Flow mediated dilation of the brachial artery: an investigation of methods requiring further standardization

Abstract

Background: In order to establish a consistent method for brachial artery reactivity assessment, we analyzed commonly used approaches to the test and their effects on the magnitude and time-course of flow mediated dilation (FMD), and on test variability and repeatability. As a popular and noninvasive assessment of endothelial function, several different approaches have been employed to measure brachial artery reactivity with B-mode ultrasound. Despite some efforts, there remains a lack of defined normal values and large variability in measurement technique.

Methods: Twenty-six healthy volunteers underwent repeated brachial artery diameter measurements by B-mode ultrasound. Following baseline diameter recordings we assessed endothelium-dependent flow mediated dilation by inflating a blood pressure cuff either on the upper arm (proximal) or on the forearm (distal).

Results: Thirty-seven measures were performed using proximal occlusion and 25 with distal occlusion. Following proximal occlusion relative to distal occlusion, FMD was larger (16.2 +/- 1.2% vs. 7.3 +/- 0.9%, p < 0.0001) and elongated (107.2 s vs. 67.8 s, p = 0.0001). Measurement of the test repeatability showed that differences between the repeated measures were greater on average when the measurements were done using the proximal method as compared to the distal method (2.4%; 95% CI 0.5-4.3; p = 0.013).

Conclusion: These findings suggest that forearm compression holds statistical advantages over upper arm compression. Added to documented physiological and practical reasons, we propose that future studies should use forearm compression in the assessment of endothelial function.

Figure 1

Representative flow-mediated response of brachial artery (forearm occlusion). Measurements are used to characterize the brachial artery response: 1) baseline diameter (D_BL), 2) maximum diameter after cuff release (D_max), 3) diameter 3 minutes after cuff release, 4) time of maximum diameter after cuff release.

Figure 2

Time course of brachial artery dilation following reactive hyperemia. Time course in 10-second intervals of brachial artery FMD expressed as percent difference from baseline diameter after upper arm cuff inflation (upper curve) and forearm cuff inflation (lower curve). Each point in the graphs represents an average of all data available in that time-point; the error bars represent one standard error to the mean.

Figure 3

Cumulative frequency of individual peak dilations after cuff deflation. (A) Upper arm cuff inflation; (B) Forearm cuff inflation. Dashed lines represent the percentage of individuals with maximal response at the average time-to-peak FMD for each of the cuff locations.

Figure 4

Reproducibility of Flow-Mediated Dilation (FMD) using Bland-Altman plots. (A) Upper arm cuff inflation; (B) Forearm cuff inflation. The x-axis illustrates the individual average values of peak-FMD. The y-axis shows the differences between two peak FMD. All values are presented as percentage.