Evaluation of direct respiratory modulation of the QT interval variability

To investigate the direct respiration-mediated vagal modulation of the QT interval variability, spectral analyses of the RTp interval (from the R wave peak to the T wave peak) variability (RTpV) and the RR interval variability (RRV) were performed in 12 subjects with normal ventricular repolarization under three conditions while the respiration frequency was kept at 0.2 Hz: during sinus rhythm, during fixed atrial pacing, and during fixed atrial pacing with autonomic blockade. The cross-spectrum between the RRV and RTpV was quantified by the squared coherence. During sinus rhythm the RRV power spectrum showed two peaks: a broad peak in the low frequency (LF) band and a sharp peak at 0.2 Hz which corresponded to the controlled respiration frequency. The RTpV power spectrum showed corresponding peaks to the RRV peaks in both the LF and high frequency (HF) bands with high coherence (mean maximum values of the squared coherence in the LF band 0.59 +/- 0.22, and in the HF band 0.74 +/- 0.14). During atrial pacing mean total power of the RTpV decreased from during sinus rhythm (from 16.3 +/- 5.6 ms2 to 12.9 +/- 5.4 ms2, P < 0.05) and the RTpV spectral peaks were abolished in both the LF and HF bands concordant with disappearance of the RRV peaks. Autonomic blockade gave no additional change to the RTpV power spectrum independently of the RRV during fixed atrial pacing. The present study suggested that the direct respiration-mediated vagal modulation may not affect the short-term variability of the QT interval in subjects without repolarization abnormality.