Chicago Fire Department Diver Juan Bucio surfaced from the murky water of the Chicago River and, when asked by his partner if he was alright, nodded in the affirmative.

It was his last communication. Moments later Bucio sank to the bottom of the river and by the time his fellow divers found him and hauled him from the water, he had no pulse.

The details of Bucio’s death, which occurred while searching for missing boater Alberto Lopez the evening of May 28, were contained in an autopsy report released Tuesday.

Ambulance outside Stroger Hospital the night CFD diver Juan Bucio died

CFD diver Juan Bucio was taken to Stroger Hospital, where he was pronounced dead on Memorial Day. | Ashlee Rezin/Sun-Times

According to the report, Bucio had a rare heart condition that caused him to become distressed. His death was caused by “asphyxia with depletion of air from diving tank due to cardiac arrhythmia due to lymphocytic myocarditis.”

His death was determined to be an accident.

Bucio and his dive partner, Michael Reyes, descended from a fire department helicopter into the river near 26th and Ashland. The two were tethered together.

At one point Bucio related to Reyes their tether line was caught on debris, Chief Ron Dorneker told an investigator for the medical examiner’s office. Reyes stopped, backed up, the line was freed and the search continued, according to Dorneker, who was in charge of the dive scene that night.

Dorneker requested a check of their dive tank air capacity, and though each man had about 10 minutes of air time before reaching the limit at which divers are required to surface, he ordered them to head up.

According to the interview with Dornecker, Reyes said he and Bucio surfaced without incident.

Chicago Fire Department diver Juan Bucio (left) died Monday while searching in the Chicago River for Alberto Lopez (right). | Bucio photo provided by Chicago Fire Department; Lopez photo from Facebook

Chicago Fire Department diver Juan Bucio (left) died Monday while searching in the Chicago River for Alberto Lopez (right), whose body was recovered days later. | Bucio photo provided by Chicago Fire Department; Lopez photo from Facebook

Once at the surface, Reyes looked at Bucio and asked if he was alright, which is protocol, Dorneker said. “Bucio had responded in the affirmative by nodding his head,” he said.

The gesture was one of three acceptable responses, the others are patting your head or verbal confirmation.

As Reyes was helped onto a boat, Chicago Police Marine Unit officers radioed a Mayday after they saw Bucio sink and were unable to get him to respond to radio messages, according to Dorneker.

A fire department spokesman was not immediately available Wednesday to provide details on the protocol of tethered divers.

Bucio, 46, was recovered from a depth of 17 to 19 feet, about 10 to 15 minutes after sinking. His diving equipment was intact, his mask sealed on his face. And his air tank was empty, the autopsy report states.

Bucio had lymphocytic myocarditis, “a rare cause of cardiovascular disease that can cause heart failure,” and in Bucio’s case it led to cardiac arrhythmia, or “the improper beating of the heart,” according to the medical examiner.

Bucio’s main oxygen supply “likely became exhausted while underwater” and he didn’t switch to a backup supply, according to a consultant’s review of the diver’s equipment that was ordered by the federal Occupational Safety and Health Administration. Bucio was “distressed due to the heart condition,” medical examiner’s spokeswoman Becky Schlikerman said.

The review of Bucio’s diving equipment showed it “used but well maintained.”

Area of Chicago River where CFD diver Juan Bucio went under

Chicago Fire Department diver Juan Bucio was pulled from the water along this stretch of the South Branch of the Chicago River late on Memorial Day. Juan Bucio was pronounced dead at Stroger Hospital. | MItch Dudek/Sun-Times

Consultant Craig Jenni wrote in a report submitted last week to OSHA that Bucio’s gear “functioned as designed, however the diver likely exhausted the primary gas supply, which created a task-loaded situation that became too overwhelming for the diver to resolve such that the diver sank to depth unable to self-rescue.”

Jenni suggested Bucio’s death “may have been averted if the diver had been using a gas-integrated dive computer with visual and audio alarms warning the diver and his dive partner when approaching minimum safe cylinder pressure.”

Lymphocytic myocarditis typically occurs in about 22 out of 100,000 cases, with a major metro-area hospital usually seeing just one or two cases per year, according to Dr. Allen Anderson, medical director of the Center for Heart Failure at Northwestern Medicine’s Bluhm Cardiovascular Institute.

The condition is typified by inflammation and white blood cells within the weakened heart muscle, and while it is linked to certain viruses, patients might not show symptoms ahead of a major event like an arrhythmia, Anderson said.

“Lots of times, people don’t know they have it,” Anderson said. “This is like a lightning strike, out of the blue.”

A routine physical would not detect the condition, Anderson said.

Firefighters lift Juan Bucio’s casket off a fire truck for his funeral at St. Rita of Cascia High School. | Ashlee Rezin/Sun-Times

The fire department is reviewing Bucio’s autopsy as part of its own ongoing investigation, CFD spokesman Larry Langford said Tuesday.

Bucio joined the department in 2003 and had been on the dive team since 2007. He is survived by his sons, ages 7 and 9, and nine siblings.

The body of 28-year-old Alberto Lopez — the man whom Bucio had been summoned to rescue — was found in the river four days later by friends who said they had been unsatisfied with Chicago police search efforts. Lopez, who fell from a boat into a river, was a father of three young children who worked in carpentry to support his family in Mexico.