BACKGROUND Every year, a significant proportion of newborn infants are affected by sudden infant death syndrome. Although the causes are still not fully understood, medical experts are generally in agreement that there is a good prospect of the child being saved provided the child receives early attention at the onset of the condition. Various forms of monitor have been proposed in an attempt to provide an early warning of this potentially fatal condition.

These include movement monitors sensitive to the infants respiratory movements, remote sound monitors and video monitors, but they are often intrusive or inconvenient to use. Furthermore, some forms of monitor are prone to generating false alarms, as a result of normal movements of the infant for example.

A fall in heart rate is a reliable indication that an infant is in distress and requires immediate attention. However, existing heart rate monitors are also inconvenient to use requiring direct physical contact with the subject. Some use electrical signals from the heart (ECG or EKG) and require electrodes connected to the body. Others rely on mechanical connections to sense the users pulse, and some use optical sensors to detect movement of blood in tiny subcutaneous blood vessels.

The present invention seeks to provide a new and inventive form of monitoring unit which is both reliable and non-intrusive.

SUMMARY OF THE iNVENT) ON The present invention provides an infant monitoring system which includes a transmitter unit having a waterproof housing, means for attaching the housing to the infant, and a cardiac detector sensitive to the infants heart beats to provide electrical output signals, in which the housing contains : - a signal processing unit for distinguishing heart beats from background noise in said electrical signals and being responsive to the rate of the heart beats to generate a warning state when the monitored heart rate falls outside predetermined safe parameters; and -a battery-powered transmitter for transmitting radio signals which distinguish between a normal quiescent state and a warning state signalled by the processing unit.

The cardiac detector is preferably an acoustic detector which converts sounds into electrical signals, which may be mounted inside the housing.

The transmitter unit will detect the heart sounds, and as long as these are present no warning signal results. However, as soon as the infant enters a dangerous condition resulting in a fall in heart rate a warning signal is sent out by the unit.

The monitoring system may conveniently include a portable receiver unit which includes a radio receiver for picking up signals from the transmitter unit, and a signal analyser for generating an alarm signal when the warning state is signalled by the transmitter unit. In a preferred form of the receiver unit, the signal analyser may be arranged to generate an alarm signal in the absence of a quiescent signal from the transmitter unit.

The transmitter unit preferably incorporates a temperature sensor, allowing the temperature of the infant to be monitored.

BRIEF DESCRIPTI OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a general view of an infant monitoring system in accordance with the invention, including transmitter and receiver units; Figure 2 is a block diagram of the transmitter unit; Figure 3 is a block diagram of the receiver unit; and Figure 4 is a top end elevation of the transmitter unit shown in Fig. 1.

DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to Fig. 1, the monitoring system incorporates a transmitter unit 1 and a receiver unit 2. The transmitter unit 1 has a heart-shaped waterproof plastic housing 6 with means for attachment to the clothing or skin of an infant such as a pad 5 of material with a multiplicity of small hooks or an adhesive applied to the rear surface of the housing, a clip etc. As shown in Fig. 4, the rear of the housing 6 may be substantially flat, but the front surface is preferably convex so as not to injure an infant or cause discomfort if it should roll over onto the unit. The front surface of the housing incorporates a small sound generator 3, which may be a piezo sounder for example. A test button 4 is also provided in any convenient position, preferably at the side of the housing, as shown, to reduce the risk of activation if the infant lies on it. The front surface of the housing 6 also carries a liquid crystal temperature indicator strip 40 by which the temperature of the infant may be visually monitored. An electronic temperature sensor may be used if desired. The receiver unit 2 is intended to be carried by a carer and could be of any shape, but in this example the receiver has a similar heart-shaped housing to the transmitter unit 1. The housing could conveniently be provided with a clip to attach to the wearers clothing. The receiver unit includes a further sound generator 7, e. g. another piezo sounder, with a green indicator light 8 and a red warning light 9, in the form of light-emitting diodes for example.

Fig. 2 shows the transmitter unit 1 in more detail. The unit is powered by a small electrical cell 10 and has a high sensitivity sound detector 11, e. g. an electret microphone, for converting sounds into electrical signals. The electrical output from the microphone is fed to a front end signal processing unit or signal amplifier 15. The high level analogue output signal then passes via an analogue-to-digital converter (ADC) 13 to a digital signal processing (DSP) unit 12. Although analogue processing may be used, DSP is preferred since it is possible to attain very good signal discrimination using known processing algorithms. By this means, the DSP unit 12 can filter out any background noise and discriminate in favourofthe low-intensity rhythmic sounds produced by the infant's heart beat, discounting any extraneous sounds due to normal movement of the infant for example. The processing unit analyses the resulting heart sounds to sense the infants mean heart rate. Provided this exceeds a predetermined normal or safe level the DSP unit 12 operates a low power radio transmitter 14 to send a quiescent radio frequency output signal. However, if the heart rate should fall below the safe level, or ceases to be detectable, the unit 12 signals the transmitter to send out an emergency warning signal which is accompanied by an audible warning from the sound generator 3. The transmitter 14 is of relatively low power, with a maximum range of 20 to 25 metres. For test purposes, depression of the test button 4 will simulate a warning condition to generate an audible alarm and operate the transmitter 14 to send out a warning signal.

Examples of suitable general purpose signal processing units which may be used in the present invention include the PIC16XX 8-bit RISC series (e. g.

PIC16F876), the MSP430 16-bit RISC family, and the HCS08 family. The total current consumption of the transmitter unit 1 may be as low as 800 uA, achieving a minimum operating life of about 280 hours from a CR3020 or similar power cell.

An electronic temperature sensor41 may be included if desired. This is also interfaced to the DSP unit 12 via front-end signal processor (amplifier) 42 and an analogue-to-digital converter43. With such an arrangement the unit 12 may generate an audible warning and/or signal the transmitter 14 to send an emergency warning signal if the infants body temperature falls outside an acceptable range.

The receiver unit 2 is shown in detail in Fig. 3. The unit 2 is powered by a long-life battery 20 and incorporates a radio receiver 21 which passes received signals to a signal analyser 22. Provided the signal analyser 22 detects a quiescent signal from the transmitter unit 1 the green light 8 is lit, but if the warning signal is detected, or if the quiescent signal from the transmitter unit 1 should fail for more than a few seconds, the green light is extinguished and the red light 9 starts to flash accompanied by an audible warning from the sound generator 7. This will immediately alert the carer, indicating that one of the following conditions has occurred: 1. The infant is no longer in range.

2. The transmitter unit has ceased to function (e. g. due to a low battery).

3. The test button has been depressed.

4. The infant is suffering physiological distress resulting in a dangerous fall in heart rate.

The cause of the alarm condition can immediately be investigated and appropriate remedial action taken.

Although the main function of the monitoring system is to guard against sudden infant death syndrome it will be appreciated that the system will also detect other events such as an attempted abduction resulting in the infant being taken out of range.

The monitoring system is therefore small and non-intrusive with no wires or electrodes, is battery operated and is simple to understand.

The transmitter and receiver units are preferably arranged to operate on different radio frequency channels allowing several monitoring systems to be used in close proximity, e. g. in a play group, nursery, hospital etc.

Common short range telemetry devices (SRDs) may be used for the transmitter 14 and receiver21, although more sophisticat@c3"Bluetooth"type systems can also be utilised. These operate at powers of between 1 mW and 100 mW at frequencies around 2. 4 GHz and use data packet transmission and spread spectrum techniques to achieve very reliable short- range communication without mutual interference.

It will be appreciated that the features disclosed herein may be present in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.