The heart doesn't contract all at once. Instead, each section waits for its cue: an electrical signal that starts at the top of the right atrium and runs downward through the ventricles. As long as nothing disturbs that signal, the heart will beat at a mostly steady, healthy pace.
Almost any part of the heart is capable of starting the electricity that drives the beat, and the heart muscle may also immediately jury-rig a new source of electrical signals. These new signals may keep the heart beating, but they disrupt the normal pace or rhythm.
A racing heartbeat ventricular tachycardia, it turns out, sent Shannon Schroeder, 37, of Poulsbo, Wash., to the emergency room. It's a good thing she went.
An echocardiogram showed that the walls of her left ventricle were thicker than they should be, and the muscle showed telltale damage of an earlier silent heart attack. The damage had weakened her heart and harmed the electric circuitry that maintains a normal rhythm.
Stimulantsincluding caffeine, nicotine, and drugscan also cause brief arrhythmias. They're usually harmless, but there are case reports of people dying from ventricular fibrillation after overdosing on controlled substances.