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The 200-millisecond threshold was set rather arbitrarily, he said. The degree of risk seen in the study was directly associated with the length of the PR segment prolongation -- the longer the interval, the greater the risk.

And the amount of prolongation was directly associated with age -- the older the person, the longer a prolonged PR interval was likely to be. The association with age is no surprise, Wang said. It's also seen in those with cardiac risk factors such as high blood pressure.

"Lengthening of the PR interval is commonly seen on routine electrocardiograms, more often in older patients, and has been considered a relatively harmless finding," Dr. Susan Cheng, a cardiology fellow at Massachusetts General Hospital and Brigham and Women's Hospital and a member of the research team, said in a statement. "But our results indicate that PR interval prolongation is not as benign as previously thought."

The event rates seen in the study "are pretty low, even over 20 years of follow-up," said Dr. Daniel Lloyd-Jones, associate professor of preventive medicine and cardiology at the Northwestern University Feinberg School of Medicine.

"And from a clinician's point of view, I wouldn't do an electrocardiogram just to look for a prolonged PR interval," Lloyd-Jones said. "But we do electrocardiograms for all sorts of reasons, and it is reasonable to look for prolonged PR intervals because it is a modest marker of risk."

The most intriguing association is with the increased risk of atrial fibrillation, he added. "The atrial fibrillation study is really an interesting one," Lloyd-Jones said.

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