When winter arrives and temperatures drop, there’s one place the mercury actually rises — in blood pressure gauges.
A survival mechanism in people and other mammals constricts blood vessels in cold weather, to conserve heat and maintain body temperature. But with less room for blood to move, pressure rises — along with the risk of fatal heart attack and stroke, which peaks during winter.
Many of these deaths could be prevented with simple precautions, says University of Florida blood pressure expert Zhongjie Sun, M.D., Ph.D., an assistant professor of physiology and medicine who has just uncovered a new facet of the mysterious mechanism after identifying a gene that triggers cold-induced high blood pressure in mice. UF researchers describe their findings in the current issue of the American Journal of Physiology — Regulatory, Integrative and Comparative Physiology.
“Everyone should bear in mind that cold temperature is a risk factor (for heart attack and stroke),” Sun said.
People especially at risk include those with high blood pressure, also known as hypertension, and patients with health conditions that require strict blood-pressure maintenance, such as diabetes and chronic kidney disease. But even comparatively healthy people aren’t entirely immune, he said.
“Patients with hypertension should be very careful when they go out (in the cold),” Sun said. “But normal people should take precautions as well.”
Blood pressure, considered a key indicator of overall health, is the force blood exerts against artery walls. The measurement has two components — systolic pressure, taken when the heart beats, and diastolic pressure, taken between beats. Blood pressure is measured in millimeters of mercury and is always expressed with systolic pressure given first.
New, stricter U.S. guidelines issued in 2003 state that a healthy person at rest should have a blood pressure below 120/80, according to the American Heart Association. High blood pressure is defined as 140/90 or more, and readings from 120/80 to 139/89 are considered prehypertensive. Fifty million Americans are believed to have hypertension, and one-third of them are unaware of it.
How much will your blood pressure increase in cold? The answer depends on variables such as the current temperature and wind chill, the temperatures you’re accustomed to, how long you’re exposed and your health, dress and activity level, Sun said. But it doesn’t take much; a previous study by Sun and his colleagues showed just five minutes’ exposure to a temperature of 52 degrees Fahrenheit can cause pressure to rise substantially. And a common medical evaluation known as the cold pressor test shows that a person who plunges one hand into freezing water for one minute will experience a rise in blood pressure lasting up to two hours.
People — and their furry mammalian counterparts — living in warmer climates aren’t immune to cold-induced blood-pressure change, either, he said. UF researchers found that a control group of 12 healthy mice kept at a constant 41 degrees Fahrenheit around the clock experienced a 50 percent increase in blood pressure after five weeks, he said.
“That’s not terribly cold,” he said. “It’s about the average temperature of a Gainesville winter.”
The study’s major finding was that 12 genetically engineered mice that lacked a receptor activated by the vessel-constricting hormone angiotensin II experienced only an 11 percent blood pressure increase under the same conditions, Sun said. The results show, for the first time, that the receptor plays a key role in cold-induced blood pressure increases. The finding could point the way toward new treatment and prevention strategies that may eventually save lives, he added.
The receptor, known as angiotensin II receptor type 1A, or AT1A, is part of the renin-angiotensin system, which regulates blood pressure in mammals, he said. AT1A receptors are located in the heart, blood vessels, kidneys and brain. The drug losartan blocks the receptors from receiving angiotensin and commonly is prescribed to treat hypertension.
“We plan to collaborate with clinicians to look at blood pressure changes in hypertensive patients in all four seasons and see if renin-angiotensin system control is a good way to control cold-induced hypertension,” Sun said.For now, physicians treating hypertensive patients with medication should be alert to seasonal changes in blood pressure, he said.
“You want to watch them (patients) more closely, because if you use the same dose of antihypertensive drugs as in the summer, they may not be able to control hypertension in cold weather,” Sun said.
People can take other simple precautions to help lower risk, Sun added. Cold-weather care includes dressing in layers to conserve body heat, easing into outdoor physical activity to minimize sudden changes in the heart’s workload and avoiding extreme exertion or heavy lifting. Wearing a hat, scarf and gloves will minimize the amount of skin exposed, important because blood pressure increases don’t require full-body exposure, he said.
People whose jobs require prolonged or repeated exposure to cold, such as farmers, construction workers, meat cutters and law enforcement officers, should be particularly mindful of precautions, he said.
Sun conducted the study with collaborators Xiuqing Wang, Ph.D., a UF assistant scientist in medicine, Charles Wood, Ph.D., a UF professor and chairman of physiology, and J. Robert Cade, M.D., a UF professor emeritus of medicine. The research was funded by a four-year, $260,000 grant from the American Heart Association.
The UF findings suggest that further research would help explain whether cold-induced blood pressure increases are controlled by AT1A receptors located in blood vessels, or elsewhere, said angiotensin expert Irving H. Zucker, Ph.D., a professor and chairman at the University of Nebraska Medical Center’s department of cellular and integrative physiology in Omaha, Neb.
“One of the things that would be useful to do is use models that target the deletion or overexpression of the angiotensin receptors to specific organs,” said Zucker, who studies angiotensin’s role in heart failure. “I’m sure Dr. Sun is thinking about that.”
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