In a cardiac arrest, seconds matter. Rookies often freeze when the monitor shows a squiggly rhythm and the team asks: “Is it shockable?” The answer determines whether you reach for the defibrillator or resume compressions. Knowing how to instantly distinguish shockable vs non-shockable rhythms is a critical resuscitation skill.
The Four Cardiac Arrest Rhythms
In ACLS, cardiac arrest rhythms fall into two groups:
Shockable
- Ventricular Fibrillation (VF)
- Pulseless Ventricular Tachycardia (pVT)
Non-Shockable
- Asystole
- Pulseless Electrical Activity (PEA)
How to Tell Shockable Rhythms Quickly
1. Ventricular Fibrillation (VF)
- Chaotic, irregular, no organized QRS or P waves.
- Looks like “scribbles” or coarse squiggly baseline.
- Defibrillate immediately.
Tip: Coarse VF = higher success with shock. Fine VF can mimic asystole—always check leads, zoom in.
2. Pulseless Ventricular Tachycardia (pVT)
- Wide QRS complexes, regular rhythm, very fast (>150 bpm).
- May look like sine waves or uniform “tombstones.”
- If patient pulseless → defibrillate.
Tip: If unsure between wide complex SVT vs VT in arrest → treat as VT.
How to Tell Non-Shockable Rhythms
3. Asystole
- Flatline or nearly flatline, no QRS complexes.
- Confirm in two leads to avoid mistaking fine VF.
- Do not shock. Continue CPR, give epinephrine, search for reversible causes.
4. Pulseless Electrical Activity (PEA)
- Organized rhythm on monitor (sinus, brady, wide QRS) but no palpable pulse.
- Heart’s electrical system works, but no effective mechanical output.
- Do not shock. Continue CPR, give epinephrine, fix underlying cause (H’s & T’s).
Rookie Algorithm (5-Second Rule)
- Look at rhythm.
- Chaotic squiggles or fast wide complexes? → Shock.
- Flatline or organized rhythm without pulse? → No shock.
- Check for pulse quickly (<10 sec).
- Proceed:
- Shockable (VF/pVT): Defibrillate → CPR → epi after 2nd shock → amiodarone after 3rd.
- Non-shockable (PEA/asystole): CPR immediately → epi every 3–5 min → treat reversible causes.
Common Rookie Mistakes
- Confusing fine VF with asystole → failing to shock.
- Forgetting to confirm asystole in two leads.
- Spending too long analyzing rhythm → interrupts CPR.
- Attempting synchronized cardioversion instead of defibrillation.
- Looking for a pulse too long → delays compressions.
Rookie Pearls
- In cardiac arrest, time off the chest kills. Rhythm checks ≤10 seconds.
- When in doubt between VF vs asystole → treat as VF (shock).
- Keep defibrillator pads on throughout resus—saves time.
- Always follow rhythm interpretation with CPR + meds + search for H’s & T’s.
Take-Home Message
Shockable vs non-shockable is the first critical branch in the cardiac arrest algorithm. For rookies:
- VF/pVT = shockable.
- Asystole/PEA = non-shockable.
- Decide in seconds, act immediately, keep CPR going.
Remember: “Squiggles and tombstones → shock. Flatline or organized no pulse → no shock.”
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