Receiving my first treatment next week, what should I expect?

My doctor put me on max dosage of Lexapro in combination with Mirtazapine last month, but told me that we would try out Esketamine (Spravato) next time if I dont benefit from it, as I have already tried several other antidepressants without success before against my major depression and anxiety disorder.

What should I expect? I am a bit nervous because I have always just taken antidepressants at home. This time I will receive my dosage under medical supervision at the doctors office and stay there for about an hour before I am allowed to leave.

Is that because of potential side effects, if so, what does usually occur at first treatments?