"Although our study does not demonstrate a causal relationship between blood pressure and external temperature, the observed linkage has potentially important consequences for pressure control in the elderly," the authors wrote in the Archives of Internal Medicine.
High blood pressure, or hypertension, can cause stroke, heart and kidney failure, and heart attacks. The condition affects more than 1 billion adults worldwide.
The French team observed the relationship between blood pressure and temperature in more than 8,800 men and women aged 65 or older. The volunteers were from three cities and the pressure was measured at regular intervals in 1999 and then two years later.
Both the systolic and diastolic pressure differed in the four seasons and in the face of the variation of the external temperatures.
Hypertension, defined by a systolic reading of 160 or more or a diastolic reading of more than 95, was detected in about a third of the volunteers during the winter and a quarter in the summer.
On average, the pressure of each person fell between the initial measurement and the follow-up and the decrease was strongly related to the outside temperature, the experts said.
The average systolic pressure was 5 millimeters of mercury higher in winter than in summer, they added.
"The higher the temperature at follow-up, compared to the base, the more pronounced was the decrease in blood pressure," the researchers wrote.
The team said it does not know the reason for this, but believes that a hormone related to stress, which is generated during cold weather, could increase the pressure by accelerating the heart.
The results suggest that doctors should consider closer monitoring of elderly patients who take medicines for pressure when the ambient temperature decreases, the scientists added.
The study "would explain the well-established seasonal variations in stroke and CVD death, aneurysm ruptures and other vascular ailments," the team concluded.
