Defibrillation and cardioversion both deliver electrical shocks to correct abnormal heart rhythms, but they differ in three fundamental ways: how the shock is timed, which heart rhythms they treat, and how much energy they use. Understanding these differences matters because each technique exists for a specific, life-threatening situation, and using the wrong one can make things worse.
1. Shock Timing: Synchronized vs. Unsynchronized
The most important technical difference is whether the shock is timed to the heart’s own electrical activity. Your heart produces a small electrical signal with every beat, visible on a monitor as a series of waves. One of those waves, called the QRS complex, represents the moment the main pumping chambers contract. Cardioversion synchronizes the shock to land precisely on that QRS complex. This timing is critical because delivering electricity at the wrong moment in the heartbeat cycle can actually trigger ventricular fibrillation, the most dangerous rhythm of all. By syncing to the QRS complex, cardioversion avoids that vulnerable window.
Defibrillation, by contrast, delivers an unsynchronized shock. It fires the moment the button is pressed, with no regard for where the heart is in its electrical cycle. This isn’t careless. It’s necessary. In ventricular fibrillation, the heart’s electrical activity is so chaotic that there is no recognizable QRS complex for the device to lock onto. The American Heart Association explicitly warns that synchronized cardioversion must not be used for ventricular fibrillation, because the machine may never detect a QRS wave and may simply never fire.
2. The Rhythms Each One Treats
Because of that timing difference, each procedure targets a completely different set of heart rhythm problems.
Defibrillation is reserved for the two most immediately lethal rhythms: ventricular fibrillation (where the heart quivers chaotically instead of pumping) and pulseless ventricular tachycardia (where the heart beats so fast it produces no effective blood flow). Both are cardiac arrest rhythms. The patient has no pulse and is either unconscious or seconds away from it. The goal of defibrillation is to depolarize a critical mass of heart muscle all at once, essentially hitting the electrical reset button so the heart’s natural pacemaker can regain control.
Cardioversion treats rhythms that are abnormal but not immediately fatal. The heart is still producing a pulse, even if it’s beating too fast or irregularly. The most common reasons for cardioversion are atrial fibrillation (a rapid, irregular rhythm originating in the upper chambers), atrial flutter, and certain types of supraventricular tachycardia. In these conditions, the heart’s electrical signals are stuck in a loop. Cardioversion works by delivering a precisely timed shock that interrupts that loop and allows the heart to return to its normal sinus rhythm. Rather than resetting the entire heart, it targets the specific reentry circuit causing the problem through local depolarization in the path of the circulating electrical wavefront.
3. Energy Levels
Cardioversion starts at much lower energy levels than defibrillation. The recommended range for synchronized cardioversion is 50 to 200 joules with modern biphasic devices, and clinicians typically begin at the lowest setting (50 joules) and increase only if the first attempts fail. In a stubborn case, they may escalate to 200 joules after several shocks.
Defibrillation uses higher energy from the start. Because the heart is in complete electrical chaos during ventricular fibrillation, a stronger shock is needed to depolarize enough heart muscle to break the cycle. Most protocols call for starting at 120 to 200 joules with biphasic defibrillators, and going to maximum energy on subsequent shocks if the first one doesn’t restore a rhythm. The 2025 AHA guidelines recommend using the maximum energy setting if your specific device’s ideal dose isn’t known.
This energy gap matters for the patient’s body. Higher energy means more potential for skin irritation or minor burns at the pad sites, which is one reason cardioversion uses the minimum effective dose and escalates gradually.
How the Patient Experience Differs
These three technical differences create very different experiences for the person receiving the shock. Defibrillation happens during cardiac arrest. The patient is unconscious, has no pulse, and the shock is delivered immediately with no preparation beyond placing the pads. Speed is everything. Every minute without defibrillation during ventricular fibrillation reduces the chance of survival.
Cardioversion is typically a planned or semi-planned procedure performed on a conscious patient whose heart is still beating. Because the electrical shock is intensely painful (comparable in stimulus to a surgical incision), patients receive deep sedation or brief general anesthesia before the shock is delivered. The goal is to prevent any memory of the experience and to blunt the body’s stress response. Short-acting sedatives are used so the patient wakes up quickly afterward. The entire process, from sedation to recovery, is relatively brief.
In urgent situations where a patient with a pulse is deteriorating rapidly from a fast rhythm, cardioversion can be performed emergently. But even then, the synchronized timing and lower initial energy distinguish it from defibrillation.
Why These Differences Matter
Using the wrong approach can be dangerous. Delivering an unsynchronized shock to a patient who has a pulse and an organized rhythm risks landing the shock during the heart’s vulnerable period, potentially converting a survivable rhythm into ventricular fibrillation. On the other hand, attempting synchronized cardioversion on a patient in ventricular fibrillation wastes precious seconds, because the device may sit waiting for a QRS complex that never comes.
Modern defibrillators used in hospitals can switch between both modes. A button or setting on the device toggles synchronization on or off. Automated external defibrillators (AEDs) found in public places are designed only for defibrillation and analyze the rhythm automatically, delivering an unsynchronized shock only when they detect ventricular fibrillation or pulseless ventricular tachycardia.
For patients undergoing elective cardioversion for atrial fibrillation, there’s an additional consideration that doesn’t apply to emergency defibrillation: blood clot risk. When the upper chambers of the heart have been fibrillating for more than 48 hours, blood can pool and form clots. Restoring normal rhythm can dislodge those clots, so patients are often placed on blood thinners before and after the procedure.