A defibrillator cannot restart a flatlined heart, whatever television suggests
On screen, a shock jolts a flatlining patient back to life. In reality, defibrillation works only on specific chaotic rhythms, such as ventricular fibrillation, and does nothing for a heart that has stopped completely. It resets the muscle so the heart's own pacemaker can take over again.
The treatment targets two dangerous arrhythmias: ventricular fibrillation and ventricular tachycardia in which the heart is no longer pumping blood effectively. A burst of current, sometimes called a counter-shock, depolarises a large share of the heart muscle at once. Exactly why this works is not fully understood, but the effect is to wipe the disordered rhythm, letting the sinoatrial node, the body's natural pacemaker, re-establish a normal beat.
That mechanism explains the limits. In asystole, the flatline, or in pulseless electrical activity, there is no rogue rhythm to interrupt, so shocking is not appropriate; care relies on CPR and drugs, with a shock only if the heart shifts into a shockable rhythm. Shocks are also not given to someone conscious or with a pulse, and a badly timed one can itself trigger fibrillation. A related technique, synchronised cardioversion, times the shock to the cardiac cycle and is used for rhythms such as supraventricular tachycardia.
Devices come in several forms. Manual external units, used with an electrocardiogram, leave diagnosis and dose to trained clinicians; every NHS ambulance in the UK carries one. Automated external defibrillators, the AEDs found in public places, analyse the rhythm themselves and talk the user through each step with recorded voice prompts, so bystanders can use them with little or no training. Implanted cardioverter-defibrillators sit inside the body, and during heart surgery such as a bypass, surgeons can apply paddles directly to the heart muscle.
The rhythm matters enormously for outcomes. In North America, survival after cardiac arrest outside hospital is often under 10 percent, and around 20 percent for arrests in hospital. People whose hearts are in a shockable rhythm do markedly better, with survival figures ranging between 21 and 50 percent.
Source: Defibrillation
