Relation Between Autonomic Nerve Activity and QT Interval in Patients with Congenital Long QT Syndrome: Analysis Using 24‐Hour Holter ECG Monitoring

Background The aim of this study was to determine the relation between the autonomic nerve activity and the QT interval in patients with the long QT syndrome (LQTS) using spectral analysis of the heart rate variability from 24‐hour Holter ECG recordings. Methods Seven patients with LQTS (QT c interval > 0.46 s) who had had no recent episode of syncope and nine control subjects without QT prolongation were analyzed. Autoregressive spectral analysis was performed to determine spectral power of low (LF: 0.04–0.15 Hz) and high (HF: 0.15–0.40 Hz) frequency components. The QT interval and RR interval were measured from ECGs that were selected from 24‐hour Holter tape at 1‐hour intervals. Results In patients with LQTS and control subjects, there was a positive correlation between the QT interval and RR interval (LQTS: r 2 = 0.76 ± 0.18, Control: r 2 = 0.90 ± 0.10, mean ± SD). The slope of the linear regression lines (QT/RR) did not differ between LQTS and control subjects (LQTS: 0.18 ± 0.05, Control: 0.18 ± 0.04), but the intercept of the regression lines in LQTS was significantly larger than in control (LQTS: 0.28 ± 0.05, Control: 0.21 ± 0.02). In LQTS, positive correlation between the QT interval and InHF (r 2 = 0.53 ± 0.09) and inverse correlation between the QT interval and In(L/H) were identified (r 2 = 0.50 ± 0.14) as in control subjects. There was no significant difference in the slopes of the regression lines of the QT interval with heart rate variability indices (InHF, In[L/H]) between LQTS and control subjects. The correlation coefficient of the QT interval with RR interval was higher than that with heart rate variability indices both in LQTS (P < 0.01) and control subjects (P < 0.05). In three patients with recent syncopal episodes, the prolongation of QT interval was exaggerated at short RR intervals, and the significant correlations of the QT interval with RR interval and heart rate variability indices observed from the recordings without episodes of syncope could not be identified. Both 24‐hour averaged value of InHF and L/H did not show any significant difference between LQTS and control. Conclusions These results indicate that in LQTS without having recent episodes of syncope, the QT interval was regulated mainly through the change in RR intervals as in control subjects. Exacerbation of QT prolongation during syncopal episodes may be due to different modulation of the QT interval by autonomic nerve system and heart rate.