A cellular basis for the primary long Q-T syndromes.

In several syndromes sudden death is associated with a long Q-T interval on the electrocardiogram. Current treatment is aimed at correcting a notional imbalance of cardiac sympathetic drive. Although this approach can be effective, the primary disorder may be a defect in the cellular mechanism that alters the length of the ventricular action potential, and hence the Q-T interval, in response to a change of heart rate. Such defects may be underdiagnosed because the long Q-T syndromes are defined simply in terms of a prolonged Q-T interval, without consideration of the fact that susceptibility to arrhythmias is likely to be due, not to a lengthened Q-T per se, but to an absence or alteration of the normal Q-T shortening in response to an increase of heart rate. People susceptible to R-on-T related arrhythmias could be identified by measurement of the dependence of the Q-T interval on heart rate and how quickly the Q-T interval adapts to a change in rate.