BRADYCARDIA-INDUCED ABNORMAL QT PROLONGATION IN PATIENTS WITH COMPLETE ATRIOVENTRICULAR-BLOCK WITH TORSADES-DE-POINTES

Fourteen patients with complete atrioventricular block with or without torsades de pointes (TdP) were included in this study. They were divided into 2 groups, 6 patients with TdP (TdP[+] group) and 8 patients without TdP (TdP[-] group). The patients were evaluated at 2 different periods, before (acute period) and after (chronic period) pacemaker implantation. In the acute period, the QRS and heart rate during the escape rhythm were not significantly different between the 2 groups; however, the QT and QTc intervals were significantly longer in the TdP(+) group than in the TdP(-) group: 753 +/- 57.5 vs 635 +/- 78.4 ms (p < 0.01) and 585 +/- 44.8 vs 476 +/- 58.3 ms (p < 0.01). In the chronic period (> 2 months after pacemaker implantation), we changed the pacemaker rate from 90 or 100 beats/min to 50 beats/min and examined the QT interval changes in relation to the heart rate. The QT interval in the TdP(+) group was significantly prolonged compared with the TdP(-) group when the pacing rate was decreased less-than-or-equal-to 60 beats/min: 551 +/- 40 vs 503 +/- 36 ms at 60 beats/min (p < 0.05), and 700 +/- 46 vs 529 +/- 43 ms at 50 beats/min (p < 0.001). Patients with complete atrioventricular block with TdP had a brady-cardia-sensitive repolarization abnormality and this characteristic remained after pacemaker implantation. The critical heart rate that induced abnormal QT prolongation in the TdP(+) group was less-than-or-equal-to 60 beats/min.