Prolonged QT interval and the cardiac conduction tissues

The QT interval, which marks the “vulnerable” period of ventricular repolarization, is related to heart rate and can be affected by a broad range of physiopathologic conditions. The interval is considered to be prolonged when, once normalized for heart rate (QTc), it exceeds 0.44 to 0.45 second [American Heart Association criterion 1956). In view of its association with sudden death, particularly in young children, there has been a call for a collective prospective registry to assess the efficacy of therapeutics available to patients with the long QT syndrome. 1 Since the long QT syndrome is recognized as a clinical entity, general interest has focused on its causes. Anomalies of the cardiac conduction tissues have been suggested, 2–4 as have conditions such as metabolic disorders 5 and neurologic imbalance. 6 The conduction tissues were implicated after clinicopathologic investigations that concentrated on the specialized myocardium and its neurons and ganglia. Some studies pointed to degenerative processes within and in the immediate environs of the conduction system. 3,4