Whether it's possible to start cardiac surgery residency at the age of 45 depends on the residency program. Some might take a chance on you, but many probably won't.
The more important question is whether you should start a surgical residency at age 45 even if you could be accepted. General surgery and then a cardiothoracic fellowship is going to be a very long and grueling path!
If you like the OR and procedures, maybe you can try for anesthesiology instead? It would probably be less of an issue to start an anesthesiology residency at age 45, though some programs might still not want to take a chance on you. You should be able to do nonprocedural specialties that aren't competitive like FM or IM though.