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 sensitivity 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°, 0° (supine), 10°, 30°, 45°, and 60°. Instant RR-interval and finger blood pressure (BP) were continuously recorded, and venous blood was collected at the end of each 8 min position for catecholamines determination. Group average heart rate, noradrenaline and diastolic BP (DBP) increased linearly with head-up tilt angle from 10°. Systolic BP (SBB) ranked only two distinct series −10°, 0°, 10° versus 30°, 45°, 60°, as did the number of spontaneous baroreflex (SBR) sequences. The spectral power of the low-frequency (LF) and high-frequency (HF) of RR variability and the ratio LF/HF changed rather abruptly from either 30° or 45°, depending on each individual. Both HF/tot i.e. the ratio of HF to total spectral RR variability and the slope of SBR decreased markedly from 10° to 30° and less but more gradually from 30° to 60°. Thus, our observations argue for gradual adjustments of vascular tone as reflected by highly consistent changes in plasma noradrenaline and diastolic arterial pressure, contrasting with a main discontinuous autonomic setting of cardiac activity as reflected by changes in the harmonic components of spectral RR variability and in the slope of cardiac baroreflex. The pattern of changes in systolic arterial pressure attested the discontinuous cardiac autonomic control rather than the gradual setting of arterial tone. We submit that these different patterns of autonomic adjustments should be considered when assessing pathophysiological states.