The schoolboy who was fatally struck by a football during recess died because his heart stopped while he was suffering from blunt force trauma.

The fifth-grader, identified as 11-year-old Austin Sergeev, was hit by the ball as he blocked a punt while playing flag football at Palisades Elementary School.

It was initially reported that the ball struck him in the throat, but a medical examiner said Wednesday it hit him in the chest.

Christopher Young, the deputy state medical examiner who performed the autopsy, said the blunt force trauma stopped his heart.

"It's an extremely unusual injury," he said.

Dr. Seshadri Balaji, a pediatric cardiac specialist at OHSU Hospital, agreed that's it's "very, very, very rare."

Balaji told The Oregonian newspaper that as the heart relaxes in its regular beat, "it is electrically very vulnerable," Balaji said. "If an object hits (over the heart) at that very time, it causes the heart to go into electrical chaos. It essentially stops," Balaji said.

The doctor said the injury occurs more frequently in children than adults and is more common in baseball than football.

"Defibrillation is the key to this," Balaji said.

All 13 Lake Oswego schools have portable defibrillators, which use an electric shock to restart the heart. The school staff did not use it, said Lake Oswego Superintendent Bill Korach.

Firefighters, who arrived in three minutes, used a defibrillator without success, said Gert Zoutendijk, Lake Oswego deputy fire marshal.

The Lake Oswego School District suspended recess football games at Palisades until officials further examine the incident.

"We want to make sure at Palisades that we are not doing something that is causing more psychological harm," Korach said.

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