Graded vascular autonomic control versus discontinuous cardiac control during gradual upright tilt

Indexes of heart rate variability (HRV) and the slope of cardiac baroreflex are extensively used for non invasive assessment of circulatory autonomic control in pathophysiology. We performed this study (1) to assess the sensi tivity of these indexes towards small graded postural stimulations and (2) to delineate the informations provided about the settings of both vascular tone and cardiac activity. Twenty heathly subjects were randomly tilted for eight minutes at each of the six angles: -10 degrees, 0 degrees (supine), 10 degrees, 30 degrees, 45 degrees, and 60 degrees. Instant RR-interval and finger blood pressure (BP) were continuously recorded, and venous blood wa s collected at the end of each 8 min position for catecholamines determinat ion. Group average heart rate, noradrenaline and diastolic BP (DBP) increas ed linearly with head-up tilt angle from 10 degrees. Systolic BP (SBB) rank ed only two distinct series -10 degrees, 0 degrees, 10 degrees versus 30 de grees, 45 degrees, 60 degrees, as did the number of spontaneous baroreflex (SBR) sequences. The spectral power of the low-frequency (LF) and high-freq uency (HF) of RR variability and the ratio LF/HF changed rather abruptly fr om either 30 degrees or 45 degrees, depending on each individual. Both HF/t ot i.e. the ratio of HF to total spectral RR variability and the slope of S BR decreased markedly from 10 degrees to 30 degrees and less but more gradu ally from 30 degrees to 60 degrees. Thus, our observations argue for gradua l adjustments of vascular tone as reflected by highly consistent changes in plasma noradrenaline and diastolic arterial pressure, contrasting with a m ain discontinuous autonomic setting of cardiac activity as reflected by cha nges in the harmonic components of spectral RR variability and in the slope of cardiac baroreflex. The pattern of changes in systolic arterial pressur e attested the discontinuous cardiac autonomic control rather than the grad ual setting of arterial tone. We submit that these different patterns of au tonomic adjustments should be considered when assessing pathophysiological states. (C) 2000 Elsevier Science B.V. All rights reserved.