Low-frequency fluctuations in heart rate, cardiac output and mean arterial pressure in humans: what are the physiological relationships?

Objective: Cardiovascular variability is a complex physiological phenomenon associated with the outcome of cardiovascular diseases. Blood pressure oscillations may cause cardiovascular complications, which, however, are also claimed to have antihypertensive effects. The physiological understanding is limited. This study evaluates whether oscillations in heart rate (HR) and cardiac output (CO) buffer fluctuations at approximately 0.1 Hz in arterial blood pressure (Mayer waves).

Method: We recorded mean arterial pressure (MAP), left cardiac stroke volume (SV), and HR in 10 healthy humans during autonomic blockade in supine and tilted (30 degrees) position. Variability in the cardiovascular variables at 0.04-0.15 Hz and phase angles (time lags) between the variables were calculated by spectral analysis.

Results: Fluctuations in cardiovascular variables at 0.1 Hz decreased after removal of HR variability (HRV) by propranolol and atropine in the supine position. Tilting from supine did not change fluctuations in MAP or total peripheral resistance (TPR), whereas variations in CO decreased. Variations in CO remained decreased in tilt after atropine compared to supine control, whereas variations in MAP and in TPR were unchanged. HRV were in phase with oscillations in CO. Variations in CO were in inverse phase with variations in TPR.

Conclusion: TPR oscillations produce fluctuations in MAP at 0.1 Hz. HRV produces CO variations, but CO variations do not efficiently buffer MAP variations during supine rest and mild ortostasis. Both feedback and feedforward mechanisms are responsible for the interaction between HR and MAP.