Possible mechanisms of the arrhythmias in the long QT syndrome.

Torsades de pointes (TP) are observed in the setting of four different clinical syndromes. Two are easily recognized by prolongation of the QT interval: acquired and congenital long QT syndromes (LQTS). Two have no patent long QT, but they do have many characteristics in common with the LQTS: catecholamine-dependent ventricular tachycardias, and short-coupled TP. The mechanism of TP is undetermined. Re-entry phenomena as well as early after depolarizations, occurring in a single or several arrhythmogenic areas may equally explain the curious pattern and behaviour of TP, and the lack of an experimental model makes difficult the progress of our knowledge. A fundamental myocardial disorder might be common to the different clinical syndromes. The existence of a substrate for the arrhythmia was reflected in one of our patients by the large discrepancy between the endocardial and epicardial refractory period duration. A common substrate, with different inputs and outputs, would explain consistently (a) why the autonomic nervous system is always involved in triggering TP, but in variable and even opposite ways according to the syndrome in cause, and (b) why the TP response to drugs is also variable and often paradoxical.