Characteristics of the Normal Electrocardiogram

The normal electrocardiogram (see Figure 11-1) is composed of a P wave, a QRS complex, and a T wave. The QRS complex is often, but not always, three separate waves: the Q wave, the R wave, and the S wave.

The P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction begins. The QRS complex is caused by potentials generated when the ventricles depolarize before contraction, that is, as the depolarization wave spreads through the ventricles. Therefore, both the P wave and the components of the QRS complex are depolarization waves.

The T wave is caused by potentials generated as the ventricles recover from the state of depolarization. This process normally occurs in ventricular muscle 0.25 to 0.35 second after depolarization, and the T wave is known as a repolarization wave.

Thus, the electrocardiogram is composed of both depolarization and repo-larization waves. The principles of depolarization and repolarization are discussed in Chapter 5. The distinction between depolarization waves and repo-larization waves is so important in electrocardiography that further clarification is needed.

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