Asystole (New Latin, from Greek privative a "not, without" + systolē "contraction"[1][2]) is the absence of ventricular contractions in the context of a lethal heart arrhythmia (in contrast to an induced asystole on a cooled patient on a heart-lung machine and general anesthesia during surgery necessitating stopping the heart).

Asystole should not be confused with very brief pauses below 3 seconds in the heart's electrical activity that can occur in certain less severe abnormal rhythms.

Asystole is found initially in only about 28% of cardiac arrest cases in hospitalized patients,[3] but only 15% of these survive, even with the benefit of an intensive care unit, with the rate being lower (6%) for those already prescribed drugs for high blood pressure.

Even in those cases where an individual suffers a cardiac arrest with asystole and it is converted to a less severe shockable rhythm (ventricular fibrillation, or ventricular tachycardia), this does not necessarily improve the person's chances of survival to discharge from the hospital, though if the case was witnessed by a civilian, or better, a paramedic, who gave good CPR and cardiac drugs, this is an important confounding factor to be considered in certain select cases.

After many emergency treatments have been applied but the heart is still unresponsive, it is time to consider pronouncing the patient dead.