Role of sympathovagal interaction in diurnal variation of QT interval.

To elucidate the role of sympathovagal interaction in diurnal variation of QT interval, 24-hour ambulatory electrocardiographic recordings from 56 subjects (23 control subjects, 18 patients with atherosclerotic coronary artery disease, and 15 patients with diabetes mellitus) were studied. The QT interval at a heart rate of 60 beats/min (QT60) was determined for each of the day and night periods by regression analysis. Sympathetic and parasympathetic activities were assessed by spectral analysis of heart rate variability and represented by the low- and high-frequency components, respectively. The proportion of high-frequency component to the sum of low- and high-frequency components was used as an index of sympathovagal balance. The relative increase in QT60 at night (delta QT60 [%]) was larger in control subjects (4.2 +/- 2.1%) than in patients with coronary artery disease (2.2 +/- 1.8%; p less than 0.01) and diabetes mellitus (-1.5 +/- 4.0%; p less than 0.001). When the data from the 3 subject groups were pooled and analyzed, delta QT60 was correlated with the change in the sympathovagal balance (r = 0.554; p less than 0.001). Low-frequency component in the day alone was also related with delta QT60 (r = 0.554; p less than 0.001), but the ratio or difference of the high-frequency component value between day and night was not. These results indicate that although change in sympathovagal balance was responsible for the diurnal variation in QT interval, the enhanced sympathetic activity in the day was a major determinant of this phenomenon.