Case Author(s): Scott Winner, M.D. and Henry Royal, M.D. , 12/06/96 . Rating: #D3, #Q4

Diagnosis: Intrapulmonary right to left shunt

Brief history:

75-year old man with increasing shortness of breath and dyspnea.

Images:

Pulmonary Perfusion Images

View main image(vq) in a separate image viewer

View second image(vq). Pulmonary Ventilation Images

View third image(xr). Chest radiograph

Full history/Diagnosis is available below

Diagnosis: Intrapulmonary right to left shunt

Full history:

75-year old man with chronic renal insufficiency and congestive heart failure with increasing shortness of breath and dyspnea. A comparison chest radiograph dated 12-2-96 demonstrates an infiltrate in the left mid lung consistent with pneumonia.

Radiopharmaceutical:

12.9 mCi Xe-133 gas by inhalation and 4.2 mCi Tc-99m MAA i.v.

Findings:

The Xe-133 ventilation images show markedly reduced ventilation to the left lung. The perfusion images show only minimally reduced perfusion to the left lung. Quantitative analysis demonstrates that the right lung receives 59% and the left lung receives 41% of total pulmonary blood flow. The right lung contributes 84% and the left lung contributes 16% of total pulmonary ventilation. The above subjective and quantitative findings are consistent with a significant intrapulmonary right-to-left physiologic shunt involving the left lung.

Discussion:

In patients with pneumonia, lung scintigraphy commonly demonstrates matched ventilation and perfusion abnormalities. Regional alveolar hypoxia in areas of infected lung usually results in reflex hypoxic pulmonary vasoconstriction. This effectively shunts blood to normal well ventilated areas of lung. This physiology results in matched ventilation/perfusion defects. Less commonly in patients with pneumonia, a reverse mismatch is present (perfusion is maintained and ventilation is reduced or absent). This is thought to be related to local release of inflammatory mediators with vasodilatory properties which overcome the hypoxic reflex vasoconstriction. The result is an intrapulmonary right to left physiologic shunt which can render the patient hypoxic and short of breath. Pulmonary emboli do not present with reverse mismatches.

References: Li et al. Scintigraphic Appearance in Patients With Pulmonary Infection and Lung Scintigrams of Intermediate or Low Probability for Pulmonary Embolism. Clin Nucl Med 19:1091-1093, 1994.

Followup:

None.

Major teaching point(s):

None.

Differential Diagnosis List

Reverse mismatch can be caused by pneumonia, bronchial obstruction, collapse, pleural effusion, metabolic alkalosis and pulmonary hypertension.

ACR Codes and Keywords:

References and General Discussion of Ventilation Perfusion Scintigraphy (Anatomic field:Lung, Mediastinum, and Pleura, Category:Inflammation,Infection)

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Case number: vq022

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