Pulmonary Artery Thrombosis

Pulmonary artery thrombosis is thrombus formed in situ within the pulmonary arteries, in contrast to embolism from the deep veins. It arises on a damaged or abnormal vessel wall, with slow or turbulent flow, or in a prothrombotic state, and often no leg thrombosis is found.

Conditions include pulmonary hypertension (chronic thromboembolic, idiopathic, Eisenmenger syndrome with congenital heart disease, in which in situ thrombosis occurs in up to 20 to 30%), pulmonary artery aneurysm, sickle cell disease, severe COVID-19 pneumonia (immunothrombosis), Behçet disease, antiphospholipid syndrome, vasculitis (Takayasu arteritis), lung cancer or pulmonary artery sarcoma, nephrotic syndrome, lung surgery (lobectomy, pneumonectomy stump), transplant, infection (aspergillosis, tuberculosis), catheters and inflammatory bowel disease.

Pathophysiology. Endothelial injury and inflammation, low flow in dilated or hypertensive vessels, hypercoagulable states and local lung disease promote thrombus within the vessel, with fibrous organisation. Thrombus propagates and may embolise distally, and chronic organised thrombus contributes to pulmonary hypertension.

The presentation overlaps with embolism: progressive dyspnoea, pleuritic chest pain, haemoptysis, cough, and signs of pulmonary hypertension and right heart failure (loud pulmonary second sound, raised jugular venous pressure, oedema). In Eisenmenger syndrome and sickle cell disease, worsening cyanosis or acute chest syndrome. In Behçet disease, haemoptysis from associated pulmonary artery aneurysm is dangerous.

Massive haemoptysis (a sign of pulmonary artery aneurysm, particularly in Behçet disease), syncope or right heart failure needs urgent assessment.

• CT pulmonary angiography: filling defect within an enlarged or abnormal artery, often central or in a dilated vessel, with mural thrombus that is eccentric, calcified and adherent to the wall, and absence of deep vein thrombosis. In Behçet disease, aneurysms with thrombosis are visible.
• Ventilation perfusion scan or MRI angiography for chronic obstruction, and echocardiography for right ventricular strain and pulmonary pressure.
• Look for DVT: compression ultrasound of the legs (usually negative).
• Right heart catheterisation and pulmonary angiography in pulmonary hypertension, with caution in aneurysm.
• Cause: thrombophilia testing (antiphospholipid antibodies including lupus anticoagulant, anticardiolipin and anti-beta-2-glycoprotein I, repeated after 12 weeks), inflammatory markers, ANCA, haemoglobin electrophoresis, cardiac imaging for shunts, and CT for tumour.
• Biopsy or PET imaging when sarcoma or vasculitis is suspected.

Differential diagnosis.

• Acute or chronic thromboembolism from the veins
• Pulmonary artery sarcoma
• Takayasu arteritis or other large vessel vasculitis
• Pulmonary artery aneurysm or dissection
• Flow artefact on contrast CT

• Anticoagulation, unless bleeding or aneurysm: apixaban 10 mg twice daily for 7 days then 5 mg twice daily, or rivaroxaban 15 mg twice daily for 21 days then 20 mg daily, or enoxaparin 1 mg/kg twice daily with warfarin to an INR of 2 to 3. In antiphospholipid syndrome (especially triple positive), use warfarin (INR 2 to 3) and avoid direct oral anticoagulants. Duration at least 3 months, and indefinite when the risk persists (pulmonary hypertension, Eisenmenger syndrome, antiphospholipid syndrome).
• Behçet disease and aneurysm: immunosuppression is the first treatment, with prednisolone 1 mg/kg daily with cyclophosphamide 600 mg/m2 monthly (or infliximab 5 mg/kg), and anticoagulation is used with caution only after immunosuppression and exclusion of aneurysm because of the risk of fatal haemoptysis. Embolisation or surgery for aneurysm.
• Eisenmenger syndrome: balance bleeding risk (haemoptysis) against thrombosis, and anticoagulate only for clear thrombosis, with specialist care. Pulmonary hypertension therapy (bosentan 62.5 mg twice daily increasing to 125 mg twice daily, or sildenafil 20 mg three times daily) as the specialist directs.
• Sickle cell disease: oxygen, hydration, analgesia, exchange transfusion to reduce haemoglobin S below 30% in acute chest syndrome, hydroxycarbamide 15 to 35 mg/kg daily, and anticoagulation for confirmed thrombosis.
• Severe COVID-19: prophylactic or therapeutic anticoagulation (enoxaparin 40 mg daily, or 1 mg/kg twice daily when thrombosis is confirmed).
• Thrombolysis (alteplase 100 mg over 2 hours) for instability, and pulmonary thromboendarterectomy or balloon pulmonary angioplasty for organised thrombus in chronic thromboembolic disease.
• Cancer related thrombosis: low-molecular-weight heparin or apixaban, and tumour treatment.
• Avoid inferior vena cava filter, since the source is not venous.
• Oxygen when PaO2 is 7.3 kPa or less, diuretics for right heart failure.

Complications and prognosis. Right heart failure, pulmonary hypertension, pulmonary infarction, haemoptysis, recurrence and death. The outlook depends on the underlying disease (poor in Behçet disease with aneurysm, Eisenmenger syndrome and sarcoma).

Monitoring. Review at 4 to 6 weeks and 3 months with echocardiography, imaging at 3 to 6 months to assess resolution, and INR or renal function monitoring while anticoagulated.

Referral and escalation. Refer to a pulmonary hypertension centre, haematology and, for Behçet disease, rheumatology. Refer urgently for haemoptysis or right heart failure.