Recent clinical studies using a vasovagal syncope suggest that endothelial function is a key factor controlling blood pressure (BP) regulation during acute hypotension. Acute increase in BP is known to attenuate brachial flow-mediated dilation (FMD). However, the influence of acute hypotension on peripheral FMD remains unclear.
PURPOSE: To test the hypothesis that acute hypotension blunts brachial FMD, an index of endothelial function, in young healthy humans. METHODS: Twelve healthy men (21.8±1.6 yrs; body mass index: 22.2±1.6 kg/m², mean±standard deviation) underwent 3 trials of brachial FMD measurement. The trials involved a standardized FMD protocol (control trial), a trial with an abrupt decrease in BP via both thigh-cuff inflation and the deflation method (hypotension trial), and a trial with a decrease in shear rate (SR) via a shortened occlusion time for 3 min without a change in BP (SR reduction trial). Trials were randomized with 30-min intervals during a single visit. Duplex ultrasound was employed to measure brachial diameter and blood velocity. BP and heart rate were continuously measured using a non-invasive beat-to-beat BP monitoring system. To adjust the effect of SR area under the curve (SR_AUC) on FMD, normalized FMD were calculated.
RESULTS: There was a marked fall in mean arterial pressure during reactive hyperemia in the hypotension trial (−24±6 mmHg), but not in the control and SR reduction trials. SR_AUC was attenuated in the SR reduction trial (p<0.001), but not in the hypotension trial (p=0.316), compared with the control trial. Thus, FMD was attenuated in the hypotension and SR reduction trials compared with the control trial (p=0.003 and p=0.043, respectively). The attenuation in FMD was greater in the hypotension trial than in the SR reduction trial (p=0.006; control, 6.9%±3.5%; hypotension, 3.5%±1.7%; SR reduction, 5.0%±2.2%). After adjusting FMD using SR_AUC, the normalized FMD remained attenuated in the hypotension trial (p=0.014), but not in the SR reduction trial (p>0.05). CONCLUSION: Current findings indicate that BP could be an important determinant of FMD. Blunted FMD of the peripheral arteries may be a physiological response to restore BP and/or prevent a further reduction in BP following acute hypotension in healthy humans.