What are the diagnostic and treatment approaches for pulmonary embolism?

Diagnostic Approach to Pulmonary Embolism

The diagnosis of pulmonary embolism requires a structured algorithm beginning with clinical probability assessment combined with D-dimer testing in non-high-risk patients, followed by computed tomography pulmonary angiography (CTPA) as the definitive imaging test. 1

Initial Risk Stratification

Begin by determining if the patient has hemodynamic instability (shock or hypotension with systolic blood pressure <90 mmHg). 1

High-Risk PE (Hemodynamically Unstable)

If the patient is hemodynamically unstable and CT is not immediately available, perform bedside echocardiography to assess for right ventricular overload. 1

• If echocardiography shows RV overload in a hemodynamically unstable patient, PE-specific treatment is justified without further imaging 1
• CT is considered "not immediately available" if the patient's critical condition allows only bedside diagnostic tests 1
• Do not delay treatment to obtain imaging in unstable patients 1

Non-High-Risk PE (Hemodynamically Stable)

For stable patients, use a systematic diagnostic algorithm:

Step 1: Clinical Probability Assessment

Assess clinical probability using either:

• Three-level scheme: low, intermediate, or high probability 1
• Two-level scheme: "PE unlikely" or "PE likely" 1

Step 2: D-Dimer Testing (Selective Use)

Measure D-dimer only in patients with low or intermediate clinical probability, or those classified as "PE unlikely." 1

• Do NOT measure D-dimer in patients with high clinical probability, as a normal result does not safely exclude PE in this population (Class III recommendation) 1
• D-dimer is also less useful in hospitalized patients due to low specificity 1
• If D-dimer is negative in low/intermediate probability patients, reject the diagnosis of PE without further testing (Class I recommendation) 1
• This approach safely excludes PE in approximately 30% of patients, with 3-month thromboembolic risk below 1% 1

Step 3: Computed Tomography Pulmonary Angiography (CTPA)

CTPA is the imaging test of first choice and should be performed in patients with:

• Elevated D-dimer levels 1, 2
• High clinical probability (as first-line test, bypassing D-dimer) 1

Interpreting CTPA Results

Accept the diagnosis of PE if CTPA shows a segmental or more proximal filling defect in a patient with intermediate or high clinical probability (Class I recommendation). 1

Reject the diagnosis of PE without further testing if CTPA is normal in a patient with low or intermediate clinical probability, or if the patient is "PE unlikely" (Class I recommendation). 1

• CTPA has high sensitivity and specificity for PE diagnosis 2
• The presence or absence of PE can be determined with sufficient certainty after CTPA without additional imaging tests 2
• Even with low contrast injection rates (2.0-2.5 mL/s), CTPA remains diagnostic in most patients 3

Important Caveat for High Clinical Probability

If CTPA is negative in a patient with high clinical probability, further investigation may be considered before withholding PE-specific treatment, though this situation is infrequent and the 3-month thromboembolic risk remains low. 1

Alternative and Adjunctive Imaging

Ventilation/Perfusion (V/Q) Scintigraphy

Reject the diagnosis of PE without further testing if the perfusion lung scan is normal (Class I recommendation). 1

V/Q scintigraphy is a valid option when:

• CTPA is contraindicated 1
• Avoiding radiation exposure is preferred (younger patients, females at risk for breast cancer) 1
• The chest X-ray is normal (increases proportion of diagnostic scans) 1

V/Q scanning is diagnostic in 30-50% of emergency patients with suspected PE. 1

Compression Ultrasonography (CUS)

Accept the diagnosis of venous thromboembolism if CUS shows a proximal deep vein thrombosis in a patient with clinical suspicion of PE (Class I recommendation). 1

• CUS is particularly useful in patients with concomitant symptomatic deep vein thrombosis 2
• A negative proximal lower limb CUS combined with low clinical probability and non-diagnostic V/Q scan has high negative predictive value 1

Tests NOT Recommended

Do NOT perform:

• CT venography as an adjunct to CTPA (Class III recommendation) 1
• MR angiography to rule out PE (Class III recommendation) 1
• Magnetic resonance pulmonary angiography is not currently a suitable alternative for CTPA 2

Common Diagnostic Pitfalls to Avoid

1. Measuring D-dimer in high clinical probability patients – this wastes time and resources, as a normal result cannot exclude PE in this population 1

2. Performing CTPA as the first test in all patients – approximately one-third of patients can have PE safely excluded with clinical probability assessment and D-dimer alone 1

3. Assuming a negative CTPA in intermediate-probability V/Q scan patients definitively excludes PE – in this specific setting, CTPA sensitivity is only 57%, and conventional pulmonary angiography may be necessary 4

4. Delaying treatment in hemodynamically unstable patients to obtain imaging – bedside echocardiography showing RV overload is sufficient to justify treatment 1

5. Using D-dimer indiscriminately in hospitalized patients – the number needed to test for a clinically relevant negative result is high in this population 1

Summary Algorithm

For hemodynamically stable patients:

1. Assess clinical probability (low/intermediate/high or PE unlikely/likely)
2. If low/intermediate probability or "PE unlikely": measure D-dimer
   • If negative: stop (PE excluded)
   • If positive: proceed to CTPA
3. If high clinical probability or "PE likely": proceed directly to CTPA
4. CTPA showing segmental or larger filling defect = PE confirmed
5. Normal CTPA in low/intermediate probability = PE excluded

For hemodynamically unstable patients:

• If CT immediately available: perform CTPA
• If CT not immediately available: bedside echocardiography for RV overload assessment
• Do not delay treatment for imaging 1