention consists in an infant respiratory alarm system comprising a capacitive element connected in an oscillator circuit so that the instantaneous oscillation frequency depends on the instantaneous value of the capacitance of said element, wherein said element is so located and constructed that its capacitance varies in response to respiratory movements of the infant, and wherein means are connected to said oscillator circuit to activate an alarm in response to the oscillation frequency remaining constant for a predetermined period of time.

From a more general aspect the invention consists in an alarm system comprising a capacitive element connected in an oscillator circuit so that the instantaneous oscillation frequency depends on the instantaneous value of the capacitance of said element, means connected to said oscillator circuit to detect the length of time for which said oscillator frequency remains constant and means for activating an alarm if said time exceeds a predetermined limit.

It will be understood that one of the most important components of a system in accordance with the invention is the capacitive element, and accordingly from yet another aspect the invention consists in a capacitive element comprising two layers of synthetic resin material, each incorporating electrically-conductive material or provided with an additional layer of electrically-conductive material, said two synthetic resin layers being separated by compressible foamed synthetic resin material and said electrically-conductive material being connected in an oscillator circuit to control the operating frequency thereof in dependence on pressure applied between said two synthetic resin layers to compress said compressible material.

Various means may be used to detect the absence of variations in the oscillator frequency, but the invention also provides one particularly advantageous method of doing this.

Accordingly from yet another aspect the invention consists in an alarm system comprising a pressure-responsive capacitive element connected in an oscillator circuit to determine the operating frequency thereof, a tuned amplifier receiving the output of said oscillator and providing an AC signal, the amplitude of which varies with the operating frequency of the oscillator circuit, a rectifier providing a DC signal, the level of which varies with the amplitude of said AC signal, means for producing a pulse train in response to variations in the level of said DC signal, and means for activating an alarm if the time interval between pulses in said pulse train exceeds a predetermined limit.

It is of course to be understood that the means for activating the alarm responds not only to a time interval between two successive pulses exceeding the predetermined limit, but also to the complete absence of pulses in the pulse train.

As already mentioned, an important component of a system in accordance with the invention is the capacitive element, and in one particular embodiment this element comprises two sheets of synthetic resin material coated with aluminium and separated by foamed synthetic resin material to form a sandwich. If desired, this sandwich may be contained in a further protective cover consisting of synthetic resin material such as polyvinyl chloride. Two wires are connected to the two aluminium coatings and lead to the remainder of the system. The whole capacitive element may be in the form of a mat or blanket of suitable size and shape to be placed between an infant and a mattress or the like.

The alarm which is activated by the system may be an audible alarm, and preferably the arrangement is such that a succession of audible tones are produced when the alarm is activated. In addition the alarm system may be arranged to provide a visible alarm and, when the system is used in domestic conditions, the alarm may be arranged to switch on lights to assist in alerting the infant's parents.

It has been stated that the alarm is acti -vated when changes in the oscillator frequency are absent for a predetermined period of time. Preferably this time is variable under the control of the user of the system so that the system may be used with infants who are suffering from breathing disorders such as asthma.

One embodiment of the invention will now be described with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a block diagram of a system in accordance with the invention; Figure 2 illustrates one type of capacitive element for use in the system of Figure 1; and Figure 3 is a diagram of a circuit which may be used in conjunction with the element of Figure 2 to form a system as illustrated in Figure 1.

The system illustrated in Figure 1 includes a capacitor 10 which may be, for example, a capacitive blanket of the kind illustrated in Figure 2. In any case the capacitor comprises at least two flexible electrically-conductive elect rodes separated by a compressible dielectric.

It is known that the capacitance of a capacitor depends on the distance between the electrodes and on the dielectric constant of the dielectric separating the electrodes. Since the electrodes and the dielectric are flexible, pressure on either of the electrodes decreases the spacing between the electrodes, and hence increases the capacitance of the capacitor.

The system illustrated also includes an oscillator 11, and the capacitor 10 forms part of the oscillator circuit so that the operating frequency of the oscillator depends on the capacitance of the capacitor, and hence varies with any pressure applied to the capacitor.

The output of the oscillator is applied to a tuned amplifier 12 which has a relatively high Q. The centre frequency of the tuned circuit, or circuits, of the amplifier is slightly displaced from the range of frequencies over which the oscillator operates. Thus as the frequency of the oscillator is changed, the amplitude of the output of the amplifier also changes.

The alternating current signal from the amplifier is applied to a rectifier 13 which converts the alternating current signal to a direct current signal, the level of which again represents the operating frequency of the oscillator, and hence the magnitude of any pressure applied to the capacitor.

The DC signal is applied to a comparator 14 where its level is compared with a reference level. Changes of the level of the DC signal compared with the reference level cause a train of pulses to be generated at the output of the comparator.

The train of pulses is applied to a missing pulse detector 15, if necessary through an invertor 16. The missing pulse detector 15 is arranged to provide an output signal when the time interval between two successive pulses exceeds a predetermined limit.

The output signal from the missing pulse detector is applied to an alarm 17 which may be an audible alarm, a visible alarm or any combination of different alarms.

Figure 3 illustrates one possible circuit which may be used to implement the system illustrated in Figure 1.

An 18V supply is provided by two 9V batteries connected in series. This is fed through a regulator (IC1) to provide a 9V supply to the rest of the circuit. R2 adjusts the output voltage. R1 provides an output sample to the adjust pin. I adj is typically 508 A. Cl and C2 provide smoothing. The output remains at 9V until the battery voltage drops below 11V.

The oscillator is basically an amplifier based on a BC109 transistor (TR1) with feedback to make it oscillate. Feedback is provided by the (a) winding of L1. The frequency of the oscillator is controlled by the tuned circuit L1 and CB (the capacitive blanket). Therefore when pressure is applied to the blanket, pushing its plates closer together, its value changes and this changes the frequency of the oscillator.

R3 and R4 provide base biasing for Tor 1.

and C3 provide decoupling for the LC circuit.

C6 is emitter decoupling. C5 is the coupling capacitor between the oscillator and tuned amplifier.

The output from the oscillator is fed to a tuned amplifier which looks for a high Q.

Slope detection is used to increase the registration value of a small change in capacitance from the blanket.

The amplifier is designed to operate on the slope of the response of the oscillator. R7 and R6 provide base biasing for TR2. L2, C8 and C9 control the frequency at which the amplifier will operate. R9 and C7 decouple the LC circuit. C10 is emitter decoupling. The output from the tuned amplifier is a 4MHz sine wave which varies in amplitude when the frequency of its input changes.

The signal is then rectified by a rectifier comprising C11, D1, R11, R12 and C12. C11 is a coupling capacitor, Rl 1 is a reservoir, R12 is the load resistor and C12 is a smoothing capacitor. The output from the rectifier is a DC level which varies by 1 volt per 2 volt change in input amplitude.

The output from the rectifier is fed to a 741 op-amp (1C2) based comparator. R13 clamps the non-inverting input of the amplifier at approximately 4.5V though this can be adjusted.

R14 and R15 control the gain of the amplifier.

The output of the comparator is a positivegoing pulse with short rise and fall times.

The signal is then fed to a 741 (1C3) based invertor which provides negative-going pulses suitable for triggering the 555 (1C4). R17 and R18 provide the reference voltage to the noninverting input. R16 and R19 control the gain.

The negative-going pulses are then fed to the 555 (1C4) based missing pulse detector. If the circuit does not receive a pulse in less than 20 secs from the previous pulse, then the output at pin 3 goes to OV sounding the alarm X1. The maximum time delay allowed between pulses before the alarm sounds is set by R22 and C14. R20 and C13 are AC coupling for trigger input pin 2.

Claims (10)

1. An alarm system comprising a variable capacitance element connected in an oscillator circuit so that the instantaneous oscillation frequency depends on the instantaneous value of the capacitance of the element, means connected to the oscillator circuit to detect the length of time for which the oscillator frequency remains constant, and means for activating an alarm if the time exceeds a predetermined limit.

2. An alarm system according to Claim 1, wherein the means for activating the alarm includes means for varying the time during which the oscillation frequency must remain constant for the alarm to be activated.

3. An alarm system according to Claim 1 or Claim 2, wherein the variable capacitance element is so constructed that, when placed beneath a prone or supine infant, the capacitance of the element varies in response to respiratory movements of the infant.

4. An alarm system according to Claim 3, wherein the variable capacitance element comprises two layers of synthetic resin material either incorporating, or coated with, electrically conductive material, the two synthetic resin layers being separated by a resilient compressible insulating material, and the capacitance of the element varying when pressure is applied to compress the insulating material.

5. An alarm system according to any of the preceding Claims, wherein the means connected to the oscillator circuit comprise a tuned amplifier receiving the output of the oscillator circuit and providing an AC signal whose amplitude varies with the operating frequency of the oscillator circuit, a rectifier producing a DC signal whose level varies with the amplitude of the AC signal, and means for producing a varying binary output signal in response to variations in the level of the DC signal, and the means for activating the alarm does so when the binary output signal remains constant for a predetermined time interval.

6. A variable capacitance element suitable for use in an alarm system according to any of the preceding Claims, wherein two flexible layers of synthetic resin material incorporating, or coated with, an electrically conductive material are separated by a compressible insulating material, the capacitance of the element being varied by pressure applied to compress the insulating material.

7. A variable capacitance element according to Claim 6, wherein the insulating material is a synthetic resin foam.

8. A variable capacitance element according to Claim 6 or Claim 7, wherein an impervious flexible cover entirely envelopes the element, connecting leads being provided leading from the respective electrically conductive layers through the cover to the exterior.

9. An alarm system substantially as described herein with reference to Figures 1 or 3 of the accompanying drawings.

10. A variable capacitance element substantially as described herein with reference to Figure 2 of the accompanying drawings.