Pulmonary Embolism
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Direct answer
Pulmonary embolism is occlusion of a pulmonary artery by material — most often thrombus — that has travelled there, classically from the deep veins of the legs. Because the lung has a dual blood supply and the clot load varies, most emboli are clinically silent and resolve, but large saddle emboli straddling the bifurcation can kill within minutes by acute right heart failure, while smaller emboli in patients with left heart failure or lung disease produce wedge-shaped, pleural-based haemorrhagic pulmonary infarcts. Venous stasis, endothelial injury and hypercoagulability — Virchow's triad — explain every risk factor.
What you must remember
- Source and triad: over 90 per cent arise from deep vein thrombosis of the lower limbs; Virchow's triad of stasis, endothelial injury and hypercoagulability explains postoperative immobility, malignancy, pregnancy, oral contraceptive use, factor V Leiden and long journeys.
- Fate: most small emboli are asymptomatic and lyse via fibrinolytic activity; recurrent showers cause thromboembolic pulmonary hypertension with webs and intimal fibrosis.
- Saddle embolus: a large clot astride the main pulmonary bifurcation, producing sudden dyspnoea, hypotension, syncope or death from acute cor pulmonale; the classic gross photograph.
- Pulmonary infarct: wedge-shaped, pleural-based, haemorrhagic, red-blue and raised; occurs mainly with coexistent left heart failure or compromised circulation; heals by organisation into a fibrous scar, since infarcted lung cannot regenerate.
- Histology: lines of Zahn — alternating pale platelet-fibrin and red cell layers — prove the clot formed in flowing blood before embolising.
- Clinical triad: dyspnoea, pleuritic chest pain and haemoptysis with tachycardia; the presentation merges with infarction when all three coexist.
- Diagnosis and prevention: D-dimer to exclude in low-probability cases, computed tomographic pulmonary angiography as the confirmatory modality (ventilation-perfusion scan when contrast is contraindicated); prophylaxis with early mobilisation, graduated stockings and pharmacological thromboprophylaxis per current guidance.
Common confusion
Pulmonary embolism is often mistaken for myocardial infarction in stems — both cause sudden chest pain and collapse — but embolism lacks ECG infarct patterns and troponin rises of coronary occlusion, instead showing right ventricular strain. Pulmonary infarct differs from other organ infarcts by being haemorrhagic (dual circulation) and wedge-shaped with the apex pointing medially. Fat embolism syndrome, appearing after long-bone fractures with petechiae and cerebral signs, and amniotic fluid embolism of labour, with disseminated intravascular coagulation, are separate entities with distinct settings.
Exam-focused takeaway
Questions present a postoperative or long-journey patient with sudden breathlessness and clear radiograph, or an autopsy with a saddle clot; ask for the source, the Virchow element present, or the infarct appearance. One-liners test lines of Zahn, haemorrhagic versus pale infarcts, and the diagnostic algorithm of D-dimer followed by CT angiography. Answer by linking risk factor to triad, then triad to outcome.
Frequently asked questions
From where do most pulmonary emboli originate?
The deep veins of the lower limbs, particularly calf and popliteal-femoral veins, dislodged by stasis or trauma.
Why are pulmonary infarcts haemorrhagic?
The lung's dual pulmonary and bronchial circulation leaks blood into necrotic parenchyma through the intact bronchial supply.
What are lines of Zahn?
Alternating laminations of platelets-fibrin (pale) and erythrocytes (dark) indicating thrombus formation in flowing blood, seen in emboli and arterial thrombi.
What is a saddle embolus?
A large embolus lodged across the pulmonary artery bifurcation, abruptly raising pulmonary vascular resistance and often causing sudden death.
Which imaging confirms pulmonary embolism?
Computed tomographic pulmonary angiography in most patients, with ventilation-perfusion scanning when renal impairment or contrast allergy precludes it.