Although it is often postulated that it takes 48 h for a clot to form in a fibrillating atrium, there are no conclusive studies to back this claim (1). In fact, several studies challenge this presumption by suggesting that atrial thrombus may form within 48-72 h of AF.

A study of patients with acute AF (86% for 48 h or less) reported that 14% of patients developed atrial thrombus within 72 h (2). Another study found that ~1% of patients with AF for 48 h or less had an embolic event (3). A 2016 study of patients undergoing electrical cardioversion within 48 h of AF onset also found a risk of neurological events of ~1% among those who underwent cardioversion without therapeutic AC; none had a CHA2DS2-VASc score <2 or had postoperative AF, however (4).

The 2014 ACC guidelines state that for patients with AF or atrial flutter of less than 48 h duration at “high risk of stroke” (often defined as CHA2DS2-VASc score 2 or greater, or history of stroke or transient ischemic attack), AC is recommended as soon as possible before or immediately after cardioversion, followed by long-term AC therapy (Class I, level of evidence C) (5). For those who are at lower thromboembolic risk, AC before cardioversion is optional without the need for its continuation post-cardioversion (Class IIa, level of evidence C).

Bonus Pearl: Did you know that up until 1980s, it was generally held that only freshly formed thrombi broke off and embolized and that “organized” clot developing subsequently within a few days was unlikely to embolize! Another 180!

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References

1. Kaufman E, Lagu T, Hannon NS, et al. Mythmaking in medical education and medical practice. Euro J Intern Med 2013;24:222-26. https://www.ncbi.nlm.nih.gov/pubmed/23312964   
2. Stoddard MF, Dawkins PR, Prince CR, et al. Left atrial appendage thrombus is not uncommon in patients with acute atrial fibrillation and a recent embolic event: a transesophageal echocardiographic study. J Am Coll Cardiol 1995;25:452-9. https://www.ncbi.nlm.nih.gov/pubmed/7829800  
3. Weigner MJ, Caulfield TA, Danias PG, et al. Left atrial appendage thrombus is not uncommon in patients with acute atrial fibrillation and and a recent embolic event: A transesophageal echocardiographic study. J Am Coll Cardiol 1995;25:452-9. https://www.ncbi.nlm.nih.gov/pubmed/9103128  
4. Garg, A, Khunger M, Seicean S, et al. Incidence of thromboembolic complications within 30 days of electrical cardioversion performed within 48 hours of atrial fibrillation onset. JACC: Clinical Electrophysiology 2016;2:487-494. https://cwru.pure.elsevier.com/en/publications/incidence-of-thromboembolic-complications-within-30-days-of-elect
5. Wann LS, Curtis AB, January CT, et al. 2011 CCF/AHA/HRS focused update on the management of patients with atrial fibrillation/American Heart Association task force on practice guidelines. J Am Coll Cardiol 2011;57;223-42. https://www.ncbi.nlm.nih.gov/pubmed/21324629