Effect of simulated microgravity via head-down tilt on intracranial compliance, brain oscillations and cardiovascular autonomic modulation.

Spaceflight and ground-based models cause fluidshift to the upper part of the body, especially the head. Intracranial compliance (ICC) and cardiovascular autonomic modulation (CAM) can be impacted, impairing cerebral blood flow. ICC is poorly explored but important for astronauts' health. Also, microgravity can reduce brain activity and affect cerebral functions. Fluidshift models are relevant for understanding its effects and the most used one is head-down tilt (HDT). Thus, this study aimed to investigate the immediate effects of HDT at -6º and - 15º on ICC, CAM and brain oscillations in healthy individuals. Sixty-one subjects (22 females) participated in the study (age 32.7 ± 6.2 years). The 30-minute HDT protocol was performed at -6º and - 15º. ICC was assessed non-invasively through the strain gauge sensor, from brain4care system, along with CAM and cortical activity during cognitive and motor tests. Participants had an increase in the ICP P2/P1 ratio when comparing pre and HDT at -6º (p = 0.004) and a trend toward elevation at -15º (p = 0.058). The sympathetic component of CAM was predominant in both HDT, and brain oscillations were reduced in most tests. The study suggests that acute HDT can reduce ICC, altering sympathovagal balance and causing cortical inhibition.