Two cardiac rhythms should be defibrillated immediately: ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT). These are the only “shockable” rhythms in cardiac arrest. The other two arrest rhythms, asystole and pulseless electrical activity (PEA), should never be shocked. Every minute without defibrillation during VF or pVT reduces the chance of survival by 7 to 10 percent when no CPR is being performed.
Ventricular Fibrillation
Ventricular fibrillation is the most common rhythm identified at the start of sudden cardiac arrest, and it’s the one most responsive to a shock. During VF, the heart’s lower chambers quiver in rapid, chaotic electrical activity instead of contracting in a coordinated way. No blood is pumped. On a heart monitor, VF looks like a disorganized, jagged waveform with no recognizable pattern, no distinct peaks, and no regular spacing.
VF is the textbook reason defibrillators exist. The electrical shock works by simultaneously resetting the vast majority of heart muscle cells, making them temporarily unresponsive. This brief pause wipes out the chaotic electrical waves bouncing through the heart and gives the heart’s natural pacemaker a chance to regain control and restart a normal, organized rhythm. The longer VF continues without a shock, the more the heart’s electrical activity degrades, eventually deteriorating into asystole (flatline), which is far harder to recover from.
Pulseless Ventricular Tachycardia
Pulseless ventricular tachycardia is the second shockable rhythm. In this case, the heart’s lower chambers beat extremely fast, typically well above 100 beats per minute, but the contractions are so rapid that the chambers never fill with enough blood between beats. The result is the same as VF: no pulse, no consciousness, no blood reaching the brain or organs.
On a monitor, pVT looks different from VF. It shows wide, regular, repeating peaks instead of chaotic squiggles. The key identifying features are a fast rate, wide electrical complexes lasting more than 120 milliseconds, and an absence of the normal smaller waves that indicate the upper chambers are working. It’s more organized than VF, but just as deadly without treatment. Defibrillation interrupts this rapid circuit the same way it stops VF, giving the heart a chance to reset.
Why Asystole and PEA Are Not Shocked
The two non-shockable arrest rhythms are asystole and pulseless electrical activity. Understanding why they don’t respond to a shock helps clarify what defibrillation actually does.
Asystole is the complete absence of electrical activity in the heart. The monitor shows a flat line with no waves of any kind. Defibrillation works by stopping chaotic electrical signals, but in asystole there are no signals to stop. Shocking a flatline does nothing. Treatment for asystole relies on CPR and medications to try to restart any electrical activity at all.
Pulseless electrical activity is more deceptive. The monitor shows what looks like an organized rhythm, sometimes even a normal-looking one, but the heart isn’t actually contracting effectively enough to produce a pulse. The electrical system is firing, but the muscle isn’t responding. Because the electrical pattern is already organized, a defibrillation shock has nothing to correct. PEA is treated by addressing the underlying cause (severe blood loss, a blood clot in the lungs, or other reversible problems) while performing CPR. PEA can also deteriorate into asystole.
How Quickly the Shock Needs to Happen
Speed is everything. When someone collapses in VF and bystanders perform CPR, survival drops by roughly 3 to 4 percent for every minute defibrillation is delayed. Without CPR, the decline is steeper: 7 to 10 percent per minute. This is why public access defibrillators (AEDs) are placed in airports, gyms, and office buildings. The goal is to deliver a shock within the first few minutes.
For adults, a biphasic defibrillator (the type found in most modern AEDs and hospital equipment) delivers an initial shock between 120 and 200 joules, following the manufacturer’s settings. Older monophasic devices use 360 joules. For children, the initial dose is 2 joules per kilogram of body weight, increasing to 4 joules per kilogram for subsequent shocks, up to a maximum of 10 joules per kilogram or the standard adult dose.
What Happens Immediately After the Shock
A successful shock doesn’t mean the heart immediately snaps back to a normal rhythm with a strong pulse. In a study of 376 defibrillation attempts during out-of-hospital cardiac arrests, the majority of patients remained pulseless for over two minutes after the shock. Post-shock asystole lasted a median of 20 seconds, and the average time to return of spontaneous circulation was over four minutes. In about 25 percent of cases, asystole persisted for more than two minutes beyond the shock.
This is why current guidelines call for resuming chest compressions immediately after a shock is delivered, without pausing to check for a pulse. The heart needs continued support during this recovery window. Rhythm and pulse checks are deferred for two minutes, or about five cycles of compressions and breaths, to avoid interrupting blood flow during a critical transition period.
How AEDs Detect Shockable Rhythms
If you’re a bystander using an AED, you don’t need to identify the rhythm yourself. The device analyzes the heart’s electrical activity and decides whether a shock is appropriate. International standards require AEDs to detect coarse ventricular fibrillation with greater than 90 percent sensitivity and to correctly identify non-shockable rhythms with greater than 95 percent specificity. In practice, performance varies by device. One study testing four commercial AEDs found sensitivity for shockable rhythms ranged from 68 to 98 percent, while specificity ranged from 74 to 100 percent. No tested device simultaneously achieved high sensitivity for fast ventricular tachycardia and high specificity for other fast heart rhythms that shouldn’t be shocked.
The practical takeaway: AEDs are highly reliable and far better than no intervention. If the device says “shock advised,” trust it. If it says “no shock advised,” continue CPR.
Safety Steps Before Delivering a Shock
A few precautions matter whether you’re using an AED or assisting someone who is. The electrode pads must make direct skin contact. Excessive chest hair can prevent a good seal and should be quickly shaved or removed with the razor included in most AED kits. If the person’s chest is wet from sweat or water, wipe it dry before placing the pads. Poor contact can cause the electrical current to travel across the skin surface rather than through the heart, making the shock ineffective and increasing the risk of sparks.
Remove any medication patches (such as nitroglycerin) from the chest before placing pads, and wipe the area clean. You can safely defibrillate on wet ground or near metal surfaces as long as the pads themselves aren’t touching the wet or conductive surface. Before pressing the shock button, verbally warn everyone to stand clear and visually confirm that no one is touching the person.