Definition

• Occlusion of a pulmonary artery by an embolic thrombus.

 Epidemiology

• Common condition with incidence rates of 100 per 100 000 people years.

 Aetiology

 • As pulmonary emboli originate from deep vein thromboses, the risk factors are the same as for that condition, i.e. immobility, acute medical illness, recent surgery, malignancy, pregnancy, and congenital and acquired thrombotic disorders.

 Pathogenesis

• A fragment of a detached thrombus from a deep vein thrombosis embolizes via the right side of the heart into the pulmonary arterial circulation and lodges in a pulmonary artery.

Presentation

 • Blockage of a major pulmonary artery usually may cause instant death due to a sudden huge rise in pulmonary arterial pressure, acute RVF, and cardiac arrest.

• Blockage of medium- sized arteries causes an area of ventilation/ perfusion mismatch in the lungs with breathlessness.

 • Smaller pulmonary emboli may lead to subtle symptoms of breathlessness, chest pain, and dizziness; these can easily go undiagnosed.

Macroscopy

• emboli are visible as fragments of thrombi within pulmonary arteries.

 • thrombi are firm and brown and the cut surface may show visible bands (lines of Zahn).

 Histopathology

• Fresh thromboemboli are composed of a mixture of fibrin and enmeshed blood cells, often arranged in alternating linear bands that correspond to the macroscopic lines of Zahn.

• thromboemboli organize after 2– 3 days with ingrowth of granulation tissue composed of fibroblasts and capillaries from the vessel wall.

• Old thromboemboli may be evident as fibrous nodules projecting from the vessel wall or fibrous bands crossing the lumen of a pulmonary artery.

 Prognosis

 • Mortality rates range from 3% to 25%.

• Risk of death is higher for larger emboli or if the diagnosis is made late.