The Associated Press

BLOOMINGTON, Ind. — An Indiana University study that measured firefighters' heart and breathing rates found that their cardiovascular stress hit peak levels at fires where civilians or other firefighters faced imminent danger.

During one fire, in which firefighters worked to rescue a mother and three children trapped in a burning house, five firefighters had heart rates at or above their predicted maximum for more than 30 minutes, project leader Jim Brown said.

"If I hadn't seen the data myself, I would have a hard time — as a physiologist — believing it was true," Brown said.

Researchers hope the findings can help create ways to lower the risk of heart attack among firefighters, The Herald-Times reported. Nearly half of the 100 firefighters who die in the line of duty every year in the U.S. are killed by heart attacks, according to the National Fire Protection Association.  

The seven-month IU study, funded by a $1 million grant from the Federal Emergency Management Agency, involved 56 members of the Indianapolis Fire Department. Firefighters wore vests that monitored heart and breathing rates while tracking their movements.

Cardiovascular stress — caused by a combination of adrenaline and physical exertion — was pushed to high levels at fire scenes, the study found. Heart rates increased rapidly when firefighters learned en route to a fire that people may be trapped in a burning building.

Bloomington Fire Chief Roger Kerr said he doesn't notice his heart pounding away while he's responding to a fire, but he feels exhausted moments after it has been extinguished.

"When your heart rate and adrenaline drop, you realize how exhausted you are," he said. "You are completely worn out."

The study also found that volunteer firefighters are at a higher risk for heart attacks than paid firefighters, and that inexperienced firefighters showed more signs of cardiovascular stress than veterans.

Brown said firefighters should perform both aerobic exercise and anaerobic exercise, such as weightlifting, to mimic the physical demands of firefighting. He hopes the study will lead to improved fire fighting strategies, such as rotating firefighters more frequently during a blaze.

The study also found that firefighters struggled to get quality sleep because they worked 24 hours straight followed by 48 hours of free time.

"If you don't sleep well or long enough, you're at higher risk for not only cardiovascular disease, but things like stroke and hypertension," Brown said.

Firefighters are often woken up by fire alarms, Kerr said, forcing their heart rate to go quickly from a resting to a rapid rate.

"Most athletes warm up before they compete, but we have to go from bed rest to full exertion in a matter of seconds, which puts a big strain on the heart," he said.