While daytime blood pressure (BP) tends to be lower during the hot days of summer than during cold weather, elderly patients treated with antihypertensive drugs have higher BP at night when weather is hot, a study shows. This suggests that their medication should not be reduced in the summer, even if BP measured in the doctor’s office is normal.

Previous research has shown that BP values are lower in hot months than in cold months, and that cardiovascular mortality follows a similar pattern. However, the effects of air temperature on BP in older hypertensive patients are unknown.

To investigate, Dr. Pietro Amedeo Modesti, from the University of Florence, and colleagues reviewed the records of roughly 6400 patients referred to high BP clinics between 1999 and 2003.

For the group as a whole group, researchers observed that an increase in daily mean outdoor air temperature was inversely related to BP, but conversely, nighttime air temperature was positively associated with higher nighttime BP.

When subjects younger than age 50 were compared with those over age 65, the authors found that air temperature was an independent negative predictor of daytime BP and a positive predictor of nighttime BP only among the older group.

Elderly subjects were also more likely to experience a surge in morning BP during cold weather compared with intermediate temperatures, but not during hot weather.

Air temperature was an independent positive predictor of ambulatory blood pressure at night only in older subjects with treated hypertension (high BP). Older subjects treated for hypertension took more antihypertensive drugs per day during cold weather than during hot weather.

“These modifications may be potentially dangerous, because they may adversely affect the risk of cardiovascular events both during winter and summer, through different mechanisms,” the authors point out in the journal Hypertension.

They advocate the use of 24-hour ambulatory BP monitoring to assess treatment effects in elderly patients.

SOURCE: Hypertension February 2006.