Characteristics of flow-mediated brachial artery vasodilation in human subjects.

Abstract

In an effort to determine whether arterial conductance vessels dilate in response to increased blood flow stimuli, brachial artery area (cm 2 ) and diameter (cm) were derived by simultaneous measurement of forearm blood flow (ml/min-100 ml) and brachial artery blood flow velocity (cm/sec) following the release of arterial occlusion. Measurements were made at rest and at the time of maximal flow after the release of graded periods of forearm arterial occlusion (20 seconds to 10 minutes). These studies showed a graded large vessel dilation following occlusions of up to 1 minute (baseline diameter, 0.33±0.01; after 1 minute occlusion, 0.45±0.02 cm; /?<0.05) after which time diameter plateaued (after 10 minutes of occlusion, 0.48±0.02 cm). In addition, the time course of diameter and flow changes after 3 minutes of arterial occlusion were examined. Flow was maximal at 5 seconds but diameter was maximal at 15-30 seconds after release. Furthermore, the half time for the return of diameter to baseline was longer than that for blood flow. We also measured the diameter after forearm heating (42° C) and noted a substantial increase in diameter (before heating, 0.32±0.01; after heating, 0.39±0.02 cm; p<0.05). Finally, we applied pressure to the venous side of arteriovenous fistulae in five hemodialysis patients. This maneuver was associated with large reductions in forearm blood flow (baseline flow, 63.3±10.6; venous compression flow, 36.0±4.4 ml/minl00 ml;/?<0.05) and a decrease in brachial artery size (baseline diameter, 0.63±0.07; venous compression diameter, 0.58±0.06 cm; p<0.05). We conclude that 1) the human brachial artery size changes in response to changes in blood flow, and 2) the maximal dilation occurs after maximal flow is noted. This process has been termed reactive dilation, 1 and it has been suggested that the process is truly localized to the large vessel and may require endothelial integrity.5 A relation between increased blood flow and large vessel dilation in human subjects has not been previously described.Recently, we have developed a noninvasive technique that allows us to calculate brachial artery diameter at frequent intervals (10-15 seconds) from simultaneously measured forearm blood flow (straingauge plethysmography) and brachial artery blood velocity (a transcutaneous Doppler probe). With this technique, we make use of the equation, vessel area=flow (ml/min)-^ velocity (cm/min). In this paper, we hypothesized that acute alterations in forearm blood flow would be associated with directionally similar changes in brachial artery size. Our results would suggest that the process of reactive dilation does indeed exist in human subjects. Subjects and Methods DesignAfter signing an informed consent, 11 young healthy subjects (mean age of 23 years) had simultaneous measurements of forearm blood flow and brachial artery blood velocity in order to calculate brachial artery diameter. The study was divided into five major parts: Part A (n=8) consisted of measurement of blood flow and blood velocity and calcu...

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