Bruno Sammartino died of multiple organ failures due to heart issues, according to FWOnline.
The record-setting WWWF Champion reportedly had a damaged heart, which was caused by a battle with rheumatic fever age 12.
Sammartino was apparently not aware of his heart problems until the final years of his life.
Meltzer's report also states that Bruno had been diagnosed with diverticulosis, and didn't want anyone to know about his failing health.
It is believed that the decades of training broke down the functioning piece of Sammartino's heart, until it could no longer compensate for the damage.