Abstract 075: Seasonal Variation and Day-by-day Variability in Nocturnal Blood Pressure Fall: The Nagahama Study

Abstract

Backgrounds: Abnormalities in circadian blood pressure (BP) variation have been suggested to be associated with cardiovascular diseases and mortality. Factors affecting the variability need to be clarified to precisely evaluate the risk of circadian BP abnormalities. Given the seasonal differences in casual BP, it was hypothesized that nocturnal BP may also differ by season. Here we aimed to clarify seasonal and day-by-day variability of circadian BP variation in a large-scale general population. Methods: We analyzed a dataset describing in the Nagahama study. Study participants were 4,792 community residents. Evening, sleep, and morning BP values were measured for 5 days at home using an automatic cuff-oscillometric device (HEM-7080IC). Participants were required to sleep with putting a cuff on the upper arm. The BP monitor was programmed to automatically measure BP at 0, 2, and 4 o’clock, and all readings were recorded in a built-in memory of the device. Sleeping period was objectively defined by actigraphy. Results: Overall frequency of extreme-dipper, dipper, non-dipper, and riser were 6.5, 34.9, 45.1, and 13.5%, respectively. Nocturnal systolic BP fall was significantly smaller in individuals whose BP was measured during the summer season (summer, –5.8 ± 7.8%; middle, –8.2 ± 7.5%; winter, –11.1 ± 7.7%; p <0.001), resulting in higher frequencies of riser (summer, 19.9; middle, 12.8; winter, 7.5%) and non-dipper (summer, 51.5; middle, 46.2; winter, 36.7%) in summer season ( p <0.001). The mean ambient temperature showed significant association with nocturnal SBP dipping level (r = 0.259, p <0.001). Results of linear regression analysis identified middle (beta = 0.156, p <0.001) and summer season (beta = 0.263, p <0.001) as strong positive determinants for decreasing nocturnal SBP fall, independently of other possible covariates. No marked seasonality was observed in day-by-day variability of the dipping pattern (Kendall’s coefficient: winter, 0.590; middle, 0.602; summer, 0.581). Conclusion: Nocturnal BP fall was largely different by season with higher frequency of riser and non-dipper in summer. The seasonality might not be due to the seasonal difference in day-by-day variability of nocturnal BP changes.