The prolonged QT interval--a frequently unrecognized abnormality.

The clinical importance of the long QT interval syndrome lies in its association with malignant ventricular dysrhythmias which are usually amenable to treatment with either drugs or cardiac pacing. The QT interval extends from the beginning of the QRS complex (whether the initial component is a Q or an R wave) to the end of the T wave. It therefore represents the algebraic sum of the individual action potentials of the ventricular myocytes, including both depolarization (the QRS complex or QJ interval) and repolarization (the T wave or JT interval). The rapid process of depolarization is rarely sufficiently prolonged to make any clinically significant difference to the QT interval duration and the term effectively refers to changes in the duration ofrepolarization, that is the JT interval.