I can recall the two worst calls of my career. #2 involved a maternity (eww, the grossest thing to begin with), but in this case, the head started to come out, and went back in. and it started to come out, and went back in. at which point, I asked my partner "that's not supposed to happen, right???" additional help was called, ALS and the supervisor, and we drove really really fast to the ER.
probably the worst involved a family of 4 who were hacked up by a then unknown person. Mom was missing half her arm, half her wrist, unconc and bleeding, 4 year old was stabbed down her collarbone (and surprisingly calm and not bleeding), baby was slashed across her face, and the other daughter hid in the closet. It ended the career of 20+ year veteran (she transferred to billing and never stepped foot on an ambulance again), and to this day is probably my worst call ever.
CISD were called for both ALS crews, the other BLS crew, the Ped's ER, Adult ER, and trauma team. and Since my unit wasn't invited (we had a non-emergency transport to handle), I was very close to quitting on the spot as a result, and I had some choice words with the supervisor. afterwards he asked me if there was anything he could do, and I still remember my response: "I could really go for a beer right now."
To be honest, alcohol typically is one of those things that you don't want to mix. But I always found that having the entire shift with you, all sharing a drink and talking about what happened can help time heal all those newly opened wounds.
And that's my non-official, based on my own experience only, way to handle the really bad calls.