Weather related changes in 24-hour blood pressure profile: Implications for hypertension management

Background/Aims. A downward titration of antihypertensive drug regimens in summertime is often performed on the basis of seasonal variations of clinic blood pressure. However, little is known on the actual interaction between outdoor air temperature and the effects of antihypertensive treatment on ambulatory blood pressure. Methods. The combined effects of aging, treatment and daily mean temperature on clinic and ambulatory blood pressure were investigated in 6404 subjects referred to our Units between October 1999 and December 2003. Days were was stratified according to percentiles of air temperature in hot (>90th) cold (<10th) and intermediates. Results. Office and mean 24-h systolic blood pressure as well as morning pressure surge were significantly lower in hot (p<0.05 for all), and higher in cold days (p<0.05 for all) when compared with intermediate days. At regression analysis, 24-hour and day-time systolic pressure were inversely related to temperature (p<0.01 for all). Conversely, night-time systolic pressure was positively related with temperature (p<0.02), hot days being associated with higher night time pressure. Air temperature was an independent predictor of night time systolic pressure increase in the subgroup of elderly treated hypertensive subjects only. Conclusions: Our results show for the first time that hot weather is associated with an increase in systolic pressure at night in treated elderly hypertensive subjects.This may be due to a nocturnal blood pressure escape from the effects of a lighter summertime drug regimen, and may have important implications for seasonal modulation of antihypertensive treatment.