Anesthesia side effect: impaired mental function, mental slowness, disorientation 2 weeks after surgery?

My father-in-law had an angioplasty in early May with "twilight sleep" and we thought he seemed a little slower mentally after that. About 3 weeks ago, he had knee replacement surgery and is still slow - or slower - mentally. Seems sometimes disoriented on the phone, out of bed and dressed at 4 a.m., etc. Blood tests reveals anesthesia still in the bloodstream but doctor said to come back for a recheck in 2 weeks and if its still there, I suppose they will do something more. We thought at first that he had a stroke but apparently its leftover anesthesia or a side effect of it. I suppose someone could even mistake it for early dementia. Has anyone seen this before? Wondering if it just goes away or if the experience is a permanent sort.