Defect baroreceptor function in females prone to vaso‐vagal reactions

Abstract

Objective There seems to be increased ortostatic susceptibility in females when compared to males. The mechanism behind this gender difference is unclear. The present study was designed to compare vagal females (VF) with non vagal females (NF) during hypovolemic circulatory stress. Design ands Methods: Lower body negative pressure (LBNP) up to 60 H2O was used on healthy females (22.8±0, 4 years, n=17), to create graded hypovolemic circulatory stress. Venous compliance, capacitance in the leg and arm as well as forearm blood flow (FBF) were measured with volumetric technique. Blood pressure and heart rate was monitored. Five females developed vagal reactions and were compared with the other twelve. Results The VF had lower resting pulse pressure. There were no significant differences in venous capacitance, venous compliance or net capillary filtration in the calf during LBNP. Pulse pressure decreased more in VF during higher LBNP levels, p<0.01. Despite this, the increase in peripheral resistance was attenuated in VF, p<0.05. The compensatory arm capacitance response to LBNP was decreased in VF, p<0.05. Conclusions The hypovolemic stress seemed to be of similar magnitude in both non-vagal and vagal females. However, pulse pressure during hypovolemic stress was more markedly reduced in the VF. Despite the increased baroreceptor stimulus, they responded with less increase in peripheral resistance and mobilised less blood from the peripheral circulation during the hypovolemic stimulus. This suggests a less effective baroreceptor activation-sympathetic discharge-effector response in vagal females.