Last revised by Raymond Chieng on 14 Sep 2022

The diagnosis of asystole refers to a cardiac arrest rhythm with no electrical activity of the heart. It is the cardiac arrest rhythm with the poorest prognosis and is often irreversible 1. Asystole is one of the non-shockable rhythms, the other being pulseless electrical activity (PEA).

Asystole is one of the four possible states found in an unconscious patient who has undergone a cardiac arrest. It is characterized by the complete and sustained absence of electrical activity of the heart, resulting in no contraction of the heart muscle tissue and therefore no cardiac output to the body. The ECG monitor demonstrates an undulating almost flat line 1,2

The underlying causes have been classically described as the four Hs and Ts 3:

  • hypoxia
  • hypovolemia
  • hyperkalemia/hypokalemia/hypoglycemia/other metabolic disorders
  • hypothermia


Asystole is often irreversible, and prognosis is poor. Survival to discharge from in-hospital asystolic cardiac arrest is 11% 4, whilst survival from out-of-hospital asystolic cardiac arrest is less than 2% 5.

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