Paradoxical effects of exercise on the QT interval in patients with polymorphic ventricular tachycardia receiving type Ia antiarrhythmic agents

We analyzed the results of exercise testing performed in the absence of all antiarrhythmic drugs in 11 case patients with newly documented polymorphic ventricular tachycardia in response to type Ia antiarrhythmic agents. These results were compared with those found in 11 control patients matched for age, sex, and heart disease to determine whether the response of the QT interval to exercise testing was abnormal in patients who developed worsening of arrhythmia while taking antiarrhythmic drugs. QT, RR, and QT_c intervals (by Bazett's method) were evaluated at rest and at 3 minutes of exercise in both groups. At rest, there was no significant difference in the QT interval (410±13 vs. 386±11 msec), RR interval (890±56 vs. 781±43 msec), or corrected QT interval (438±10 vs. 438±4 msec) in the case patients and the control patients. Both groups demonstrated a similar chronotropic response to exercise. The QT interval shortened in both groups with exercise (p<0.001), but the degree of shortening tended to be greater in the control patients (to 310±9 msec) than in the case patients (to 357±11 msec) (p=0.06). Thus, there was a paradoxical increase in the QT_c interval in the patients who experienced a proarrhythmic effect of type Ia drugs but not in the control patients (to 482±8 vs. 431±5 msec; p<0.001). Ten of 11 case patients but only one of 11 control patients had an increase in QT_c interval of more than 10 msec with exercise (p<0.001). We conclude that patients who develop polymorphic ventricular tachycardia in response to type Ia antiarrhythmic agents have an abnormal repolarization response to exercise.