Postoperative Fever and a Ventilation-Perfusion Scan in a Patient with COPD
A 55-year-old woman is 4 days post total knee arthroplasty and develops sudden dyspnea, pleuritic chest pain, and tachycardia to 118. She has a history of COPD (FEV1 55% predicted) and hypertension. Oxygen saturation is 90% on room air. D-dimer is elevated. She has a contrast allergy to iodinated contrast (urticaria and bronchospasm) and an eGFR of 50. The team requests a ventilation-perfusion (V/Q) scan rather than CT pulmonary angiography because of the contrast allergy and renal function. Chest radiograph shows hyperinflation but no infiltrate or effusion.
1. How should the V/Q scan be interpreted in a patient with COPD, and what are the pitfalls?
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COPD produces heterogeneous ventilation and matched perfusion defects from air trapping and parenchymal destruction, which lowers the specificity of V/Q scanning and can create intermediate-probability interpretations. The key is to look for mismatched defects: perfusion defects with normal or near-normal ventilation in a segmental or subsegmental distribution, which are more specific for pulmonary embolism. A normal perfusion scan effectively excludes PE. In COPD, a high-probability pattern (multiple mismatched segmental defects) remains meaningful, but intermediate patterns are common and should prompt further testing. Because the chest radiograph is clear, the study is interpretable, but the interpreter must compare ventilation and perfusion images carefully and avoid overcalling matched defects as PE. If the result is nondiagnostic, alternative imaging (e.g., MR angiography or catheter angiography) or empiric anticoagulation may be considered, weighing bleeding risk after recent arthroplasty.
2. The V/Q scan is reported as intermediate probability with a single subsegmental mismatched defect. What is your management, and how do you weigh the risks?
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An intermediate-probability scan with a single subsegmental mismatched defect is nondiagnostic and does not exclude PE. In a postoperative patient 4 days after knee arthroplasty with tachycardia, hypoxemia, and pleuritic pain, the pretest probability is high. Management options are empiric therapeutic anticoagulation while pursuing definitive imaging, or further imaging such as MR angiography or catheter pulmonary angiography if anticoagulation is deemed too risky. The bleeding risk from recent arthroplasty must be balanced against the mortality of untreated PE; if anticoagulation is started, the surgical team should be involved and the wound monitored. If PE is excluded, the dyspnea should be attributed to COPD exacerbation, atelectasis, or another cause and treated accordingly. The judgment is that a nondiagnostic V/Q in a high-pretest-probability patient should not be treated as negative.