Ryan...I am sure you know smoking
is bad for you...and with your mechanical valve it is still bad for you...smoking anything encourages plaque
build-up in your arteries and you are more likely to end up needing a stent
or a bypass...you don't really want that to happen do you. Weed and blood thinners wont have a bad effect on each other, its just a really dumb thing to do after you have had Open Heart surgery.
I wouldn't usually recommend anyone take up drinking but a beer or two is a much safer option for you than taking up weed again.
You might like to have a look on this site that's especially for those of us who have mechanical heart valves...