Baroreflex and oscillation of heart period at 0.1 Hz studied by alpha-blockade and cross-spectral analysis in healthy humans

1. Parameters derived from frequency-domain analysis of heart period and bl ood pressure variability are gaining increasing importance in clinical prac tice. However, the underlying physiological mechanisms in human subjects ar e not fully understood. Here we address the question as to whether the low frequency variability (similar to0.1 Hz) of the heart period may depend on a baroreflex-mediated response to blood pressure oscillations, induced by t he alpha -sympathetic drive on the peripheral resistance. 2. Heart period (ECG), finger arterial pressure (Finapres) and respiratory airflow were recorded in eight healthy volunteers in the supine position wi th metronome respiration at 0.25 Hz. We inhibited the vascular response to the sympathetic vasomotor activity with a peripheral alpha -blocker (urapid il) and maintained mean blood pressure at control levels with angiotensin I I. 3. We performed spectral and cross-spectral analysis of heart period (nn) a nd systolic pressure to quantify the power of low- and high-frequency oscil lations, phase shift, coherence and transfer function gain. 4. In control conditions, spectral analysis yielded typical results. In the low-frequency range, cross-spectral analysis showed high coherence (>0.5) and a negative phase shift (-65.1 +/- 18 deg) between RR and systolic press ure, which indicates a 1-2 s lag in heart period changes in relation to pre ssure. In the high-frequency region, the phase shift was close to zero, ind icating simultaneous fluctuations of RR and systolic pressure. During urapi dil + angiotensin II infusion the low-frequency oscillations of both blood pressure and heart period were abolished in five cases. In the remaining th ree cases they were substantially reduced and lost their typical cross-spec tral characteristics. 5. We conclude that in supine rest conditions, the oscillation of nn at low frequency is almost entirely accounted for by a baroreflex mechanism, sinc e it is not produced in the absence of a 0.1 Hz pressure oscillation. 6. The results provide physiological support for the use of non-invasive es timates of the closed-loop baroreflex gain from cross-spectral analysis of blood pressure and heart period variability in the 0.1 Hz range.