MUNICH, Germany – People with high blood pressure seem to be more vulnerable to heart attacks when the temperature drops, new research shows.

The findings, presented Monday at a conference of the European Society of Cardiology, are not surprising because cold weather makes the blood vessels constrict, making it harder for blood to move through, but the study is the first to document that variations in the weather increase the occurrence of heart attacks in people with high blood pressure.

The two-year study, conducted by scientists at the University of Burgundy in France, examined 748 people admitted to local hospitals with a heart attack. The researchers matched the hospital admission with weather information covering the same period.

Of those in the study, 50 percent were being treated for high blood pressure or had at some time suffered from the condition.

Overall, heart attacks were more frequent when the temperature dropped below 39.2 degrees. However, a closer analysis revealed that this was only true for people who had high blood pressure. There were double the number of heart attacks among these people when the temperature dropped below the threshold.

Heart attacks also went up in hypertensive people when the temperature dropped by more than nine degrees on the day of their attack, regardless of how cold it was.

Blood pressure rises when it gets colder because the blood vessels narrow to preserve body heat. The hypertensive starts at a higher blood pressure than healthy people and ends up with an even higher blood pressure when the weather turns. Higher blood pressure is more strenuous for the heart.

The study highlights the special vulnerability that people with high blood pressure have to cold weather and underlines the importance of getting their blood pressure down to normal levels, said Dr. David Faxon, chief of cardiology at the University of Chicago and a former president of the American Heart Association.

Barometric pressure, or air pressure, was also an important influence on heart attacks in people with high blood pressure, the study found, but not in those with normal blood pressure. A shift of more than 8 points increased the chances of an attack, the investigators found.

“A peak in (heart attack) onset in this population was observed at the passage of a cold front, but not a warm one,” the study found. Cold fronts have denser air than warm fronts.

How the weather is linked to heart attacks is unclear, but several theories exist. One is that the inflammation caused by winter respiratory infections could make the blood vessels more vulnerable. Another is simply that the extra narrowing of the blood vessels is enough to trigger a heart attack.

“This makes sense because when you get chilly weather, the blood pressure increases,” said Dr. Lars Ryden, professor of Cardiology at the Karolinska Institute in Stockholm, Sweden, who was not connected with the study. “It happens in everyone but people with hypertension are particularly sensitive. The reason people get hypertension is that they have overreactive blood vessels and if the blood vessels contract in a hypertensive patient, you get an even more narrow vessel than if they contract in a normal patient.”

The best protection, Ryden said, is to dress warmly when going out in the cold.

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