Respiratory modulation of human autonomic rhythms

We studied the influence of three types of breathing [spontaneous, frequenc y controlled (0.25 Hz), and hyperventilation with 100% oxygen] and apnea on R-R interval, photoplethysmographic arterial pressure, and muscle sympathe tic rhythms in nine healthy young adults. We integrated fast Fourier transf orm power spectra over low (0.05-0.15 Hz) and respiratory (0.15-0.3 Hz) fre quencies; estimated vagal baroreceptor-cardiac reflex gain at low frequenci es with cross-spectral techniques; and used partial coherence analysis to r emove the influence of breathing from the R-R interval, systolic pressure, and muscle sympathetic nerve spectra. Coherence among signals varied as fun ctions of both frequency and time. Partialization abolished the coherence a mong these signals at respiratory but not at low frequencies. The mode of b reathing did not influence low-frequency oscillations, and they persisted d uring apnea. Our study documents the independence of low-frequency rhythms from respiratory activity and suggests that the close correlations that may exist among arterial pressures, R-R intervals, and muscle sympathetic nerv e activity at respiratory frequencies result from the influence of respirat ion on these measures rather than from arterial baroreflex physiology. Most importantly, our results indicate that correlations among autonomic and he modynamic rhythms vary over time and frequency, and, thus, are facultative rather than fixed.