Healthy firefighters who were exposed to extreme heat and physical exertion had changes in physiological measures of cardiovascular function that could increase their risk of an MI, according to an open-label, randomized, crossover study.

Participants in a fire simulation had experienced an activation of platelets, an increase of thrombus formation and an impaired vascular vasomotor function, which the researchers mentioned were mechanisms in the pathogenesis of acute MI.

Lead researcher Nicholas L. Mills, PhD, of the University of Edinburgh in the United Kingdom, and colleagues published their results online in Circulation on April 3.

“Limiting the duration of exposure, active cooling and effective rehydration are simple and inexpensive ways that might mitigate the risk posed by fire suppression,” the researchers wrote. “These findings suggest pathogenic mechanisms to explain the association between fire suppression activity and acute myocardial infarction in susceptible firefighters.”

The researchers mentioned that cardiovascular events account for approximately 45 percent of on-duty fatalities for firefighters in the U.S. each year. They added that coronary artery disease death is 12 to 136 times more likely to occur during or after fire suppression, which is often characterized by high ambient temperatures, extreme physical exertion, noxious air pollutants and psychological stress.

This study enrolled 19 healthy, non-smoking firefighters who were part of the Scottish Fire and Rescue Service. They had no symptoms of respiratory tract infection in the four weeks before the study and did not have cardiovascular disease, arrhythmias, diabetes, hypertension or asthma. The mean age was 41 years old, and 16 of 19 firefighters were males.

The firefighters were randomized to a training exercise in a fire simulation facility or undertook light duties. They were off duty for 48 hours before attending the sessions, so that the researchers could limit confounding from other occupational exposures.

After each session, the researchers performed cardiovascular assessments in a quiet, temperature-controlled room. The participants abstained from alcohol for 24 hours and from food, tobacco and caffeine-containing drinks for at least four hours before each vascular study.

During the fire simulation exposure, participants entered the facility as part of a team of four firefighters, ascended stairs while dragging a water-filled hose throughout the facility, located and attempted to extinguish the fire located on the first floor and then tried to identify and rescue an 80 kg dummy who represented a dead person.

In all, 17 of the 19 firefighters had both exposures and were included in the final analysis. They had their heart rate and blood pressure monitored for 30 minutes before the exercises and for 24 hours after the exercises.

The mean maximum temperature in the fire simulation facility was 406.2 degrees Celsius, while the mean core temperature at baseline was 37.4 degrees Celsius. During the simulation exposure, the firefighters’ heart rate and temperature rose rapidly and were associated with asymptomatic ST-segment depression.

After the fire simulation exposure, thrombus formation increased 73 percent in the low-shear chamber and 66 percent in the high-shear chamber, while platelet-monocyt binding increased 7 percent. Meanwhile, systolic and diastolic blood pressures were lower immediately before the vascular studies compared with the control period.

“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,” Mills said in a news release. “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 researchers mentioned the study’s main limitation was that fire simulation exposure does not accurately represent real-life fire suppression. They also mentioned the firefighters in this study were familiar with the fire simulation center and the exercise undertaken.

“Exposure to extreme heat and physical exertion during simulated fire suppression increases thrombogenicity, impairs vascular function, and causes myocardial injury in healthy firefighters,” the researchers wrote. “Our findings suggest the pathogenic mechanisms to explain the association between fire suppression activity and acute myocardial infarction in susceptible firefighters.”