I am not a cardiac physiotherapist, however i dont see that there should be a problem with using your arms to sit to stand after an aortic valve surgery. The little I know about about cardiac rehab, overhead activities especially resisted is the concern. Sit to stand is a natural functional process, for as long as it is done correctly with no valsava maneovres i honestly dont see a problem.

I would let the physios who are more knowledgeable in cardiac rehab give you something more evidence based...Im curious to know what they have to say

cheers