Blood pressure levels can change by season

November 7, 2007

Antihypertensive treatment that works in the summer may not be sufficient for wintertime blood pressure (BP) control.

Antihypertensive treatment that works in the summer may not be sufficient for wintertimeblood pressure (BP) control.

That is the surprising result from a five-year Veterans Affairs (VA) study that found fewerpatients with good BP control during the winter months than in summer. Weather alone could notaccount for the difference as the study was conducted in 15 cities with a wide variation intemperature and climactic conditions.

"Whether you're in Anchorage, Alaska, or Juan Juan, Puerto Rico, there is a difference inhigh blood pressure returning to normal in the winter compared to the summer," said Ross Fletcher,MD, chief of staff at the VA Medical Center in Washington, DC.

Only 50% to 60% of patients showed the seasonal change in blood pressure, Dr Fletcher said,but the trend for variability was consistent across racial and ethnic groups.

The study resulted from an observation that patient BP tended to be higher in winter than insummer, Dr Fletcher said. That led to a study to determine if the effect could be seen across theentire VA system.

Researchers identified 443,632 hypertensive patients from the VA's 1.2 million patientelectronic database covering 15 hospitals across the nation. Individuals with readings higher than140 mm Hg systolic or more than 90 mm Hg diastolic on three separate days were identified ashypertensive.

The hypertensive patients were followed for five years with monthly BP measurements. The averagesignificant difference in percent of patients returning to normal BP was 7.76% between winter andsummer, Dr Fletcher said.

The study was not designed to determine possible causes for the difference, but Dr Fletcherspeculated that weight and exercise could be factors. In general, he said, people gain weight inwinter and lose weight in summer. They also tend to exercise more in summer than in winter.