CHICAGO (CBS) — A Chicago Fire Department diver suffered from a rare heart condition that contributed to his death while he was trying to rescue a boater in the Chicago River on Memorial Day.

Juan Bucio vanished in the river near 26th and Ashland around 8 p.m. He was pronounced dead after being pulled out several minutes later.

The Cook County Medical Examiner said Bucio had a condition called lymphocytic myocardis–which caused an irregular heart beat.

Chicago Fire Department diver Juan Bucio. (Photo supplied to CBS)

That condition caused distress that led him to run out of air in his tank.

The manner of death has been ruled an accident.

The man divers were looking for, Alberto Lopez, 28, died.

His body was found several days later in the river.

Lymphocytic myocarditis is an inflammatory reaction in heart muscle – it usually happens after a viral infection as the heart tries to fight a virus, according to Dr. Kousik Krishnan, a Rush University Medical Center cardiologist.

“His heart went in to this sudden arrhythmia and made incapacitated to do what he was trained to do and lost consciousness,” explained Dr. Krishnan.

That response can weaken heart muscle, give signs of heart failure and also make someone at risk of dangerous rhythm problems, Krishnan said. The rare condition, combined with an irregular heartbeat, could prove fatal.

“While the Scuba equipment functioned properly, the decedent likely exhausted his primary air supply at some point during the dive. This in turn caused him to not be able to activate other safety equipment that would have kept him afloat, and thus he sank below the surface,” stated the Cook County Medical Examiner.