OSTENSIBLE DAY-NIGHT DIFFERENCE OF QT PROLONGATION DURING LONG-TERM TREATMENT WITH ANTIARRHYTHMIC DRUGS - REAPPRAISAL OF THE LAW OF REGRESSION TO THE MEAN

This study was designed to test whether the law of regression to the m ean explains the diurnal variation in the modulation of electrocardiog raphic variables during the treatment with antiarrhythmic agents. In p art 1, in 34 subjects, ambulatory ECG monitorings were repeated twice, and the collected QT interval (QT(c)) at a heart rate of 60 beats/min was calculated separately for the daytime and night. The individual d iurnal QT(c) variation (day-night difference) of the first recording ( 4.4 +/- 3.3%) was significantly correlated with that of the second rec ording (5.0 +/- 3.1%; r = 0.61; p < 0.0001), and naturally, the second measurement tended to be lower than the first value in those with rel atively greater baseline diurnal QT(c) variation and vice versa (p < 0 .005). In part 2, 30 subjects undertook ambulatory ECG recordings befo re and during treatment with class Ia antiarrhythmic drugs. Mean QT(c) changes in the daytime and in the night with the drugs were comparabl e (18 +/- 17 ms and 19 +/- 15 ms). However, the day-night difference o f postdrug QT(c) changes in each subject was inversely correlated with baseline diurnal QT(c) variation (r = -0.64; p < 0.0001). These obser vations in parr 2 were comparable with those in part 1, and individual day-night difference in QT prolongation with antiarrhythmic drugs see med to be a chance oo occurrence. It was suggested that the law of reg ression to the mean is appreciably reflected in the ostensible intrada y Variation of pharmacologic modulation of electrocardiographic variab les.