When the electrical system that keeps your heart beating stops, the resulting condition is called asystole (pronounced “ay-SIS-tuh-lee”).

It’s sometimes referred to as a “flatline” because on an electrocardiogram, the activity of the heart is depicted as a steady flat line, rather than the wavy line associated with a beating heart.

Asystole is also the most severe of the four heart rhythm problems (arrhythmias) that can lead to cardiac arrest. Cardiac arrest is when the heart stops beating altogether or begins to beat so insufficiently that the drop in blood flow to the brain and rest of the body can be fatal.

The electrical activity that controls the beating of your heart starts at the top of your heart in the sinoatrial (SA) node — the heart’s natural pacemaker. Under normal circumstances, the sinus node generates an electrical stimulus around 60 to 100 times per minute.

The SA node sends an electrical impulse through a special pathway that allows each cell in the pathway to transmit the electrical charge to the next cell. This impulse travels down through the heart and first stimulates the two atria (the heart’s upper chambers), which causes them to contract.

Next, the two ventricles (the heart’s lower chambers) are triggered, which causes them to contract, allowing blood to be pumped to your lungs and the rest of your body.

The SA node then sends another signal, and the whole process starts over again.

Asystole can occur from events that lead to cardiac arrest or that significantly disrupt your heart’s electrical system. During cardiac arrest, your heart isn’t pumping, your blood isn’t circulating, and you’re not breathing. This state is known as clinical death.

Your brain cells can survive for about 5 minutes without blood or oxygen. After that, your brain cells will begin to die. Your entire brain, including the part that controls your heartbeat, will be unable to function again. This state is known as brain death.

Besides heart-related issues, some common causes of asystole include:

Heart-related causes

As durable as your heart is at beating 24 hours a day, its electrical system is vulnerable to interference and damage. Several types of heart problems can lead to asystole or other forms of cardiac arrest.

Some of the most common heart-related causes include:

  • Heart attack: A heart attack is more of a “heart plumbing” problem than an electrical issue. This is because a heart attack is usually the result of a blockage in an artery that supplies blood to the heart muscle. But heart tissue damage caused by a heart attack can involve the electrical pathways within the heart. It’s not unusual for an arrhythmia to result from a heart attack.
  • Ventricular fibrillation (VFib): VFib is an arrhythmia in which the ventricles quiver instead of contracting fully to pump blood out to the body. Research suggests that a prolonged episode of VFib is one of the most common causes of asystole.
  • Ventricular tachycardia: Ventricular tachycardia is an arrhythmia that occurs when the ventricles beat too fast to allow the heart to efficiently pump oxygenated blood throughout the body. Like VFib, ventricular tachycardia can become so serious that it leads to a complete stop in the heart’s electrical activity.
  • Atrial fibrillation (AFib): When the heart is in AFib, the atria quiver instead of contracting and relaxing in rhythm with the ventricles. A 2020 study suggests that AFib may be a risk factor for heart attack, an established cause of asystole. In addition, AFib can lead to other complications, including ventricular tachycardia.
  • Congenital heart disease: Some heart conditions are present at birth. Congenital heart defects significantly increase the risk of cardiac arrest, according to a 2018 study. Most children born with some type of congenital heart disease live long, healthy lives. However, their heart health and risk of cardiac arrest and heart attack must be monitored throughout their lives.

CPR is considered the primary method of immediately treating a person in asystole.

Unlike VFib or ventricular tachycardia, which can be treated by shocking a person with a defibrillator, asystole is considered a “nonshockable” rhythm. However, effective CPR may convert asystole into a shockable rhythm.

A 2020 analysis of more than 67,000 cases of cardiac arrest treatment suggests that when a nonshockable rhythm such as asystole is converted into VFib or ventricular tachycardia, the odds of surviving significantly increase.

The key is for the conversion to occur as soon as possible after a person goes into cardiac arrest.

The key to managing asystole is to try to determine whether the cause is reversible. For instance, if high potassium levels are causing asystole, it could possibly be reversed and potentially lead to a shockable rhythm.

Another example is if the cause is identified as a blood clot in the lungs. In this instance, administering a fast-acting clot-busting medication could break up the clot and help restore the heart’s rhythm.

One other treatment that may help revive a heart in asystole is the administering of medications known as vasopressors. These drugs, which include epinephrine, can help increase blood flow to the heart, brain, and throughout the body.

Epinephrine may be administered while the person is also receiving CPR.

Any form of cardiac arrest carries a high risk of death. About 70% to 90% of people who experience cardiac arrest outside a hospital die before reaching the hospital, according to the Centers for Disease Control and Prevention (CDC).

But asystole is an especially dangerous condition. The American Medical Resource Institute reports that less than 2% of people who experience asystole outside a hospital setting survive, even if trained paramedics or other healthcare professionals intervene.

When there are no longer electrical impulses in your heart and it stops pumping, the resulting condition is called asystole. It’s also known as a “flatline” because of the straight, flat line that’s depicted on an electrocardiogram.

Asystole can occur from events that lead to cardiac arrest or that significantly disrupt your heart’s electrical system.

Asystole is the most urgent and serious type of cardiac arrest. Survival rates are low, but immediately starting vigorous CPR and injecting epinephrine may help improve the odds of survival.