Although related to max heart rate but not a human. Some years ago we were training our horse for endurance riding,useing a Polar monitor under the girth, the horse ,Chaka , has a normal Hr of 30 at rest,and I was riding at about 90 bpm,in a local forest. A railway line passes through it, I was crossing the line via a bridge,when a train appeared and blew the horn, obviously the horse spooked and hurted off for a good few yards before I got him settled ,as I looked down at the monitor it read 300,and he was calming by then. Us humans don't have much of a heart range eh? 30 to 300.