When a heart stops and the monitor goes flat with that long beep, the doctors grab the paddles, shout 'clear', and the jolt restarts the heart. Every medical drama runs on it: flatline, shock, heart leaps back to life. The defibrillator is the magic restart button.
A defibrillator cannot restart a stopped heart, and shocking a flatline does nothing. The clue is in the name: it de-fibrillates. It is designed to stop a heart that is quivering chaotically, in rhythms called ventricular fibrillation or pulseless ventricular tachycardia, by hitting it with a controlled jolt that briefly stops all electrical activity, giving the heart's natural pacemaker a chance to resume a normal beat. It works by stopping chaos, not by sparking a dead muscle back to life. A true flatline, asystole, where there is no electrical activity at all, is not a shockable rhythm; the treatment there is CPR and drugs like adrenaline, not the paddles. So the TV staple of shocking a flat green line is medically backwards. The paddles are for a heart beating wrongly, not a heart that has gone silent. Hollywood has been pressing the wrong button, dramatically, for decades.