PHYSIOLOGICAL AND PATHOLOGICAL RESPONSES OF TU WAVES TO CLASS IA ANTIARRHYTHMIC DRUGS

Abnormal repolarization associated with torsades de pointes is express ed as QT prolongation. The physiological response to class Ia antiarrh ythmic drugs is also reflected in prolongation of the QT interval. How ever, the essential difference between pathological and physiological prolongation is not clear. The purpose of this investigation was to di fferentiate between pathological and physiological changes in the repo larization waves of surface electrocardiograms (ECG) induced by class Ia drugs. In 18 patients without a history of torsades de pointes or s yncope (control group), TU waves were compared before and after the ad ministration of class Ia drugs (physiological response). In eight pati ents with torsades de pointes induced by class Ia drugs (torsades de p ointes group), the TU waves at torsades de pointes were compared with those before drug administration (pathological response). In the contr ol group, although the QTc (measured in lead II and corrected for hear t rate by Bazett's formula) was increased significantly (0.04 +/- 0.04 to 0.44 +/- 0.05 s, P < 0.001), the U-amp (amplitude of the U wave me asured in a precordial lead where the T and U waves were clearly diffe rentiated) remained unchanged. In the torsades de points group, howeve r, the QTc was increased (0.42 +/- 0.04 to 0.54 +/- 0.07 s, P < 0.02); the U-amp was also increased, significantly (0.09 +/- 0.07 to 0.27 +/ - 0.18 mV, P < 0.05). Thus, enlargement of the U wave may help to diff erentiate between the physiological and pathological responses to clas s Ia drugs.