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Get the datashseet of AD620 Instrumentation Amplifier from www.analog.com . You'll find schematic of ECG on page 12. Connect the output of this schematic to ADC IC. Inetrface ADC to parallel port and read the digital data and plot it or do any manipulations required.
An alternative - purchase or borrow a Heart Rate Monitor that an athelete might use - at least the chest strap portion. It transmits a pulse each time it sees a heartbeat. I did some tinkering and it appeared that the transmitted frequency wasn't particularly high (50 kHz or something like that) - a small coil pickup was sensitive enough for me to see the pulses on a scope. I could also hear the pulses on my AM radio if I wasn't too far away. With a filter you'd be able to keep noise out and trigger something that could easily adapt to the input of a computer.
Sort of related - immunity to electrical noise was a problem for my HRMs so I called several manufacturers to see if they had anything that was more immune to electrical noise than other models. It was tough getting any kind of answer but one response was "we all use the same IC in the receiver so I doubt that any one mfr will be better..." That suggested that someone makes an IC receiver - no doubt the transmitter too. Can anyone identify the ICs?
A much simpler idea: construct only the analog ECG using AD620 and connect its output to sound card LINE IN. Now you can read the digital data from your sound card with good accuracy and sampling rate.