Asystole is a form of cardiac arrest in which the heart shows no electrical activity: the ECG displays a flat or near-flat line, indicating that the heart’s electrical system has completely ceased to function. Asystole is a non-shockable cardiac arrest rhythm, meaning defibrillation is not an appropriate treatment, and resuscitation focuses entirely on CPR and addressing any reversible underlying cause.
Distinction from ventricular fibrillation and PEA
There are three broad categories of cardiac arrest rhythm:
- Ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT): shockable rhythms where defibrillation can restore normal function
- Pulseless electrical activity (PEA): organised electrical activity is present, but there is no effective cardiac output or pulse
- Asystole: no electrical activity at all
Why defibrillation does not work in asystole
Defibrillation works by delivering a large electrical charge that simultaneously depolarises all cardiac muscle cells, allowing the sinus node to re-establish a coordinated rhythm. This only works when the heart’s electrical system is disorganised but still active. If there is no electrical activity at all, defibrillation cannot restart it.
Causes
Asystole can be the initial rhythm of cardiac arrest (particularly in hypoxic arrest, such as following drowning or respiratory failure) or it can develop as a terminal rhythm when VF degenerates and the heart exhausts its energy reserves. It can also arise from severe heart block.
Prognosis
Asystole has a poor prognosis compared with VF. Survival depends on rapid CPR and on identifying and reversing any treatable underlying cause using the "4 Hs and 4 Ts" framework. Without a reversible cause, sustained asystole is a marker of prolonged cardiac arrest with very poor survival prospects.
« Back to Glossary Index