What is the best course of action to rule out pulmonary infarct in a patient with diaphragmatic atelectasis and low lung volumes?

Diagnostic Approach for Suspected Pulmonary Infarct with Diaphragmatic Atelectasis

If there is high clinical concern for pulmonary embolism (PE) and infarct despite atelectasis on imaging, proceed directly to V/Q scanning as recommended by the radiologist, as this is the most appropriate next step to definitively rule out PE in this clinical scenario. 1

Understanding the Clinical Context

The imaging findings describe diaphragmatic atelectasis in both lower lobes with low lung volumes, which creates diagnostic uncertainty because:

• Atelectasis and PE can present with identical symptoms (dyspnea, hypoxemia) but through different pathophysiologic mechanisms—atelectasis causes ventilation-perfusion mismatch through hypoventilation, while PE causes it through perfusion defects 2
• Atelectasis is extremely common in patients being evaluated for PE, occurring in 19% of patients undergoing CT pulmonary angiography for suspected PE, making it as frequent as pneumonia as an alternative diagnosis 2
• Low lung volumes reduce the sensitivity of CT imaging for detecting PE, as compressed lung parenchyma can obscure small emboli 2

Why V/Q Scanning is the Appropriate Next Step

V/Q scanning is specifically recommended when CT findings are equivocal or when there are technical limitations (such as low lung volumes) that reduce diagnostic confidence: 1

• A normal perfusion scan definitively excludes PE without need for further testing, regardless of the presence of atelectasis 1
• A high-probability V/Q scan confirms PE in patients with intermediate or high clinical probability 1
• V/Q scanning can distinguish between atelectasis and PE because atelectasis typically shows matched ventilation-perfusion defects (both reduced), while PE shows mismatched defects (normal ventilation with reduced perfusion) 1, 3

Clinical Probability Assessment Must Guide Decision-Making

Before proceeding to V/Q scanning, assess the clinical probability of PE using validated tools: 4, 5

• Apply Wells score or revised Geneva score to stratify pretest probability as low, intermediate, or high 4, 5
• Consider D-dimer testing if not already performed: A negative D-dimer (using age-adjusted cutoff of age × 10 ng/mL for patients >50 years) safely excludes PE in low or intermediate probability patients 1, 5
• If D-dimer is elevated or clinical probability is high, proceed directly to V/Q scanning as recommended 4

Alternative Diagnostic Considerations

Lower extremity compression ultrasonography (CUS) can be performed before or alongside V/Q scanning: 1

• Finding a proximal deep vein thrombosis (DVT) on CUS confirms venous thromboembolism and justifies anticoagulation without need for further PE imaging 1
• This approach is particularly useful if V/Q scanning is not immediately available or if there are contraindications 1, 3
• However, a negative CUS does not exclude PE, as only 30-50% of PE patients have detectable DVT 1

Interpreting V/Q Scan Results in This Context

The presence of atelectasis does not invalidate V/Q scanning, but interpretation requires careful correlation: 3

• Normal perfusion scan: PE is excluded; atelectasis explains the clinical presentation 1
• High-probability scan: PE is confirmed; atelectasis may be secondary to infarction or coexist independently 1
• Non-diagnostic scan with low clinical probability: Combine with negative proximal CUS to exclude PE 1
• Non-diagnostic scan with intermediate/high clinical probability: Consider CT pulmonary angiography if technically feasible, or pulmonary angiography if CT remains non-diagnostic 1

Common Pitfalls to Avoid

Do not assume atelectasis excludes PE—pulmonary infarction occurs in approximately 30% of acute PE cases and can present as peripheral consolidation that may be mistaken for atelectasis or pneumonia 6, 7

Do not rely solely on CT imaging when lung volumes are low—the sensitivity of CT for PE is reduced in the setting of atelectasis and hypoventilation, making V/Q scanning the superior modality in this scenario 2

Do not delay anticoagulation if clinical suspicion remains high—failure to follow evidence-based diagnostic strategies when withholding anticoagulation despite clinical suspicion is associated with increased VTE episodes and sudden death 4

Recognizing Pulmonary Infarction

If PE is confirmed, look for radiologic features of pulmonary infarction: 6, 7

• Peripheral, pleural-based consolidations with convex margins toward the hilum (not the classic "wedge" shape) 6
• Focal areas of hyperlucency within the consolidation on CT 6
• Clinical presentation includes pleuritic chest pain (most common), with or without hemoptysis 6, 7
• Younger age, taller height, and active smoking are independent predictors of infarction in acute PE 6