Buffering Effects of the Heart Rate Reflexes

Background Despite constant fluctuations in cardiac preload caused by the effects of respiration and changes in posture on venous return to the heart, arterial blood pressure remains remarkably constant. The effects of instantaneous lung volume (ILV) and variations of central venous pressure (CVP) on blood pressure (BP) were studied by use of frequency domain techniques to quantify the contribution of heart rate (HR) reflexes to attenuation of the effects of changes in right ventricular preload on arterial pressure. Methods and Results Random independent variation of ILV, then CVP (obtained using lower-body negative pressure), was performed in eight humans in the supine position. HR, ILV, CVP, and systolic (SBP) and diastolic (DBP) BPs were recorded during control periods and after complete blockade obtained by use of 0.04 mg/kg atropine and 0.2 mg/kg propranolol. A frequency-domain analysis was performed on pairwise relations by the cross-spectral technique. During autonomic blockade, fluctuations in CVP were induced up to 0.14 Hz but caused corresponding changes in arterial pressure only up to 0.08 Hz (P<.02), indicating a mechanical damping effect of the heart and pulmonary vasculature. Fluctuations of BP were also delayed from CVP by 1.55 to 2.10 seconds. At frequencies <0.1 Hz, relations of CVP to all indices of BP