Asystole on the ECG
Asystole is the complete absence of ventricular electrical activity on the ECG, appearing as a flat or near-flat line with no QRS complexes, and representing cardiac arrest.
ECG criteria
| Feature | What you see in Asystole |
|---|---|
| Rate | Zero. No ventricular complexes. |
| Rhythm | None. |
| P wave | Usually absent. Occasionally P waves persist alone — P-wave asystole, where the atria still depolarise but nothing conducts. |
| PR interval | Not applicable. |
| QRS | Absent. Any complexes at all mean this is not asystole. |
| ST-T | Not applicable. |
How to spot it
- Confirm the patient. Asystole is a clinical diagnosis with an ECG correlate, never the other way round — check responsiveness and a central pulse.
- Check the leads are connected and the gain is up. A disconnected electrode produces a perfect flat line.
- Confirm in more than one lead. Fine ventricular fibrillation can look flat in one lead and be obvious in another, and that distinction changes the treatment.
- Look for P waves. Persisting P waves with no QRS complexes mean the atria are still going and the block is complete — this may respond to pacing.
- Look for any slow wide complexes. An agonal or escape rhythm is not asystole and is managed differently.
What it gets confused with
| Looks like | How to tell them apart |
|---|---|
| Fine ventricular fibrillation | Low-amplitude chaotic activity that can look flat in a single lead. Check a second lead — this one is shockable and asystole is not. |
| Lead disconnection or artefact | A perfectly flat line with a well patient. Check the electrodes and the monitor gain before anything else. |
| Agonal rhythm | Occasional very slow, broad complexes. Not asystole, though the prognosis is grim. |
| Pulseless electrical activity | Organised complexes on the monitor with no palpable pulse. The tracing looks nothing like asystole. |
Traps
- Always check a second lead before calling asystole. Fine ventricular fibrillation can appear flat in one lead. Calling it asystole withholds a defibrillation that might work.
- Check gain and lead connection first. The commonest cause of a flat line on a monitor is a detached electrode, not a dead patient.
- Asystole is not a shockable rhythm. Defibrillating it wastes time and interrupts compressions.
- P-wave asystole is worth spotting: the atria are still depolarising, the failure is entirely in conduction, and it is among the few presentations where pacing may help.
Why it happens
All spontaneous pacemaker activity has ceased, or nothing that does fire is able to depolarise the ventricular myocardium. It is usually the end state of prolonged hypoxia, profound acidosis or extensive myocardial damage, in which the tissue can no longer sustain an action potential — which is why asystole so often follows other arrest rhythms rather than starting them.
Why it matters
Asystole carries the worst prognosis of the cardiac arrest rhythms, and it is not shockable. Management is high-quality chest compressions, adrenaline, and a hard search for a reversible cause. Its main ECG teaching point is negative: never accept a flat line at face value without confirming the patient, the leads and a second lead.
Questions
Why must asystole be confirmed in more than one lead?
Because fine ventricular fibrillation can appear almost flat in a single lead depending on the direction of the fibrillatory vectors. Ventricular fibrillation is shockable and asystole is not, so the distinction changes management immediately.
Is asystole shockable?
No. Defibrillation works by depolarising myocardium simultaneously so an organised rhythm can resume, which requires electrical activity to interrupt. In asystole there is none. Treatment is compressions, adrenaline and reversible causes.
What is P-wave asystole?
A pattern in which atrial depolarisation continues, producing visible P waves, but no impulse reaches the ventricles, so there are no QRS complexes. It reflects complete conduction failure rather than complete electrical failure, and it is one of the few arrest patterns where pacing may be of use.
Reading about asystole is not the same as calling it on a tracing you have never seen.
Practise on real cases in ECG Pro