Only two cardiac rhythms are treated with defibrillation: ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT). These are called “shockable rhythms” because they involve disorganized or dangerously fast electrical activity that a controlled shock can reset. The other two cardiac arrest rhythms, asystole and pulseless electrical activity, do not respond to electrical shock.
Ventricular Fibrillation
Ventricular fibrillation is the most common reason for defibrillation. During VF, the heart’s lower chambers quiver chaotically instead of contracting in a coordinated way. There is no effective pumping, no pulse, and no blood flow to the brain or organs. On a heart monitor, VF looks like a rapid, irregular, jagged waveform with no recognizable pattern.
VF can appear as “coarse” or “fine.” Coarse VF has taller, more obvious waves and generally responds better to a shock. Fine VF produces very small, low-amplitude waves that can look almost like a flat line. This distinction matters because fine VF is sometimes mistaken for asystole, which is not shockable. If the monitor appears to show a flat line, checking a second lead or rotating the monitoring pads 90 degrees can reveal VF hiding in a different electrical plane. Research on animal models found that in two-thirds of cases, one lead showed no electrical activity while obvious VF was visible in other leads.
Pulseless Ventricular Tachycardia
Ventricular tachycardia (VT) is a dangerously fast heart rhythm originating in the lower chambers. The heart beats so rapidly that it cannot fill with blood between contractions. When VT produces no detectable pulse, it is treated identically to VF: immediate defibrillation.
On a monitor, pulseless VT shows a wide, regular pattern with rapid, uniform-looking complexes and no visible normal “P waves” that typically signal the upper chambers firing. The electrical complexes are wider than 0.12 seconds, reflecting the abnormal path the signal takes through the heart muscle. The key clinical question isn’t the exact appearance on the monitor but whether the patient has a pulse. No pulse plus a fast, wide rhythm means defibrillation.
Why These Two Rhythms Respond to Shock
Defibrillation does not restart a stopped heart. Contrary to what movies suggest, it does the opposite: it briefly stops all electrical activity at once. The shock depolarizes a critical mass of heart muscle simultaneously, creating a momentary electrical silence. This gives the heart’s natural pacemaker, a small cluster of cells near the top of the heart, a chance to take over and fire a normal, organized signal. Under the right conditions, that reset leads to a coordinated heartbeat and a pulse.
This mechanism explains why defibrillation only works on VF and pulseless VT. Both rhythms involve electrical activity that is present but chaotic or too fast to produce effective pumping. The shock clears that electrical chaos. In asystole, there is no electrical activity to reset. In pulseless electrical activity (PEA), the electrical signals are actually organized, but the heart muscle fails to respond to them, often because of a reversible cause like massive blood loss or a collapsed lung. Shocking an organized rhythm serves no purpose and can make things worse.
How Timing Affects Survival
Speed is the single biggest factor in whether defibrillation saves a life. A large study of witnessed cardiac arrests found that when the first shock was delivered within six minutes, over 93% of shocks successfully terminated ventricular fibrillation. Every additional minute of delay reduced the chance of successful defibrillation by about 6%. That same per-minute delay also reduced the probability of surviving to hospital discharge by 6%.
This is why automated external defibrillators (AEDs) are placed in airports, gyms, schools, and office buildings. AEDs analyze the heart rhythm automatically and will only deliver a shock if they detect VF or rapid VT. Their algorithms are designed with high sensitivity for shockable rhythms and high specificity for non-shockable ones, meaning they are very unlikely to shock a rhythm that shouldn’t be shocked. You do not need medical training to use one safely.
Defibrillation vs. Synchronized Cardioversion
Defibrillation delivers its shock at a random point in the heart’s electrical cycle because in VF and pulseless VT, there is no organized cycle to worry about. Synchronized cardioversion is a related but different procedure used when a patient still has a pulse but has an abnormally fast rhythm that is causing dangerous symptoms like chest pain, difficulty breathing, low blood pressure, or altered consciousness.
In synchronized cardioversion, the machine times its shock to land at a specific safe point in the heartbeat. This avoids triggering VF in someone who still has some organized cardiac function. The distinction is critical: an unsynchronized shock delivered to a patient with a pulse and an organized rhythm could push them into cardiac arrest. Synchronized cardioversion applies to unstable tachycardias of any type, whether the fast rhythm originates in the upper or lower chambers, as long as the patient still has a pulse.
Energy Levels for Shock Delivery
Modern defibrillators used in hospitals and by paramedics allow the operator to select the energy level in joules. For adults in VF or pulseless VT, initial shock energy typically ranges from 120 to 200 joules for biphasic defibrillators (the current standard), with subsequent shocks at the same or higher energy if the first is unsuccessful.
For children, energy is calculated by weight. The American Heart Association recommends 2 joules per kilogram for the first shock, 4 joules per kilogram for the second, and at least 4 joules per kilogram for subsequent shocks, up to a maximum of 10 joules per kilogram or the adult dose, whichever is lower. AEDs designed for public use handle energy selection automatically, and many include a pediatric mode or pediatric pads that reduce the delivered energy.
Pad Placement
Where the pads go affects how effectively the electrical current passes through the heart. The standard position places one pad on the upper right chest, below the collarbone, and the other on the lower left side of the chest, under the armpit. This is called the anterior-lateral position. An alternative is anterior-posterior placement, with one pad on the front of the chest over the heart and the other directly behind on the back.
Some emergency protocols recommend anterior-posterior placement as the first choice when feasible, with a switch to the alternative position after three consecutive failed shocks. Changing the pad position alters the direction of current flow through the heart, which can sometimes terminate a rhythm that resisted shocks in the original configuration.