Evaluation of direct respiratory modulation of the QT interval variability

To investigate the direct respiration-mediated vagal modulation of the QT i nterval variability, spectral analyses of the RTp interval (from the R wave peak to the T wave peak) variability (RTpV) and the RR interval variabilit y (RRV) were performed in 12 subjects with normal ventricular repolarizatio n 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 fr equency. The RTpV power spectrum showed corresponding peaks to the RRV peak s in both the LF and high frequency (HF) bands with high coherence (mean ma ximum 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 RT pV decreased from during sinus rhythm (from 16.3 +/- 5.6 ms(2) to 12.9 +/- 5.4 ms(2), P < 0.05) and the RTp V spectral peaks rr ere abolished in both the LF and HF bands concordant with disappearance of the RRV peaks. Autonom ic blockade gave no additional change to the RTpV power spectrum in depende ntly of the RRV during fixed atrial pacing. The present study suggested tha t the direct respiration-mediated vagal modulation may not affect the short -term variability of the QT internal in subjects without repolarization abn ormality.