Where is the most likely site of the thrombus in a patient with atrial fibrillation (AF) and acute onset of symptoms in the right leg?

The Atrium (Left Atrium) is the Most Likely Site of Thrombus

In a patient with atrial fibrillation (indicated by the irregularly irregular left radial pulse) presenting with acute limb ischemia, the thrombus originates in the left atrium, specifically the left atrial appendage, and embolizes into the systemic arterial circulation to cause the acute leg ischemia. The answer is D. Atrium.

Clinical Reasoning

Identifying Atrial Fibrillation as the Source

• The irregularly irregular pulse is pathognomonic for atrial fibrillation (AF), which creates the substrate for thrombus formation 1
• The acute onset of a pale, pulseless right leg represents acute arterial occlusion from an embolus, not in-situ thrombosis 1
• In patients with AF, thrombus arises most frequently in the left atrial appendage (LAA) due to blood stasis, endothelial dysfunction, and hypercoagulable state 1

Why the Left Atrium/Left Atrial Appendage

• More than 90% of thrombi in AF patients form in the left atrial appendage, which cannot be visualized on standard transthoracic echocardiography 2
• The pathophysiology involves reduced LAA flow velocities from loss of organized mechanical contraction during AF, creating stasis that promotes thrombus formation 1
• Transesophageal echocardiography studies demonstrate LAA thrombus in 5-15% of AF patients, and up to 43% of patients presenting with acute thromboembolism have residual left atrial thrombi 1, 3

Embolic Pathway to the Leg

• Thrombi from the left atrium embolize through the left ventricle into the systemic arterial circulation, causing stroke and peripheral arterial emboli 4
• This is fundamentally different from pulmonary embolism, which requires right-sided thrombi that travel through the right ventricle to the pulmonary circulation 4
• The patient's presentation with unilateral leg ischemia is classic for arterial embolism from a cardiac source in AF 1

Why Other Options Are Incorrect

Ventricle (Option C)

• While left ventricular thrombi can occur (typically post-myocardial infarction or in dilated cardiomyopathy), they are not the primary source in AF patients 5
• LV thrombi have different embolic characteristics and are far less common than LAA thrombi in AF 5
• The clinical scenario specifically points to AF as the underlying rhythm disturbance, making LAA the overwhelmingly more likely source 1

Abdominal or Thoracic Aorta (Options A & B)

• Aortic thrombi or atheromatous disease account for up to 25% of AF-associated strokes but are secondary contributors, not the primary source 1
• The acute presentation with AF makes cardiogenic embolism from the left atrium the most likely mechanism 1
• Aortic sources would not explain the clear association with new-onset or uncontrolled AF 1

Critical Clinical Implications

Immediate Management Considerations

• This patient requires urgent vascular imaging (CTA or catheter angiography) to confirm arterial occlusion and plan revascularization 1
• Echocardiography (particularly TEE) should be performed to identify the cardiac source and assess for residual thrombus, as 43% of patients with acute thromboembolism have persistent LAA thrombi 3
• Therapeutic anticoagulation must be initiated immediately (unless contraindicated by bleeding risk) to prevent recurrent embolism 1

Long-term Stroke Prevention

• The CHA₂DS₂-VASc score should be calculated to determine ongoing anticoagulation needs, with scores ≥2 mandating long-term oral anticoagulation 6
• Direct oral anticoagulants (DOACs) are preferred over warfarin for stroke prevention in AF patients 6
• Patients with LAA thrombi despite anticoagulation have significantly higher rates of future ischemic events (7.1% vs 4.4% at 12 months) 7