Sex differences in autonomic and cardiovascular responses during lower-body negative pressure
Authors: Jarrard Caitlin , Watso Joseph , Foster Josh , Mckenna Zachary , Atkins Whitley , Huang Mu , Moralez Gilbert , Crandall Craig 
Published: May 2023

Female adults have a lower tolerance to central hypovolemia, simulated by tilt testing and lower body negative pressure (LBNP; PMID: 26109186). However, the mechanisms responsible for this observation remain unknown. Previous studies investigated sex differences at baseline and presyncope, but the intermediate responses are not fully understood, particularly when expressed as a percentage of tolerance. Therefore, the purpose of this study was to compare autonomic and cardiovascular responses to a simulated hemorrhage between female and male adults at the same relative percentage of their tolerance. We tested the hypothesis that females would have lower blood pressure (BP) and muscle sympathetic nerve activity (MSNA) at the same relative percentage of their tolerance. Eight female (28±8 years old, 171±8 cm, 77±6 kg, BMI: 27±3 kg/m2) and eight male (age: 32±7 years old, height: 178±9 cm, mass: 86±10 kg, BMI: 27±2 kg/m2) adults completed a progressive LBNP protocol that started at -40 mmHg and was further reduced by 10 mmHg every 3 minutes until the participant reached pre-syncope. Presyncope was based on the following criteria: continued reports by the subject of feeling faint and/or nauseous; a rapid decline in blood pressure resulting in systolic blood pressure < 80 mmHg; and/or a relative bradycardia accompanied by narrowing of pulse pressure. LBNP tolerance was quantified as a cumulative stress index (CSI) (mmHg·min) and was compared between sexes using an unpaired t-test. MSNA (microneurography - radial nerve), beat-to-beat blood pressure (BP; Finometer), and heart rate (HR) were obtained at baseline, at 33% and 66% of maximum CSI (60-second averages for both), and the last 30 seconds before 100% CSI (i.e., presyncope). Data were statistically analyzed between the sexes using mixed effects analysis and are presented as mean ± SD. Consistent with prior findings, tolerance to LBNP was lower in females (female: 607±369 vs. male: 960±339 mmHg·min, p = 0.067). There were no differences in systolic, mean, or diastolic BP or HR between the sexes (e.g., main effect of sex, p > 0.199 for all indices) throughout the protocol. MSNA burst frequency (bursts/min) (baseline: 18±8 vs. 25±11; 33% CSI: 30±13 vs. 50±5; 66% CSI: 38±11 vs. 56±4; 100% CSI: 28±10 vs. 40±12; sex: p = 0.004, stage: p < 0.0001, interaction: p = 0.704) and burst incidence (bursts/100 cardiac cycles) (baseline: 29±9 vs. 41±20; 33% CSI: 38±21 vs. 58±10; 66% CSI: 35±18 vs. 52±10; 100% CSI: 32±18 vs. 37±12; sex: p = 0.047, stage: p = 0.022, interaction: p = 0.952) were lower in females than males throughout the protocol, respectively. However, there was no interaction between sexes and relative LBNP stages for MSNA burst frequency or burst incidence. These findings suggest a lack of sex differences in the magnitude of the increase of MSNA throughout LBNP and that lower basal sympathetic activity may be related to reduced tolerance to central hypovolemia. Department of Defense - US Army W81XWH1820012 (CGC) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.