Effects of calf venous capacitance initiation rate on tolerance to lower body negative pressure in women with and without vasovagal syncope (858.5)

Abstract

Objective: Lower limb venous blood pooling has been considered an important factor in the pathophysiology of vasovagal syncope (VVS). However, not only the pooled volume, but also the rate by which the hypovolemic stimulus is instituted may be of importance. The aim of this study was to determine the influence of calf venous capacitance (Vcap initiation rate on tolerance to lower body negative pressure (LBNP) in women with and without VVS. Design and Methods: Graded LBNP to presyncope was used to determine LBNP tolerance index (LTI) in 13 young women with VVS and 15 matched controls. During LBNP, Vcap and the initiation rate of Vcap was evaluated with volumetric technique and overall cardiovascular responses were measured. Results: No differences in Vcap were found. In controls, the initiation rate of Vcap was negatively related to LTI (r = ‐0.54, P < 0.05), and a rapid initiation of Vcap was related to a lesser decline in stroke volume (r = ‐0.68, P < 0.05) and pulse pressure (r = ‐0.52, P < 0.05) during LBNP. In VVS, the initiation rate of Vcap was positively related to LTI (r = 0.82, P < 0.01), and a slower initiation was related to lesser decline in stroke volume (r = 0.61, P < 0.05). Conclusions: In healthy women, a rapid institution of Vcap was related to a more effective cardiovascular regulation and a higher LTI, possibly explained by greater baroreceptor deactivation due to a more rapid onset of the hypovolemic stimulus. However, the opposite was presented in VVS women, highlighting fundamental differences in cardiovascular regulation.Grant Funding Source: Supported by the Swedish Heart and Lung Foundation