Asystole is the complete cessation of all electrical and mechanical activity in the heart. Often called “flatline” because of how it appears on a heart monitor, asystole means the heart has stopped generating the electrical signals it needs to beat. Without immediate intervention, it is fatal.
What Happens in the Heart During Asystole
Your heart beats because of a built-in electrical system. A cluster of cells near the top of the heart fires an electrical impulse, which travels through the heart muscle in a specific sequence, triggering each chamber to contract in the right order. This cycle repeats roughly once per second and produces the familiar pattern of waves on a heart monitor.
In asystole, that entire system shuts down. No impulse fires, no chambers contract, and no blood moves. On a heart monitor, the normal peaks and valleys disappear completely, replaced by a flat or nearly flat line. There are no P waves (which represent the upper chambers firing), no QRS complexes (which represent the main pumping squeeze), and no T waves (which represent the heart resetting for the next beat). The person has no pulse, loses consciousness within seconds, and stops breathing.
Why Defibrillation Doesn’t Work on a Flatline
In movies, a defibrillator shock restarts a flatlined heart. In reality, shocking asystole is not part of any resuscitation protocol, and for good reason. A defibrillator works by delivering a jolt of electricity that resets a heart rhythm that has gone chaotic, like in ventricular fibrillation, where the heart quivers uselessly instead of pumping. The shock stops the chaos so the heart’s normal pacemaker cells can take over again.
With asystole, there is no chaotic rhythm to reset. The electrical system has gone silent entirely. Shocking a heart with no activity is like rebooting a computer that has no power source. There’s nothing to reorganize. This is why asystole is classified as a “non-shockable” rhythm in emergency medicine.
One important clinical step before declaring asystole: the monitor lead must be checked in more than one orientation. A very fine ventricular fibrillation can sometimes look like a flat line if the electrical activity happens to run perpendicular to the monitoring electrodes. Rotating the leads 90 degrees or switching to a different view can reveal subtle fibrillation that would actually respond to a shock.
Common Causes
Asystole is rarely a primary event. It usually results from another condition that has spiraled out of control. Emergency medicine groups these causes into two categories, sometimes called the “Hs and Ts,” which serve as a checklist for treatable problems:
- Hypovolemia: severe blood or fluid loss that leaves the heart with nothing to pump
- Hypoxia: dangerously low oxygen levels, which can shut down the heart’s electrical cells
- Acidosis: a critical buildup of acid in the blood that disrupts normal cell function
- Potassium imbalance: either too much or too little potassium, both of which can halt the heart’s electrical signals
- Hypothermia: severe drop in body temperature
- Tension pneumothorax: a collapsed lung that puts pressure on the heart
- Cardiac tamponade: fluid compressing the heart from outside
- Toxins: drug overdoses or poisoning
- Thrombosis: a massive blood clot blocking flow in the lungs or coronary arteries
Sometimes asystole is the final progression of another cardiac arrest rhythm. A heart that starts in ventricular fibrillation, for instance, may deteriorate into asystole if resuscitation efforts are unsuccessful. In other cases, a heart with severe underlying disease simply stops firing.
How Asystole Is Treated
Because defibrillation is off the table, treatment centers on high-quality CPR and medication. Chest compressions manually push blood through the body to keep the brain and organs alive while the medical team works to identify and reverse whatever caused the arrest. Epinephrine, a drug that stimulates the heart and tightens blood vessels, is given at regular intervals to try to restart electrical activity.
Equally important is finding the underlying cause. If the heart stopped because of a massive potassium spike from kidney failure, for example, correcting that imbalance gives the heart a reason to start firing again. If blood loss caused the arrest, replacing that volume is critical. Without addressing the root problem, CPR and medication alone are unlikely to bring the heart back.
Survival Rates and Neurological Outcomes
Asystole carries the worst prognosis of any cardiac arrest rhythm. When it occurs outside a hospital, survival to discharge is roughly 2 to 3 percent. A large study of over 35,000 patients with out-of-hospital asystole found that about 22 percent achieved a return of heartbeat at some point during resuscitation, but only 6.2 percent of those patients were still alive at 30 days. And of that group, fewer than 1 percent had a favorable neurological outcome, meaning they could function independently or with only moderate disability.
These numbers reflect a harsh reality: by the time the heart has reached asystole, the brain and other organs have often already suffered significant damage from lack of blood flow. Even when the heart can be restarted, the period without circulation frequently leaves lasting neurological injury.
When Resuscitation Efforts Stop
Because prolonged asystole has such poor outcomes, medical guidelines include frameworks for deciding when further resuscitation is unlikely to help. The American Heart Association’s 2025 guidelines describe rules based on a few key factors: whether emergency personnel witnessed the arrest, whether any shock was delivered, and whether the heart showed any return of spontaneous circulation.
After approximately 15 to 20 minutes of resuscitation with no response, these rules become increasingly reliable at predicting futility. A retrospective analysis found that applying these criteria at the 20-minute mark identified over 99 percent of patients who would have survived or had good neurological outcomes, meaning very few viable patients were missed. Another tool involves measuring carbon dioxide levels in exhaled breath during CPR. Very low readings after 20 minutes of advanced resuscitation strongly suggest that blood is not circulating meaningfully, though guidelines caution against using this measurement alone to end efforts.
These are not rigid cutoffs. The decision to continue or stop resuscitation depends on the full clinical picture, including the patient’s age, what caused the arrest, and whether a reversible cause has been identified but not yet fully treated. Hypothermia, for instance, can protect the brain and warrants longer resuscitation attempts than most other causes.