An electrocardiogram (e-lek"tro-kar'de-o-gram") (ECG) is a recording of the electrical changes that occur in the myocardium during a cardiac cycle. (This pattern occurs as action potentials stimulate cardiac muscle fibers to contract, but it is not the same as individual action potentials.) Because body fluids can conduct electrical currents, such changes can be detected on the surface of the body.

To record an ECG, electrodes are placed on the skin and connected by wires to an instrument that responds to very weak electrical changes by moving a pen or stylus on a moving strip of paper. Up-and-down movements of the pen correspond to electrical changes in the myocardium. Because the paper moves past the pen at a known rate, the distance between pen deflections indicates time elapsing between phases of the cardiac cycle.

As figure 15.21a illustrates, a normal ECG pattern includes several deflections, or waves, during each cardiac cycle. Between cycles, the muscle fibers remain polarized, with no detectable electrical changes. Consequently, the pen does not move and simply marks along the baseline. When the S-A node triggers a cardiac impulse, the atrial fibers depolarize, producing an electrical change. The pen moves, and at the end of the electrical change, returns to the base position. This first pen movement produces a P wave, corresponding to depolarization of the atrial fibers that will lead to contraction of the atria (fig. 15.21b-e).

When the cardiac impulse reaches the ventricular fibers, they rapidly depolarize. Because the ventricular walls are thicker than those of the atria, the electrical change is greater, and the pen deflects more. When the electrical change ends, the pen returns to the baseline, leaving a mark called the QRS complex, which usually consists of a Q wave, an R wave, and an S wave. This complex appears due to depolarization of the ventricular


The muscle fibers within the ventricular walls are arranged in patterns of whorls. The fibers of groups (a) and (b) surround both ventricles in these anterior views of the heart.


The muscle fibers within the ventricular walls are arranged in patterns of whorls. The fibers of groups (a) and (b) surround both ventricles in these anterior views of the heart.

fibers just prior to the contraction of the ventricular walls (fig. 15.21f and g).

The electrical changes occurring as the ventricular muscle fibers repolarize slowly produce a T wave as the pen deflects again, ending the ECG pattern (fig. 15.21h). The record of the atrial repolarization seems to be missing from the pattern because the atrial fibers repolarize at the same time that the ventricular fibers depolarize. Thus, the QRS complex obscures the recording of the atrial repolarization.

Physicians use ECG patterns to assess the heart's ability to conduct impulses. For example, the time period between the beginning of a P wave and the beginning of a QRS complex (P-Q interval, or if the initial portion of the QRS wave is upright, P-R interval) indicates the time for the cardiac impulse to travel from the S-A node through the A-V node. Ischemia or other problems affecting the fibers of the A-V conduction pathways can increase this P-Q interval. Similarly, injury to the A-V bundle can extend the QRS complex, because it may take longer for an impulse to spread throughout the ventricular walls (fig. 15.22).

U What is an electrocardiogram?

Which cardiac events do the P wave, QRS complex, and T wave represent?

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