As I was surfing around today, I ran across this great Shockwave animation showing the waves of an electrocardiogram in the three appendicular leads as the electrical vector is plotted on the heart:
It automatically plays at an appropriate speed to see what's going on . . . as long as you watch it several times, each time following a different aspect. But it also has a "step" button that allows you to step through the animation in tiny increments to follow the simultaneous processes even more carefully. It's brilliantly done!
One could link to the animation during a lecture and play it through as you explain the electrical activity of the heart. Or provide the link to students for self-study.
But even if we don't get to this level of discussion in our classes, I think it's great for reinforcing our own visualization of the electrical activity of the heart and its relationship to the "normal" ECG waves.