Cardioversion

Feature	Synchronized Cardioversion	Unsynchronized Cardioversion (Defibrillation)
Patient	Has a PULSE c	PULSELESS
Rhythm	Unstable Tachyarrhythmias: - A-fib, A-flutter, SVT - Monomorphic VT	Lethal Arrhythmias: - Ventricular Fibrillation (VF) - Pulseless VT (e.g. torsades) c
Mechanism	Interrupts a single reentrant circuit, allowing the SA node to resume pacemaking.	”Resets” the entire myocardium by depolarizing all cells at once to terminate chaotic electrical activity.
Timing	Shock synchronized to R-wave (avoids T-wave) c	Shock is unsynchronized (delivered immediately)
Energy	LOW Energy (50-200 J)	HIGH Energy (≥200 J for biphasic)
Sedation	YES (patient is conscious)	NO (patient is unconscious)

Rhythm recognition

Shockable Rhythms

Rhythms where the heart has disorganized electrical activity that can be “reset” by immediate defibrillation.

• Types

  • Ventricular Fibrillation (VF): Chaotic, irregular, undulating baseline without recognizable QRS complexes.
  • Pulseless Ventricular Tachycardia (pVT): Wide, regular QRS complexes (>120 ms) without a palpable pulse. (Note: If VT has a pulse, it is managed via tachycardia algorithm, NOT cardiac arrest algorithm).

• Management Algorithm

  1. Initiate: High-quality CPR + O2 + Attach monitor/defibrillator.
  2. Check Rhythm: Identify VF/pVT.
  3. First Shock: Defibrillate (unsynchronized cardioversion).
  4. Resume CPR: 2 mins immediately post-shock. Establish IV/IO access.
  5. Check Rhythm/Pulse: If still VF/pVT → Second Shock.
  6. Resume CPR + Epinephrine: 1 mg Epi IV/IO q3-5 mins. Consider advanced airway.
  7. Check Rhythm/Pulse: If still VF/pVT → Third Shock.
  8. Resume CPR + Antiarrhythmic: Amiodarone (First dose 300 mg, second 150 mg) OR Lidocaine. Treat reversible causes.

• High-Yield Exam Points

  • DO NOT stop CPR to check a pulse immediately after shocking. Resume CPR immediately.
  • Unsynchronized shock = Defibrillation (used for pulseless arrest).
  • Synchronized shock = Cardioversion (used for unstable tachyarrhythmias with a pulse).

Nonshockable Rhythms

Rhythms where defibrillation is ineffective and contraindicated. Focus is on CPR, early Epinephrine, and identifying the underlying cause.

• Types

  • Asystole: “Flatline.” Complete absence of ventricular electrical activity.
  • Pulseless Electrical Activity (PEA): Any organized electrical activity (excluding VF/pVT) observed on ECG in a patient without a palpable pulse.

• Management Algorithm

  1. Initiate: High-quality CPR + O2 + Attach monitor.
  2. Check Rhythm: Identify Asystole/PEA.
  3. Epinephrine ASAP: 1 mg Epi IV/IO q3-5 mins (administer as early as possible).
  4. Resume CPR: 2 mins. Establish IV/IO access, advanced airway.
  5. Check Rhythm/Pulse: If still Nonshockable → Continue CPR.
  6. Treat Underlying Cause: Rapidly assess and intervene on the 5 H’s and 5 T’s.

• High-Yield Exam Points

  • NEVER shock Asystole or PEA.
  • Early administration of Epinephrine is the most critical pharmacological intervention.
  • True asystole must be confirmed in 2 leads to rule out fine VF (though modern monitors do this automatically, Step 2 may test the concept).