Internal jugular venous flow responses to graded lower body negative pressure: a sex-stratified analysis.

Background: The internal jugular vein (IJV) is the primary pathway for cerebral venous drainage in the supine position. Although lower body negative pressure (LBNP) is widely used to simulate central hypovolemia, the dose-response relationship between graded LBNP and jugular venous flow, and potential sex differences therein, remain incompletely characterized. We hypothesized that graded LBNP would produce a progressive, dose-dependent reduction in jugular venous flow, and examined whether this response differed between sexes. Methods: Forty-four healthy young adults (22 female, 22 male) underwent graded LBNP (-10 to -40 mmHg) in the supine position. IJV cross-sectional area and blood flow velocity were assessed using B-mode ultrasound and pulsed-wave Doppler at the mid-neck (J2 segment). Volumetric flow was calculated as the product of mean area and mean envelope velocity. Relative flow changes (normalized to individual supine baseline) were log-ratio transformed prior to analysis. Data were analyzed using linear mixed-effects models with state, sex, and their interaction as fixed effects. Results: Female participants showed significant jugular venous flow reductions at LBNP30 and LBNP40 (bothp< 0.001); males showed significant reductions from LBNP20 onward (p ≤0.0004). The main effect of sex was not significant (p = 1.00), and no state× sex interaction reached significance across LBNP levels or during recovery. A non-significant trend toward a larger flow reduction in females at LBNP40 (p = 0.056) did not persist in a completers-only sensitivity analysis (p = 0.16). Baseline IJV cross-sectional area was larger in males (p = 0.031) but did not differ after indexing to body surface area (p = 0.23). Conclusion: Graded LBNP induces a progressive, dose-dependent reduction in jugular venous flow from moderate levels onward. Despite sex differences in Systemic responses, the jugular venous flow response was broadly similar between females and males, and baseline morphological differences were largely Attributable to body size. These observations are consistent with jugular venous outflow during central hypovolemia being governed predominantly by mechanical factors, although the modest sample size and exploratory analyses warrant confirmation. These findings inform the physiological understanding of jugular venous responses to central hypovolemia and are relevant to spaceflight countermeasures.