DC Cardioversion in Atrial Fibrillation Must Be Synchronized
Direct-current cardioversion for atrial fibrillation must always be performed with R-wave synchronization to prevent ventricular fibrillation—unsynchronized shocks (defibrillation) are only appropriate for ventricular fibrillation where R-wave synchronization is not feasible. 1, 2
Why Synchronization Is Mandatory
Synchronized cardioversion delivers the electrical shock timed to the R wave of the QRS complex, ensuring the shock does not occur during the vulnerable phase of ventricular repolarization (the T wave). 1
Delivering a shock during the T wave can induce ventricular fibrillation, a potentially fatal complication. 1, 2, 3
The term "defibrillation" specifically refers to unsynchronized discharge, which is contraindicated for atrial fibrillation. 1, 2
Technical Implementation of Synchronization
Select an ECG lead that clearly displays both the R wave amplitude and atrial activity before initiating the procedure. 1
The shock must be synchronized to the peak of the QRS complex, and under no circumstances should it be delivered on the T wave. 1
For low-energy shocks, ideally synchronize to an R wave preceded by a long R-R interval (>500 ms) to ensure the shock does not fall within the T wave of the previous beat. 1, 4, 5
Verify that a clearly visible artifact indicates the timing of the shock in relation to the QRS complex on the monitor before delivering energy. 1
Critical Pitfall: Pre-Excited Atrial Fibrillation
In patients with pre-excited atrial fibrillation (Wolff-Parkinson-White syndrome), distinguishing the QRS from the T wave can be extremely difficult due to abnormal, irregular, and varying QRS complexes. 3
Case reports document iatrogenic ventricular fibrillation caused by inappropriate synchronization with the T wave instead of the R wave in pre-excited AF due to varying R wave amplitude affected by the accessory pathway. 3
In these high-risk cases, carefully verify proper R-wave synchronization before each shock delivery, as the varying morphology increases the risk of misidentification. 3
Energy Levels and Waveforms
Start with an initial energy of 200 J for monophasic waveforms, as initial shocks of 100 J are often too low (only 14% success rate versus 95% with 360 J). 1
Biphasic waveforms achieve higher success rates (94%) compared to monophasic waveforms (79%) and require lower total energy delivery. 2, 6
For biphasic defibrillators, begin with 150-200 J and increase energy for subsequent shocks if the first attempt fails. 7, 6
Hemodynamically Unstable Patients
When atrial fibrillation causes hemodynamic instability (ongoing myocardial ischemia, symptomatic hypotension, angina, or heart failure not responding to pharmacological measures), perform immediate R-wave synchronized cardioversion without delay. 1, 2, 7, 6
Do not waste time on pharmacological therapies in severe hemodynamic compromise—proceed directly to synchronized electrical cardioversion. 2, 6
This is a Class I recommendation (highest level of evidence) from the ACC/AHA/ESC guidelines. 1, 2
Safety Data Supporting Synchronization
Studies of transvenous low-energy cardioversion demonstrate that well-synchronized shocks during narrow-complex rhythms carry minimal risk of ventricular proarrhythmia, even in patients with a history of ventricular tachycardia and left ventricular dysfunction. 4
However, inappropriate T wave sensing during these same studies resulted in rare inductions of ventricular fibrillation, confirming the critical importance of proper synchronization. 4
The mechanism of proarrhythmia relates to the temporal relationship between shock delivery and the repolarization time of the previous QRS complex, not to ventricular rate alone. 4
Contraindications to Cardioversion
Never perform cardioversion in patients with digitalis toxicity or hypokalemia, as these conditions dramatically increase the risk of ventricular arrhythmias. 1, 7, 6