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 the ventricles begin to contract to pump blood out. The major part of the current flow is the high, thin upstroke of the "R" wave; the thin downstroke is simply return to the base line. Then follows the heavy, hump-shaped "T" wave that marks the repolarization of the muscle surface; some subjects also show a short dip called "S" (not pictured here) before the "T."

In the heart-attack phase, Lead I shows a deepened "Q" wave and a distorted "step" pattern in the downstroke of the "R" wave: the current takes a path around the area of dying muscle. This merges into an abnormal (inverted) "T" wave. After recovery from a mild attack, Lead I tracings return to normal except for the deepened "Q" wave. A more dramatic picture of changes is obtained from the V4 Lead, in which the electrode is almost directly over the damaged front wall of the heart.

This file is automatically generated by a robot program, so reader's discretion is required.