PHYSIOLOGY AND PATHOPHYSIOLOGY OF HEART-RATE AND BLOOD-PRESSURE VARIABILITY IN HUMANS - IS POWER SPECTRAL-ANALYSIS LARGELY AN INDEX OF BAROREFLEX GAIN

1. It is often assumed that the power in the low- (around 0.10 Hz) and high-frequency (around 0.25 Hz) bands obtained by power spectral anal ysis of cardiovascular variables reflects vagal and sympathetic tone r espectively. An alternative model attributes the low-frequency band to a resonance in the control system that is produced by the inefficient ly slow time constant of the reflex response to beat-to-beat changes i n blood pressure effected by the sympathetic (with or without the para sympathetic) arm(s) of the baroreflex (De Beer model). 2. We have appl ied the De Beer model of circulatory variability to patients with vary ing baroreflex sensitivity and one normal subject, and have shown that the main differences in spectral power (for both low and high frequen cy) between and within subjects are caused by changes in the arterial baroreflex gain, particularly for vagal control of heart rate (R-R int erval) and left ventricular stroke output. We have computed the power spectrum at rest and during neck suction (to stimulate carotid barorec eptors). We stimulated the baroreceptors at two frequencies (0.1 and 0 .2 Hz), which were both distinct from the controlled respiration rate (0.25 Hz), in both normal subjects and heart failure patients with eit her sensitive or poor baroreflex control. 3. The data broadly confirm the De Beer model. The low-frequency (0.1 Hz) peak in either R-R or bl ood pressure variability) was spontaneously generated only if the baro reflex control of the autonomic outflow was relatively intact. With a large stimulus to the carotid baroreceptor it was possible to influenc e the low-frequency R-R but not low-frequency blood pressure variabili ty. This implies that it is too simplistic to use power spectral analy sis as a simple measure of autonomic balance; its underlying modulatio n is more complex than generally believed. 4. It may be that power spe ctral analysis is more a sensitive indicator of baroreflex control, pa rticularly of vagal control, than direct evidence of autonomic balance , Of course, there is often a correlation between the gain of the refl ex and the autonomic balance of vagus and sympathetic. These considera tions may help our understanding of some conditions, such as exercise or heart failure, when the power spectral analysis method fails to ide ntify increased sympathetic discharge; this failure may partly be expl ained by the decrease in baroreflex sensitivity which occurs in these two conditions.