Respiratory · Year 1 · from Respiratory
Case 1: Pulmonary Embolism with V/Q Mismatch
Clinical Image
Source: Wikimedia Commons - Pulmonary Embolism V/Q Scan - CC BY 2.5
Case Presentation
A 58-year-old woman presents to the emergency department with sudden onset of dyspnea and right-sided pleuritic chest pain that began 6 hours ago. She recently returned from a 14-hour flight from Australia. She has a history of hypertension and takes oral contraceptives. On examination, she appears anxious and mildly dyspneic. Vital signs show heart rate 108 bpm, blood pressure 135/85 mmHg, respiratory rate 24/min, temperature 37.4C, and oxygen saturation 91% on room air.
Arterial blood gas on room air shows pH 7.48, PaCO2 30 mmHg, PaO2 62 mmHg. The calculated A-a gradient is elevated at 38 mmHg (expected for age approximately 17 mmHg). D-dimer is elevated at 2,400 ng/mL. CT pulmonary angiography reveals filling defects in the right lower lobe segmental and subsegmental pulmonary arteries consistent with acute pulmonary embolism.
The V/Q abnormalities in PE explain the blood gas findings. The obstructed pulmonary arteries create regions of high V/Q (ventilated but not perfused - dead space). Blood is redirected to remaining perfused areas, potentially creating regions of low V/Q (relatively overperfused). The net effect is hypoxemia with an elevated A-a gradient. The hypocapnia and respiratory alkalosis reflect compensatory hyperventilation driven by hypoxemia and stimulation of J-receptors. Notably, the hypoxemia in PE typically responds well to supplemental oxygen because the primary problem is V/Q mismatch, not true shunt.
A V/Q scan (shown in the image) demonstrates the classic pattern of PE: segmental perfusion defects in areas with preserved ventilation (mismatched defects). This pattern has high specificity for PE. The patient is anticoagulated with direct oral anticoagulant therapy and recovers uneventfully.
Key Learning Points
- Pulmonary embolism creates high V/Q regions (dead space) by obstructing perfusion to ventilated lung
- Blood redistribution to non-obstructed regions creates low V/Q areas, causing hypoxemia
- The A-a gradient is elevated in PE, distinguishing it from hypoventilation
- Hypocapnia from hyperventilation is typical because the linear CO2 dissociation curve allows compensation
- V/Q scan shows mismatched defects (perfusion defect with preserved ventilation) characteristic of PE