Endothelium dependent vasodilation is not enhanced in healthy women with varying degrees of orthostatic intolerance
The purpose of this study was to determine if the vasodilator capacity of the popliteal artery in women was augmented or the vasoconstrictor capacity in the lower extremity was attenuated in women with orthostatic intolerance. Orthostatic tolerance was assessed using a graded lower body negative pressure test and cumulative stress index (CSI). Popliteal artery diameter and velocity (Doppler ultrasound) was measured in 13 orthostatically tolerant and 7 orthostatically intolerant (OT: CSI range, -633 to -1875 mmHg; OI: CSI range, -80 to -552 mmHg·min; P = 0.015) women after 5 min of distal calf occlusion (FMD), 3 min of cold pressor test (CPT), and 5 min of distal calf occlusion combined with CPT (FMD+CPT). Peak popliteal diameter, measured during FMD+CPT was not different from peak popliteal diameter during FMD. Popliteal FMD, normalized to the shear stimulus, was not different between OT and OI women or between FMD and FMD+CPT. Despite similar vasoconstrictor responses to CPT, assessed by the reduction in peak popliteal artery conductance (OT: -11.0±15.6%; OI: -9.1±23.8%), the magnitude of change in peak popliteal conductance actually increased during FMD+CPT and was similar in OT vs. OI women (OT: 5.8±31.5%; OI: 47.2±60.1%). In conclusion, endothelium dependent vasodilation of the popliteal artery at rest is not enhanced in healthy women with varying degrees of orthostatic intolerance. The increase in popliteal vascular conductance during the combined stimulus (FMD+CPT) may suggest inhibition of sympathetic vasoconstriction via nitric oxide mediated vasodilation (or other dilators) and/or differential control of sympathetic activation in the arms and legs.