CHICAGO (CBS)–Chicago Fire Department diver Juan Bucio 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, authorities said.

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

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

The Cook County Medical Examiner’s report said Bucio had a rare heart condition that led to his death. The official cause of death as ruled by the medical examiner is asphyxia with depletion of air from his diving tank due to cardiac arrhythmia.

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.

The Chicago Fire Department is continuing its own investigation into the circumstances surrounding Bucio’s death and will examine whether protocols were followed.