Peripheral vascular responses of men and women to LBNP

This study examined the hypothesis that women exhibit smaller vasoconstrictor responses in the calf during graded lower body negative pressure (LBNP).

Selective deep oxygenated hemoglobin (Hb) assessed by near-infrared spectroscopy (NIRS) was used to examine blood flow changes in the calf. Eleven men and nine women volunteers underwent graded LBNP up to -60 mm Hg. Cardiovascular responses were measured by: NIRS on the forearm and superficial and deep calf for oxygenated and deoxygenated Hb; mercury strain gauge plethysmography for calf blood pooling; electrocardiogram for heart rate; and, photoplethysmography for blood pressure, cardiac output, and total peripheral resistance.

Cardiac output was lower and total peripheral resistance higher in women; however, both men and women had similar decreases in cardiac output and increases in total peripheral resistance with LBNP. Forearm oxygenated Hb decreased from baseline throughout LBNP, and no difference was found between men and women. Female subjects had greater rates of increased blood pooling with LBNP than their male counterparts. Men had greater selective deep calf oxygenated Hb reductions compared to women during LBNP. Moreover, when the oxygenated Hb response as a function of calf blood pooling was examined with regression analyses, men had greater slopes (-0.62 +/- 0.05) than women (-0.33 +/- 0.04).

The greater slopes in oxygenated Hb at given blood pooling and at each negative pressure in male subjects led us to conclude that men had greater vasoconstrictor responses in the calf during graded LBNP.