Sex-based differences in central and cerebral hemodynamic responses to lower body negative pressure (LBNP): an experimental study

Purpose The effectiveness of lower body negative pressure (LBNP) as an in-flight countermeasure against cardiovascular deconditioning is still debated, with significant gaps in understanding cerebral hemodynamics and sex-related differences. This study investigates acute central, peripheral, and cerebral responses to graded LBNP during exposure to -6° head-down tilt posture (HDT) in 30 subjects (15 females, 15 males). Methods Central and peripheral hemodynamic variables (heart rate, blood pressure, cardiac output, and total peripheral resistance) were continuously and non-invasively measured using finger photo-plethysmography along with NIRS parameters from the forearm and the forehead, and cerebral blood velocities within the middle and posterior cerebral arteries (MCA and PCA). The protocol included a 15-min baseline phase in -6° HDT followed by three 5-min levels of LBNP (from -20 to -40 mmHg). Results LBNP primarily affected central and peripheral hemodynamics. At the cerebral level, significant changes in cerebral blood velocity and Beer-Lambert NIRS parameters were detected, while spatially resolved spectroscopy NIRS variables did not show significant changes. Sex differences were observed in blood pressure, total peripheral resistance, and forearm NIRS parameters, whereas at the cerebral level, sex had a significant effect only on the cerebrovascular conductance of the PCA (CVCPCA). Conclusion Our main findings highlight: (i) comparable anterior and posterior cerebral responses, as both blood velocities in the MCA and PCA exhibited a similar decreasing trend at higher LBNP values; and (ii) sex differences occurring primarily in systemic and peripheral cardiovascular responses.