A long-term rise in a man's resting heart rate increases his risk of dying, French researchers report.

Their study of more than 4,300 men, aged 42 to 53, found that those whose resting heart rate increased over five years were nearly 50 percent more likely to die over a 20-year span than men whose rate stayed the same or decreased.

A long-term decrease in resting heart rate reduced death risk by nearly 20 percent, the study found. The findings were to be presented at the American Heart Association's annual meeting in Chicago.

Resting heart rate is an indicator of how hard the heart is working to maintain adequate blood flow. A resting heart rate of 60 to 80 beats per minute (bpm) is normal. People in excellent physical condition have a resting heart rate of about 40 to 50 bpm.

The researchers concluded that resting heart rate and its changes may be an independent risk factor for death.

"We don't know why resting heart rate does down or up over time. It might be related to lifestyle changes, such as less activity," study author Dr. Xavier P. Jouven, of the Hopital Europeen Georges Pompidou INSERM in Paris, said in a prepared statement.

"We also cannot say for sure whether the increase in resting heart rate is only a marker for some other disease process or whether it is directly associated with mortality," Jouven said.

More research needs to be done to learn more about resting heart rate's role as a health indicator, he said.