Dose-Effect Relationship Between Mild Levels of Hypergravity and Autonomic Circulatory Regulation

INTRODUCTION: The dose-effect relationships between different levels of hypergravity (> + 1.0 G(z)) and steady-state hemodynamic parameters have been reported in several studies. However, little has been reported on the dose-effect relationship between hypergravity levels and estimates of autonomic circulatory regulation, such as heart rate variability, arterial pressure variability, and spontaneous cardiac baroreflex sensitivity. We investigated dose-effect relationships between hypergravity levels from + 1.0 G(z) to + 2.0 G(z) (Delta 0.5 G(z)) and autonomic circulatory regulation to test our hypothesis that autonomic circulatory regulation has a linear relationship with hypergravity levels. METHODS: Using a short-arm human centrifuge, 10 healthy seated men were subjected to + 1.0 G(z), + 1.5 G(z), and + 2.0 G(z) hypergravity. We evaluated steady-state hemodynamic parameters and autonomic circulatory regulation indices. Heart rate variability, arterial pressure variability, and spontaneous cardiac baroreflex sensitivity between arterial pressure and R-R interval variabilities were assessed by spectral analysis, sequence analysis, and transfer function analysis. RESULTS: Steady-state heart rate, stroke volume, and sequence slope (indicating spontaneous cardiac baroreflex sensitivity in response to rapid changes in arterial pressure) showed linear correlations with increases in gravity (from + 1.0 G(z) to + 2.0 G(z)). On the other hand, steady-state cardiac output, steady-state systolic arterial pressure, and low-frequency power of diastolic arterial pressure (indicating peripheral vasomotor sympathetic activity) remained unchanged with gravity increases. CONCLUSION: Contrary to our hypothesis, the present study suggested that autonomic circulatory regulations show complex changes with hypergravity levels. Spontaneous cardiac baroreflex sensitivity reduces in a dose-dependent manner from + 1.0 G(z) to + 2.0 G(z), whereas peripheral vasomotor sympathetic activity seems to be maintained.