Outdoor temperature and blood pressure appear to be correlated in the elderly, with higher rates of hypertension in cooler months, according to a report in the Jan. 12 issue of the Archives of Internal Medicine professional journal.

Seasonal variations in blood pressure have been recognized among the general population for 40 years, according to background information in the article. Few previous studies, however, have looked specifically at older adults.

"Elderly persons may be particularly susceptible to temperature-related variations in blood pressure," the authors write. "The baroreflex, which is one of the mechanisms of blood pressure regulation, is modified in elderly subjects, and it has been hypothesized that disorders of baroreflex control and enhanced vasoreactivity (sensitivity of blood vessels) could contribute to the aging-associated increase in cardiovascular morbidity (illness)."

Dr. Annick Alprovitch, of the Institut National de la Sant et de la Rcherche Mdicale, in Paris, and colleagues assessed the relationship between blood pressure and temperature in nearly 9,000 people 65 and older. All were part of a study conducted in three French metropolitan areas. Participants' blood pressure was measured at the beginning of the study, starting in 1999, and again about two years later. Outdoor temperatures on the day of measurement were obtained from local meteorological offices.

Both systolic (top-number) and diastolic (bottom-number) blood pressures differed across the four seasons and across the distributions of outdoor temperatures. Average systolic blood pressure was 5 millimeters of mercury higher in winter than in summer. High blood pressure—defined as a systolic blood pressure of 160 millimeters of mercury or higher, or a diastolic blood pressure of 95 millimeters of mercury or higher—was detected in 33.4 percent of participants during winter and 23.8 percent during summer.

On average, each individual's blood pressure decreased between the initial and follow-up measurements. This decrease also was correlated strongly with outdoor temperature.

"The higher the temperature at follow-up compared with baseline, the greater the decrease in blood pressure," the authors write. These differences over time were larger in participants age 80 and older.

"Mechanisms that could explain the association between blood pressure and temperature remain undetermined," the authors continue. The sympathetic nervous system, which helps control involuntary actions, such as stress response, is activated and the hormone catecholamine is released in response to cold temperatures. That physiological response to cooler weather might increase blood pressure by speeding the heart rate and decreasing the responsiveness of blood vessels, they suggest.

"Although our study does not demonstrate a causal link between blood pressure and external temperature, the observed relationship nevertheless has potentially important consequences for blood pressure management in the elderly," the authors write.

It might help explain well-established seasonal variations in illness and death from stroke, aneurysm ruptures, and other vascular diseases, their report says.

"Because the risk of stroke or aneurysmal rupture is highest in the elderly, improved protection against these diseases by close monitoring of blood pressure and antihypertensive medication when outdoor temperature is very low could be considered."