Morning attenuation of endothelium-dependent, flow-mediated dilation in healthy young men: Possible connection to morning peak of cardiac events?

Background and hypothesis: Brachial artery flow-mediated dilation (FMD), a noninvasive, widely used clinical index of endothelial function and magnitude of FMD, has been report ed to be closely related to many coronary risk factors and coronary atherosclerosis. However, there has been no study that examines the diurnal change of FMD. We designed this study to reveal the diurnal variation of FMD in healthy volunteers. Methods: We examined FMD in response to reactive hyperemia by high resolution ultrasound in 13 healthy young men (age 25-32) at four different times over the course of a day. Results Mean measures of brachial artery FMD was 4.0% at 8:00, 5.3% at 12:00, 9.7% at 17:00, and 6.9% at 21:00 hours. Flow-mediated dilation at 8:00 and at 12:00 hours was significantly lower than that at 17:00 (p < 0.05). Conclusions: These results show that endothelial function has diurnal variation and is significantly attenuated in the morning. Morning attenuation of endothelial function should be recognized in clinical research and may play an important role in the circadian variation of the occurrence of acute cardiovascular events.