Obesity and Endothelial Function: Investigating the Importance of the Shear Stress Stimulus Profile Used for FMD Assessment

Abstract

Endothelial function is important for the maintenance of vascular health and it can be assessed by measuring endothelial dependent flow mediated dilation (FMD) in response to an imposed increase in shear stress (SS). While the 5 min reactive hyperemia FMD test (5 min RH-FMD) of the brachial artery (BA) remains the most commonly employed method of increasing shear stress, other methods, including handgrip exercise (HGEX), also exist. The purpose of this study was to determine whether obesity is associated with a uniform, or a SS profile specific impact on FMD. 19 healthy controls (21.42 +/- 2.46 years) and 9 obese subjects (27 +/- 5.43 years old) participated in this study. Each subject performed two 5 min RH-FMD tests, two HGEX-FMD tests, and one 15 min RH-FMD test. BA diameter and blood velocity (BV) were assessed using ultrasound. Data are means ± SD. No differences in mean SS experienced during HGEX were observed between groups (p>0.05). %HGEX-FMD was significantly higher in the controls (11.34 ± 4.72% vs. 5.19 ± 3.00%)(p<0.001), while no differences in 5 min or 15 min RH-FMD were observed in the full study sample (p>0.05). To account for higher SS area under the curve (SS AUC) experienced by the obese group in both the 5 min RH- (p=0.055) and 15 min RH-FMD tests (p=0.004), a subset of the 9 controls with the highest SS AUC in each test was compared to the obese group. In this subset, the lower RH-FMD in the obese group approached significance (5 min RH- FMD: controls 8.52 ± 2.0 % vs. obese 6.22± 2.5%, p=0.052; 15 min RH-FMD: controls 17.75 ±5.03% vs. obese 13.28± 5.14%, p =0.08). The findings demonstrated for the first time, that HGEX-FMD was more than 50% lower in an obese group vs. a control group with a matched SS stimulus. It appears that the larger RH SS stimulus in the obese group played a role in masking impaired RH-FMD. When the SS magnitude was considered, the ability of the endothelium to respond to all stimulus profiles was more similarly attenuated in obese participants.