I've had one of these; I got a suppository and instructions to hold it in for 20 minutes.
20 minutes doesn't sound like a long time but after 5 I realised this would be a bit of a challenge. I managed 15 before going off like a rocket (I had to cling on to the seat to stop myself from taking off).
They wouldn't give me the sedation because of my sleep apnoea so I endured the rest of the procedure whilst totally compost mentis (well as much as usual anyway).
They pump you up with air to keep things open and I watched the whole thing on the monitor.
A little uncomfortable but not painful and after I stopped farting an hour later everything was back to normal.

I once had a Barium meal and afterwards went for a couple of pints of Guinness. The following morning I produced a turd that resembled a pedestrian crossing.

Good luck

PP