Does Brachial Artery Flow Mediated Vasodilation Reflect Systemic Vascular Function?

Abstract

Flow-mediated vasodilation (FMD) has served as a noninvasive method to assess vascular health. However, recently both the importance of the FMD-induced shear stimuli and potential limb differences in terms of vascular function have come to light. PURPOSE: We sought to examine the FMD response in both the arm (brachial artery) and lower leg (popliteal artery). METHODS: To assess vessel diameter, blood velocity, and shear rate in six subjects, ultrasound Doppler measurements were made in the brachial artery and popliteal artery. Interestingly, preliminary studies revealed a large difference in the post-ischemic hyperemia in the leg following the standard 5 minute cuff occlusion. Thus, to more closely equate the shear stimulus an occlusion of 3 min and 5 min was adopted for the arm and leg, respectively. In both cases blood velocity was assessed at 0–15, 20–40, 50–70, 80–100 seconds post-occlusion while diameter was assessed at 80–100 seconds and compared to rest. RESULTS: Resting diameters were significantly larger in the popliteal artery than the brachial (0.59 ± .03 and 0.46 ± 0.01 cm, respectively) which also contributed to differences in post-cuff shear rate. A similar and significant FMD was observed both in the arm (5.4 ± 0.5%) and in the leg (5.2 ± 1%) while, despite experimental design, the mean change in shear rates were greater in the brachial (102 ± 9 s−1) than popliteal (68 ± 29 s−1) artery. To fully account for the remaining difference in shear rate, data were normalized (percent change in diameter/ average shear rate change) which revealed no difference between arm and leg vessels. CONCLUSIONS: These data reveal an attenuated post-ischemic hyperemia and larger vessel diameter in the popliteal artery which combine to reduce post-ischemic shear stimuli when compared to the brachial artery. However, when this difference in shear rate is normalized, the vessels of the arm and leg appear to respond in a similar fashion to a given shear stimulus.