Relationship between brachial artery blood flow and total [hemoglobin plus myoglobin] during post-occlusive reactive hyperemia

The associations between macrovascular and microvascular responses reported previously during post-occlusive reactive hyperemia have been inconsistent. The purpose of this study was therefore to determine the temporal relationship between the reactive hyperemic responses within a conduit artery and the downstream microvessels. Conduit artery blood flow was measured in the brachial artery with pulsed Doppler ultrasound. A potential analog of microvascular flow, changes in skeletal muscle total[hemoglobin + myoglobin] (T[Hb + Mb]), was assessed with near-infrared spectroscopy (NIRS). We found a high degree of correlation between these two measures (r = 0.91). Cross-correlation analysis revealed two distinct response patterns. In 10 of our 15 subjects there was time displacement between peak brachial artery blood flow (BABF) and T[Hb + Mb] responses; in the remaining 5 the peaks were coincident. Granger causality testing suggested that reactive hyperemia in the macrovessel determined hyperemia in the downstream microvessels in all 15 study subjects. Time constants for the on (T-1) and off (T-2) kinetics of each response were calculated; our initial hypothesis was that T-1 and T-2 for T[Hb + Mb] would correlate with T-1 and T-2 for BABF, respectively. However, only for T-2 was this observed (r = 0.52; p < 0.05). No similar relationship was observed for T-1. Adipose tissue thickness did not influence either time constant for T[Hb + Mb]. Taken together, our results show that the temporal characteristics of the hyperemic response in the conduit artery are qualitatively reflected in the downstream microvasculature, but mechanisms for quantitative differences remain to be identified. (C) 2013 Elsevier Inc. All rights reserved.