Pat, once your heart reaches a certain rate its efficiency decreases (cardiac output drops). Using a HRM can keep you from getting into that zone. When cardiac output drops the amount of blood being pumped to your myocardium drops as well. If you dont get onto the wrong side of the supply and demand balance you will not have a heart attack. Those guys passing out on the intense rides are obviously suffering from a supply/demand imbalance of oxygen to the brain. Lets just say they get into into that zone where cardiac output (supply) drops but thier demand is constant or increasing. If demand (oxygen consumption) is high, and cardiac output drops your oxygen saturation will gradually fall. Perhaps their oxygen saturation is dropping to a level thats causing them to pass out.