<s>[INST] My grandfather is having a heart bypass tomorrow due to him having clots in his legs, the doctors told him he would not survive the surgery on his legs in he did not have this heart bypass, how risky is this operation going to be? He gave up smoking last year after smoking for 40+ years. What chance does he have of surviving this operation? They have already put it off twice before because they didnt think he would make it through!! [/INST] Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Risk of operation can be calculated on the basis of various test reports including the heart function. Smoking can damage the vessels of heart, brain, lung and kidney. It also puts you under increased risk of spontaneous clotting in arteries of body. Hope I have answered your query. Let me know if I can assist you further. </s>