Short-term 6° head-down tilt bed rest simulating microgravity ameliorates blood pressure and heart rate variability:...

As humanity ventures into space exploration, understanding the physiological effects of microgravity becomes increasingly important. Beyond its relevance to astronaut health, the application of altered gravity is gaining attention in clinical and sports settings. While most studies have explored long-term adaptive changes to microgravity, the acute effect of short-term microgravity exposure on hemodynamics and cardiac autonomic function still remains unexplored. Therefore, this study aimed to investigate the effect of 30 min 6° head-down tilt (HDT) bed rest simulating microgravity on blood pressure (BP) and cardiac autonomic functions. In this open-label quasi-experimental study, 27 healthy males (aged 18-35 years), selected from an initial pool of 45, were enrolled and divided into control (n = 13, seated rest) and 6° HDT (n = 14) groups. Pre- and post-intervention BP and heart rate variability (HRV) were assessed and paired t-tests were done. The 6° HDT for 30 min showed a significant decrease in SBP (-5.06 %; p < 0.01; d = 0.82) and MAP (-5.06 %; p < 0.05; d = 0.72), with no significant changes in control group. Significant increase in mean RR (4.02 %; d = 0.22), pNN50% (15.71 %; d = 0.14), HRVTi (26.52 %; d = 0.51), and HF power (11.64 %; p < 0.05), along with significant decrease in HR (-5.23 %; d = 0.30) and stress index (-21.6 %; d = 0.49) were also observed post-6° HDT (p < 0.05), indicating enhanced parasympathetic modulation. After applying the FDR correction, only SD2 (FDR_correctedp = 0.004) and SDNN (FDR_correctedp = 0.008) maintained statistical significance in the HDT group, whereas no parameters in the control group achieved statistical significance. No significant changes were observed in LF power and LF/HF ratio in both the groups. Therefore, 30 min of 6° HDT exposure elicited a stronger parasympathetic response. Beyond astronauts' health, these findings may offer therapeutic insights for populations prone to autonomic dysfunction, including the elderly, immobilized patients, and those at risk of orthostatic intolerance.