Patients with atrial fibrillation are at increased risk of thrombus formation in the left heart, especially in the auricula and the atrium. Their presence increases the risk of ischemic stroke, especially during cardioversion, performed either by drug or by external electric shock. The gold standard to exclude this thrombus and consider cardioversion is the transesophageal echocardiography.

D-dimers, products of the degradation of fibrin, have been studied several times to assess the presence of thrombi in the body, especially in pulmonary embolism. In cardiology and despite several studies on the matter, there is currently no blood D-dimers value allowing to detect the presence of a thrombus with a good predictive value. Correlating the D-dimer values with age, as has been done for some years in pulmonary embolism, would give the opportunity to create a simple, inexpensive and widely applicable tool to eliminate the presence of a thrombus in the left heart.

If the results are satisfactory, the investigators might consider not using transesophageal echocardiography as this examination is unpleasant for the patient and operator dependant, unlike a standard blood sampling.

This study aims to test the following hypothesis: D-dimers values superior or equal to ten times the patient's age is predictive of a thrombus.

Inclusion Criteria:

• Patients with de novo or recent atrial fibrillation requiring cardioversion, either by drug, by an electric shock or an intervention.

Exclusion Criteria:

• valvulopathy
• infection
• neoplasia
• inflammatory disease
• recent surgery
• pregnancy
• aorta aneurism
• deep vein thrombosis
• pulmonary embolism