Thanks to the elaborate radio communication set-up used on the field during the re-enactment, the man received medical help in a matter of minutes, said Dennis E. Frye, co-chairman of the Antietam Commemoration Committee.
Frye said heart attacks are not unusual during a re-enactment because of the physical stress involved in the hobby, which requires endurance and conditioning.
"The re-enactors do the best they can to police themselves. Most re-enactors are aware of the danger of physical stress on the body," he said.
Still, news of the man's death affected fellow re-enactors, Frye said.
"The armies in both camps put their flags at half-staff in his honor," he said. "We certainly express our condolences to the family."
A Rochester, N.Y., woman was seriously burned about 11 a.m., when chemicals used in Civil War-era photography exploded in front of her, said her friend Rebecca Davenport, who witnessed the accident.
Elizabeth Gibson, wife of photographer re-enactor Rob Gibson, dropped to the ground and rolled until the fire on her antebellum dress was extinguished.
Several women re-enactors stood around the wounded woman, spreading their hoop skirts to hide her view in the busy sutler area.
Gibson was flown by helicopter to Medstar in Washington, D.C., said Alan Matheny, emergency medical services coordinator for Halfway Volunteer Fire Company.
She was listed in critical but stable condition Sunday night, according to the hospital.
Emergency medical crews treated more than 300 people during the weekend event, mostly for minor scrapes, sprains and exhaustion, DeHaven said.
The first-aid tent was busy Sunday, with close to 70 people treated by 4 p.m., Matheny said.
Many of them were there for heat-related problems, he said.
At least six people were transported to Washington County Hospital on Sunday, Matheny said.
Two were re-enactors who suffered cardiac-related problems during the dawn battle, he said. Other battlefield casualties included minor powder burns and twisted ankles.