Summer does not always mean lower: seasonality of 24 h, daytime, and night-time blood pressure

Objective: Evaluation of seasonal influences on ambulatory blood pressure monitoring (ABPM) values in a very large population living in a mild-climate geographic area. Methods: Among patients referred to our Hypertension Center between September 2002 and January 2011 with a reliable ABPM, we considered those in the two hottest (July and August) vs. those in the two coldest (January and February) months. Results: Seven hundred and forty-two men (53.2%) and 653 women (46.8%) were studied; 1245 (89.3%) were hypertensive patients of which 795 (63.9%) were drug-treated. In winter, mean daytime SBP and DBP were higher (P=0.001 and P<0.001, respectively), but only 24-h DBP was significantly higher (P=0.012). On the contrary, higher night-time SBP and pulse pressure were recorded in summer (P=0.005 and P=0.023, respectively). Uncontrolled hypertensive patients had the highest mean difference between winter and summer night-time SBP (127.1 +/- 13.4 vs. 131.0 +/- 12.6 mmHg; P=0.001). In winter a dipping pattern was prevalent (58.2%), whereas in summer a nondipping pattern prevailed (61.9%; P<0.001). Isolated nocturnal hypertension (INH) was present in 9.8% in winter vs. 15.2% in summer (P=0.003). Conclusion: Our data on a very large ABPM sample confirmed that hottest summer months are associated with lower daytime BP and also lower 24-h DBP. However, we found an inverse relationship regarding night-time BP, dipping pattern, and INH that were higher or more common in summer. These findings were even more evident in treated patients, especially when not at target. Different sleeping behaviors or improper dose reduction of drug therapy in summer may explain the findings.