A spokeswoman at the Washington D.C. chief medical examiner's office told The Associated Press that an autopsy revealed Kalas, 73, had high blood pressure and suffered from atherosclerotic cardiovascular disease. The disease is a major cause of heart attacks and strokes because of plaque buildup restricting blood flow in arteries.
Kalas collapsed inside the Phillies broadcast booth at Nationals Park and died about an hour later.
Kalas, who was the Hall of Fame's Ford C. Frick Award recipient in 2002, is survived by his wife, Eileen, and sons Todd, Brad and Kane.
Private funeral and burial services are planned for early next week.
Todd Zolecki is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.