Yes, with any blockage over 70% there is a significant risk of a heart attack. However, with anticoagulants the severity of MI events are reduced. With a heart attack, a blockage usually breaks open, releasing the material trapped in the artery lining. This causes a blockage further down the vessel. You then have the secondary problem, a clot forms where the breakage occurred. Aspirin or other anticoagulants help to prevent this second blockage, which causes most deaths.
Is Angioplasty safe? Everything in life has a risk, even walking down the street. Angioplasty is safe when you look at the statistics, in fact very safe. I would rather Angioplasty than open heart surgery because there is then a large risk for infection. With antibiotics no longer having the effects they used to have, I would rather a much less invasive procedure like Angioplasty.