Is the QT interval an indicator of autonomic state?

Prolonged QT interval is suggested to indicate an increased risk of sudden cardiac death in certain clinical conditions such as diabetes mellitus. We investigated whether the individual QT interval is an indicator of an auton omic state. An ambulatory 24-hour ECG was recorded in 53 subjects from diff erent clinical backgrounds. Power spectral components of heart rate variabi lity (HRV) and the QT interval were regressively obtained at a heart rate o f 60, 70, 80, 90, or 100 beats per minutes (bpm). Log values of the high-fr equency component of HRV (HF: 0.15-0.50 Hz, a scale of cardiac parasympathe tic tone) failed to show a relationship with the QT interval. In contrast, the QT interval at a heart rate of 90 bpm and I00 bpm showed a significant correlation with the log Values of the low-frequency component (LF: 0.04-0. 15 Hz) and the log[LF / HF], i.e., a putative scale of sympathetic tone (10 0 bpm: QT vs logLF: r = 0.414, p < 0.005, QT vs log[LF / HF]: r = 0.416, p < 0.002). Also, attenuated rate-dependent QT shortening was associated with greater logLF and log[LF / HF] values at a heart rate of 80, 90, or 100 bp m. These results suggest that the QT interval at a moderate heart rate (app roximately 90-100 / min) and the degree of rate-dependent QT shortening are related to individual sympathetic tone.