I recently had an echocardiogram that revealed either an asd or pfo. No further tests have been done yet. The md wants me to have a nuclear stress test. Is this safe?

Yes. The stress test will demonstrate if your heart is able to tolerate the physiologic leak that you have in your heart. The study should be done where treatment for angina can be given quickly. The test would be stopped if any serious changed in your ekg were seen during exercise.
Nuclear. This test is safe. There is radiation but if you haven't had large radiation xposure, won't be a big deal.