Heart rate variability adaptation due to head-down tilt bed rest: insights from pooled data of six campaigns.

Microgravity challenges cardiovascular regulation, necessitating adaptive responses to maintain homeostasis. This retrospective study examines the changes in heart rate variability (HRV) induced by head-down tilt (HDT) bed rest, a ground-based analog for microgravity. Standardized 24-h Holter electrocardiograms (ECG) were collected from 66 male volunteers across six ESA HDT campaigns of different durations (5, 21, and 60 days). HRV indices, including time, frequency, and nonlinear metrics, were computed to evaluate autonomic modulation during HDT and following re-exposure to orthostatic stress. Results showed decreased high- and low-frequency power during HDT, indicating impaired autonomic modulation. Additional alterations were observed at HDT discontinuation, likely reflecting cardiovascular deconditioning and gravity readaptation. The use of harmonized protocols and a large pooled dataset provided valuable insights into cardiovascular adaptation to HDT. These findings emphasize the importance of standardized protocols and large pooled datasets in future studies investigating cardiovascular responses to simulated microgravity.