Firefighters’ exposure to extreme heat and physical exertion may trigger blood clots and impair blood vessel function — increasing risk of heart attack, according to a Scottish study.

Cardiovascular events are the leading cause of death among firefighters, representing about 45 percent of on-duty fatalities annually in the United States.

“This new study should encourage practitioners to aggressively evaluate and treat firefighters for cardiovascular disease risk factors, and when indicated, perform additional studies — such as exercise stress testing, coronary artery calcium scans or echocardiography — to detect atherosclerosis or cardiac enlargement,” said Stefanos N. Kales, M.D., M.P.H., associate professor at Harvard Medical School and Harvard School of Public Health, The Cambridge Health Alliance – Occupational Medicine in Cambridge, Massachusetts.

For the study, 17 nonsmoking, healthy firefighters randomly selected from the Scottish Fire and Rescue Service participated in two simulation exercises, each a week apart. The firefighters were exposed to temperatures reaching upward of 400 degrees Celsius (752°F) as they retrieved a “victim” weighing about 176 pounds from a two-story structure.

For 30 minutes before the exercises and for 24 hours following, researchers monitored blood pressure, heart rate, heart rhythm and the strength and timing of electrical impulses passing through each part of the heart.

“In this setting, an increase in blood clotting is likely an exaggerated normal physiological reaction to both these stressors (extreme heat and physical exertion),” said Nicholas Mills, M.D., Ph.D., chair of cardiology and consultant cardiologist at the University of Edinburgh in Scotland. “Lower blood pressure immediately following fire suppression is likely due to dehydration and an increase in blood being diverted to the skin to help the body cool down. We discovered the core body temperature increased, on average, nearly 2 degrees Fahrenheit over 20 minutes. And increases in hemoglobin occur as the body loses water and the blood gets more concentrated.”

The study, published in the American Heart Association journal Circulation, suggests that people exercising in extremely high temperatures should keep well hydrated and allow time to cool down afterward.