The findings suggest that people with high blood pressure may need higher doses of medication or even different drugs in the winter months, says researcher Ross D. Fletcher, MD, chief of staff at the VA Medical Center in Washington, D.C.
The researchers analyzed the electronic health records of 443,632 veterans with high blood pressure treated at 15 VA hospitals throughout the U.S. over a five-year period.
Blood pressure was nearly 8% less likely to return to normal in the winter than in the summer, the study showed.
"In all cities, there was a seasonal variation that didn't seem to be related to outside temperature," Fletcher says.
Whether you're in San Juan, Puerto Rico, or Anchorage, Alaska, "every summer it gets better and every winter it gets worse," he tells WebMD.
One hopeful trend: In each of the 15 cities studied, the number of people with hypertension whose levels returned to normal rose an average of 4% per year, Fletcher says.
The findings were reported at the American Heart Association's Scientific Sessions 2007.
(Do you find that your blood pressure is higher in the wintertime? What do you do differently? Talk about it on WebMD's Hypertension: Support Group board.)
Understanding Your Blood Pressure Reading
"Normal" blood pressure is a measurement of less than 120/80; prehypertension is a blood pressure reading in which the top number is in the range of 120-139 and the bottom number is in the range of 80-89.
In the study, people with readings of more than 140 over 90 on three separate days were considered to have high blood pressure.
Explaining the Seasonal Gap
Fletcher says that weight and exercise may play a role in the seasonal variations. "People gain weight in the winter and lose weight in the summer.Â People tend to exercise more in the summer and less in the winter," he says.
Halperin notes that other studies have shown that blood pressure and heart disease rates can vary over the course of a day.
Heart attacks, for example occur more commonly in the morning, he says, while blood pressure tends to drop at night. "But to my knowledge, this study is the first to show seasonal variations," Halperin tells WebMD.
Blood Pressure in Summer and Winter
Halperin advises doctors and patients to be more attentive to blood pressure in winter months.
"If a person has borderline readings in the summer, he should think about being screened again in six months to make sure blood pressure hasn't dropped further," he says.
If blood pressure does dip in the winter, a more potent treatment regimen should be considered, Fletcher says.
"Our goal is to get as many people to below 140 over 90 as possible," he says.
The other 13 VA hospitals studied were in Baltimore, Boston, Chicago, Fargo, N.D., Honolulu, Houston, west Los Angeles, Miami, Minneapolis, New York, Philadelphia, Portland, Ore., and Washington, D.C.Â