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You could take a look at the manual for a commercial device - The Polhemus Fastrack manual has a reasonable theory of operation section. Look here hxxp://www.polhemus.com/support.htm.
You could also try doing a patent search at www.uspto.gov. Many patents contain simplified schematics.
I have made some great looking panels by inkjet printing on good quality paper and then laminating in an adhesive backed lamination pouch. Check out the lamination supplies at your local office supply store.
I suggest you visit ti.com, analog.com, linear.com etc. and do a search on ECG and EKG ( electrocardiogram ). Many of these semiconductor vendors have app. notes for simple ECG amplifiers and heart rate determination.