Incidence of thrombi prior to ablation
BALTIMORE, MARYLAND. Catheter ablation for atrial fibrillation (AF) carries a low (0.3 – 0.7%), but still significant, risk of ischemic stroke and transient ischemic attack (TIA). The two major causes of stroke or TIA occurring during the procedure are the formation and subsequent embolization (blocking of a blood vessel) of char or thrombi (blood clots) formed on the ablation catheter and the disturbance of existing thrombi in the left atrium or left atrial appendage (LAA). This disturbance can be caused by catheter manipulation or be related to the more forceful heart beat accompanying restoration of normal sinus rhythm. The risk of a stroke or TIA during the procedure can clearly be reduced by ensuring that there are no clots in the left atrium or LAA prior to starting the procedure. This is the reason why most major ablation centers insist that patients scheduled for an ablation undergo a TEE (transesophageal echocardiogram) shortly before the procedure. A TEE is the "gold standard" for determining if clots are present in the left atrium or LAA, but the procedure is invasive, uncomfortable for the patient and relatively costly. Thus, researchers at Johns Hopkins University School of Medicine recently undertook a study to determine if all afib patients scheduled for ablation actually needed a pre-procedural TEE.

Their study involved 585 patients who underwent a total of 732 ablation procedures (repeat rate of 25%). All patients were anticoagulated with warfarin for at least 4 weeks prior to their procedure except for the last 5 days when warfarin was replaced by enoxaparin (a low molecular weight heparin). All patients also underwent a TEE within 24 hours prior to the procedure. The researchers found thrombi in the LAA in 12 cases (1.6%) requiring cancellation of the procedure. All 12 patients had been on warfarin for at least 6 months prior to the procedure and all had a left atrial diameter equal to or greater than 4.5 cm (45 mm). Nine of the patients had persistent afib and 3 had paroxysmal AF.

Analysis of all the TEE data collected revealed that the risk of finding a clot in the left atrium or LAA was significantly associated with left atrial diameter and stroke risk as measured with the CHADS2 score. NOTE: The CHADS2 score assigns a risk of 1 point each for congestive heart failure, hypertension, age 75 years or older and diabetes, and 2 point score if having suffered a previous stroke or TIA. Thrombi were present in 0.3%, 1.4% and 5.3% in patients with CHADS2 scores of 0, 1 and 2 or greater. None of the cases where thrombi were observed had a left atrial diameter less than 4.5 cm. In contrast, no thrombi were observed in patients with a CHADS2 score of 0 and a left atrial diameter of less than 4.5 cm.

The researchers suggest that a pre-procedural TEE may be unnecessary in this group of patients provided they have been properly anticoagulated prior to the procedure.

Scherr, D, et al. Incidence and predictors of left atrial thrombus prior to catheter ablation of atrial fibrillation. Journal of Cardiovascular Electrophysiology, Vol. 20, April 2009, pp. 379-84
Cappato, R. Searching for left atrial thrombi prior to catheter ablation of atrial fibrillation. Journal of Cardiovascular Electrophysiology, Vol. 20, April 2009, pp. 385-87

Editor's comment: This study confirms that warfarin is by no means 100% effective in preventing clot formation in the left atrium and, on a more positive note, that healthy afibbers with no stroke risk factors and a left atrial diameter less than 4.5 cm have a very low risk of forming clots in the left atrium.