Sex Differences In Cardiovascular Responses To Exercise Under Moderate Hypovolemic Stress

Due to higher metabolic demands to the contracting muscles during exercise, cardiovascular adjustments occur, resulting in increased arterial blood pressure (BP). Women exhibit blunted pressor responses to exercise compared to men. In response to central hypovolemia, triggered by lower body negative pressure (LBNP), women appeared to have lower tolerance. In both cases, exercise and LBNP, redistribution of blood flow from central to peripheral is required. However, how cardiovascular alterations to sustained LBNP and exercise differ in women is unclear. PURPOSE: To investigate sex differences in cardiovascular responses during exercise and LBNP. METHODS: A total of 17 men and women completed the study. Heart rate (HR) from ECG, beat to beat BP from Finapres, diameter, velocity, and flow of the brachial artery (BBF) using Doppler ultrasound on the contracting arm were continuously measured during 5 min of baseline with and without LBNP (@-20 mmHg), and 5 min of handgrip exercise at 35% and 50% of maximal voluntary contraction with and without LBNP. RESULTS: Compared to baseline, both men and women had a significant increase in HR and reduction in BBF to LBNP (M; 88 ± 40@rest, 65 ± 23@LBNP, W; 58 ± 27@rest, 28 ± 17@LBNP, mL/min), while mean BP did not change. During exercise, women had blunted increases in BBF and mean BP. When LBNP was applied to exercise, BBF remained high to the level of exercise in both men and women with an additional increase in HR only in women (M; Δ4.1 ± 3.7 vs. W; Δ7.4 ± 3.2, bpm). CONCLUSION: Women may exhibit greater cardiac compensation (via augmented tachycardia) when exposed to similar sustained hypovolemic stress during exercise. However, LBNP does not appear to evoke redistribution of the blood flow from contracting limb regardless of sex or exercise intensity. This may be due to insufficient LBNP pressure or a higher priority in blood flow towards the contracting muscles to meet the metabolic demands. Supported by: This work is supported by National Institute of Health, NIGMS Grant (SC2GM144165 to A.K. Jensen)