Should a shock be synchronized in ventricular tachycardia (VT) or ventricular fibrillation (VF)?

Shock Synchronization in VT and VF

For ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT), unsynchronized shocks (defibrillation) must be used, while synchronized cardioversion should be used for monomorphic VT with a pulse.

Ventricular Fibrillation (VF)

• Synchronized cardioversion must never be used for VF as the device may not sense a QRS wave, potentially resulting in no shock delivery 1, 2
• VF requires immediate high-energy unsynchronized shocks (defibrillation) 1
• The American College of Cardiology recommends delivering an immediate unsynchronized shock for witnessed VF, with an initial energy of 200J (monophasic) or manufacturer's recommended dose (biphasic) 2
• After shock delivery, CPR should be resumed immediately to minimize interruptions in chest compressions 2

Ventricular Tachycardia (VT)

The approach to VT depends on whether the patient has a pulse and the morphology of the VT:

Pulseless VT

• Pulseless VT is treated exactly like VF with unsynchronized high-energy shocks (defibrillation doses) 1
• Immediate CPR should be initiated with minimal interruptions 2
• The shock energy and protocol are the same as for VF 1, 2

VT with Pulse

The approach depends on the morphology of the VT:

Monomorphic VT (regular form and rate) with pulse

• Synchronized cardioversion is recommended 1
• Initial energy for monomorphic VT with pulse: 100J for biphasic or monophasic waveforms 1
• If there's no response to the first shock, it may be reasonable to increase the dose in a stepwise fashion 1
• Synchronization helps avoid shock delivery during the relative refractory period when a shock could produce VF 1

Polymorphic VT (irregular) with pulse

• Polymorphic VT should be treated as VF using unsynchronized high-energy shocks 1
• Synchronization is usually not possible with polymorphic VT due to the irregular QRS complexes 1

Timing Considerations

• Synchronized shocks delivered shortly after the peak of the QRS complex in rapid VT appear to offer significant advantages, with a 93% success rate compared to 42% for shocks outside the QRS complex 3
• Unsynchronized shocks should be delivered immediately for pulseless rhythms to avoid delays in treatment 2

Common Pitfalls to Avoid

• Never use synchronized cardioversion for VF as it may result in no shock delivery 1, 2
• Never use synchronized cardioversion for pulseless VT as it delays treatment of a cardiac arrest rhythm 1
• Never use synchronized cardioversion for polymorphic (irregular) VT even with a pulse, as synchronization is usually not possible 1
• Avoid excessive delays in shock delivery for pulseless rhythms 2
• After defibrillation attempts, most patients remain pulseless for over 2 minutes, so chest compressions should be resumed immediately after shock delivery 4

Pediatric Considerations

• For pediatric patients with VF or pulseless VT, use 2 J/kg for the first shock, 4 J/kg for the second shock, and 4-10 J/kg for subsequent shocks 2, 5
• The initial biphasic external shock dose of 2 J/kg may be inadequate; appropriate doses appear to be in the range of 3-5 J/kg 5