referring to fig1 a heart monitoring apparatus comprises a housing 1 which encloses the circuitry of the apparatus ( not shown ). an ic card 2 is received within a slot 3 in one wall of the housing 1 . a four digit 7 - segment liquid crystal display ( lcd ) 4 is mounted in another wall of the housing 1 together with a user input means , in the form of a double pole press - to - make switch 5 , and a led display 6 comprising a yellow led 6a and a green led 6b . a first electrode 7 and a second electrode 8 are coupled to the circuitry within the housing 1 by means of respective leads 9 , 9a which go to a common plug 10 . referring to fig2 the first and second electrodes 7 , 8 are coupled to the input of a differential amplifier 11 . the output of the differential amplifier 11 is fed to a filter 12 . the filter 12 is a low - pass filter having a - 3db point at approximately 40 hz . the output of the filter is coupled to a first input of a multiplexing and analogue - to - digital converter circuit 13 . the output of the filter 12 is also amplified by an amplifier 14 before being supplied to a second input of the multiplexer and analogue - to - digital converter circuit 13 . the output of the multiplexing and analogue - to - digital converter circuit 13 is fed to a microprocessor 15 which controls the operation of the apparatus . the first and second electrodes 7 , 8 are also coupled to the terminals of an impedance measuring circuit 16 which is driven by a signal from the microprocessor 15 . an output of the microprocessor 15 is coupled to a memory interface 17 which is arranged to receive an ic card 2 . the microprocessor 15 is also coupled to the led display 6 , an eprom 18 , a ram 19 and a real time clock circuit 20 . the real time clock 20 is powered by its own battery 21 and outputs time signals to a display interface 22 which drives the display 4 . the display interface 22 is also coupled directly to the microprocessor 15 . operating power for the apparatus is provided by a battery 23 . power is supplied to the functional blocks of the apparatus , including the microprocessor 15 , from the battery 23 via a power control circuit 24 . the power control circuit 24 is a latching circuit which operates to supply power to the microprocessor 15 on momentary operation of the switch 5 , a first pole of which is connected between the battery 23 and a control input of the power control circuit 24 . a battery voltage sensing circuit 25 is arranged for detecting the voltage of the battery 23 and for providing a signal indicative of this voltage to the microprocessor 15 . the operation of the apparatus will now be described . the ic card 2 is inserted into the slot 3 until it is in operating relation with the memory interface 17 . the subject then places the first and second electrodes 7 , 8 on respective wrists . operation of the switch 5 causes the power control circuit 24 to supply power to the microprocessor 15 . the microprocessor 15 then begins to perform a control program stored in the eprom 18 . the program stored in the eprom 18 may make use of data in the ic card to control the operation of the apparatus . for instance , the microprocessor 15 may read sampling rate data from the ic card and use this to set the sampling rate of the analogue - to - digital converter circuit 13 . the device hardware is initialized by the microprocessor 15 which also lights the yellow led 6a . there is then a short delay of approximately a third of a second , after which the microprocessor 15 applies a signal to the impedance measuring circuit 16 . the impedance measuring circuit 16 injects a small current at 10 hz through the electrodes 7 , 8 . the signal thus generated is amplified by the differential amplifier 11 , filtered by the filter 12 and fed directly to the multiplexing and analogue - to - digital converter circuit 13 . the microprocessor 15 monitors the output from the multiplexing and analogue - to - digital converter circuit 13 and determines an impedance value which is stored in the ram 19 . the impedance measurement is then terminated . the microprocessor 15 , then tests for the presence of an ic card 2 in the slot 3 . if no card is detected , the yellow led 6a is caused to flash for a period and the monitoring cycle of the device is terminated . if a card is , however , detected the microprocessor accesses the ic card 2 to read certain items of card specific data and any control data . in the embodiment described the card is pre - programmed with data representing a card id number , a recording interval , the card size and a check sum . however , other data such as an a - to - d sampling rate may also be included . the recording interval sets the duration of an ecg monitoring operation . if the card has already received a recording it will include data indicating the number of recordings which have been made . the microprocessor 15 first checks the check sum to ensure that the data read from the ic card 2 has not been corrupted and that the ic card 2 is of an appropriate type . from the card size and the number of recordings on the card , the microprocessor 15 calculates the number of recording spaces remaining on the card . this number is then sent , by the microprocessor 15 , via the display interface 22 to be displayed on the display 4 . if the ic card is found to be full , operation of the device is terminated and the yellow led 6a is caused to flash for a period . the microprocessor 15 , then accepts a signal from the battery voltage sensor 25 , indicating the initial battery voltage . if this voltage is below a predetermined level the microprocessor 15 causes the green led 6b to flash to signal a low battery warning . the ecg signals picked up by the electrodes 7 , 8 are amplified by the differential amplifier 11 filtered by the filter 12 and then amplified by the amplifier 14 before being supplied to the multiplexer and analogue - to - digital circuit converter circuit 13 . the microprocessor 15 takes the data output from the multiplexer and analogue - to - digital converter circuit 13 and stores it in the ram 19 . the microprocessor 15 monitors the passage of time by monitoring the real time clock 20 and after a period of approximately three seconds from the operation of the switch 5 , it checks whether the switch 5 is still being operated . this is performed by monitoring an input which is coupled to the battery 23 via a second pole of the switch 5 . if the switch 5 has been released the microprocessor 15 sends a signal to the power control circuit 24 which causes the power control circuit 24 to interrupt the supply of power to the microprocessor 15 , returning it to its dormant state . if the switch 5 is indeed still being operated after three seconds , the ecg monitoring is continued until the end of the period , defined by the recording interval read from the ic card 2 . the microprocessor 15 determines when the monitoring period is over with reference to the real time clock 20 . once the ecg monitoring has been completed the impedance between the electrodes 7 , 8 is again determined and the result stored in the ram 19 . a calibration operation is then carried out . the microprocessor 15 supplies a signal to the calibrating circuit 26 which supplies a 1 millivolt peak - to - peak 2 hz square wave to the input of the filter 12 . the microprocessor 15 then detects the signal output by the multiplexing and analogue - to - digital converter circuit 13 and stores it in the ram 19 . this value gives an indication of the calibration state of the apparatus . the final battery voltage is then determined and stored in the ram 19 . finally , the microprocessor 15 transfers the data stored in the ram 19 , together with time and date data from the real time clock 20 and an instrument id code from the eeprom 18 to the memory interface 17 which outputs it to the ic card 2 . the data output to the ic card 2 comprises the ecg signal data , calibration data , the initial impedance value , the final impedance value , the instrument id code , a number representing the final battery voltage , a date code and a time code . other data may be included as is deemed desirable . for instance , the state of the ic card battery may be monitored and data representing this may be stored on the ic card . the subject can repeat the monitoring operation as the card capacity permits . the subject may then remove the ic card from the housing 1 , whereupon , the card can be transferred to a central station , e . g . by post , where it is read by a microcomputer system . the operation data , including the calibration data can be used as necessary to normalize the ecg data . the ecg data read from the ic card can be processed , stored , displayed or printed as required using conventional techniques . the impedance data and battery voltage data can be used in the management of a remote subject . for instance , if the battery voltage is low the subject may be instructed to replace the batteries . the impedance data indicates whether a subject is applying the electrodes properly . therefore , if the measured impedance is high , it may indicate that the subject requires further tuition in how to use the apparatus . in an alternative embodiment , the ic card 2 is preprogrammed with data defining a monitoring regime . this data may define an interval to separate periods of ecg monitoring and the number of monitoring operations to be performed . this embodiment operates substantially in the same manner as the embodiment described above . however , following recording of a first block of ecg data , the microprocessor 15 continues to monitor the real time clock 20 and when it determines that a period , corresponding to the interval read from the ic card 2 , has passed , starts a further ecg monitoring cycle . this process is repeated until the number of recordings made corresponds to the number read from the ic card 2 . wriststrap electrodes 27 , 28 will now be described in more detail with reference to fig3 and 5 . each electrode 27 , 28 comprises a silicone rubber strip 30 . the strip 30 is approximately 240 mm long . a first portion 30a of the strip 30 is approximately 25 mm wide and extends along approximately a third of the length of the strip from one end thereof . a second portion 30b , comprising the remainder of the strip , is approximately 16 mm wide . a buckle 31 is secured at the free end of the first portion 30a by means of a small tab 30c , extending therefrom . a conductive portion 31 , of substantially the same dimensions as the first portion 30a of the strip 30 , is bonded to the first portion 30a of the strip 30 . the conductive portion is formed from nickel loaded rubber . a stud fastener 32 is located towards the free end of the first portion 30a of the strip 30 and holds the tab 30c in a loop so as to secure a buckle 34 to the strip 30 . a rivet 33 is located towards the other end of the first portion 30a . the electrode is formed into a loop by passing the free end of the second portion 30b of the strip 30 through the buckle . in use , a subject places the strip 30 about a wrist and tightens it by drawing the second portion 30b of the strip 30 through the buckle 34 so that the conducting portion 31 is in contact with his skin .
an ambulatory heart monitoring apparatus is provided with a slot for receiving an ic card . a user places electrodes on his body . operation of a switch causes the apparatus to record egg signals from the electrodes onto the ic card . operation data , including , for example , battery voltage data , data representing the impedance between the electrodes and calibration data , are stored on the ic card in association with the recorded ecg signal . the operation data is useful for managing remote subjects . also the calibration data can be used to normalize the recorded ecg data when it is processed at a remote processing station .