If I had to guess, I'd say we were in for another strip wherein John had trouble coping with one of his patients. Let's see what sort of mutant we have to deal with now, eh?
Well, so much for the patient being the mutant.
Panel 1: We see John singing "Country Roads" off-key to a patient who I initally believed to be a boy of roughly 10 years of age sitting in the chair with the nitrous mask over his (or her) face.
Panel 2: Said patient is confused for a second as John adjusts the flow of the gas. Perhaps it's less because he stopped singing and more because the words "Adjust! Adjust!" are floating in mid-air. After all, if s/he starts seeing sound effects, s/he may be getting too much anesthetic.
Panel 3: In any event, John resumes signing horribly as he starts working on the patient's mouth.
Panel 4: He then tells Jean that he believes if a patient can stand his singing voice, he or she is adequately sedated.
Hypothesis: I do believe we'll have a wrap-up to this tomorrow. Also, the patient is a young girl in her early teens.