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
Images:
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.
Radiopharmaceutical:
Tc-99m DTPA aerosol by inhalation and Tc-99m MAA i.v.
Findings:
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.
Discussion:
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.
Followup:
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
Copyright by Wash U MO
