Cardioversion and defibrillation

Cardioversion vs. Defibrillation - Shocking The System

A timed, lower-energy shock to "reset" an unstable, perfusing tachyarrhythmia versus a high-energy, untimed shock to "reboot" a pulseless, life-threatening one.

⭐ Synchronization is Key: Cardioversion delivers a shock synchronized with the R-wave to avoid the vulnerable T-wave period. An unsynchronized shock on the T-wave (R-on-T phenomenon) can induce lethal ventricular fibrillation.

The Procedure - Paddles & Placement

• Paddle Size:
  • Adults: 8-12 cm diameter.
  • Pediatrics: 4.5 cm for infants (<10 kg), 8-12 cm for children (>10 kg).
• Conductive Medium:
  • Essential to reduce transthoracic impedance and prevent skin burns.
  • Use pre-made pads or apply conductive gel.
• Placement Options:
  • Anterolateral (Standard):
    • Sternal paddle: Right of the upper sternum, below the clavicle.
    • Apex paddle: 5th-6th intercostal space, left mid-axillary line.
  • Anteroposterior (AP).

⭐ AP Placement: Often preferred for cardioverting atrial fibrillation/flutter and is necessary for patients with an implanted pacemaker or ICD to avoid delivering energy through the device.

Synchronized Cardioversion - Timing is Everything

• Principle: A low-energy shock timed precisely to the QRS complex's R-wave.
• Goal: Resets the heart's electrical activity, allowing the SA node to regain control.
• Mechanism: The "sync" mode avoids the vulnerable T-wave period, preventing the dangerous R-on-T phenomenon which can trigger Ventricular Fibrillation.

• Indications: Unstable tachyarrhythmias with a pulse (e.g., A-fib, A-flutter, VT with pulse).
• Energy: Lower than defibrillation; start at 50-100 J (biphasic).

⭐ If the patient becomes pulseless, abandon cardioversion and proceed immediately to high-energy defibrillation and CPR.

Defibrillation - The Un-Syncable Shock

• Principle: An unsynchronized, high-energy electrical shock used for pulseless life-threatening arrhythmias.
• Indications:
  • Ventricular Fibrillation (VF)
  • Pulseless Ventricular Tachycardia (pVT)
• Mechanism: Depolarizes the entire myocardium simultaneously, terminating the chaotic rhythm and allowing the sinus node to regain control.
• Energy: Biphasic (120-200 J); Monophasic (360 J).

⭐ The most critical factor for survival in VF/pVT is minimizing the time to defibrillation. Minimize interruptions in chest compressions before and after the shock.

Risks & Reminders - Handle With Care

• Thromboembolism: High risk in AF/Aflutter >48h. Mandates 3-4 weeks of anticoagulation before and after, OR a pre-procedure TEE to exclude thrombus.
• Complications: Includes skin burns (use gel/pads correctly), transient myocardial stunning, and post-shock arrhythmias like bradycardia.

⭐ Post-cardioversion ST-segment elevation can be a benign, transient finding-not always acute MI.

High‑Yield Points - ⚡ Biggest Takeaways

• Synchronized cardioversion treats unstable tachyarrhythmias with a pulse (e.g., Afib, VT with pulse), avoiding the vulnerable T-wave period.
• Unsynchronized defibrillation is for pulseless VT and VF only, delivering a high-energy, non-timed shock.
• PEA and asystole are non-shockable rhythms; treat with high-quality CPR and epinephrine.
• Defibrillation uses higher energy (e.g., 120-200J biphasic) than cardioversion (e.g., 50-100J for SVT/Afib).
• Administer sedation/analgesia before cardioverting a conscious patient.