What Causes Premature Ventricular Contractions Post Retina Repair Surgery?

I have had pvcs for several years. I had a bout 4 years ago that lasted for days. I saw my cardiologist and he said pvs are benign. He did order all the usual tests : ekg, sonogram stress test, etc. All tests came back normal. my Dr. put me on beta blocker (metoprolol) and pvcs stopped for the most part ( what a relief!). My Dr. reassured me the benign nature of pvs. 3 weeks ago I underwent eye surgery to repair a detached retina and was administered general anesthesia. 5 days ago the pvcs returned with a vengeance. I am currently taking 100 mg of metoprolol and is not as effective as before....any explanation? thank you