Why constant nausea after open heart surgery?

Dear Doctor,

After it has been "three months" since emergency open heart surgery for single CABG, there is still a major and disabling symptom of NAUSEA. Unrelenting nausea that was present from the first conscious moment after awakening from the anesthesia...and on a steadily increasing basis has remained, along with other first-ever symptoms like belching, an uncomfortable fullness after small meals [like upper stomach is holding food too long while pushing diaphragm up toward lungs which then causes shortness of breath] which, altogether, impedes any quality of life, literally day and night.

So, while this is happening to a [at least before the MI] 'healthy' 64 y/o male who doesn't smoke, drink, have heart burn or food allergies since for years, he has eaten the Mediterranean diet, used targeted dietary supplements [including probiotics], been active, and slept well--what biochemical imbalance or organ damage could be causing this reign of nausea? By now, we can't imagine that the anesthesia drugs [fluoride-based including Amiodarone] are still in his system and with the Rx protocol for CAD, he has intermittently been on a daily beta blocker, ACEi, diuretics, and aspirin since the surgery...plus, blood tests yesterday reveal there is no H. pylori. So far, both injected and oral Zofran have helped, but for reasons of not wanting to stay on such prophylactic indefinitely and rather, to learn what is the inciting CAUSE for sake of restoring better health, do trust that I am already eagerly awaiting your reply. Thanks kindly.