Discuss the risk factors, clinical features, investigations and management of pulmonary thromboembolism.

Definition

Pulmonary embolism (PE) is obstruction of the pulmonary arterial tree by thrombus, usually from deep vein thrombosis of the lower limbs or pelvis. PE and DVT are manifestations of one disease — venous thromboembolism. It is the most commonly missed life-threatening diagnosis in hospital medicine, because the clinician must think of it to find it.

Risk Factors — Virchow's Triad

Pathophysiology

• Respiratory: obstruction creates alveolar dead space; surfactant loss causes atelectasis; V/Q mismatch and shunting → hypoxaemia; hyperventilation → hypocapnia and respiratory alkalosis
• Haemodynamic: raised pulmonary vascular resistance → acute RV pressure overload → RV dilatation → septal shift impairing LV filling → reduced output, hypotension and shock
• Pulmonary infarction occurs in only ~10% because of the dual bronchial blood supply — typically with small peripheral emboli

Clinical Features

• Sudden breathlessness (commonest symptom) and tachypnoea (commonest sign)
• Pleuritic chest pain and haemoptysis — suggest peripheral infarction
• Tachycardia; SYNCOPE or shock indicates MASSIVE PE
• Signs of DVT in only a minority; a normal examination does not exclude PE
• Massive PE: raised JVP, parasternal heave, loud P2, RV gallop

Risk classification: HIGH-RISK (massive) — hypotension or shock; INTERMEDIATE — normotensive with RV dysfunction and/or raised troponin; LOW-RISK — neither.

Diagnostic Approach — Pre-test Probability First

WELLS SCORE: clinical signs of DVT (3.0); PE most likely diagnosis (3.0); HR > 100 (1.5); immobilisation ≥ 3 days or surgery within 4 weeks (1.5); previous VTE (1.5); haemoptysis (1.0); malignancy (1.0). Score ≤ 4 = "PE unlikely"; > 4 = "PE likely".

• PE UNLIKELY → D-DIMER. Negative excludes PE; positive → imaging
• PE LIKELY → PROCEED DIRECTLY TO IMAGING. Do not order a D-dimer
• Age-adjusted D-dimer (age × 10 µg/L over 50) improves specificity in the elderly

Investigations

• D-DIMER — sensitive, not specific; raised in infection, malignancy, pregnancy, trauma, surgery and old age. Useful only to RULE OUT PE in low-probability patients
• CT PULMONARY ANGIOGRAPHY — the investigation of choice; also identifies alternative diagnoses
• V/Q scan — preferred in pregnancy, renal impairment and contrast allergy
• ECG — sinus tachycardia is the commonest finding; also T inversion V1–V4, RBBB, right axis deviation, P pulmonale, and the classic but uncommon "S1 Q3 T3"
• Chest X-ray — usually normal; Hampton's hump and Westermark's sign are rare
• Blood gas — hypoxaemia with hypocapnia and a raised A–a gradient; a normal gas does not exclude PE
• ECHOCARDIOGRAPHY — RV dilatation, septal flattening, McConnell's sign; invaluable at the bedside in the unstable patient who cannot go to CT, and excludes tamponade and dissection
• Compression ultrasound of the legs; troponin and BNP for risk stratification

Management

1. Immediate: oxygen, cautious fluids (excess worsens RV failure), noradrenaline for hypotension.

2. ANTICOAGULATION — the cornerstone

• Start immediately on clinical suspicion in high or intermediate probability, before confirmatory imaging, unless bleeding risk is high
• DOACs (apixaban, rivaroxaban, edoxaban, dabigatran) are first-line for most patients
• LMWH in cancer-associated thrombosis and in pregnancy (throughout — warfarin and DOACs are contraindicated)
• Warfarin where DOACs are unsuitable — severe renal failure, antiphospholipid syndrome with triple positivity, mechanical valves
• Duration: at least 3 months; extended or indefinite for unprovoked PE, recurrence, active cancer or persistent risk factors

3. REPERFUSION — for high-risk (massive) PE

• SYSTEMIC THROMBOLYSIS for haemodynamic instability; contraindications mirror those for stroke thrombolysis
• Catheter-directed thrombolysis or thrombectomy; surgical embolectomy in specialist centres
• In intermediate-risk PE, routine thrombolysis is NOT recommended — monitor closely, with rescue thrombolysis for deterioration

4. IVC filter — only for absolute contraindications to anticoagulation, or recurrence despite adequate anticoagulation.

5. Follow-up: assess for chronic thromboembolic pulmonary hypertension at 3–6 months if dyspnoea persists; thrombophilia testing only in selected patients, and never during the acute event or on anticoagulation; age-appropriate cancer screening in unprovoked VTE.

6. PREVENTION — the most effective strategy: risk-assess every hospitalised patient and provide mechanical and pharmacological thromboprophylaxis; early mobilisation; extended prophylaxis after major orthopaedic surgery.