What is an EKG?

What does that durned squiggle mean?

Thankfully, the nature of the electical singals of the heart are such that they can be monitored noninvasively through leads on the skin. A lead samples the potential at two points, compares them, and then itself carries a signal showing the difference. So, we all know that lead I shows the difference in potential between the left and right arm.

But leads also have polarity. Remember how a cell when it depolarizes becomes less negate or more positive? Well, a depolarization (change to more positive) towards the direction of the positive half of a lead (such as the left arm for lead I) makes the lead show a positive value. When plotted on the infamous EKG paper, this shows as a positive deflection.