that sounds like happened to me (I was offered a treadmill but my knees would give out before my heart did, so they decided on the injection).
You have doctors all round you monitoring you, and you have a cannula so that they can regulate the drugs and if necessary add other ones - as my heart didn't slow down again quickly enough they did it with a beta blocker.
It did feel uncomfortable; but it's absolutely nothing to be scared of. If, God forbid, you ever had a heart attack, the best time would be when you're in a room full of heart specialists. (But you won't have one, they could see it coming if one was threatening.)