Pulmonary Arteriovenous Fistulae Workup
- Author: Barry A Love, MD; Chief Editor: Howard S Weber, MD, FSCAI more...
With chronic hypoxemia, the hemoglobin and hematocrit rise. The rise is roughly proportional to the degree of cyanosis.
However, because may patients with pulmonary arteriovenous malformations (AVMs) also have hereditary hemorrhagic telangiectasia (HHT), bleeding from epistaxis and GI telangiectasias may lead to anemia.
Pulse oximetry is a useful tool in initial screening for pulmonary arteriovenous malformations. Pulse oximetry should be performed in the supine and upright position. Oxygen saturations less than 95% are suggestive of either right-to-left shunting, or pulmonary disease. With significant pulmonary arteriovenous malformations, the oxygen saturation typically decreases in the upright position.
Echocardiogram is a useful tool for excluding other sources of intracardiac right-to-left shunt.
Echocardiogram with bubble contrast
A peripheral intravenous catheter is inserted and a solution of 8 mL of saline mixed with 1 mL of the patient's blood and 1 mL of air is agitated to produce microbubbles in the solution. The solution is then injected rapidly while the heart is imaged, preferably in a 4-chamber view. The microbubbles produce a bright echo reflection as they enter the echo field of view.
Normally, the right atrium and right ventricle are brightly opacified and then the microbubbles, which are less than 50 microns in diameter, are filtered out at the pulmonary capillary bed. and no bubbles are seen in the left atrium or left ventricle. In the presence of pulmonary arteriovenous malformations, the bubbles pass through the arteriovenous malformations and are seen in the left atrium and left ventricle.
Care needs to be taken not to inject any visible air, as this could lead to systemic air embolization. The classic teaching was that pulmonary arteriovenous malformations could be differentiated from intracardiac shunt by the timing of when bubbles first appear in the left atrium. The dogma is that bubbles appearing within one cardiac cycle are due to intracardiac shunt, whereas bubbles that appear within one cardiac cycles after first seen in the right atrium represent pulmonary arteriovenous malformations. Unfortunately, because of various factors, this does not appear to be the case.
Examples are shown in the images below.
Chest radiographs reveal some abnormality in many patients with large arteriovenous malformations. 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
Examples are shown in the images below.
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%.
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.
The finding of an intrapulmonary shunt by means of contrast echocardiography warrants further evaluation with standard pulmonary angiography or contrast-enhanced CT scanning.
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.
Similarly, a study by Karam et al indicated that transthoracic contrast echocardiography can be used to effectively screen pediatric patients with hereditary hemorrhagic telangiectasia (HHT) for pulmonary arteriovenous malformations. The report, which involved 92 children, found the sensitivity and specificity of this modality for the detection of these malformations to be 100% and 95.1%, respectively, with positive and negative predictive values of 96% and 100%, respectively.
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.
An example is shown in the image below.
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 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.
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%.
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. See the Oxygen Consumption calculator.
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. See the Cardiac Output calculator.
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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