Absolute

Relative

A left atrial diameter greater than 4.5 cm or AF duration greater than 6 months is associated with a low likelihood of maintaining sinus rhythm. Patients with sick sinus syndrome or sinoatrial node block should not undergo cardioversion until a pacemaker has been placed. Ectopic or multifocal atrial, junctional, and sinus tachycardias do not normally respond to cardioversion. These rhythms have an automatic focus arising from cells that are depolarizing at a rapid rate; delivery of a shock may actually increase the rate of the tachyarrhythmia. Patients with a history of minimal hemodynamic or symptomatic improvement while in sinus rhythm should not undergo cardioversion because of an increased risk-to-benefit ratio. Inadequately anticoagulated patients with AF of longer than 48 hours’ duration have an increased risk of thromboembolic stroke or arterial embolization (5% rate in the first 2 weeks after cardioversion).

Choice of Cardioversion Unit: Biphasic versus Monophasic

Biphasic waveform automated external defibrillators (AEDs) and cardioversion units were developed in the late 1990s and have been shown to provide superior atrial fibrillation conversion rates with lower-energy shocks compared with traditional monophasic units currently in use—86% of patients with AF returned to sinus rhythm after a single biphasic shock, versus 51% with the first monophasic shock. Most commercially available AEDs in the United States use biphasic waveforms, whereas most cardioversion units in hospitals and emergency departments continue to provide monophasic waveform shock. Cost and retraining of medical staff are likely barriers to wider use of biphasic cardioversion units.

Electrical versus Chemical Cardioversion

Given the procedural sedation (see Chapter 2, Procedural Sedation and Analgesia) required for nonemergent electrical cardioversion, much effort has been exerted to find an ideal antiarrhythmic medication for chemical cardioversion. The most effective agents, as recommended by the American College of Cardiology and American Heart Association (ACC/AHA), are the following:

Unfortunately, each of these agents has also been shown to be proarrhythmic—especially in the setting of decreased left ventricular ejection fraction (a relatively common finding in patients with AF). A less effective agent, amiodarone, has been shown to be safer in patients with a low ejection fraction, but it has delayed cardioversion effects in AF and is not currently approved for chemical cardioversion.

The advantage of chemical cardioversion is primarily related to the avoidance of procedural sedation in patients at high risk for airway compromise during procedural sedation (e.g., obstructive sleep apnea, known tracheal stenosis, prior tracheostomy, macroglossia). Like electrical cardioversion, chemical cardioversion in a nonurgent setting requires full anticoagulation for 3 weeks to avoid thromboembolic complications.

Rhythm Control versus Rate Control in Persistent Atrial Fibrillation

Five recent studies have demonstrated little difference in clinical outcome between rate and rhythm control strategies for patients with AF. In fact, rate control strategy is associated with fewer hospitalizations and less adverse drug effects when compared with rhythm control. Based on this evidence, some authors now recommend rate control and anticoagulation as primary therapy for persistent AF. The American Academy of Family Physicians and the American College of Physicians Joint Commission on Atrial Fibrillation recommends rate control with chronic anticoagulation for most patients with AF (Snow and colleagues, 2003). Drugs recommended for rate control are metoprolol, atenolol, diltiazem, and verapamil, with digoxin being moved into second-line therapy. The Joint Commission further recommends that most patients converted to sinus rhythm from AF not be placed on rhythm maintenance medications because of the risks outweighing any potential benefits. For patients who are unable to take warfarin, aspirin 325 mg orally daily or clopidogrel 75 mg may be used. These drugs have the advantage of not requiring monitoring of the international normalized ratio (INR), although aspirin and clopidogrel are clearly inferior to warfarin for preventing cardioembolic events. The role of oral direct thrombin inhibitors in place of warfarin remains controversial.

Echocardiography

Transesophageal echocardiography is helpful before elective cardioversion to estimate left atrial diameter (see Chapter 90, Echocardiography). TEE can identify small atrial thrombi, especially in the left auricular appendage, that are not visible with transthoracic echocardiography in patients for whom anticoagulation with warfarin poses high risk.