Influence of Nitric Oxide on Flow Mediated Dilation

Overview

Flow-mediated vasodilation (FMD) has become one of the most widely assessed parameter to analyze endothelial function. The peripheral endothelial function as assessed by FMD correlates with coronary artery endothelial function and moreover, impaired FMD has been revealed to be good predictor for cardiovascular events in patients beyond cardiovascular risk factors and in patients with coronary artery disease. This non-invasive technique measures the ability of the arteries to respond with endothelial release of vasoactive factors during reactive hyperemia. In this study the goal is to analyze in a clinical trial to what extend nitric oxide contribute to FMD by measuring FMD before and after blocking the release of nitric oxide. FMD is measured using a semi-automatic device named UNEX, recently developed in Japan. This semi-automatic ultrasound system represents a new development that overcomes the limitation of classic systems. The hypothesis of this study is that FMD response is mostly nitric oxide dependent. To prove this hypothesis, overall 20 healthy male volunteers are included in this study. Total duration of this study for each volunteer is 2-4 weeks with total 3 visits at the Clinical Research Unit (CRC) of the Department of Nephrology, University of Erlangen-Nuremberg. This study is important to better understand the mechanism of any kind of impaired FMD by cardiovascular risk factors and diseases and to better interpret impaired FMD assessed by UNEX.

The assessment of endothelial function is crucial, as its dysfunction is described as a key pathological condition associated with many diseases leading to arteriosclerosis. Flow-mediated vasodilation (FMD) has become one of the most widely assessed parameter to analyze endothelial function. Celermajer et al. was the first to measure the FMD-response in vivo by ultrasound. This non-invasive technique measures the ability of the arteries to respond with endothelial release of vasoactive factors during reactive hyperemia. The peripheral endothelial function as assessed by FMD correlates with coronary artery endothelial function. Impaired FMD has been revealed to be good predictor for cardiovascular events in patients beyond cardiovascular risk factors and in patients with coronary artery disease. It is also predictive for the extent and severity of coronary atherosclerosis. Beyond the predictive nature of FMD, several studies have been reported on the effect of pharmacologic or physiologic interventions on FMD. For example weight loss and exercise increased FMD in overweight and obese patients with coronary heart disease. Impaired FMD is mainly characterized by reduced bioavailability of different vasodilators due to oxidative stress. Of these vasodilators nitric oxide plays the major role, but its contribution to FMD is inconclusive. Studies to clarify the role of nitric oxide on FMD have been done with various methods, which require extensive training and standardization. Operator, study preparation, image acquisition and site selection, sphygmomanometer probe position, cuff occlusion time, the accurate use of edge-detection software as well as the correct characterization of the FMD response are all factors, which influence FMD measurement. In this study the goal is to analyze in a clinical trial to what extent nitric oxide contribute to FMD by measuring FMD before and after blocking the release of nitric oxide. FMD is measured using a semi-automatic device named UNEX, recently developed in Japan. FMD measurements using UNEX implement the standards described in guidelines. This semi-automatic ultrasound system using an H-type ultrasound probe represents a new development that overcomes the limitation of classic systems (in particular the investigator-dependency of conventional, "hand-held" FMD measurements). This system comprised a 7.5-megahertz linear array transducer and a novel stereotactic probe-holding device (UNEX Co., Nagoya, Japan). Another advantage of this device is the continuous recording of B-mode images and A-mode waves of the artery in the longitudinal plane, so that continuous measurement of arterial diameter in the 4.5 min following cuff deflation occurs. A meta-analysis described that the nitric oxide dependency of the FMD response was the most in studies with FMD measurements at brachial artery with distal placement of the cuff and 5-minute occlusion. These criteria are fulfilled by UNEX. However, no data on nitric oxide dependency are available in the literature. Finally, this semi-automatic assessment of FMD has been shown to improve the precision and repeatability of the measurements. The hypothesis of this study is that FMD response is mostly nitric oxide dependent and this could not be proven so far due to less sensitive conventional tool of FMD measurement.