Understanding Pulmonary Embolism
A pulmonary embolism occurs when a blood clot or other material lodges in the pulmonary arteries, obstructing blood flow to the lungs. Most commonly, the clot originates in deep leg veins and travels through the venous system to lodge peripherally in the lungs. Less frequent sources include fat emboli from long bone fractures, amniotic fluid during labour complications, air from central line placement, tumour fragments, or foreign bodies.
The obstruction raises pulmonary artery pressure and increases right ventricular workload. Depending on the clot size and location, haemodynamic consequences range from subclinical to sudden cardiovascular collapse. This is why rapid risk assessment and appropriate diagnostic confirmation are critical in emergency medicine.
Clinical Presentation and Diagnostic Challenge
Pulmonary embolism presents with non-specific symptoms that overlap significantly with other cardiopulmonary conditions. Common presentations include:
- Dyspnoea – the most frequent finding, though subjective and present in many diseases
- Pleuritic chest pain – sharp pain worsened by breathing
- Tachycardia – elevated heart rate due to sympathetic compensation
- Cough – often dry, sometimes with haemoptysis if infarction occurs
- Syncope – can indicate massive PE with haemodynamic instability
Because these signs appear in pneumonia, myocardial infarction, asthma, and other conditions, clinical gestalt alone cannot reliably exclude PE. This diagnostic uncertainty drove development of the Wells criteria.
Wells Score Calculation
The Wells score aggregates seven weighted clinical parameters into a total point score. Each feature carries a specific point value based on its association with confirmed PE in the original derivation cohort.
Wells Score = DVT Signs + Heart Rate + Post-operative Status + Immobilisation History + Haemoptysis + Malignancy + Clinical PE Probability
Scoring:
DVT Signs (leg swelling ≥3 cm or unilateral pitting oedema): 3 points
Heart Rate >100 bpm: 1.5 points
Surgery or immobilisation in last 4 weeks: 1.5 points
History of DVT or PE: 1.5 points
Haemoptysis: 1 point
Malignancy (active or treated in last 6 months): 1 point
Clinical PE Probability (judge vs. alternative diagnosis): 0, 2, or 3 points
DVT Signs— Clinical evidence of deep vein thrombosis: calf pain, swelling, or unilateral leg circumference difference ≥3 cmHeart Rate— Resting heart rate measured in beats per minutePost-operative Status— Surgery or immobilisation (bed rest, cast, long flight) within 4 weeks prior to assessmentImmobilisation History— Prolonged reduced mobility including hospitalisations, long flights, or leg immobilisationHaemoptysis— Blood in sputum, indicating possible pulmonary infarction from PEMalignancy— Active cancer diagnosis or cancer treatment within the past 6 monthsClinical PE Probability— Clinician's subjective assessment: PE less likely than alternative (0 pts), PE and alternatives equally likely (2 pts), or PE more likely (3 pts)
Risk Stratification and Clinical Interpretation
The original Wells criteria divide patients into three risk categories. A score of 0–1 indicates low risk (approximately 10% probability of PE); 2–6 points represents moderate risk (~30% probability); and ≥7 points indicates high risk (~65% or greater probability). These estimates derive from the derivation study population and may vary with local epidemiology.
The two-tier simplified Wells scheme collapses scores into PE-unlikely (≤4 points) and PE-likely (>4 points) categories, useful when combined with D-dimer testing. In low-risk patients with a negative D-dimer, PE can be safely excluded without imaging. Moderate- or high-risk patients typically require computed tomography pulmonary angiography (CTPA) regardless of D-dimer result.
Practical Considerations When Using the Wells Score
The Wells score is most useful in haemodynamically stable patients; unstable presentations warrant immediate imaging and treatment regardless of score.
- Subjective probability assessment carries weight — The 'clinical probability' component (0, 2, or 3 points) significantly influences the final score. This forces explicit clinical reasoning rather than rote calculation. Overestimating PE probability inflates score and may trigger unnecessary imaging; underestimating delays diagnosis. Document the clinical reasoning clearly.
- Immobilisation definitions matter — Not all reduced mobility counts equally. Standard examples include hospitalisation, bedrest, long-haul flights (>4 hours), leg casts, or wheelchair use. Short car journeys or routine office work do not qualify. Be precise about duration and type.
- Malignancy timing is specific — Only active malignancies or those treated within 6 months contribute points. Remote cancer history (>6 months post-treatment) does not increase PE risk per Wells criteria, though clinical judgment may vary by cancer type.
- D-dimer thresholds vary by Wells category — Low Wells score patients with negative D-dimer can safely avoid imaging. Conversely, high-risk patients need CTPA even with negative D-dimer. Do not treat D-dimer as a standalone decision tool—it works only within the Wells stratification framework.