eMedicine Specialties > Pediatrics: Cardiac Disease and Critical Care Medicine > Cardiology

Arteriovenous Fistulae, Pulmonary: Differential Diagnoses & Workup

Author: Sat Sharma, MD, FRCPC, Professor and Head, Division of Pulmonary Medicine, Department of Internal Medicine, University of Manitoba; Site Director, Respiratory Medicine, St. Boniface General Hospital
Contributor Information and Disclosures

Updated: Apr 28, 2009

Differential Diagnoses

Other Problems to Be Considered

Consider the diagnosis of pulmonary arteriovenous malformations (PAVM) in individuals with any of the following presentations: (1) 1 or more pulmonary nodules associated with typical chest radiographic findings of pulmonary arteriovenous malformations; (2) mucocutaneous telangiectases; (3) unexpected findings such as dyspnea, hemoptysis, hypoxemia, polycythemia, clubbing, cyanosis, cerebral embolism, or brain abscess.

Workup

Laboratory Studies

• Patients may have anemia because of the ongoing blood loss caused by arteriovenous malformations (AVMs) in the GI tract.
• However, hypoxemic patients without GI bleeding is usually present with polycythemia.

Imaging Studies

• Chest radiography (see Media files 2-3, Media files 7-8)
  
  

  

  Left lower lobe arteriovenous malformation (AVM).

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  Left lower lobe arteriovenous malformation (AVM).

  
  

  

  Lateral radiograph showing a left lower lobe arteriovenous malformation (AVM).

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  Lateral radiograph showing a left lower lobe arteriovenous malformation (AVM).

  
  

  

  Small arteriovenous malformations (AVMs) in the right and left lower lobes.

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  Small arteriovenous malformations (AVMs) in the right and left lower lobes.

  
  

  

  Lateral radiograph shows a left lower lobe arteriovenous malformation (AVM).

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  Lateral radiograph shows a left lower lobe arteriovenous malformation (AVM).

  
  
  • Chest radiographs reveal some abnormality in approximately 98% of patients. The classic abnormal radiographic finding is a round or oval mass of uniform opacity. The mass is frequently lobulated and most commonly appears in the lower lobes. A chest radiograph can reveal features that may be undetectable on plain chest radiographs; examples include a feeding vessel, an artery radiating from the hilus, and the vein deviating toward the left atrium.
  • In a patient who has clinical features suggestive of pulmonary arteriovenous malformation but normal chest radiographic findings, further evaluation with other modalities should be performed. Patients with microscopic pulmonary arteriovenous malformations may have normal chest radiographic findings. Pulmonary arteriovenous malformations should also be considered in the differential diagnosis of a pulmonary nodule. A cautious approach to these patients is suggested before diagnostic needle biopsy is undertaken.
• Contrast-enhanced CT scanning (Media files 4-5, Media files 9-12)
  
  

  

  Large left lower lobe arteriovenous malformation (AVM) showing a feeding vessel to the left atrium.

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  Large left lower lobe arteriovenous malformation (AVM) showing a feeding vessel to the left atrium.

  
  

  

  Another view of the infused CT scan of the left lower lobe arteriovenous malformation (AVM) shown in Media file 4.

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  Another view of the infused CT scan of the left lower lobe arteriovenous malformation (AVM) shown in Media file 4.

  
  

  

  Contrast-enhanced CT scan showing a left lower lobe arteriovenous malformation (AVM).

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  Contrast-enhanced CT scan showing a left lower lobe arteriovenous malformation (AVM).

  
  

  

  Right lower lobe arteriovenous malformation (AVM).

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  Right lower lobe arteriovenous malformation (AVM).

  
  

  

  CT scan obtained after coil embolotherapy.

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  CT scan obtained after coil embolotherapy.

  
  

  

  Left lower lobe embolotherapy performed at the same sitting as the coil embolotherapy depicted in Media file 11.

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  Left lower lobe embolotherapy performed at the same sitting as the coil embolotherapy depicted in Media file 11.

  
  
  • The presence of a pulmonary arteriovenous malformation and its vascular anatomy can also be evaluated by means of contrast-enhanced ultra-fast CT. CT allows for the detection of 90% of pulmonary arteriovenous malformations, whereas, in one study, angiography allowed for the detection of only 60% of pulmonary arteriovenous malformations. The superior sensitivity of CT is attributed to the absence of superimposition of lesions on CT views.
  • Three-dimensional (3D) helical CT scanning produces images of vascular structures that are continuously reconstructed by a helical CT scanner. The accuracy of 3D helical CT scanning is reported to be 95%.
• Tests for confirmation of an intrapulmonary right-to-left shunt
  • These tests should be performed initially. The shunt is best calculated by using the 100% oxygen method. Contrast echocardiography and radionuclide scanning have nearly 100% sensitivity and are used to confirm clinically significant pulmonary arteriovenous malformations. However, the 100% oxygen method for shunt calculation is the least expensive and readily available.
  • In patients who have a shunt fraction of more than 5%, as determined with the 100% oxygen method, further assessment and management is recommended. In some patients with a shunt fraction of less than 5% but a high clinical suspicion for pulmonary arteriovenous malformations, additional evaluation with contrast echocardiography or radionuclide scanning is recommended.
• Contrast echocardiography
  • Contrast echocardiography is an excellent tool for evaluating cardiac or intrapulmonary shunts. This technique involves the injection of 5-10 mL of agitated saline into a peripheral vein while simultaneously imaging the right and left atria with 2-dimensional echocardiography. In patients without right-to-left shunting, contrast is rapidly visualized in the right atrium and then gradually dissipates. In patients with intracardiac shunts, contrast is visualized in the left heart chambers within 1 cardiac cycle, after its appearance in the right atrium. In patients with pulmonary arteriovenous malformations, contrast is visualized in the left atrium after a delay of 3-8 cardiac cycles. Contrast echocardiography is almost 100% sensitive in detecting clinically important pulmonary arteriovenous malformations.
  • In one case series, pulmonary arteriovenous malformations were visible in 11 of 14 patients with positive contrast echocardiographic findings who underwent pulmonary angiography. Six had abnormal chest radiographic results, and 8 had an increased A-a gradient. Contrast echocardiography had 100% sensitivity in this study. The finding of an intrapulmonary shunt by means of contrast echocardiography warrants further evaluation with standard pulmonary angiography or contrast-enhanced CT scanning.
• Radionuclide perfusion lung scanning
  • Radionuclide perfusion lung scanning is also useful in the diagnosis of pulmonary arteriovenous malformations, particularly if contrast echocardiography is not available.
  • In patients without an intrapulmonary shunt, the peripheral intravenous injection of technetium 99m–labeled macroaggregated albumin results in the filtering of these particles by the lung capillaries. However, anatomic shunts with dilated pulmonary vascular channels allow these particles to pass through the lung, with subsequent filtering by the capillaries in the brain and kidneys.
• Pulmonary angiography (see Media file 6)⁸
  
  

  

  Pulmonary angiographic findings are required not only to confirm the diagnosis but also to plan therapeutic embolization.

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  Pulmonary angiographic findings are required not only to confirm the diagnosis but also to plan therapeutic embolization.

  
  
  • Despite advances in noninvasive diagnostic techniques, contrast-enhanced pulmonary angiography remains the criterion standard in the diagnosis of pulmonary arteriovenous malformations. This test is usually necessary if embolotherapy is being considered. Perform pulmonary angiography in all lobes of the lungs to look for unsuspected pulmonary arteriovenous malformations.
  • Currently, digital subtraction angiography appears to be replacing conventional angiography. Whether CT or MRI can replace standard pulmonary angiography in the diagnosis of pulmonary arteriovenous malformations requires further comparative studies. Presently, CT and MRI are appropriate noninvasive modalities for the follow-up evaluation of patients with proven pulmonary arteriovenous malformations.
• MRI: MRI has been reported to be useful in the diagnosis of pulmonary arteriovenous malformations. Rapidly flowing blood results in an absent or minimal MR signal, a so-called flow void. However, pulmonary arteriovenous malformations may be indistinguishable from adjacent air-filled lungs on MRI, a significant limitation in screening for small lesions. Therefore, spin-echo MRI has reduced sensitivity and specificity for detection of pulmonary arteriovenous malformations, compared with those of other techniques. Better results are obtained with phase-contrast cine sequences, and MR angiography can be used to define the vascular anatomy of a pulmonary arteriovenous malformations. A combination of MR techniques may be useful in differentiating pulmonary arteriovenous malformations from various other lesions, but more comparative data are required before the routine use of MRI is recommended.

Other Tests

• Pulmonary function tests: Oxygenation is commonly affected in individuals with PAVM. Most patients have saturation levels of less than 90% at rest. Orthodeoxia is a decrease in PaO₂ or SaO₂ that occurs when one assumes an upright position from the supine position. Patients with this finding have normal spirometric findings and a mildly reduced diffusing capacity. Recent case series have indicated that 80-100% of patients with pulmonary arteriovenous malformations have either a PaO₂ of less than 80 mm Hg or an SaO₂ of less than 98% on room air.
• Shunt fraction measurement: The shunt fraction is most accurately assessed by using the 100% oxygen method, which involves the measurement of PaO₂ and SaO₂ after the patient breathes 100% oxygen for 15-20 minutes. The fraction of cardiac output that shunts right-to-left circulation is elevated in patients with pulmonary arteriovenous malformations; normal values are less than 5%. A shunt fraction of more than 5%, as determined by using the 100% oxygen method, has a sensitivity of 87.5% and a specificity of 71.4%.
• Exercise testing: Patients with pulmonary arteriovenous malformations have reduced exercise tolerance. In most patients, incremental exercise testing results in decreased saturation. One case series of patients showed that the average maximum oxygen consumption was 61% of the predicted value; saturation decreased from 86% at rest to 73% with peak exercise.

Procedures

• Right heart catheterization
  • Most patients with pulmonary arteriovenous malformations have normal or low pulmonary arterial pressure. Despite severe oxygen desaturation, the mean pulmonary arterial pressure is low in most patients.
  • Their cardiac output is generally normal to moderately elevated.
  • Patients may develop new pulmonary hypertension or increased baseline pulmonary hypertension after embolization or resection of a large pulmonary arteriovenous malformation.
• The radionuclide method of shunt calculation is expensive and not routinely available at most hospitals; however, it has several advantages compared with the 100% oxygen method.
  • ABG sampling is not needed.
  • The 100% oxygen method may overestimate intrapulmonary shunt.
  • The radionuclide method is more suitable for shunt measurement during exercise.

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