Comparing The Dynamic Response Characteristics Of Flow-mediated Dilation In The Brachial And Radial Arteries

Abstract

Recent research indicates that smaller arteries tend to dilate more when exposed to shear stress (flow mediated dilation (FMD)) vs. larger arteries. However, the dynamic characteristics of FMD have not been compared in differently sized vessels. PURPOSE: To compare the Phase 1 (initial dilation) dynamics of the FMD response in the radial artery (RA) and the brachial artery (BA). METHODS: N = 15 healthy male subjects. BA and RA diameter and mean blood flow velocity (MBV) were measured by echo and Doppler ultrasound respectively. Rhythmic handgrip exercise was used to create a shear rate (shear rate (SR) estimate of shear stress = vessel diameter/MBV) of 60 s-1 for 6 minutes in the BA and RA. Subjects performed 3 exercise trials at the 60 s-1 SR target in each artery over three visits. These trials were averaged to provide one mean RA and one mean BA response. Curve fitting allowed determination of the phase 1 response dynamics (time delay (TD1) and time constant (tau 1)). Data are reported as means ± SD. RESULTS: RA diameter was 0.215 ± 0.0445 mm; BA diameter was 0.368 ± 0.0664 mm (P=0.002). The shear rate achieved in the RA (56.027 ± 3.935 s-1) and BA (57.939 ± 5.850 ss) was not significantly different (P = 0.120). The Phase 1 %FMD and %FMD at the end of the 6 min exercise bout were significantly greater in the RA (RA phase 1 = 6.25 ± 3.42%; 6 min 9.84 ± 3.99 %; BA 2.59 ± 1.69%; 6min 2.526 ± 1.96 %)(phase 1 P=0.005; 6min P=<0.001)). The RA had a significantly shorter TD1 (9.60 ±15.42 s) than BA (31.80 ± 26.15 s) (P = 0.017). However there was no significant difference in the speed of the phase 1 response (tau1) between the BA (51.30 ± 43.26 s) and RA (65.75 ± 38.40s) (P = 0.461). TD1 was modestly correlated with percent FMD in the RA (r2 = 0.146, P = 0.039) but not in the BA (r2 =.205, P = 0.104). Tau1 was not correlated with FMD in either the RA (P = 0.583) or the BA (P = 0.104). CONCLUSION: When experiencing a SR of 60 s-1, the RA began to dilate sooner (shorter TD1) and had a greater phase 1 %FMD magnitude than the BA. However, no differences could be detected in the speed of the phase 1 response (tau1). FMD response dynamics were largely unrelated to FMD magnitude. The distinct phase I FMD magnitudes and time delays in response to the same shear rate indicate vessel specific differences in the initiation of the FMD response. Supported by NSERC and CFI.