WO2001085019A2

WO2001085019A2 - Heart monitoring apparatus and method

Info

Publication number: WO2001085019A2
Application number: PCT/GB2001/002014
Authority: WO
Inventors: Thomas John Harris; Philip Needham
Original assignee: Cardionetics Limited
Priority date: 2000-05-09
Filing date: 2001-05-08
Publication date: 2001-11-15
Also published as: GB2362216A; GB0011196D0; AU2001252426A1; WO2001085019A3

Abstract

A heart monitoring apparatus and method is disclosed in which an electrocardiograph signal is obtained from a patient and digitized. The digitized electrocardiograph signal is stored directly into a low power memory device which has a capacity to store the electrocardiograph signal for only a most recent period of time. Thus the low power memory device continuously receives the electrocardiograph signal and continuously overwrites the oldest stored electrocardiograph signal in the memory device. A processor is provided for analysing the stored electrocardiograph signals to generate processed cardiac data. The processed cardiac data and the electrocardiograph signals used to obtain the processed cardiac data are stored in a main memory device. The processor is responsive to a processing instruction signal to enter a high power mode of operation to read and process the electrocardiograph signals stored in the low power memory device and to store the processed cardiac data and the associated electrocardiograph signals in the main memory device. Once processing of the electrocardiograph signals over a period of time has been completed, the processor returns to a low power mode of operation to wait for the next processing instruction signal.

Description

HEART MONITORING APPARATUS AND METHOD

The present invention generally relates to apparatus and a method for monitoring the operation of the heart of a patient. More specifically, the present invention relates to the storing and processing of electrocardiograph signals obtained from a patient in a power efficient manner.

The electrical signals associated with muscular expansion and contraction of the heart and its chambers are frequently monitored to determine diseased conditions of the heart as well as deterioration and improvement of the condition of the heart, in particular following surgery or other treatment of the disease.

Electrical signals of the heart are usually detected by means of conductive pads or contacts attached to the external chest wall and directly wired to a suitable machine which provides a graphical trace of the waveform, for a suitable display device on which the graphic waveform is represented for analysis by a suitably trained person. This however requires a patient to be permanently attached by wires to the monitoring apparatus and further requires the interpretation of a suitably trained person.

For long term monitoring of the heart of a patient, a portable battery powered device is known which allows the periodic recording of the electrocardiograph signal. At the end of the monitoring period, e.g. after seven days, the portable device can be removed from the patient and the recorded signal can be transmitted to a suitable machine for providing a graphical trace for analysis by a suitably trained person, or for the automatic analysis of the recorded signal.

US patent number 4223678 discloses a recorder for recording an electrocardiograph signal in which the signal is recorded in a low power FIFO memory and only transferred to permanent storage when a cardiac event occurs. This reduces the total amount of storage required since the signal is only stored when a cardiac event is detected to have occurred. This system still however only records the signal and does not perform any analysis and does not store processed cardiac parameters to aid diagnosis.

It is known in the prior art that the electrocardiograph signal can be automatically analysed. For example, in European patent no. 0850016 in the name of the present Applicants, the content of which is hereby incorporated by reference, a technique for analysing an electrocardiograph signal is described in which the signal is pre-processed to enhance the salient features of the electrocardiograph signal and suppress the noise and to generate a plurality of values representative of features of the electrocardiograph signal. These features are compared with reference features using a neural network. The neural network can determine whether the values representative of the features defined as a vector are within or beyond a threshold range of a reference vector defined by the values of the reference features. Thus in this way it is possible to use reference features obtained during an initial monitoring phase and to detect a threshold change in the structure of the electrocardiograph signal which can be indicative either of an improvement or a deterioration in the functioning of the heart of a patient. Also, the reference features can be obtained to be indicative of any one of a number of possible heart conditions. Thus when the measured features come within a range of the reference features, it is possible for the apparatus to give a warning that the patient is possibly suffering from a particular heart condition.

Under certain circumstances, it may be desirable for a patient to be monitored over a longer period of time, e.g. over seven days. Such monitoring is provided for in the conventional recordal technique whereby the electrocardiograph signals are periodically recorded over a long period of time. However, no analysis is provided for. However, the inventors of the present invention have determined that although the arrangement disclosed in European patent no. 0850016 is capable of providing analysis in a portable module, improvements are required to provide a more compact and efficient long term monitoring device.

Thus one aspect of the present invention provides a heart monitoring apparatus and method in which electrocardiograph signal is obtained from a patient and digitized. The digitized electrocardiograph signal is stored directly into a low power memory device which has a capacity to store the electrocardiograph signal for only a most recent period of time. Thus the low power memory device continuously receives the electrocardiograph signal and continuously overwrites the oldest stored electrocardiograph signal in the memory device. A processor is provided for analysing the stored electrocardiograph signals to generate processed cardiac data. The processing is performed as a neural network to generate processed cardiac data comprising cardiac fitness and/or stress indicators. The processed cardiac data and the electrocardiograph signals used to obtain the processed cardiac data are stored in a main memory device. The processor is responsive to a processing instruction signal to enter a high power mode of operation to read and process the electrocardiograph signals stored in the low power memory device and to store the processed cardiac data and the associated electrocardiograph signals in the main memory device. Once processing of the electrocardiograph signals over a period of time has been completed, the processor returns to a low power mode of operation to wait for the next processing instruction signal.

This aspect of the present invention provides a device which stores an electrocardiograph signal continuously for a recent period of time which is always available for processing. Power consumption of the device is kept to a minimum by keeping the processor in a "sleep" mode until processing of the electrocardiograph signal is required. The power consumption is only increased when processing of the electrocardiograph signal is required and a permanent recording of the electrocardiograph signal is also required.

This aspect of the present invention benefits from providing a device which is capable not only of efficiently and periodically recording the electrocardiograph signal, but also processing the recorded electrocardiograph signal to generate processed cardiac data over a long monitoring period in a power efficient manner. This greatly aids the job of the healthcare professional who has to review the stored data at the end of a long term monitoring period since the processed cardiac data can include indications of specific cardiac conditions or events whilst also allowing the healthcare professional to look at and analyse the raw electrocardiograph signal to confirm the automatic analysis. In other words, the automatic analysis can direct the healthcare professional to important passages in the recorded electrocardiograph signal which should be studied.

The present invention is ideally suited to a battery powered portable device that can be worn by the patient. The low power consumption during the quiescent periods greatly enhances the battery life. Further, the periodic recordal of the electrocardiograph signal rather than the continuous recordal of the electrocardiograph signal greatly reduces the memory requirements of the device thus reducing cost and size.

In one embodiment the low power memory device comprises a volatile memory medium into which the electrocardiograph signal is input by direct memory access. In order to record the electrocardiograph signal for only a most recent period of time, the low power memory device conveniently comprises a cyclical memory device wherein once the memory device is full, newly input signals overwrite the oldest signals in the memory device.

In an embodiment of the present invention the main memory device comprises a non-volatile memory medium such as flash memory. The processor used to carry out the processing operation can thus be controlled by processing instructions which are stored in the non-volatile memory.

The processing of the electrocardiograph signals is preferably carried out using the technique disclosed in European patent no. 0850016. The electrocardiograph signals are pre-processed to extract features of the electrocardiograph signal. A neural network is implemented having the extracted features as inputs as well as reference features. Thus the processed cardiac data can include the similarities or differences between the features extracted from the electrocardiograph signals and the reference features. The processed electrocardiograph data can also include other data which is useful to a healthcare professional such as heart rate, ST segment depression and QRS duration. The processed cardiac data can also identify ectopic beats and sinus arrhythmia for example. In a preferred embodiment, the low power memory device has a capacity to store data over a first period of time and the processor processes data over a second period of time which is longer than the first period of time. Thus, the processor reads all of the data out of the low power memory device and then continues to read data from the low power memory device which has been received after the start of processing. The reason for this is that processing of the data can be carried out quite quickly compared to the receipt of data and thus once the data has been processed it can be overwritten in the low power memory reducing the required capacity of the low power memory device.

In one embodiment the processing instruction signal is generated when a patient feels unwell. This is thus a manually triggered operation which, because a patient may collapse before being able to trigger processing and only trigger processing after recovery, the continuous storage of the electrocardiograph signal for a previous period of time is necessary in order to capture the events which led up to the collapse of the patient.

In an embodiment of the present invention, during processing of the electrocardiograph signal, if a patient tries to trigger processing again by indicating that they feel unwell, the request for processing is ignored and instead a time marker can be generated and stored in association with the processed cardiac data and the associated electrocardiac signal. Thus any stored time markers can be used to indicate to a healthcare professional that the patient was feeling particularly unwell at this period of time and the healthcare professional can correlate this to the corresponding recorded electrocardiograph signal and associated processed cardiac data.

In other embodiment of the present invention, the processing instruction signal can alternatively or in addition be generated automatically and periodically. This automatic recordal of the electrocardiograph signal and the processing of it is particularly useful for monitoring the reaction of the patient to drug therapy.

In one embodiment the apparatus includes a patient output device such as a display or audible output device for outputting a message to the patient for example when the main memory device is full, the monitoring period is over, or the battery power supply for the apparatus is low. A single warning message could be used to inform the patient that they should seek an appointment with the healthcare professional. Alternatively, each individual circumstance could be separately indicated.

In an embodiment of the present invention the apparatus includes an output port for outputting the data stored in the main memory device, i.e., the processed cardiac data and the associated electrocardiograph signals for each event, either triggered automatically or manually by the patient.

The second aspect of the present invention provides a heart monitoring apparatus and method in which an electrocardiograph signal is received from the patient and stored for a recent period of time in a low power storage means during a low power operation mode. The patient response signal can be input from a patient when they feel unwell. A processor operates in a low power mode of operation until a patient response signal is received. When the patient response signal is received and the processor is in a low power mode of operation, it enters a high power mode of operation wherein the electrocardiograph signal stored in the low power storage is read and processed to generate processed cardiac data and the processed cardiac data and associated electrocardiograph signal are stored in the main storage means. Once the processing of the electrocardiograph signal is completed, the processor returns to the low power mode of operation to await a next patient response signal. If during the high power mode of operation, a patient response signal is received, a time marker is generated and stored in the main storage means in association with the processed cardiac data and associated electrocardiograph signal.

Thus in accordance with this aspect of the present invention, a patient response determines when the electrocardiograph signal is stored and processed. The same response can also be used by the patient to not only trigger the storage and processing of the electrocardiograph signal, but also the marking of the electrocardiograph signal to indicate when the patient feels unwell. This provides additional feedback information to the medical practitioner. An embodiment of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of the apparatus according to an embodiment of the present invention;

Figure 2 is a flow diagram illustrating the operation of the apparatus according to the embodiment of the present invention; and

Figure 3 is a schematic diagram of the processing performed by the microprocessor on the electrocardiograph signal in the embodiment of the present invention.

Figure 1 schematically illustrates the components of a small portable heart momtoring device which can be worn by a patient for a relatively long monitoring period, e.g. seven days and in which the patient 1 has electrodes 2 placed on their chest for picking up the electrocardiograph signal. The electrocardiograph signals are amplified by an amplifier 3 and bandpass filtered by the bandpass filter 4 to remove noise. An analogue-to-digital converter (ADC) 5 digitizes the electrocardiograph signal from the patient 1 and inputs the digitized electrocardiograph signal into a low power memory device 6 using direct memory access (DMA). The analogue-to-digital converter samples the electrocardiograph signal at 100 Hz at 16 bit (2 byte) resolution. The low power memory device 6 comprises volatile random access memory (RAM) which has a low power consumption for writing data. The low power memory device 6 has a capacity to store 30 minutes of data input from the analogue-to-digital converter 5. Conveniently, the capacity of the low power memory device 6 is thus 512 kbytes. The data is stored in the low power memory device 6 from the analogue-to-digital converter 5 in a cyclical addressing manner so that the oldest stored data in the low power memory device 6 is overwritten by newly input data.

A microprocessor 7 is provided to control the analogue-to-digital converter 5 to perform the conversion and to read the data from the low power memory device 6. The microprocessor 7 is also provided with a main memory device 8 in which processor instructions are stored for controlling the microprocessor to carry out the processing and control operations. The main memory device also stores processed cardiac data obtained by the microprocessor 7 reading and processing the electrocardiograph signals stored in the low power memory device 6. The main memory device 8 also stores the electrocardiograph signal associated with the processed cardiac data.

The main memory device 8 comprises a flash memory device for the non- olatile storage of the processed cardiac data, the electrocardiograph signal associated with a number of events and the program code for controlling the microprocessor. Conveniently, the main memory device 8 has a capacity of 24 Mbytes which will permit the storage of 48 events

In this embodiment the processing of the electrocardiograph signal by the microprocessor 7 can be triggered either automatically or manually. A symptom button 10 is provided on the device to allow a patient to press the button when they feel unwell. This will cause a latch to be set in a latch circuit 11. The microprocessor 7 will periodically, e.g. every minute, "wake up" to read and reset the latch circuit 11 to determine whether the symptom button 10 has been pressed. If it has been pressed, the microprocessor stays in the "woken up" mode and begin to read and process the data in the low power memory device 6 to store the processed cardiac data and the electrocardiograph signal in the main memory device 8. If on the other hand, the latch is not set, the microprocessor will return to its "sleep" mode i.e. re-enter the low power mode of operation. Once the microprocessor 7 has completed the processing of the data for a period of time, the microprocessor 7 will "go to sleep" i.e. enter a low powered mode of operation. During the processing operation carried out by the microprocessor 7, the microprocessor 7 continues to monitor the latch circuit 11 to detect any further depressions of the symptom button 10. If the symptom button 10 is depressed during processing by the microprocessor 7, the microprocessor 7 logs the timing of the event for storage in the main memory device 6 in association with the electrocardiograph signal being processed and the processed cardiac data. This enables a healthcare professional to note that at a particular point during the recording and processing of the data the patient felt particularly unwell and this can be correlated by the healthcare professional to specific events in the cardiac data. The initiation of processing by the microprocessor 7 can also be automatic in response to signals from a timer circuit 12. The timer circuit can cause a regular generation of a processing instruction signal to cause the microprocessor 7 to wake up and process the stored electrocardiograph signal in the low power memory device 6. However, in order to avoid anxiety by the patient predicting when the next measurement period is going to take place, preferably the timer circuit 12 is operable to generate a periodic processing instruction signal which is not absolutely regular and therefore less predictable by the patient.

A patient display 14 is provided for displaying information to the patient, e.g. low battery (when the battery power is low), memory full (when the main memory device 8 has reached capacity), or test over (when the monitoring period for the patient has been completed). The patient display can take the form of light emitting diodes or of a small liquid crystal display. The patient display 14 can also display a warning to instruct the patient to seek medical advice. This can occur when the processed cardiac data for an event indicates that there has been a significant cardiac event which requires medical advice. Alternatively, the patient display 14 can simply display a message for the patient to return to the healthcare professional when any of the conditions mentioned above, i.e. low battery, memory full, or end of test, has been reached.

An input/output port 13 is provided to allow the microprocessor 7 to output the data stored in the main memory device 8 in a report form when required, e.g. at the end of the test period. The input/output port 13 can simply comprise an interface to a printer to allow the report to be printed for analysis by a healthcare professional. Alternatively, the input output port 13 can allow the apparatus to be interfaced to a computer device in which the report data can be further analyzed. The input/output port 13 can also include a communication device such as a modem or mobile communication device, e.g. a GSM transmitter to allow the data to be transmitted to a remote device. Where such a capability is provided, the microprocessor 7 can also be programmed to respond when the processed cardiac data indicates a significant event has occurred to contact a remote device to inform them that such a significant event has occurred and to identify the patient. This will enable the remote mobile monitoring of a patient by a healthcare professional centre which can react by contacting the patient to request that they seek medical advice urgently.

The apparatus of Figure 1 includes a battery power supply which is not shown which is of limited capacity and thus of reduced weight in view of the general low power consumption of the apparatus. Higher power consumption only occurs during the periodic processing periods following an event such as the pressing of a symptom button or the automatic generation of the processor instruction signal.

The operation of the apparatus will now be described in more detail with reference to the flow diagram of Figure 2.

In step SI the apparatus is initialized and the process started. In step S2 the processor is "put to sleep" and thus enters the low power mode of operation. The analogue-to- digital converter stores the electrocardiograph data in the low power memory device 6 in step S3. If at any time during the storage of data a stop signal is detected in step S4, the storage of data is terminated and in step S 12 the processor is set to a standby mode and in step S 13 the apparatus waits for re-initialization. A stop signal may be detected when a stop button is pressed or when the apparatus is required to output a report e.g. when connected to a host device such as a printer or a computer or at the end of a predetermined monitoring period such as seven days.

If, in step S4, there is no detection of a stop signal, electrocardiograph data is continuously recorded until a processing event is detected in step S5. A processing event can either comprise the manual depression of the symptom button 10 or the automatic generation of the processor instruction signal from the timer circuit 12. The processor then wakes up (at step S6) and reads and processes the data stored in the low power memory device in step S7. The electrocardiograph signal read from the low power memory device 6 is then stored by the microprocessor 7 in the main memory device 8 together with the processed cardiac data. This continues until a number n of bytes of data has been read and processed. In this embodiment 30 minutes- worth of data stored in the low power memory device 6 prior to the event is read which comprises 360 kbytes. After the event, a further 10 minutes of data (120 kbytes) of data is read from the low power memory device 6 as it overwrites the previously processed data. Thus in all 480 kbytes of data is read from the low power memory device 6 and processed. It is then determined in step S10 whether the main memory device is full, i.e. is there capacity for recordal of a further event. If not, the process returns to step S2 wherein the processor is "put to sleep", i.e. enters the low power mode of operation to await the next event. If, however, the main memory device is detected as being full, in step SI 1 the microprocessor 7 is set to a standby mode of operation in step SI 1 and in step S13 awaits re-initialization or to request to output the data.

At any time whilst the processor is in the low power mode, or when it is in the standby mode, it is possible for the data to be output via the input/output port 13 from the main memory device 8. The data can either be output as raw data or in the form of a report. If a report is required to be output, the microprocessor 7 processes the raw stored data generate the report format.

The processing of the electrocardiograph signal, when an event is triggered, will now be described in more detail with reference to Figure 3.

The processing carried out on the electrocardiograph signal is described in more detail in European patent no. 0850016. The functions performed by the programmed microprocessor comprise a preprocessor function 20 which extracts features of the electrocardiograph signal as measurements of peak heights and widths. The preprocessor function 20 can include measurement of the interval between R waves to enable the calculation of the heart rate in beats per minute. The features identifying the shape of the electrocardiograph pulse are input as a series of values defining a feature vector into a neural network 21. The neural network 21 also receives reference feature vectors which define reference heart conditions, e.g. normal or abnormal. The neural network can thus compare the input feature vectors with the reference feature vectors to determine the proximity of the input feature vectors to the reference feature vectors and therefore classify the input feature vectors accordingly. For example, the reference feature vectors could indicate one or a number of specific heart conditions. If the input feature vectors lie within the range of any one of these reference feature vectors, the output of the neural network can include an indication that the patient may be suffering from a specifically-identified heart condition. Alternatively, the reference feature vectors can simply be obtained from an initial measuring phase and if the input feature vectors differ from the reference feature vectors, this indicates a change in heart condition which can be indicated. This change may be an improvement or a deterioration in the condition of the heart. The analysis performed by the neural network can not only indicate the condition of the heart, but also the levels of stress experienced by the heart.

The output of the neural network 21 can be subject to post processing in a post processor 22. The post processor can for example comprise a further neural network for performing further analysis using the output of the neural network and other features obtained from the preprocessor 20.

The final function performed by the microprocessor is a report generator function 23 for collating the processed cardiac data into a report format when requested to do so for output over the input/output port 13.

Although the present invention has been described hereinabove with reference to a specific embodiment, the present invention is not limited to this embodiment and modifications which lie within the spirit and scope of the present invention will be apparent to a skilled person in the art.

For example, although an embodiment a latch circuit is used for inputting the processor instruction signal to the microprocessor 7, alternatively, interrupts could be used whereby the processor need not periodically wake up to read the latch. Instead, when an interrupt is received, the processor wakes up to perform processing.

It can be seen that the present invention provides a compact and efficient heart monitoring apparatus capable of periodic recordal and processing of electrocardiograph data from a patient. The use of a low power temporary memory device and a low power mode of operation by the processor during quiescent periods reduces power requirements and the use of periodic monitoring and processing reduces memory requirements.

Claims

CLAIMS:

1. Heart monitoring apparatus comprising: receiving means for receiving and digitizing an electrocardiograph signal from a patient; low power storage means for storing the electrocardiograph signal received in a recent period of time; processing means for processing the stored electrocardiograph signal to generate processed cardiac data; and main storage means for storing the processed cardiac data and associated electrocardiograph signal; wherein said processing means is adapted to be responsive to a processing instruction to enter a high power mode of operation to read and pre-process the electrocardiograph signal stored in said low power storage means to extract features, to implement a neural network having the extracted features as inputs to generate processed cardiac data comprising cardiac fitness and/or stress indicators as outputs of said neural network, and to store the processed cardiac data and associated electrocardiograph signal in said main storage means, said processing means being further adapted to enter a lower power mode of operation once processing of the electrocardiograph signal over a period of time has been completed to await a said processing instruction.

2. Heart monitoring apparatus according to claim 1, wherein said apparatus is battery powered, portable and can be worn by the patient, the apparatus including electrocardiograph leads having electrodes for placement on the patient's chest to obtain the electrocardiograph signal for input to said receiving means.

3. Heart monitoring apparatus according to claim 1 or claim 2, wherein said low power storage means comprises a volatile memory medium.

4. Heart monitoring apparatus according to claim 3, wherein said receiving means is adapted to input said electrocardiograph signal to said volatile memory medium by direct memory access.

5. Heart monitoring apparatus according to any preceding claim, wherein said low power memory means comprises a cyclical memory device.

6. Heart monitoring apparatus according to any preceding claim, wherein said main memory means comprises a non- volatile memory medium.

7. Heart monitoring apparatus according to claim 6, wherein said non- volatile memory medium comprises flash memory.

8. Heart monitoring apparatus according to any preceding claim including instruction storage means for storing process implementable instructions, wherein said processing means comprises a microprocessor for reading and implementing the instructions stored in said instruction storage means.

9. Heart monitoring apparatus according to claim 8, wherein said instruction storage means comprises part of said main storage means.

10. Heart monitoring apparatus according to any preceding claim, wherein said processing means is adapted to process the electrocardiograph signal over said period of time which is greater than said recent period of time defining the capacity of said low power storage means by allowing said low power storage means to continue to store said electrocardiograph signal after processing has begun in storage regions in said low power storage means from which the electrocardiograph signal has already been read and processed by said processing means.

11. Heart monitoring apparatus according to any preceding claim, including patient response means for use by a patient to indicate when they feel unwell and for consequently generating said processing instruction.

12. Heart monitoring apparatus according to claim 11 , wherein said patient response means is adapted to generate said processing instruction only when said processing means is in said low power mode and to generate a time marker when said processing means is in said high power mode, said processing means being adapted to store any said time markets in said main storage means in association with said processed cardiac data and associated electrocardiograph signal.

13. Heart monitoring apparatus according to claim 11 or claim 12, wherein said patient response means includes latch means which is triggerable by a patient when they feel unwell, and said processing means is adapted to periodically read said latch means to obtain said processing instruction during said low power mode.

14. Heart monitoring apparatus according to claim 13 and claim 12, wherein said processing means is adapted to periodically read said latch means to obtain the or each time marker during said high power mode.

15. Heart monitoring apparatus according to any preceding claim, including automatic means for automatically and periodically generating said processing instruction during said low power mode.

16. Heart monitoring apparatus according to any preceding claim, including patient output means for outputting a message to the patient when at least one of the following conditions exists: said main memory means is full, a monitoring period is over, and a battery power supply for said apparatus is low.

17. Heart monitoring apparatus according to any preceding claim, including data outputting means for outputting said processed cardiac data and associated electrocardiograph signal stored in said main storage means.

18. A method of monitoring a patient' s heart, comprising: receiving and digitizing an electrocardiograph signal from the patient; storing the electrocardiograph signal received in a recent period of time in low power storage means; operating a processor in a low power mode of operation until a processing instruction is received; responding to a received processing instruction by entering a high power mode of operation wherein the electrocardiograph signal stored in said low power storage means is read and pre-processed to extract features, the features are input to a neural network, the outputs of the neural network comprise processed cardiac data in the form of cardiac fitness and/or stress indicators, and the processed cardiac data and associated electrocardiograph signal are stored in main storage means; and once processing of the electrocardiograph signal over a period of time has been completed, returning the processor to the low power mode of operation to await a said processing instruction.

19. A method according to claim 18 carried out by an apparatus which is portable and can be worn by the patient, wherein said low power storage means, said processor, and said main storage means are provided in said portable apparatus, the method including placing electrodes with electrocardiograph leads on the patient's chest to transmit the electrocardiograph signal to the apparatus.

20. A method according to claim 18 or claim 19, wherein the electrocardiograph signal is stored in said low power storage means comprising a volatile memory medium.

21. A method according to claim 20, wherein the electrocardiograph signal is input to said volatile memory medium by direct memory access.

22. A method according to any one of claims 18 to 21 , wherein said electrocardiograph signal is stored cyclically in said low power memory means.

23. A method according to any one of claims 19 to 22, wherein the processed cardiac data and associated electrocardiograph signal is stored in said main storage means which comprises a non-volatile memory medium.

24. A method according to claim 23, wherein said processed cardiac data and associated electrocardiograph signal is stored in said non-volatile memory medium which comprises flash memory.

25. A method according to any one of claims 18 to 24, including storing processor implementable instructions in instruction storage means, and the processing is carried out by a microprocessor reading and implementing the instructions stored in said instruction storage means.

26. A method according to claim 25, wherein said instructions are stored in said instruction storage means which comprises a part of said main storage means.

27. A method according to any one of claims 18 to 26, wherein the processing step processes the electrocardiograph signal over said period of time which is greater than said recent period of time defining the capacity of said low power storage means by allowing said low power storage means to continue to store said electrocardiograph signal after processing has begun in storage regions in said low power storage means from which the electrocardiograph signal has already been read and processed.

28. A method according to any one of claims 18 to 27, including receiving a patient response signal from a patient when they feel unwell and consequently generating said processing instruction.

29. A method according to claim 28, wherein said processing instruction is only generated when said processor is in said low power mode and a time marker is generated when said processor is in said high power mode, any said time markers being stored in said main storage means in association with said processed cardiac data and associated electrocardiograph signal.

30. A method according to claim 28 or claim 29, wherein the patient input signal triggers latch means and said processor periodically reads said latch means to obtain said processing instruction during said low power mode.

31. A method according to claim 30 and claim 29, wherein said latch means is periodically read by said processor to obtain the or each time marker during said high power mode.

32. A method according to any one of claims 18 to 31 , including automatically and periodically generating said processing instruction during said low power mode.

33. A method according to any one of claims 18 to 32, including outputting a message to the patient when at least one of the following conditions exists: said main memory means is full, a monitoring period is over, and a battery power supply for said apparatus is low.

34. A method according to any one of claims 18 to 33, including outputting said processed cardiac data and associated electrocardiograph signal stored in said main storage means.

35. Heart monitoring apparatus comprising: receiving means for receiving and digitizing an electrocardiograph signal from a patient; low power storage means for storing the electrocardiograph signal received in a recent period of time; processing means for processing the stored electrocardiograph signal to generate processed cardiac data; and main storage means for storing the processed cardiac data and associated electrocardiograph signal; wherein said processing means is adapted to be responsive to a processing instruction to enter a high power mode of operation to read and process the electrocardiograph signal stored in said low power storage means, and to store the processed cardiac data and associated electrocardiograph signal in said main storage means, said processing means being further adapted to enter a lower power mode of operation once processing of the electrocardiograph signal over a period of time has been completed to await a said processing instruction; the apparatus further including patient response means for use by a patient to indicate when they feel unwell and to generate a said processing instruction when said processing means is in said low power mode and to generate a time marker when said processing means is in said high power mode, said processing means being further adapted to store any said time markets in said main storage means in association with said processed cardiac data and associated electrocardiograph signal.

36. Heart monitoring apparatus according to claim 35, wherein said apparatus is battery powered, portable and can be worn by the patient, the apparatus including electrocardiograph leads having electrodes for placement on the patient's chest to obtain the electrocardiograph signal for input to said receiving means.

37. Heart monitoring apparatus according to claim 35 or claim 36, wherein said low power storage means comprises a volatile memory medium.

38. Heart monitoring apparatus according to claim 37, wherein said receiving means is adapted to input said electrocardiograph signal to said volatile memory medium by direct memory access.

39. Heart monitoring apparatus according to any one of claims 35 to 38, wherein said low power memory means comprises a cyclical memory device.

40. Heart monitoring apparatus according to any one of claims 35 to 39, wherein said main memory means comprises a non- volatile memory medium.

41. Heart monitoring apparatus according to claim 40, wherein said non- volatile memory medium comprises flash memory.

42. Heart monitoring apparatus according to any one of claims 35 to 41 including instruction storage means for storing process implementable instructions, wherein said processing means comprises a microprocessor for reading and implementing the instructions stored in said instruction storage means.

43. Heart monitoring apparatus according to claim 42, wherein said instruction storage means comprises part of said main storage means.

44. Heart monitoring apparatus according to any one of claims 35 to 43, wherein said processing means is adapted to pre-process said electrocardiograph signals to extract features, and to implement a neural network having the extracted features as inputs, said processed cardiac data comprising the output of said neural network and comprising cardiac fitness and/or stress indicators.

45. Heart monitoring apparatus according to any one of claims 35 to 64, wherein said processing means is adapted to process the electrocardiograph signal over said period of time which is greater than said recent period of time defining the capacity of said low power storage means by allowing said low power storage means to continue to store said electrocardiograph signal after processing has begun in storage regions in said low power storage means from which the electrocardiograph signal has already been read and processed by said processing means.

46. Heart monitoring apparatus according to any one of claims 35 to 45, wherein said patient response means includes latch means which is triggerable by a patient when they feel unwell, and said processing means is adapted to periodically read said latch means to obtain said processing instruction during said low power mode.

47. Heart monitoring apparatus according to claim 46, wherein said processing means is adapted to periodically read said latch means to obtain the or each time marker during said high power mode.

48. Heart monitoring apparatus according to any one of claims 35 to 47, including automatic means for automatically and periodically generating said processing instruction during said low power mode.

49. Heart monitoring apparatus according to any one of claims 35 to 48, including patient output means for outputting a message to the patient when at least one of the following conditions exists: said main memory means is full, a monitoring period is over, and a battery power supply for said apparatus is low.

50. Heart monitoring apparatus according to any one of claims 35 to 49, including data outputting means for outputting said processed cardiac data and associated electrocardiograph signal stored in said main storage means.

51. A method of monitoring a patient' s heart, comprising: receiving and digitizing an electrocardiograph signal from the patient; storing the electrocardiograph signal received in a recent period of time in low power storage means; allowing a patient response signal to be input from a patient when they feel unwell; operating a processor in a low power mode of operation until a patient response signal is received; responding to a received patient response signal by entering a high power mode of operation wherein the electrocardiograph signal stored in said low power storage means is read and processed to generate processed cardiac data and the processed cardiac data and associated electrocardiograph signal are stored in main storage means; and once processing of the electrocardiograph signal over a period of time has been completed, returning the processor to the low power mode of operation to await a said patient response signal; wherein when said patient response signal is input when said processor is in said high power mode a time marker is generated, any said time markers being stored in said main storage means in association with said processed cardiac data and associated electrocardiograph signal.

52. A method according to claim 51 carried out by an apparatus which is portable and can be worn by the patient, wherein said low power storage means, said processor, and said main storage means are provided in said portable apparatus, the method including placing electrodes with electrocardiograph leads on the patient's chest to transmit the electrocardiograph signal to the apparatus.

53. A method according to claim 51 or claim 52, wherein the electrocardiograph signal is stored in said low power storage means comprising a volatile memory medium.

54. A method according to claim 53, wherein the electrocardiograph signal is input to said volatile memory medium by direct memory access.

55. A method according to any one of claims 51 to 54, wherein said electrocardiograph signal is stored cyclically in said low power memory means.

56. A method according to any one of claims 52 to 55, wherein the processed cardiac data and associated electrocardiograph signal is stored in said main storage means which comprises a non- volatile memory medium.

57. A method according to claim 56, wherein said processed cardiac data and associated electrocardiograph signal is stored in said non-volatile memory medium which comprises flash memory.

58. A method according to any one of claims 51 to 57, including storing processor implementable instructions in instruction storage means, and the processing is carried out by a microprocessor reading and implementing the instructions stored in said instruction storage means.

59. A method according to claim 58, wherein said instructions are stored in said instruction storage means which comprises a part of said main storage means.

60. A method according to any one of claims 51 to 59, wherein the processing step comprises preprocessing said electrocardiograph signals to extract features, implementing a neural network having the extracted features as inputs, and outputting from the neural network said processed cardiac data as cardiac fitness and/or stress indicators.

61. A method according to any one of claims 51 to 60, wherein the processing step processes the electrocardiograph signal over said period of time which is greater than said recent period of time defining the capacity of said low power storage means by allowing said low power storage means to continue to store said electrocardiograph signal after processing has begun in storage regions in said low power storage means from which the electrocardiograph signal has already been read and processed.

62. A method according to any one of claims 51 to 61 , wherein the patient input signal triggers latch means and said processor periodically reads said latch means to obtain said processing instruction during said low power mode.

63. A method according to claim 61, wherein said latch means is periodically read by said processor to obtain the or each time marker during said high power mode.

64. A method according to any one of claims 51 to 63, including automatically and periodically generating said processing instruction during said low power mode.

65. A method according to any one of claims 51 to 64, including outputting a message to the patient when at least one of the following conditions exists: said main memory means is full, a monitoring period is over, and a battery power supply for said apparatus is low.

66. A method according to any one of claims 51 to 65, including outputting said processed cardiac data and associated electrocardiograph signal stored in said main storage means.