Hi. The angiogram shows a critical disease plus it also mentions that he had Acute coronary syndrome, that is an event die to one of the blockages. In that case the culprit blockage which appears to unstable (as mentioned ulcerated should be addressed at the earliest) in fact we do it on table after seeing the angiogram and discussing with family, in the same sitting.
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In a stable scenario there are various options. Here I concur with your doctors advice
Later the right side blockage also may require stenting
Seeing the images will help opine better
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Need to see angiogram images and know other details,like history, echo report, other health issues, to opine correctly