The impact of mild hypohydration and sex on the cardiovascular responses to progressive lower-body negative pressure.

The arterial baroreflex maintains blood pressure during mild-to-moderate orthostatic stress, but the roles of hydration status and sex in this response are unclear. This study used progressive lower-body negative pressure (LBNP) to examine how both fluid restriction and sex influence blood pressure regulation during central hypovolemia. Twenty-eight healthy young adults (15 females) completed two visits, in a counterbalanced order: 24-hr fluid restriction (hypohydrated) or 24-hr ad libitum fluid intake (euhydrated). Hydration status was assessed by changes in body mass and urine specific gravity. Beat-by-beat blood pressure, heart rate, stroke volume, and cardiac output were measured during progressive LBNP to presyncope, defined as systolic blood pressure < 90 mmHg for three consecutive cardiac cycles. 24-hr fluid restriction reduced body mass (Δ - 1.27 ± 0.70 kg; p < 0.01) and elevated urine specific gravity (Δ + 0.013; p < 0.01), confirming that participants were mildly hypohydrated after 24-hr of fluid restriction. Mild hypohydration did not affect tolerance time to presyncope (p = 0.48), blood pressure (all p > 0.39), stroke volume (p = 0.87), or cardiac output (p = 0.21), but heart rate was borderline elevated during LBNP when hypohydrated (p = 0.051). Females exhibited lower tolerance to LBNP than males (12.8 ± 4.2 min vs. 16.7 ± 5.7 min; p = 0.037), and a distinct cardiac response, characterized by a higher heart rate (~ 4.3 bpm) and a greater reduction in stroke volume (~ 2.6 mL) during LBNP (both p < 0.01). However, no significant interactions were observed between sex and hydration status for any cardiovascular responses to LBNP (all p > 0.05). These data indicate that sex and hydration status influence cardiac function in response to LBNP.