Bush, 67, had a stent placed in his heart in August at a Dallas hospital a day after the blockage was discovered, but two doctors confirmed he had a 95 percent blockage in that artery, CNN reported Tuesday.
National Journal first reported the blockage level.
"Certainly President Bush was at significant risk having a 95 percent blockage in one of his coronary arteries. That doesn't mean he was going to have a heart attack," cardiologist Warren Levy told CNN. "But certainly he was at significant risk for a heart attack and something did need to be done."
In a commentary published in USA Today in August, Dr. Marc Siegel, medical director of Doctor Radio at New York University's Langone Medical Center, said an angiogram revealed "a severe blockage in a crucial position in a coronary artery that was easily life threatening."
Last month, Bush said he was "doing fine."
"Other than the fact that I nearly bled to death when I nicked myself shaving because I'm taking blood thinner, I'm doing pretty good," he joked in an interview with ABC's "Good Morning America." "I thought you were going to have to put a tourniquet on me."