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People being treated for hypertension more often achieve blood pressure control in the summer than in the winter, according to epidemiological research conducted across Veterans Affairs (VA) hospitals and presented at the American Heart Association's 2007 Scientific Sessions, held November 4–7 in Orlando, Fla.
The pattern holds true across the warmest and coldest cities in the United States and its territories.
In the study, researchers tracked the health records of 443,632 veterans with hypertension at 15 hospitals over 5 years. To be eligible, subjects needed readings >140 mmHg systolic or >90 mmHg diastolic on three different days. Participants in the mostly male sample were 66 years old, on average; roughly half were white, just under a quarter were Hispanic, and slightly over a quarter were black.
In every city studied, participants registered higher blood pressure
readings in the winter than in the summer, despite being on standard
hypertension medications, such as angiotensin II receptor-blockers (ARBs) or
angiotensin-converting enzyme (ACE) inhibitors. Specifically, participants
achieved pressures 7.6% lower—a significant amount—in the summer
months. In winter, there were more patients with 
160 mmHg systolic and
>100 mmHg diastolic readings.
Some cities showed greater blood pressure variability than others, with Baltimore, Philadelphia, and Washington, D.C., registering the highest winter spikes of all. Comparatively, Chicago, Fargo, N.D., and Minneapolis had the lowest winter pressures. Other cities included in the sample were Anchorage, Alaska; Boston; Honolulu; Houston; West Los Angeles; Miami; New York; Portland, Ore.; and San Juan, Puerto Rico.
SLOTH AND SNACKS
Lead researcher Ross D. Fletcher, MD, isn't sure why some cities show more variation than others. But he's willing to speculate on why blood pressure rises in winter generally: People tend to eat greater amounts, gain more weight, and exercise less during cold months, says Fletcher, chief of staff at VA Medical Center in Washington, D.C.
In his view, these behavioral patterns occur at least somewhat independently of increased cold and reduced light in winter—or a city's northern or southern location—because even people in the southernmost U.S. cities—where it's warmest and the daylight hours are longest—show seasonal blood pressure variation. In fact, people in Anchorage and San Juan had practically the same amount of blood pressure variation.
"We had thought that perhaps it was the cold and lack of light that account for the blood pressure variation, but we're not seeing predictable differences in the warmest and coldest cities," he says. "Instead it seems like lifestyle factors make the difference. People eat heavier food in winter, over the holidays. And they're more sedentary."
Fletcher notes there could be a separate effect of ambient temperature—the amount of warmth people typically experience during winter, depending on heat settings in their homes and offices, daily activities, and the like. This was something the researchers could not control for. In future research, he and his team plan to explore whether a relationship exists between weight gain and blood pressure spikes in winter.
MED AND MOVEMENT
The study's findings indicate a need for increased attention to blood pressure control during the winter months, says Fletcher. At the onset of cold weather, physicians may want to intensify monitoring of hypertension patients and increase their medication if needed, he suggests.
"Physicians could up medication doses if they don't want to change the drug class," he says. "Dosages could be doubled."
Frequent blood pressure checks and communication among medical professionals, which enable doctors to quickly adjust medication according to recent readings, can also help. The VA's electronic medical records system aids this feedback loop by alerting doctors when medication needs to be adjusted based on updated blood pressure readings taken by nurses, says Fletcher.
Meanwhile, hypertension patients should strive to maintain a healthy diet
and a physical activity regimen throughout the year, adds Fletcher.
"Research shows exercise will keep pressures down," he notes.
"So I recommend going to the gym." 
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