What is the difference between synchronized and nonsynchronized (defibrillation) in a clinical setting?

Synchronized vs Nonsynchronized Defibrillation

Synchronized cardioversion delivers a shock timed with the QRS complex to avoid the vulnerable period of the cardiac cycle, while nonsynchronized defibrillation (unsynchronized shock) delivers energy at any point in the cardiac cycle without regard to the QRS complex. 1

Key Mechanism and Purpose

Synchronized cardioversion synchronizes shock delivery with the QRS complex to avoid delivering energy during the relative refractory portion of the cardiac cycle, which could induce ventricular fibrillation. 1 This timing is critical because shocks delivered during the vulnerable period (roughly corresponding to the T wave) can precipitate VF in patients who still have organized cardiac electrical activity. 1

Nonsynchronized defibrillation, by contrast, delivers the shock immediately without waiting for QRS detection, which is essential in cardiac arrest situations where every second counts and there may be no organized QRS complexes to detect. 1

Clinical Applications: When to Use Each Mode

Use Synchronized Cardioversion For:

• Supraventricular tachycardia (SVT) including atrial fibrillation, atrial flutter, and atrial tachycardia 1, 2
• Monomorphic ventricular tachycardia WITH a pulse 1, 2
• Any hemodynamically unstable tachyarrhythmia where the patient still has a pulse and organized QRS complexes 3

The initial energy for synchronized cardioversion of monomorphic VT with pulse is 100J for either biphasic or monophasic waveforms. 2, 3 For atrial fibrillation, start with 120-200J biphasic. 4

Use Nonsynchronized Defibrillation (Unsynchronized Shocks) For:

• Ventricular fibrillation (VF) 1, 2
• Pulseless ventricular tachycardia 1, 2
• Polymorphic (irregular) VT, even if the patient has a pulse 1, 2

You must NEVER use synchronized cardioversion for VF because the device may not sense a QRS wave and thus no shock will be delivered, wasting critical time during cardiac arrest. 1, 2, 3

Critical Safety Considerations and Common Pitfalls

Major Contraindications to Synchronized Mode:

Never use synchronized cardioversion for pulseless VT—treat it exactly like VF with immediate unsynchronized high-energy shocks to avoid delays in cardiac arrest treatment. 2, 3 The synchronization function will delay or prevent shock delivery in a patient who is in cardiac arrest. 1

Never use synchronized cardioversion for polymorphic VT, even with a pulse, as synchronization is usually not possible due to the irregular, constantly changing QRS morphology. 2, 3

Supporting Evidence on Shock Timing:

Research demonstrates that defibrillation shocks delivered within a 100ms window of the QRS peak in rapid VT have a 93% success rate compared to only 42% for shocks delivered outside this window, with significantly less risk of deterioration to VF (odds ratio 19.6). 5 This strongly supports the rationale for synchronized cardioversion in organized rhythms with pulses.

Practical Algorithm for Mode Selection:

1. Check for pulse:

   • No pulse → Use unsynchronized defibrillation immediately 2
   • Pulse present → Proceed to step 2

2. Identify rhythm:

   • VF or polymorphic VT → Use unsynchronized defibrillation 1, 2
   • Monomorphic VT, SVT, atrial fibrillation, or atrial flutter → Use synchronized cardioversion 1, 2

3. Assess hemodynamic stability (if pulse present):

   • Unstable (hypotension, altered mental status, chest pain, pulmonary edema) → Immediate synchronized cardioversion 3
   • Stable → Consider pharmacologic therapy first, but have cardioversion equipment immediately available 2, 3

The key distinction is that synchronized mode requires an identifiable QRS complex and a patient who can tolerate the brief delay for synchronization, while unsynchronized mode is for cardiac arrest or chaotic rhythms where immediate shock delivery is paramount. 1, 2