Three-Dimension Blood Flow Classification Scheme Better Describes NO-Mediated Arterial Vasodilation

Abstract

Although blood flow-induced shear stress is the major physiological stimulus for nitric oxide (NO)-mediated arterial vasodilation, the current blood flow classification scheme consists of only two dimensions, direction and shear rate. This simplification avoids turbulence quantification and blood's non-Newtonian liquid characteristics. PURPOSE: To investigate NO-mediated arterial vasodilation using a new blood flow classification scheme based on three dimensions, i.e. direction, shear stress and turbulence. METHODS: Eighteen apparently healthy, young men (25±4 years) were randomly assigned to one 45-minute session either sham- or active-Enhanced External Counterpulsation (EECP). Brachial (b) and femoral (f) flow mediated dilation (FMD) were assessed before and within 10 minutes after completing EECP using high resolution ultrasound and Doppler, and analyzed off-line with edge-detection software. After 20 minutes of EECP, blood flow velocity (V) and artery diameters (D) were recorded live for 2 minutes for further off-line analysis. In addition, a blood sample was drawn to determine hematocrit, blood viscosity (μ) and density (ρ), Reynolds number (Re =V•D•ρ/μ), and shear stress (SS=shear rate•μ). Two-way repeated measurements ANOVA were performed and statistical significance was set at ·=0.05. RESULTS: Femoral and brachial blood flow patterns were retrograde-turbulent without increased shear stress and antegrade-laminar with increased shear stress, respectively. Femoral retrograde turbulent Re and brachial antegrade shear stress were increased during EECP (8811±4410 vs. 274±140, and 58.1±13.2 vs. 29.9±7.3 dynes•cm-2, respectively, p<0.05), when compared to sham. Thus, EECP increased retrograde turbulent blood flow and antegrade shear stress in the femoral and brachial artery, respectively. Femoral and brachial shear rate patterns during EECP do not reflect the same shear stress patterns. This is probably due to blood's non-Newtonian fluid characteristics that significantly affect blood viscosity when shear rate is below 100 s-1. fFMD was increased after EECP compared to sham (fFMD= 13.1±1.4% vs. 7.9±1.4%, p<0.05) and bFMD was increased after EECP compared to before EECP (bFMD= 10.6±1.3% vs. 7.0±1.2%, p<0.05), despite different blood flow patterns. CONCLUSIONS: These results provide, for the first time, novel evidence that blood's non-Newtonian fluid characteristics should not be avoided. Adding turbulence and shear stress to blood flow pattern characterization allows a better understanding of blood mechanics and NO-mediated arterial vasodilation.