Original contribution
Within-subject variability of flow-mediated vasodilation of the brachial artery in healthy men and women: implications for experimental studies

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Abstract

Flow-mediated vasodilation (FMD) of the brachial artery is used as a marker of cardiovascular disease risk. It is defined as the percentage dilation from the baseline diameter in response to a provoked increase in blood flow. The within-subject variability, crucial in the design of trials with FMD as an endpoint, appears to vary widely between studies. We assessed the analytical and within-subject variability of FMD in healthy subjects and estimated the number of subjects needed to detect various treatment effects in intervention trials and observational studies. FMD was assessed with B-mode high-resolution ultrasound (US). A total of 13 volunteers were measured on six occasions, after they had fasted overnight. Within-subject variability was assessed from all six scans per subject. Analytical variation or reading variation was assessed by reading one scan of each subject twice by one observer. The mean (±SD) FMD was 5.60 ± 2.15 FMD% of the baseline diameter. The within-subject SD was 2.8 FMD%, resulting in a coefficient of variation (CV) of 2.8/5.6 × 100% = 50.3%. The CVs for the baseline and maximum diameter were much smaller: 4.8% (SD 0.193 mm at a mean of 4.060 mm) for the baseline and 5.2% (SD 0.222 mm at a mean of 4.285 mm) for the maximum. The CV for reading variation was 34%. The number of subjects needed to detect a treatment difference of 2 FMD% with a probability of 0.05 and a power of 0.80 would be 31 in a crossover design and 62 per group in a parallel design for comparison of group changes. We conclude that the within-subject variability of FMD is large, about 50% of the mean response. This includes biologic and reading variation. Repeated measurements and repeated readings of recorded measurements are recommended to reduce variability.

Introduction

Assessment of flow-mediated vasodilation (FMD) is used to evaluate endothelial function and is used as a surrogate endpoint for cardiovascular disease in observational and experimental studies. It is based on the ability of vascular endothelial cells to respond to changes in shear stress. The response consists of a release of nitric oxide, a potent vasodilator that relaxes the smooth muscle cells in the vascular wall (Joannides et al 1995), resulting in an increase in lumen diameter. FMD is generally assessed from high-resolution ultrasound (US) images of the brachial artery at baseline and at maximum dilation (Sorensen et al 1995). In healthy people, FMD is 5% to 15% of the baseline diameter but, in patients with cardiovascular disease, FMD is impaired or absent Sorensen et al 1994, Neunteufl et al 1997. An impaired FMD appears to be predictive of future events: patients with chest pain and impaired FMD were shown to be more likely to experience cardiovascular events in a 5-year period than patients with preserved FMD (Neunteufl et al 2000).

The diameter of the brachial artery can be measured reproducibly with high-resolution (7.5 to 13 MHz) US: in the literature we found coefficients of variation (CVs) (within-subject SD divided by mean baseline diameter) of 1.5% to 6%. At a mean baseline diameter of 4.0 mm, this corresponds with a within-subject SD of 0.06 to 0.24 mm. However, the reported within-subject variability of the FMD is less consistent: CVs ranging from 1.2% to 13.6% (Table) have been reported. It is possible that some of these reported CVs are, in fact, SDs, or that they represent the CVs for baseline or maximum diameter rather than for the percentual increase in diameter. Instead of CVs, some investigators report alternative measures of reproducibility that are not well-defined in the papers (Table 1). Moreover, most reproducibility studies were based on only two repeated measurements in less than 10 subjects. Therefore, we designed a study that would adequately estimate within-subject variability in FMD. This information was used to estimate sample sizes for experimental studies that compare treatment differences in flow-mediated vasodilation.

Section snippets

Subjects and methods

The Medical Ethics Committee of Wageningen University approved the study protocol. We explained aim and design of the study to the volunteers, who all signed informed consent forms.

Reading or intraobserver variance

The mean ± SD of 13 scans read twice by one observer was 4.2 ± 2.5 FMD% at the first reading and 3.3 ± 1.8 FMD% at the second reading. The correlation between the first and second reading was 0.74. The intraobserver SD was 1.27 FMD% at a mean of 3.73 FMD%, corresponding with a CV of 34%.

Within-subject variance

The number of measurements of sufficient quality ranged from 2 to 6 per subject (Table 2), with a total of 62 of 78 scans. The remaining 16 scans with low quality were not used in the analysis. The CV of the

Discussion

Within-subject variability of flow-mediated vasodilation is large: at a mean FMD of 5.60 FMD% of the baseline diameter, we found a within-subject SD of 2.82 FMD%. The corresponding CV was 50%, which is larger than those found in the literature (Table 1). However, the CVs for baseline and maximum diameter in our study were about 5%, comparable to those found by others Sinoway et al 1989, Sorensen et al 1995. Therefore, we think that previously reported CVs of FMD may either have been SD of FMD (

Acknowledgements

The authors thank the volunteers for their time and cooperation, Dr. M. van Montfort and Dr. A.A.M. Jansen for statistical advice, our sonographer, Jan Harryvan, for conducting the measurements, personnel of the Vascular Imaging Center of the Julius Centre for Health Sciences and Primare Care, Utrecht Medical Center for reading the images, and Dr. Rudy Meijer of the same center for training.

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