Doctors who use treadmill tests to diagnose heart disease could better predict the risk of death if they paid more attention to what happens to a patient’s heart after he or she steps off the treadmill, a large study found.

Treadmill tests are given to millions of Americans each year. Their hearts are analyzed while they walk at a steadily increasing pace. But doctors concentrate almost entirely on what happens while the patient is actually exercising.

The decade-long study of more than 29,000 patients showed that the presence of irregular heartbeats in the minutes after a treadmill test is a better indicator of death within five years than irregular heartbeats during the test itself.

An irregular heartbeat after exercise on the treadmill “will be considered a marker of risk,” said Dr. Daniel M. Shindler, a cardiologist at Robert Wood Johnson Medical School in New Jersey. “Before, it was not clear.”

That information could, in turn, help doctors decide which patients need invasive testing, aggressive treatment and close monitoring.

The study was reported in today’s New England Journal of Medicine.

Cardiologists at the Cleveland Clinic found that 11 percent of patients with irregular heartbeats after a stress test were dead within five years, compared with 5 percent of those who did not have such irregular rhythms.

Nine percent of patients with irregular heartbeats during the test itself died within five years, compared with 5 percent who did not have irregular beats during the exercise.