Effects of gravitational acceleration on cardiovascular autonomic control in resting humans

Purpose: Previous studies of cardiovascular responses in hypergravity suggest increased sympathetic regulation. The analysis of spontaneous heart rate variability (HRV) parameters and spontaneous baroreflex sensitivity (BRS) informs on the reciprocal balance of parasympathetic and sympathetic regulations at rest. This paper was aimed at determining the effects of gravitational acceleration (ag) on HRV and BRS. Methods: Eleven healthy subjects (age 26.6 ± 6.1) were studied in a human centrifuge at four ag levels (1, 1.5, 2 and 2.5 g) during 5-min sessions at rest. We evaluated spontaneous variability of R–R interval (RR), and of systolic and diastolic blood pressure (SAP and DAP, respectively), by power spectral analysis, and BRS by the sequence method, using the BRSanalysis® software. Results: At 2.5 g, compared to 1 g, (1) the total power (PTOT) and the powers of LF and HF components of HRV were lower, while the LF/HF ratio was higher; (2) normalized units for LF and HF did not changed significantly; (3) the PTOT, LF and HF powers of SAP were higher; (4) the PTOT and LF power of DAP were higher; and (5) BRS was decreased. Conclusions: These results do not agree with the notion of sympathetic up-regulation supported by the increase in HR and DAP (tonic indices), and of SAP and DAP LF powers (oscillatory indices). The PTOT reduction leads to speculate that only the sympathetic branch of the ANS might have been active during elevated ag exposure. The vascular response occurred in a condition of massive baroreceptive unloading.