What are the shockable and non‑shockable rhythms and their associated conditions in Advanced Cardiovascular Life Support (ACLS)?

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Last updated: March 13, 2026 • View editorial policy

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ACLS Cardiac Arrest Rhythms: Shockable vs Non-Shockable

In ACLS, there are 4 cardiac arrest rhythms divided into 2 categories: shockable rhythms (ventricular fibrillation and pulseless ventricular tachycardia) and non-shockable rhythms (pulseless electrical activity and asystole). 1

Shockable Rhythms

These rhythms require immediate defibrillation and are associated with significantly better survival outcomes:

  • Ventricular Fibrillation (VF): Represents disorganized electrical activity of the ventricular myocardium that generates no significant forward blood flow 1
  • Pulseless Ventricular Tachycardia (pVT): Represents organized electrical activity of the ventricular myocardium but produces no clinically detectable pulse or significant forward blood flow 1

Key Management Points for Shockable Rhythms:

  • Immediate defibrillation is critical - early CPR and rapid defibrillation can significantly increase survival to hospital discharge for witnessed VF arrest 1
  • Biphasic shock energy: Use manufacturer recommendation (initial dose 120-200 J); if unknown, use maximum available 1
  • Antiarrhythmic therapy: After failed initial shocks, consider either amiodarone (300 mg bolus first dose, 150 mg second dose) or lidocaine (1-1.5 mg/kg first dose, 0.5-0.75 mg/kg second dose) for shock-refractory VF/pVT 2
  • Survival rates: Patients with initial VF or VT have approximately 37% survival to hospital discharge, substantially higher than non-shockable rhythms 3

Non-Shockable Rhythms

These rhythms do NOT respond to defibrillation and require immediate high-quality CPR with epinephrine:

  • Pulseless Electrical Activity (PEA): A heterogeneous group of organized electrical rhythms associated with either absence of mechanical ventricular activity or mechanical activity insufficient to generate a clinically detectable pulse 1
  • Asystole: Absence of detectable ventricular electrical activity, with or without atrial electrical activity 1

Key Management Points for Non-Shockable Rhythms:

  • Do NOT shock - defibrillation is ineffective and delays critical CPR 4
  • Immediate high-quality CPR with epinephrine 0.01 mg/kg (maximum 1 mg) every 3-5 minutes 4
  • Search for reversible causes (H's and T's) 4
  • Survival rates: PEA has 12% survival to discharge, asystole has 11% survival to discharge - both significantly lower than shockable rhythms 3

Critical Clinical Distinctions

Rhythm Transitions During Resuscitation:

  • If shockable rhythm becomes non-shockable: Immediately transition to the asystole/PEA algorithm and continue CPR without attempting defibrillation 4
  • If non-shockable rhythm becomes shockable: Deliver shock immediately and resume CPR 4
  • Subsequent VF/VT (occurring during resuscitation for PEA/asystole) occurs in approximately 25-27% of cases and is associated with worse outcomes than PEA/asystole that remains non-shockable 3, 5

Prognostic Implications:

  • Shockable rhythms have substantially better outcomes: Adjusted odds ratio of 1.68 for survival to discharge compared to non-shockable rhythms 3
  • PEA has slightly better survival than asystole: 12% vs 11% survival to discharge (adjusted OR 1.1) 3
  • Patients progressing from non-shockable to shockable rhythm have 7% survival compared to 2% for those remaining non-shockable 5

Common Pitfalls to Avoid

Never delay CPR or defibrillation to establish vascular access - CPR and defibrillation are the only therapies proven to improve survival in VF/pVT, while medications have only shown increased ROSC rates without proven long-term survival benefit 1, 2

Minimize interruptions in chest compressions - pauses should only occur to assess rhythm, deliver shocks for VF/VT, perform pulse checks when organized rhythm appears, or place advanced airway 1

Do not empirically shock without rhythm confirmation - CPR artifact can obscure the underlying rhythm, potentially leading to inappropriate shock delivery for non-shockable rhythms 6

Avoid excessive ventilation - after advanced airway placement, provide only 1 breath every 6-8 seconds (8-10 breaths/min) with continuous chest compressions at ≥100/min 1

References

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