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. 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.