from The Hippocrates Project at the New York University School of Medicine.

http://endeavor.med.nyu.edu/courses/
physiology/courseware/ekg_pt1/

A sophisticated shockwave tutorial describing how electrical depolarization and repolarization of heart tissue drives the heart and is recorded on a standard Electrocardiogram (ECG or EKG). Professor Sayegh, Physics Department, University of Michigan also has a site which explains some of the physics of an EKG.

The Hippocrates site contains other multimedia medical tutorials available to the public.