Reference — Cardiac
ACLS Rhythm Identification
In cardiac arrest, correct rhythm identification in seconds determines whether to defibrillate or continue CPR. This reference covers the four pulseless rhythms, recognition cues, and the immediate response priority for each.
Educational use only. ACLS algorithms require formal certification and regular skills validation. This reference supports knowledge review — it does not replace ACLS training or institutional emergency protocols. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
Shockable vs Non-Shockable Rhythms
| Rhythm | Shockable? | Organized? | Pulse? |
|---|---|---|---|
| Ventricular Fibrillation (VF) | Yes | No | No |
| Pulseless VT (pVT) | Yes | Yes — wide QRS | No |
| Pulseless Electrical Activity (PEA) | No | Yes — organized | No |
| Asystole | No | No | No |
Key principle: Shockable rhythms have electrical activity that can potentially be reset to sinus rhythm by defibrillation. Non-shockable rhythms require CPR and treatment of reversible causes — shocking will not help.
Shockable Rhythms
Ventricular Fibrillation (VF)
- Recognition: Chaotic, irregular waveforms with no organized P waves, QRS, or T waves. Baseline is erratic — can be coarse (larger waves) or fine (small waves)
- Pitfall: Fine VF may resemble asystole — always confirm in two leads and check for artifact before treating as asystole
- Immediate response: CPR → charge defibrillator → clear and shock → resume CPR immediately → reassess in 2 min
- Medications: Epinephrine 1 mg IV/IO q3–5 min; amiodarone 300 mg IV/IO (or lidocaine) for refractory VF after 3rd shock
Pulseless Ventricular Tachycardia (pVT)
- Recognition: Wide, regular QRS complexes at rate > 100 bpm (often 150–250 bpm); no identifiable P waves; organized but no pulse
- Pitfall: Wide complex tachycardia with a pulse is a different treatment pathway — always confirm pulselessness first
- Immediate response: Same as VF — CPR → defibrillation → reassess
- Energy: Biphasic 120–200 J per manufacturer recommendation
Non-Shockable Rhythms
Pulseless Electrical Activity (PEA)
- Recognition: Any organized rhythm on the monitor with no palpable pulse (may look like sinus rhythm, idioventricular, or other organized patterns)
- Pitfall: Monitor looks reassuring — the error is not checking for a pulse. Always confirm pulselessness.
- Immediate response: CPR → do not shock → epinephrine 1 mg IV/IO q3–5 min → identify and treat H's and T's
- H's and T's: Hypovolemia, Hypoxia, H⁺ (acidosis), Hypo/Hyperkalemia, Hypothermia; Tension pneumothorax, Tamponade, Toxins, Thrombosis (PE or coronary)
Asystole
- Recognition: Flat or near-flat line — no discernible waveforms; no palpable pulse
- Pitfall: A disconnected or loose lead can mimic asystole. Always confirm in two leads and check all connections before treating
- Immediate response: CPR → do not shock → epinephrine 1 mg IV/IO q3–5 min → identify and treat H's and T's
- Prognosis: Worst among cardiac arrest rhythms. Discuss continuation criteria with the team if no response after adequate resuscitation
High-Quality CPR — Key Reminders
- Rate: 100–120 compressions per minute
- Depth: At least 2 inches (5 cm) in adults; allow full chest recoil between compressions
- Interruptions: Minimize pauses — target < 10 seconds for any interruption (rhythm check, shock)
- Ventilation: 10 breaths/min with advanced airway; 30:2 without advanced airway
- Switch compressors every 2 minutes to prevent fatigue-related quality decline
- Avoid hyperventilation — excessive ventilations increase intrathoracic pressure and reduce coronary perfusion pressure
Related Resources
Standards & sources
Fact-checked Jun 20, 2026This page is written to align with American Heart Association (AHA) · American College of Cardiology (ACC) · AHA ACLS Guidelines. It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →