ELECTROCARDIOGRAMS like the sample sections diagramed above represent the paths taken by electrical impulses that the heart muscle generates as it contracts and expands. The first three columns show impulses picked up from pairs of electrodes at the extremities (Lead I, wrist to wrist; Lead II, right arm to left leg; Lead III, left arm to left leg); the second three columns from single electrodes over the chest.
In the normal Lead I, a short, heavy horizontal line shows the resting heart. The short downstroke, which doctors call the "Q" wave, is formed as...
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