ST Segment and T Wave

Repolarization of the ventricles begins at the end of the QRS complex and consists of the ST segment and T wave. Whereas ventricular depolarization occurs along established conduction pathways, ventricular repolarization is a prolonged process that occurs independently in every cell. The T wave represents the uncancelled potential differences of ventricular repolarization. The junction of the QRS and the ST segment is the J junction. The T wave is sometimes followed by a small U wave, the origin of which is unclear. An inverted U wave has been associated with several clinically significant conditions, such as hypertension, coronary artery disease, valvular heart disease, and certain metabolic disorders. There may be an association between exercise- or rest-related U-wave

TABLE 34-1 -- Spontaneous electrical activity of cardiac tissues

Tissue	Intrinsic Depolarization Rate (Beats/Min)
Sinoatrial node	>60
Atrioventricular node	40–60
Purkinje fibers	<40

Figure 34-2 Einthoven triangle. (From Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.)