Contributions of sympathetic and vagal mechanisms to the genesis of heart rate fluctuations during orthostatic load: a spectral analysis.

Spectral analysis was utilized in order to determine the influence of postural change on heart rate fluctuations and respiratory frequencies and to evaluate the frequency-specific contributions of both vagal and beta-adrenergic mechanisms to the genesis of the heart rate fluctuations during change in posture. In unmedicated subjects the total power (0.01-0.5 Hz) of heart rate fluctuations and the relative power at the mid-frequency band (0.05-0.15 Hz) were significantly increased in response to postural change, while the relative power of high-frequency fluctuations (0.15-0.5 Hz) was significantly depressed. Low-frequency fluctuations (0.01-0.05 Hz) were unchanged. The respiratory frequency was non-significantly slowed. The coherence between heart rate and respiration showed a significant reduction during orthostatic load. The heart rate fluctuations above 0.05 Hz were abolished by vagal blockade, during supine rest, while beta-adrenergic blockade reduced fluctuations at the mid-frequency band. Combined blockade caused a depression of heart rate fluctuations over the entire frequency range. On standing, vagal blockade or autonomic double blockade caused a decrease in heart rate fluctuations at each frequency band. After beta-adrenergic blockade alone, mid-frequency and high-frequency fluctuations were significantly reduced. The coherence between heart rate and respiration was nearly abolished under vagal blockade in each body posture. We conclude that the increased sympathetic outflow during orthostatic load is reflected by a marked increase in heart rate spectral power densities in the mid-frequency range.(ABSTRACT TRUNCATED AT 250 WORDS)