SYSTEM AND METHOD FOR MONITORING A HEARTBEAT

Abstract: A monitor system for monitoring the heartbeat, in particular of a baby, without being attached to the living being, comprising a transducer (40, 80) responsive the the heartbeat to generate an electrical signal, the system further comprising a filter (226) for filtering the electrical signal and processing circuitry (236) to process the filtered signal. An oscillating element generates, in response to a heartbeat signal, a sinusoidal signal of decaying amplitude. The oscillating element comprises an active band-pass filter (230), which passes a selected range of frequencies centred on a frequency of between 10 Hz and 15 Hz. The processing circuitry (236) is arranged to activate an alarm if the heartbeat ceases.

Description: SYSTEM AND METHOD FOR MONITORING A HEARTBEAT The present invention relates to a system and method for monitoring the heartbeat of a living being without having to be fitted to the living being, in particular when the living being is a baby. Systems have been developed for home and hospital use to monitor the presence of a baby in a cot or a pram, for example by monitoring a heartbeat or breathing of the baby, which set off an alarm when the baby is removed by an unauthorised person. Further, medical staff monitor the patients under their care for any change relating to their well being. Monitoring a heartbeat of a new born baby can alert staff immediately to medical problems, such as a potential cot death incident. Such systems must be capable of being applied to a number of living beings simultaneously, for instance in a hospital ward where a number of cots or beds would be present. Monitoring systems have been developed which use various types of sensors which are connected to the body of the living being. The sensors are then typically connected to a suitable form of processing facility. However, in such systems it is inconvenient to have to attach and detach the living being from the sensors when they are moved. Also, the attachment of sensors to a baby invariably requires the use of wires which may become entangled in with the baby's limbs or around a baby's neck. GB 2,150,332 discloses a heartbeat monitor which is attached to the chest of a baby, for example by being fitted in a pocket of a close fitting vest which pocket lies over the chest of the baby. The monitor comprises a microphone for picking up cardiac sounds and avoids the use of wires to connect the monitor to a processing facility by using a radio link. However, this system would not immediately be able to detect the removal of a baby from a cot or pram by an unauthorised person and has the inconvenience of having to repeatedly fit the monitor to a baby. Equipment for monitoring a heartbeat has to be able to detect a signal having a very short pulse duration and if the sensor is not directly fitted to the living being will have to pick out the heartbeat signal of very small amplitude from a lot of background movement or noise including movement or noises made by the living being, such as breathing, vocal noises and limb movements an well as movement or noises emanating from the surroundings of the living being. This invariably requires complex and expensive electronics. GB 2,165,979 discloses a monitor apparatus which describes a system which monitors breathing by sensing pressure changes in an air envelope on which a baby rests. The air envelope is designed to couple the signal of interest which may also comprise a heartbeat to a transducer which communicates with the air pocket. However, GB 2,165,979 does not disclose in detail how the air envelope is designed to discriminate between the signal of interest and other movement or noises occurring in the local environment. The transducer signal is amplified, rectified and integrated and then the resultant signal is compared with a control signal. If the resultant signal drops below the control signal for a predetermined period of time then an alarm is activated. Accordingly, the present invention provides a monitor system for monitoring the heartbeat of a living being, comprising a transducer responsive to the heartbeat to generate a heartbeat signal, a filter for filtering the heartbeat signal and processing circuitry to process the filtered signal, characterised in that the system comprises an oscillating element responsive to the heartbeat signal to generate a sinusoidal signal of decaying amplitude having a half cycle which approximates to the heartbeat signal and the filter passes only those frequencies in a selected range close to the frequency of the sinusoidal signal generated by the oscillating element. The monitor system according to the present invention is capable of reliably distinguishing the heartbeat signal from a composite signal including signals generated by other noises or movements. As long as the heartbeat signal is present, the sinusoidal signal generated by the oscillator element will pass through the filter to the processing circuitry. If the heartbeat stops the sinusoidal signal generated by the oscillator element will decay to zero and so the amplitude of the signal passing through the filter to the processing circuitry will drop. The processing circuitry can be arranged to set off an alarm when the signal passing from the filter drops below a predetermined level. Preferably the oscillating element is an electrical resonator circuit with a gain of less than unity. In a preferred embodiment the sensor comprises a transducer responsive to the heartbeat to generate an electrical heartbeat signal. In this case the oscillating element may be an electrical resonator circuit, for example an active filter, with a gain approaching unity such that the oscillations generated by the oscillator element decay slowly. Conveniently, the oscillating element and the filter comprise an active filter. When exited, an active filter will oscillate at the frequencies within the bandwidth of the filter and so can perform both the functions of the oscillating element and the filter. Before reaching the oscillator element and/or filter it is preferred to pass the electrical heartbeat signal through a high pass filter which passes frequencies greater than approximately 5 to 7.5 Hz and so blocks lower frequencies in order to prevent swamping of the low amplitude heartbeat signal by high amplitude breathing and/or limb movement signals. In a preferred embodiment, which can utilise low cost passive low pass filters the active filter comprises; a first filter which passes frequencies below the bottom end of the selected range of frequencies, a second filter which passes frequencies below the top end of the selected range of frequencies, and a subtraction element for subtracting the first signal from the second signal. Alternatively, the oscillating element comprises a mechanical oscillator. A flexible board can be designed so that it oscillates in response to a heartbeat signal so that a piezo device fixed to the board generates a sinusoidal signal of decaying amplitude having a half cycle which approximates to the heartbeat signal. In this case the baby's heartbeat can provide a regular stimulus to maintain the oscillation of the board. The oscillations form a ~ continuous sinusoidal signal so long as the heartbeat is present and so are much easier to detect than the short duration intermittent pulse of the heartbeat. Similarly, the filter can comprise a mechanical filter, for example if a board as described above has a resonant frequency close to that of a sinusoidal signal having a half cycle which approximates to the heartbeat signal, it will transmit that frequency in preference to other frequencies and hence act as a filter. In order to aid detection of the heartbeat signal, the electrical signal generated by the transducer preferably passes through at least one DC amplification stage. In order to generate a sinusoidal oscillation having a half cycle which approximates to the heartbeat signal the oscillator element is preferably responsive to and the filter preferably passes frequencies in the range of 6.8Hz to 20Hz. The selected range of frequencies are preferably centred on a frequency of between 10 and 15Hz, preferably 12.5Hz. Preferably, the processing circuitry comprises a threshold detector which activates an alarm when a signal passed through the filter rises or falls past a pre-set threshold. More preferably, the threshold detector comprises a comparator for comparing the signal passed by the filter with a control signal. In order to give a good indication of the overall health of the living being, preferably, the monitor system also detects other forms of physical activity associated with a living being to generate a signal which is passed to the processing circuitry. In this case it is preferable that the processing circuitry is responsive to all the signals it receives when generating a signal to actuate an alarm. According to a second aspect of the present invention there is provided monitor system for monitoring the heartbeat of a living being, comprising a transducer responsive to the heartbeat to generate a heartbeat signal, a filter for filtering the heartbeat signal and processing circuitry to process the filtered signal, characterised in that the system comprises an oscillating element responsive to the heartbeat signal to generate a sinusoidal signal of decaying amplitude having a frequency in a selected range of- frequencies centred on a frequency of between 10 and 15 Hz, preferably 12.5Hz and the filter passes frequencies in a selected range centred between 10 and 15Hz, preferably 12.5Hz. According to a third aspect of the present invention there is provided a method for monitoring the heartbeat of a living being, remote from the living being, including the steps of: generating a heartbeat signal from a sensed heartbeat movement, filtering the heartbeat signal, and processing the filtered signal, characterised in that the method further comprises the steps of: generating a sinusoidal oscillation in response to the heartbeat signal which sinusoidal oscillation has a decaying amplitude and a half cycle which approximates to the heartbeat signal, and filtering only those frequencies in a selected range close to the frequency of the sinusoidal oscillation generated by the oscillating element. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures in which: Figure 1 shows schematically an embodiment of a system for monitoring a heartbeat of a living being; Figure 2 shows the electronic circuitry used to process the signals from a transducer to distinguish between various forms of physical activity, including a heartbeat, associated with a living being; Figure 3 shows a plurality of signals associated with the circuitry of Figure 2; and Figures 4a to 4d schematically illustrate embodiments of support boards on which ' a transducer is mounted. Referring to Figure 1, there is shown a schematic embodiment of a monitor system (10) for detecting physical activity, including heartbeat, associated with a baby. A cot (20) for a baby has underneath the mattress (30) a transducer arrangement (40). The transducer arrangement is exited by one or more physical activities associated with a baby, such as a heartbeat, breathing or limb movement, and produces a signal having components derived from or related to any such physical activities. The transducer arrangement (40) is coupled, via a suitable signal transmission medium to a cot unit (50). The cot unit (50) comprises circuitry for processing the signals received from the transducer arrangement (40) to distinguish between various forms of physical activity emanating from or associated with a baby in the cot (20). The cot unit (50) is, directly or indirectly, connected to and can trigger the activation of an alarm (60) in the event that the signal from the transducer arrangement (40) indicates that at least a selectable one of a plurality of physical activities associated with the baby has changed or terminated. For example, the cot unit (50) may activate an alarm if a determination is made to the effect that the signal no longer contains a component indicative of heartbeat. Various thresholds can be set which monitor not only the presence of a particular component of the signal but also the level or rate of that component. The alarm (60) is typically located in a central location where it has a high probability of attracting the attention of a responsible adult during home use or nursing staff for hospital use. The alarm (60) may produce an audio output or a visual indication, for example, where it is undesirable to disturb other babies or patients in a hospital. In one embodiment, the transducer arrangement comprises a suitably flexible support board (70) (see Figures 4a to 4d) having a piezo-ceramic device (80) centrally disposed thereon which produces a voltage when stressed or flexed. The piezo-ceramic device (80) is in the form of a disc. The support board (70) is constructed, for example, a flexible- plastic board as shown in Figure 1. Manufacturing the board from a suitable plastic can increase the sensitivity of the transducer arrangement as a whole to the physical activities of a baby. Preferably the support board is made from ABS plastic due to the low cost and ability to shape and work the plastic. Alternative embodiments can be realised in which the flexible board is manufactured from metal, fibre board, hard board or other forms of plastic. In a preferred embodiment mechanical means are utilised which increase the sensitivity of the transducer arrangement to the movement of the mattress or baby by locating a fulcrum (96) for the support board (70) directly below the piezo-electric element. The flexible support board such as that shown in figure 4a can be designed so that it oscillates in response to a heartbeat signal so that the piezo device generates a sinusoidal signal of decaying amplitude having a half cycle which approximates to the heartbeat signal. In this case the baby's heartbeat can provide a regular stimulus to maintain the oscillation. The oscillations form a continuous sinusoidal signal so long as the heartbeat is present and so are much easier to detect than the short duration intermittent pulse of the heartbeat. Optionally, the support board (70) is formed with, for example, a plurality of discontinuities, such as slots (90) (see Figures 4b to 4d). In a further embodiment the support board (70) utilises an arrangement of grooves on at least one of either the top or bottom surfaces thereof. The discontinuities ensure that the board is flexible and so will readily distort in response to the physical activities of the baby but is also strong enough to support the mattress and the baby. In a preferred embodiment (see in Figures 4b to 4d) the slots (90) are radially disposed with respect to a central portion of the support (70) board where the piezo device (80) is located. A further embodiment (Figure 4d) also comprises further slots (90) disposed around the periphery of the support board (70). In figure 4d the support board (70) comprises a total of eight slots having at the ends thereof a hole which reduces the stresses conventionally associated with the slots. The support board (70) is generally rectangular although other shapes are also possible, for example square or circular. Still further, a plurality of support boards may be utilised having piezo devices connected therebetween which produce electrical signals in response to relative movement of the support boards. Preferably the lower surface of the support board (70) is held in an elevated position above and not in contact with the bottom of the cot. This enables the support board to deform independently of the rigid surface of the bottom of the cot and so increases its sensitivity to physical activity. Also, in certain embodiments it will enable the board to oscillate. In the embodiments shown in Figures 4a to 4d, the degree by which the plastic support board (70) can flex is increased by using strategically placed feet (95) (shown in dotted lines) arranged to support the support board (70) such that movement thereof is not impeded by, for example, the bottom of the cot. The feet (95) are regularly disposed around the periphery of the support board (70) and one foot (96) is centrally disposed so as to provide a fulcrum support for the piezo device. Therefore, as the baby moves, including breathing and heartbeat movements, the board flexes which, in turn, causes the piezo device to produce a signal indicative of such movement. The piezo device is situated at the position on the support board (70) which flexes the most and therefore stressed the piezo device the most. This position has been found to be typically the centre of the board, particularly if a fulcrum support is provided at the centre of the board. An alternative embodiment can be realised in which the feet are omitted. According to this alternative embodiment, the plastic support board is supported at its edges by the frame of the cot such that the board if free to flex or oscillate and such movement is not impeded by the bottom surface, if any, of the cot. The monitoring system according to the present invention will detect all voluntary and involuntary movements of the living being, however, during deep sleep, the only physical activities which are present are the breathing and the heartbeat. Optionally, the support board (70) comprises a removable protective covering (not shown) to guard against damage, for example, by bodily or other fluids. The piezo-ceramic device (80) is connected to the cot unit (50) using co-axial cable (100). Co-axial cable is preferred as it is less susceptible to mains interference than most cables. However, as an alternative, suitable screened twisted pair may be utilised. With reference to Figure 2, there is shown a diagram of a circuit (200) implementing detection means for distinguishing between various components, including heartbeat, of the signal derived from the transducer arrangement (40). The circuit (200) detects signals in two main bandwidths of interest. A first circuit (202) determines whether or not the signal from the transducer arrangement (40) contains a component indicative of or derived from breathing or limb movement. A second circuit (204) determines whether or not the signal from the transducer arrangement contains a component indicative of or derived from the heartbeat of the baby. The piezo device (80) is connected in parallel with a 20 M ohm resistor (206), to a DC- voltage amplifier (208). The amplifier is arranged to amplify the DC component of the signal from the piezo device by a factor of fifteen. The first circuit (202) comprises a 5 th order low pass filter (210) designed to pass frequencies less than or equal to approximately one hertz, more particularly, frequencies below between a quarter of one hertz and one hertz. The breathing and other low frequency signals, such as those generated by limb movement, are passed by the 5 th order low pass filter (210). A DC correction is added to the signal from the DC amplifier (208) to counteract the DC signal offset introduced to the signal when it passes through the amplifier (208) and the low pass filter (210). The DC correction is generated in a conventional way by amplifying the signal from the low pass filter (210) by a factor of ten using a further DC amplifier (212) and then using an integrator (214) having an open loop gain and an RC time constant of five seconds. A feedback resistor (216) is utilised to provide a feedback current of +/- 2 micro-amps. In an embodiment of the present invention, the resistor is a 10 M ohm resistor and the capacitor is a 0.47 microfarad capacitor. A threshold detector (218) is used to ultimately determine whether or not the output from the low pass filter (210) is indicative of the presence of breathing or limb movement. The threshold detector can be realised as a single comparator using an Op-amp (222) with controlled positive feedback (224) to provide the threshold and the stability of a Shmitt trigger circuit to eliminate false signals. The output (220) of the threshold detector is connected to a microprocessor or other processing circuit within the cot unit (50) for further processing. Referring to the lower portion of Figure 2, there is shown circuitry for the detection of a heartbeat. The output from the DC amplifier (208) is fed to a high pass passive filter (226) which passes all signals having a frequency of five hertz or greater. This high pass filter blocks low frequencies and so prevents the low amplitude heartbeat signal from being swamped by the high amplitude breathing and limb movement signals. It also isolates subsequent circuitry from the DC offset signal generated when the signal passed through the amplifier 208. The output of the high pass filter (226) is amplified by a factor of two using a DC amplifier (228). The output of the DC amplifier (228) is fed to an active band-pass filter (230) which acts as an oscillating element which because of the bandwidth selected is responsive to the heartbeat signal to generate a sinusoidal signal of decaying amplitude having a half cycle which approximates to the heartbeat signal. The filter (230) passes this oscillation. Because the band-pass filter (230) is an active filter when it is exited it will oscillate at the frequencies within the bandwidth of the filter. The bandwidth is selected to incorporate those frequencies that correspond to twice the duration of a typical heartbeat pulse. Referring to Figure 3, the heartbeat signal is shown at (a). When this signal (a) enters the active band-pass filter (230) it will be masked by other signals associated with physical activity of the baby. Each pulse of the heartbeat signal (a) has a duration of (T) which will vary to some extent from baby to baby. Therefore, the active band-pass filter (230) is set to pass a bandwidth centred on a frequency of l/(2t), where t is the typical duration of a baby's heartbeat pulse. Thus, when a signal having a heartbeat signal component (a) enters the filter (230) it will cause the filter to oscillate at the frequency of 1/(2T) and so will generate a sinusoidal oscillation shown at (b) in Figure 3 which sinusoidal oscillation is passed by the filter (230) and has a frequency of 1/(2T). The active filter (230) acts as an oscillator with a gain of less than one and so between heartbeats the sinusoidal oscillation (b) will decay. However, each time a heartbeat pulse enters the filter, the sinusoidal oscillation will be reinforced, as shown in figure 3. In other words, the active band-pass filter (230) is responsive to a heartbeat signal (a) to generate a sinusoidal signal (b) of decaying amplitude having a half cycle (shaded portion in signal (b)) which approximates to the heartbeat signal (shaded portion in signal (a)) and passes only those frequencies in a selected range close to the frequency of the sinusoidal signal generated by the filter (230). It has been found that the best results are achieved when the selected range of frequencies that the oscillator is exited by and which the filter passes is centred on between 10Hz and 15Hz, preferably 12.5Hz. Preferably, the sinusoidal oscillation generated by one heartbeat pulse is still detectable when the filter (230) is exited by the next heartbeat pulse as shown in the signal (b) in Figure 3. However, the amplitude decay profile (330) is such that the signal decays to " zero within a few seconds. Using such filter (230) tuned to the heartbeat signal as described above enables the heartbeat to be reliably monitored without the need for very fast and expensive comparator circuitry. Therefore, the short duration pulses of the heartbeat signal (a), which are difficult to detect are used to generate a continuous sinusoidal signal which endures as long a heartbeat signal is present and so is easier to detect. The active band-pass filter (230) has been implemented using two low-pass filters. The first low pass filter (232) is arranged to pass frequencies of less than 6.8 Hz while the second low pass filter (234) is arranged to pass frequencies of less than 20 Hz. The active band-pass filter (230) is constructed by inverting the output of the first low pass filter (232) and combining the inverted signal with the output of the second low pass filter in order to cancel all signal frequencies except those between 6.8 Hz and 20 Hz which are thus passed by the filter (230). The frequency range 6.8Hz to 20Hz corresponds to a range of durations for a heartbeat pulse which are typical for a baby. A threshold detector (236) is used to ultimately determine whether or not the output from the active filter (230) is indicative of the presence of a heartbeat. The threshold detector can be realised as a single comparitor using an Op-amp (238) and a voltage divider (240) comprising two resistors to set the threshold level. The output (250) of the threshold detector is connected to electronic circuitry or a microprocessor within the cot unit (50) for further processing. The electronic circuitry or microprocessor is suitably designed or programmed to determine whether or not it is necessary to sound the alarm. The presence or absence of the signals relating to heartbeat, breathing or limb movement can be utilised foF monitoring the health of the baby or simply for monitoring whether or not the baby is present in the cot. For example, the alarm may be sounded as soon as a determination has been made that the breathing/limb movement and/or heartbeat signals are not present. The microprocessor and or electronic processing circuitry can be provided within the cot unit (50). However, other embodiments can be realised in which the processing is performed remotely by, for example, a computer. With further reference to Figure 1 , in this embodiment which is suitable for monitoring a number of cots in a hospital ward, the cot unit (50) is connected via communication link (102) to a computer or a system controller (107). Also, a relay output unit (110) is provided under the control of the cot unit (50) and/or the system controller (107) to perform various functions, such as to lock the doors to a maternity ward to prevent a baby being stolen, send a signal to a pager informing the holder of the pager of the problem, or to sound an alarm. The system controller (107) receives and displays status information associated with a plurality of cots, each having its own transducer arrangement (40) and associated cot unit (50). The cot units (50) and the system controller (107) can operate an alarm independently. If the system controller (107) does not receive an appropriate response after interrogating a cot unit (50) via the communication link (102) it will go into alarm immediately. Likewise each cot unit (50) detects or monitors interrogation by the system controller (107) and will sound an alarm if it fails to be interrogated regularly. This will highlight hardware failure and guard against leads being cut. Responsible adults that are authorised to remove a baby from a cot can be disable by a security device such as a magnetic swipe card device (112) or a security key-pad device. In the version of the monitor for domestic use it is useful for the cot unit to include an- audio-microphone to detect audible sounds made by the baby. The cot unit can then also incorporate a radio transmitter which transmits a radio signal carrying the audible sounds plus any alarm signal to a remote radio receiving unit which also incorporates a loud speaker. The receiving unit is mobile and can be carried by and located in the neighbourhood of an adult responsible for the baby. Therefore, if the baby cries, the crying noise will be transmitted by the receiving unit and the responsible adult can act appropriately. Further, if the heartbeat, breathing and/or limb movement activity of the baby ceases an audible alarm signal will be emitted by the receiving unit to alert the responsible adult to the danger. Although, the above embodiments have been described in relation to babies, it will be appreciated that the present invention can equally be applied to other living mammals such as adults, small children and even animals. For example, in a hospital for patients suffering from senile dementia or similar illnesses, the monitor system could be used to warn nursing staff when such patients get out of bed during the night. In this case, it may be necessary to use two transducer arrangements spaced apart under the mattress in order to reliably detect the presence of an adult patient.

Claims: CLAIMS 1. A monitor system for monitoring the heartbeat of a living being, comprising a transducer responsive to the heartbeat to generate a heartbeat signal, a filter for filtering the heartbeat signal and processing circuitry to process the filtered signal, characterised in that the system comprises an oscillating element responsive to the heartbeat signal to generate a sinusoidal signal of decaying amplitude having a half cycle which approximates to the heartbeat signal and the filter passes only those frequencies in a selected range close to the frequency of the sinusoidal signal generated by the oscillating element. 2. A monitor system according to claim 1 characterised in that the sensor comprises a transducer responsive to the heartbeat to generate an electrical heartbeat signal. 3. A monitor system according to claim 2 characterised in that the oscillating element is an electrical resonator circuit with a gain of less than unity. 4. A monitor system according to claim 3 characterised in that the oscillating element and the filter comprise an active filter. 5. A monitor system according to claim 4 characterised in that the active filter comprises: a first filter which passes frequencies below the bottom end of the selected range of frequencies, a second filter which passes frequencies below the top end of the selected range of frequencies, and a subtraction element for subtracting the inverted first signal from the second signal. 6. A monitor system according to any one of claims 2 to 5 characterised in that the electrical signal passes through at least one DC amplifier stage. 7. A monitor system according to claim 1 characterised in that the oscillating element comprises a mechanical oscillator. 8. A monitor system according to claim 1 or claim 7 characterised in that the filter comprises a mechanical filter. 9. A monitor system according to any one of the preceeding claims characterised in that the oscillator element is responsive to and the filter passes frequencies in the range of 6.8Hz to 20Hz. 10. A monitor system according to any one of the preceding claims characterised in that the selected range of frequencies which the oscillator element is responsive to and which the filter passes is centred on a frequency of between 10Hz and 15Hz. 1 1. A monitor system according to any one of the preceding claims characterised in that the processing circuitry comprises a threshold detector. 12. A monitor system according to claim 11 characterised in that the threshold detector comprises a comparitor for comparing the signal passed by the filter with a control signal. 13. A monitor system according to claim 11 or claim 12 characterised in that the processing circuitry activates an alarm in response to the output of the threshold detector. 14. A monitor system according to any one of the preceding claims characterised in that the monitor system also detects other forms of physical activity associated with a living being to generate a signal which is passed to the processing circuitry. 15. A monitor system according to claim 14 characterised in that the processing circuitry is responsive to all the signals it receives when generating a signal to actuate an alarm. 16. A monitor system for monitoring the heartbeat of a living being, comprising a transducer responsive to the heartbeat to generate a heartbeat signal, a filter for filtering the heartbeat signal and processing circuitry to process the filtered signal, characterised in that the system comprises an oscillating element responsive to the heartbeat signal to generate a sinusoidal signal of decaying amplitude having a frequency within a selected range of frequencies centred on a frequency of between lOHz and 15Hz and the filter passes only those frequencies in a selected range centred on a frequency of between lOHz and 15 Hz. 17. A method for monitoring the heartbeat of a living being, remote from the living being, including the steps of: generating a heartbeat signal from a sensed heartbeat movement, filtering the heartbeat signal, and processing the filtered signal, characterised in that the system further comprises the steps of: generating a sinusoidal oscillation in response to the heartbeat signal which sinusoidal oscillation has a decaying amplitude and a half cycle which approximates to the heartbeat signal, filtering only those frequencies in a selected range close to the frequency of the sinusoidal oscillation generated by the oscillating element.

Inventors: GIBBON PAUL WILLIAM, ROSS KEITH ALEXANDER, RUSSELL BRIAN ERIC, SMITH DEREK TRAVERS, GIBBON, PAUL, WILLIAM, ROSS, KEITH, ALEXANDER, RUSSELL, BRIAN, ERIC, SMITH, DEREK, TRAVERS
Applicants: GIBBON PAUL WILLIAM, MOVEMENT CONTROL SYSTEMS LIMIT, ROSS KEITH ALEXANDER, RUSSELL BRIAN ERIC, SMITH DEREK TRAVERS, GIBBON, PAUL, WILLIAM, MOVEMENT CONTROL SYSTEMS LIMITED, ROSS, KEITH, ALEXANDER, RUSSELL, BRIAN, ERIC, SMITH, DEREK, TRAVERS
Application year: 1997
Main classification: A61B5
IPC codes: A61B5, G01P13