DIFFERENTIAL RESPONSE OF QTU INTERVAL TO EXERCISE, ISOPROTERENOL, AND ATRIAL-PACING IN PATIENTS WITH CONGENITAL LONG QT SYNDROME

Sympathetic stimulation is well known to contribute to the genesis of QTU prolongation and ventricular tachyarrhythmias in patients with congenital long QT syndrome. In this study, we performed exercise treadmill testing, isoproterenol infusion (1-2-mu-g/min), and right atrial pacing (cycle length 500 msec) in 11 patients with congenital long QT (LQT) syndrome (LQT group) and in 12 age- and sex-matched controls (control group). The responses of the corrected QT (QT(c); Bazett's method) interval and the TU wave complex were evaluated. The QT(c) interval was prolonged from 482 +/- 63 msec1/2 to 548 +/- 28 msec1/2 by exercise in the LQT group (n = 11; P < 0.005), and this was associated with fusion of the T waves with enlarged U waves, whereas the QT(c) interval did not increase with exercise in the control group (n = 12; 402 +/- 19 msec1/2 vs 409 +/- 22 msec1/2). The QT(c) interval was also prolonged from 466 +/- 50 msec1/2 to 556 +/- 33 msec1/2 by isoproterenol in the LQT group (n = 7; P < 0.005) in association with morphological changes of the TU wave complex like those seen with exercise, whereas it was only slightly increased from 399 +/- 10 msec1/2 to 436 +/- 13 msec1/2 by isoproterenol in the control group (n = 77; P < 0.001). However, the QT(c) interval did not increase with atrial pacing in the LQT group (n = 8; 476 +/- 57 msec1/2 vs 486 +/- 59 msec1/2), whereas it was slightly increased from 400 +/- 21 msec1/2 to 426 +/- 18 msec1/2 by atrial pacing in the control group (n = 8; P < 0.005). These results suggest that sympathetic stimulation plays an important role in the QTU prolongation and marked TU wave complex abnormalities in patients with congenital long QT syndrome.