The long Q-T interval and syndromes.

The clinical syndromes characterized by an abnormally long Q-T interval in the ECG are classified at present as acquired (rate-dependent), congenital (adrenergic-dependent), and miscellaneous. The therapy of each differs depending on the cause, the principal of which are drugs (for the acquired form), excessive sympathetic activity (for the congenital form), and a variety of causes for the miscellaneous group. The long Q-T interval, ascribable in some cases to pathologic disparity of durations of the excited state, aggravated under certain conditions by increased sympathetic activity transmitted through the stellate ganglia and sympathetic cardiac nerves, are at present the most probable electrical sources of arrhythmogenesis in the LQTS. However, there are other possibilities for the development of threshold potentials during the Q-T and Q-U intervals (regarding the U wave as part of the myocardial recovery process). These include minor deviations in the time or site of ventricular activation, the onset of recovery before excitation is completed, physiologic and pathologic displacements of the S-T segment (large ventricular gradient), desynchronized depolarization and repolarization, principally endocardial (in ischemic heart disease), early afterdepolarization with arrested repolarization, and late afterdepolarization following a slow diastolic wave that may be related to the U wave. Although the basis for some of these possibilities has been observations made at the membrane level principally on the canine Purkinje fiber, evidence for the clinical validity of some has accumulated in isolated examples. It is likely that additional clinical proof will be forthcoming, and with it additional means of managing the malignant ventricular arrhythmias encountered in the long Q-T syndromes. A number of drugs with the desired selective actions are under active investigation.