Seasonal variations in endothelium-dependent flow-mediated vasodilation and endothelium-independent nitroglycerine-induced vasodilation

Cardiovascular mortality has been shown to vary seasonally. However, it has not been determined whether vascular function is affected by the season. The purpose of this study was to investigate the associations of vascular function with season and outdoor temperature. Between April 2007 and March 2022, measurements of flow-mediated vasodilation (FMD) of the brachial artery as an index of endothelial function and nitroglycerine-induced vasodilation (NID) as an index of endothelium-independent vasodilation were performed in 2190 subjects. There was no significant seasonal difference in FMD (spring, 3.9 +/- 3.1%; summer, 3.5 +/- 3.0%; fall, 3.7 +/- 3.0%; winter, 3.6 +/- 3.2%; P = 0.14). There was no significant correlation between FMD and daily mean outdoor temperature (r = -0.02, P = 0.25). Multivariate analyses revealed that neither season (beta = -0.020, P = 0.31) nor outdoor temperature (beta = 0.005, P = 0.81) was significantly associated with FMD after adjustment for other confounding factors. There were significant seasonal differences in NID (spring, 12.8 +/- 6.3%; summer, 12.0 +/- 6.1%; fall, 11.7 +/- 6.1%; winter, 12.3 +/- 5.9%; P = 0.02). However, multivariate analysis revealed that there was no significant association between season and NID after adjustment for other confounding factors (beta = -0.012, P = 0.56). There was no significant correlation between NID and daily outdoor mean temperature (r = -0.03, P = 0.17). Multivariate analysis revealed that outdoor temperature was not significantly associated with NID (beta = -0.006, P = 0.78). There was no significant association of FMD or NID with season or outdoor temperature, suggesting that it is not necessary to take into account the effects of season and outdoor temperature on vascular function when interpreting the results of FMD and NID measurements. Public trials registry number: UMIN000039512.