RELATION BETWEEN BRADYCARDIA DEPENDENT LONG QT SYNDROME AND QT PROLONGATION BY DISOPYRAMIDE IN HUMANS

Background-Recent molecular biological investigations have identified abnormal genes in familial forms of long QT syndrome, but in bradycard ia dependent acquired long QT syndrome, no such genetic abnormality ha s yet been identified. Objective-To investigate the relation between t he responses of QT interval to pacing change and to disopyramide. Meth ods-This study included 13 patients with bradyarrhythmia who had under gone pacemaker implantation. The patients were divided into two groups : group I (n = 8), patients with QT prolongation (QT interval greater than or equal to 500 ms) during bradycardia; group II (n = 5), patient s without QT prolongation (QT interval < 500 ms) during bradycardia. T he responses of QT interval caused by the change of pacing rate were d etermined and compared with the changes of the QT interval after disop yramide administration. Results-The QT interval in group I was signifi cantly longer than that in group II when the pacing rate was decreased from 110 to 50 beats/min: mean (SD) 451 (16) v 416 (17) ms at 90 beat s/min (p = 0.0033), and 490 (19) v 432 (18) ms at 70 beats/min (p = 0. 0002), respectively. The QT interval was prolonged significantly by di sopyramide in both groups, but the change was more pronounced in group I than in group II: 78 (33) v 35 (10) ms (p < 0.05). Conclusions-This study suggests that the patients showing bradycardia dependent QT pro longation are also more markedly affected by disopyramide and that abn ormal potassium channel may be the underlying mechanism.