PITTSBURGH -- Each year hundreds of thousands of people suffer a stroke, with nearly one third of patients leaving the hospital with no explanation for the event.

But doctors at Allegheny General Hospital believe they have figured out the cause thanks to a new technology.

Carl Fazio had a leaky mitral valve repaired 10 years ago, but last year he developed atrial fibrillation -- an abnormal rhythm in his heart's two upper chambers

"I was getting very weak walking, loss energy, trouble breathing, sleeping," Fazio said.

Doctors could easily detect Fazio's heart failure, but they can't always see why a stroke happened.

The cause remains a mystery in one third of patients without blood clots or atrial fibrillation.

"They are in the normal rhythm and they are never known to have atrial fibrillation," said Dr. Ashis Tayal of Allegheny General Hospital.

A new study shows atrial fibrillation could be the cause, but the duration of the irregularity was either intermittent or just 10 to 15 seconds in duration, something too brief to be measured.

Cardiac telemetry is now making the difference as it literally monitors the heart's activity.

Patients are monitored in the hospital for just two days but Tayal has a new way of doing things.

He sent one to the homes of 56 patients with strokes of unknown cause for 21 days.

"Twenty-three percent of those patients had some evidence of atrial fibrillation during monitoring," said Tayal.

Tayal's study suggests that brief episodes of atrial fibrillation could be the potential cause, and immediate treatment with blood thinners could protect the patient from a second stroke.

However more study is needed to verify the findings.