I really doubt the procedure was a "stress test". Rather, it was some sort of minor surgical procedure like a colonoscopy. For you younger guys, allow me to describe the delights of this experience, it is routine in the US upon reaching age 50.
The day before, you are instructed to eat no food and drink only clear liquids. Then at about 6pm the evening before you drink over a liter of a foul tasting solution they give you. About 90 minutes after that you have an explosive diarrhea attack lasting 2 to 3 hours.
The next morning at about 4, you drink another liter of the ambrosia and repeat the process, until whatever you pass is clear. When you arrive at the Hospital you are already dehydrated and worn out. They place an IV, check your vitals, and if all is good they roll you in for the viewing.
Within seconds of being dosed with the sedatives you are pretty much out of it, although in my case still barely conscious. It only takes about 30 min after that unless they find polyps, in which case they are removed for biopsy. In my case there were none.
After that they monitor you in the recovery room for about 90 min until your wife shows up to take you home.
The next day you feel ok, they tell you to eat but in my case I just wasn't hungry. The following day I woke up feeling like I had run a marathon and had a huge appetite. 8 hours later (56 hours after the procedure and 80 hours after the fasting began) I felt pretty normal.
Toward the end, I don't think I was experiencing the effects of the drugs; I was just dehydrated and starved. For me, I would not attempt any high performance activity before five days after the fasting starts.