Hi lph,
Yes, I had one too a few years ago and I'll probably have to have another one sometime soon. My doctor just wanted to check that there was no problem with my low resting pulse rate (about 42).
Anyway, the way it worked was that they sat me on the bike and attached all of the ECG electrodes using little sticky pads, then I had to cycle. I think they started me on about 50 Watts output and then told me to go harder every two minutes. The whole thing lasted for about half an hour - they stopped me at around 250 Watts by which time I had dripped sweat all over the floor and was a little tired! The only disconcerting thing about it was having to cycle topless because of all of the wires being stuck to my chest, but I suppose the nurses had seen it all before. Next time I am going to take a sports bra and ask if I can wear that.
In the end they decided that I was perfectly fit.
I hope yours goes equally well.
Bron