In medicine, asystole is a state of no cardiac electrical activity, hence no contractions of the myocardium and no cardiac output or blood flow. Asystole is one of the conditions required for a medical practitioner to certify death.

In asystole, the heart is not likely to respond to defibrillation because it is already depolarized, however some emergency physicians advocate the trial of defibrillation in the case that the rhythm is actually fine ventricular fibrillation, or a rhythm that is shockable but too small to be seen on the monitor. When asystole is confirmed by a physician, there are still treatments that may be effective, but asystole is often the last stage of heart disease. The only possible ways to restart an asystolic heart are the administration of epinephrine also known as adrenaline, or of atropine to block vagal tone that may be preventing the heart from beating, and finally pacing of the heart, or applying a small electric shock to the heart at a regular rate in hopes of causing it to beat.

While the heart is asystolic, there is no blood flow to the brain unless CPR or internal cardiac massage (when the chest is opened and the heart is manually compressed) is performed, and then it is still a small amount. After many emergency treatments have been applied but the heart is still unresponsive, it is time to consider pronouncing the patient dead since even in the rare case that a rhythm should reappear, the brain has been deprived of oxygen long enough that consciousness will never exist in that person again.