Evidence of low- and high-frequency oscillations in human AV interval variability: evaluation with spectral analysis.

The spontaneous beat-to-beat variation of atrioventricular (AV) interval was analyzed in time and frequency domains and compared with atrial cycle length (A-A) variability. The analysis was applied in humans at rest and tilt position during sinus rhythm and atrial pacing. The AV intervals showed spontaneous oscillations of small amplitude with a relative standard deviation of 1.8%. Spectral analysis of AV interval series showed the existence of two main oscillatory components at low frequency (LF; 0.04-0.13 Hz) and at high frequency (HF; 0.17-0.4 Hz), synchronous with those of A-A interval series. The same LF and HF fluctuations were found in AV interval variability during atrial pacing. Tilt maneuver inducing a sympathetic stimulation and vagal withdrawal increased LF power and decreased HF power of A-A interval spectra (P < 0.05). On the contrary, tilt decreased the LF (46%, P < 0.05) and HF power (29%, P = NS) of AV conduction spectra. In parallel, tilt decreased (P < 0.0001) the mean A-A interval, leaving the mean AV interval unchanged. When heart rate was held constant by atrial pacing, tilt reduced the mean AV interval, the LF power (65%, P < 0.05), and HF power (10%, P = NS). These results indicate a direct influence of the autonomic nervous system on the LF and HF oscillations of AV conduction. The decrease of AV interval oscillations during tilt demonstrates a marked reduction of autonomic modulation of AV conduction mainly attributable to parasympathetic withdrawal.