Pulmonary Embolism
A venous clot, usually from the deep leg veins, lodges in the pulmonary arterial tree and abruptly creates ventilated-but-unperfused lung and a sudden rise in right ventricular afterload, so the clinical picture is driven by gas-exchange mismatch and acute right heart strain rather than by the lung itself being diseased.
In a nutshell
A venous clot lodges in the pulmonary arteries, creating ventilated-but-unperfused dead space (hypoxaemia, breathlessness) and, if large, a sudden rise in right ventricular afterload that can cause acute right heart failure and obstructive shock. Diagnosis is risk-stratified with the Wells score before CTPA; treatment is anticoagulation, with thrombolysis reserved for haemodynamic collapse.
Classic presentation
Sudden-onset breathlessness and pleuritic chest pain with tachycardia in a patient with a provoking risk factor such as immobility, recent surgery or malignancy.
Key points
- PE causes dead space (ventilation without perfusion), the opposite mismatch pattern to pneumonia, which causes shunt (perfusion without ventilation).
- The chest X-ray and lung parenchyma are often normal: the pathology is vascular obstruction, not lung tissue disease.
- A large PE threatens life through acute right ventricular failure and obstructive shock, not hypoxia alone: hypotension or syncope signals massive PE.
- Wells score determines the diagnostic pathway: D-dimer to exclude PE if unlikely, straight to CTPA if likely.
- D-dimer is sensitive but not specific: useful only to rule out PE in low-probability patients, never to confirm it.
- Anticoagulation prevents clot propagation while the body's own fibrinolysis clears the embolus; thrombolysis is for haemodynamically unstable disease only.
First-line investigation
Wells score to establish clinical probability, followed by D-dimer (if unlikely) or CT pulmonary angiography (if likely).
First-line management
Prompt anticoagulation once PE is confirmed or strongly suspected, with thrombolysis reserved for haemodynamically unstable massive PE.
Exam traps
- A normal chest X-ray does not exclude PE: it is expected, since the lung parenchyma is usually structurally normal.
- Do not send a D-dimer in a high-probability (PE likely) patient: proceed straight to CTPA, since a normal D-dimer cannot be relied upon to exclude disease in this group.
- Hypotension in confirmed or suspected PE indicates massive PE and the need for thrombolysis consideration, not just faster anticoagulation.
- S1Q3T3 is a classic but insensitive ECG finding; sinus tachycardia is the most common ECG abnormality in PE.
Educational content pending clinical review. Not medical advice.