Case Author(s): Eric Hutchins, M.D. and Keith Fischer, M.D. , 9/10/04 . Rating: #D2, #Q4
Diagnosis: Atelectasis of the left lower lobe
Brief history:
73 year-old man with increasing shortness of breath and hypoxia. He is POD six from a CABG procedure.
Images:
Ventilation and perfusion scintigraphy
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PA chest radiograph
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Lateral chest radiograph
Full history/Diagnosis is available below
Diagnosis: Atelectasis of the left lower lobe
Full history:
79 year old male, post operative day six after coronary artery bypass graft procedure, with increasing hypoxia. Ventilation/perfusion scintigraphy requested to evaluate for pulmonary embolus.
Radiopharmaceutical:
20.7 mCi Xe-133 gas by inhalation and 4.26 mCi Tc-99m MAA i.v.
Findings:
Ventilation scintigraphy demonstrates minimal ventilation of the left lower lobe and delayed ventilation images show air trapping at the right lung base. Perfusion images are mildly patchy showing nonsegmental perfusion defects. No pleural-based, large subsegmental or segmental perfusion defects are seen. Blunting of the bilateral costophrenic angles is present, consistent with the patient's known pleural effusions. Perfusion of the left lower lobe is maintained in the area of near absent ventilation, indicating the presence of a functional right to left shunt. Chest radiograph performed the same day demonstrates bilateral pleural effusions and a retrocardiac density suggesting left lower lobe atelectasis or infiltrate.
Discussion:
Hypoxia is a common indication for ventilation/perfusion scintigraphy. Although the ordering physician is usually looking for evidence of pulmonary embolism, it is common to find a functional shunt causing hypoxia. A "reverse ventilation-perfusion mismatch" can be found in up to 43% of mechanically ventilated patients and represents a failure of the noraml vasoconstriction of pulmonary arteries in non-ventilated lung. Causes of a physiologic (i.e. functional) shunt include atelectasis, mucus plug, pneumonia, and obstructive lung disease.
Reference:
Miller, T.R. and Matthews, J.D. "Reverse Ventilation-Perfusion Mismatch" in ACR Nuclear Radiology Test and Syllabus #44 (Fifth Series) 1998.
ACR Codes and Keywords:
References and General Discussion of Ventilation Perfusion Scintigraphy (Anatomic field:Lung, Mediastinum, and Pleura, Category:Effect of Trauma)
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Case number: vq052
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