Case Author(s): Matt Jaksha, M.D. and Jerold Wallis, M.D. , . Rating: #D2, #Q4

Diagnosis: Functional Right to Left Shunting

Brief history:

Rule out pulmonary embolus


Portable aerosol ventilation images

View main image(vq) in a separate image viewer

View second image(vq). Portable perfusion images

View third image(xr). Chest radiograph 4 hours prior to VQ

Full history/Diagnosis is available below

Diagnosis: Functional Right to Left Shunting

Full history:

This is a 41 year old male with a history of subarachnoid hemorrhage and seizure approximately two weeks ago who developed tachycardia, tachypnea and hypoxia. A portable chest radiograph was obtained (shown). As the hypoxia continued, the patient was intubated and lung scintigraphy was ordered to evaluate for pulmonary embolism.


Tc-99m DTPA aerosol by inhalation and Tc-99m MAA i.v.


The aerosol ventilation images demonstrate markedly decreased ventilation of the left lung. The perfusion images are nearly normal in this region, with mild heterogeneity elsewhere. The chest radiograph shows marked volume loss on the left with retrocardiac opacity probably representing left lower lobe collapse.


A reverse mismatch, or decreased ventilation with normal perfusion, indicates functional right to left shunting. Blood passes through the pulmonary capillary bed without being oxygenated. This is frequently seen in intubated, intensive care unit patients, often the result of central mucous plugging. This can cause significant hypoxia. Certain measures can be taken in this situation. Putting the patient in the decubitus position with the mismatched side up (in this case left side up) will decrease blood flow to the area. If PEEP is being used to help ventilate the patient, this can exacerbate the problem by relatively decreasing flow to ventilated lung, while the poorly ventilated area receives a greater portion of the total pulmonary flow (since the PEEP pressures are transmitted slightly less efficiently to the obstructed region). If possible, positive end-expiratory pressure should be turned off in patients with evidence of functional right to left shunting. Finally, the primary problem should be directly addressed. Sometimes bronchoscopy is necessary to clear the mucous plug.

The chest radiograph had initially shown significant left sided atelectasis. After the patient was intubated, a repeat radiograph (not shown) showed good position of the endotracheal tube and persistence of the left sided volume loss. The patient had not improved and the lung scintigraphy was ordered.


Bronchoscopy was required to clear the obstruction to the patient's left lung. The chest radiograph approximately 12 hours after the VQ scan showed improvement in the left lower lobe. The patient was extubated one day later.

ACR Codes and Keywords:

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

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Case number: vq030

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