Hard to say... do you mean a blockage of a coronary artery - the blood vessels that feed the heart?
People can block a coronary artery 100% and still be alive - depending on where the blockage is located and how long it took for the blockage to develop. If the blockage happened slowly, the heart will make extra blood vessels to feed the parts of the heart not getting good blood flow - called collateral blood flow - so when the vessel finally blocks completely, the person will still have blood flow but may still have a heart attack.
If the coronary artery blocks off suddenly, the person will have a heart attack from the lack of blood flow but this doesn't have to kill them. You can lose part of your heart muscle and still live. But, if about 40% of the left ventricle dies due to a blocked coronary artery, then the person most likely will die from not having enough heart left to pump blood.
If the blockage is in the left main coronary artery - the main artery for the left ventricle - then, it doesn't take much blockage at all for the person to start having problems. If this part blocks, the person will die unless immediate intervention can happen. (In other words, the person would need to already be at a hospital that can do immediate heart surgery.)
What most people with coronary artery blockage die from is bad heart rhythms such as ventricular fibrillation.
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