Normalization of flow-mediated dilation to shear stress area under the curve eliminates the impact of variable hyperemic stimulus

Abstract

BackgroundNormalization of brachial artery flow-mediated dilation (FMD) to individual shear stress area under the curve (peak FMD:SSAUC ratio) has recently been proposed as an approach to control for the large inter-subject variability in reactive hyperemia-induced shear stress; however, the adoption of this approach among researchers has been slow. The present study was designed to further examine the efficacy of FMD normalization to shear stress in reducing measurement variability.MethodsFive different magnitudes of reactive hyperemia-induced shear stress were applied to 20 healthy, physically active young adults (25.3 ± 0. 6 yrs; 10 men, 10 women) by manipulating forearm cuff occlusion duration: 1, 2, 3, 4, and 5 min, in a randomized order. A venous blood draw was performed for determination of baseline whole blood viscosity and hematocrit. The magnitude of occlusion-induced forearm ischemia was quantified by dual-wavelength near-infrared spectrometry (NIRS). Brachial artery diameters and velocities were obtained via high-resolution ultrasound. The SSAUC was individually calculated for the duration of time-to-peak dilation.ResultsOne-way repeated measures ANOVA demonstrated distinct magnitudes of occlusion-induced ischemia (volume and peak), hyperemic shear stress, and peak FMD responses (all p < 0.0001) across forearm occlusion durations. Differences in peak FMD were abolished when normalizing FMD to SSAUC (p = 0.785).ConclusionOur data confirm that normalization of FMD to SSAUC eliminates the influences of variable shear stress and solidifies the utility of FMD:SSAUC ratio as an index of endothelial function.

Use of Ultrasound for Non-Invasive Assessment of Flow-Mediated Dilation

“…Emphasis is placed on 'consider' since the FMD test should not be normalized to the shear rate using conventional approaches. A number of studies have attempted to account for the effect of shear stimulus on FMD by evaluating the quotient of FMD and shear, rather than FMD alone, or by using analysis of covariance (ANCOVA), with shear stimulus as the cofactor 34,51,[91][92][93][94] . The techniques described above require use of the general linear model (GLM) for determining statistical probabilities associated with the differences found between groups or experimental treatments; however, when using a GLM, the following assumptions must hold true: 1) there must be at least a moderate correlation between the two variables (i.e., shear and FMD), 2) the relationship between shear and diameter must be linear, 3) the intercept of the regression slope must be zero, 4) variance must be similar between groups, and 5) data must be normally distributed 93,95) .…”

“…Third, blood viscosity exhibits low intra-subject variability 36) , particularly among a healthy, homogeneous group. Shear rate has been used as a surrogate measure of shear stress in a number of previous studies 36,[48][49][50][51] . Fourth, in arteries, the velocity profile will generally not develop to a full parabola as a consequence of flow unsteadiness and short vessel entrance lengths; however, in the brachial artery, under resting conditions, the underestimation is less pronounced, likely due to a more parabolic velocity profile in this artery, i.e., n (velocity profile) is closer to 2 52) .…”

“…A number of laboratories are now able to make semi-automated diameter measurements across the cardiac cycle using edge detection software 35,51,92,[101][102][103][104][105][106] . Studies using edge detection software to estimate FMD have reported intersession coefficients of variation of approximately 11-18% 101,106,107) .…”

Assessment of Endothelial Function Using Ultrasound

“…A number of laboratories, using commercial or custom edge-detection software, are now able to make semi-automated diameter measurements (Woodman & Playford et al 2001;Craiem & Chironi et al 2007;Peretz & Leotta et al 2007;Padilla & Johnson et al 2008;Pyke & Jazuli 2011;Thijssen & Tinken et al 2011). The authors of this chapter, using custom edge-detection software, are able to make thirty diameter measurements per second.…”

“…The result is higher viscosity in the center of the vessel, thereby reducing the shear stress gradients at the vessel wall. It is worth noting that shear stress assessments do not seem to result in conclusions different from shear rate assessments alone (Padilla & Johnson et al 2008). This may be explained by two factors: 1) sources of error from whole blood viscosity estimates, and 2) for a given population, viscosity changes little.…”