Background The genes for the long QT syndrome (LQTS) linked to chromosomes 3 (LQT3) and 7 (LQT2) were identified as SCN5A, the cardiac Na⁺ channel gene, and as HERG, a K⁺ channel gene. These findings opened the possibility of attempting gene-specific control of ventricular repolarization. We tested the hypothesis that the QT interval would shorten more in LQT3 than in LQT2 patients in response to mexiletine and also in response to increases in heart rate.

Methods and Results Fifteen LQTS patients were studied. Six LQT3 and 7 LQT2 patients were treated with mexiletine, and its effects on QT and QT_c were measured. Mexiletine significantly shortened the QT interval among LQT3 patients (QT_c from 535±32 to 445±31 ms, P<.005) but not among LQT2 patients (QT_c from 530±79 to 503±60 ms, P=NS). LQT3 patients (n=7) shortened their QT interval in response to increases in heart rate much more than LQT2 patients (n=4) and also more than 18 healthy control subjects (9.45±3.3 versus 3.95±1.97 and 2.83±1.33, P<.05; data expressed as percent reduction in QT per 100-ms shortening in RR). Among these patients, there is also a trend for LQT2 patients to have syncope or cardiac arrest under emotional or physical stress and for LQT3 patients to have cardiac events either at rest or during sleep.

Conclusions This is the first study to demonstrate differential responses of LQTS patients to interventions targeted to their specific genetic defect. These findings also suggest that LQT3 patients may be more likely to benefit from Na⁺ channel blockers and from cardiac pacing because they would be at higher risk of arrhythmia at slow heart rates. Conversely, LQT2 patients may be at higher risk to develop syncope under stressful conditions because of the combined arrhythmogenic effect of catecholamines with the insufficient adaptation of their QT interval when heart rate increases.