MD Nexus

Pulmonary Arteriovenous Malformation (AVM)

Epidemiology

Etiology

Pathogenesis

Diagnosis

100% FIO2 Shunt Study: assess pO2 on 100% FIO2 to assess degree of shunt
-Actually measures “physiologic shunt”, which is equal to alveolar shunt (caused by V/Q mismatch) + anatomic shunt (caused by AVM)
-Measure arterial pO2 and hemoglobin after breathing 100% FIO2 x 15-20 min (deep breaths periodically to wash out nitrogen)
-Normal: 5% of cardiac output
-Calculate shunt as % of cardiac output (assuming O2 content of 5 mL per 100 mL): this calculation may underestimate the % shunt
-At threshold <535 mm Hg: 62% sensitivity/98% specificity
-Supine and Upright A-a Gradient on 100% FIO2: only 68% sensitive in detecting a shunt from an AVM

Exercise Study: high ventilatory drive during exercise (similar to patients with congential heart disease and right-to-left shunts) with preserved work capacity

Swan: PVR is usually normal (with no pulmonary HTN)

PFT’s:
-FEV1: normal
-VC: usually normal (but may be decreased in some patients with large shunt)
-DLCO: moderately decreased (71-78% pred) with large shunt (especially in those with widespread, small AVM’s), but usually normal
-DLCO/VA: usually better preserved

CXR/Chest CT Pattern: CXR is abnormal in 60-90% of cases (but AVM may be subtle)
-Shape: round or oval, slightly lobulated, well-defined nodule (range from 1 to several cm)
-Number (most AVM’s are multiple): single AVM’s may be due to sporadic etiology/multiple AVM’s are usually due to HHT
–72% of cases have AVM’s in both lungs
-Location: lower-lobe predilection
-Calcification: occasional (probably due to phleboliths)
-Cavitation: none
-Maneuvers: AVM may change in size with Valsalva or Mueller maneuvers
-Growth: AVM may increase in size during puberty, pregnancy, or in presence of pulmonary venous hypertension (due to mitral stenosis or LV dysfunction)

Helical CT: useful to demonstrate AVM
-Contrast is usually not required

Chest MRI: less sensitive than CT or pulmonary angio, as small AVM’s with rapid flow are not visualized

Q Scan/Shunt Study: uses 99mTc-MAA and lung/right kidney imaging to evaluate % of shunt fraction (actually measures “anatomic shunt” only), as macroaggregated albumin will escape into systemic circulation if shunt exists
-AVM may produce shunt fraction of up to 60% of cardiac output
-Shunt fraction increases with standing (probably because most AVM are in lower lobes and gravity increases lower lobe flow with standing)
-Shunt fraction increases with exercise in patients with smaller AVM’s and increases with exercise in patients with larger AVM’s (due to fact that larger AVM’s do not enlarge further with exercise and smaller AVM’s are more compliant)
-At threshold of >5%: false-positive rate is 28% (with sensitivity of only 68%)

Pulmonary Angiogram: may demonstrate macroscopic, microscopic, or smaller diffuse AVM’s
-Allows identification of feeder vessels and assessment of rest of lungs prior to surgery

Transthoracic Echo with Bubble Study: best screening test for shunt from an AVM
-Immediate appearance of bubbles in left side: indicates intracardiac shunt (such as PFO, etc.)
-Delayed appearance of bubble in left side (after at least 4 cardiac cycles): indicates intrapulmonary shunt
-Sensitivity: 93-100% (negative predictive value: 97%)
-Specificity: 21-67% (lower specificity since 27% of normals have a PFO and intrapulmonary shunting may occasionally be seen in 5% of normals)
-Negative TTE + Negative CXR: excludes AVM in 100% cases
-Positive TTE with Bubble Study + Negative Spiral CT: suggests that the AVM is below the detection limit for the spiral CT -> should screen these cases with spiral CT every 1- years to periodically assess for AVM’s with >3 mm feeder vessels

Transesophageal Echo (TEE) with Bubble Study: more sensitive to exclude intracardiac shunt
-Immediate appearance of bubbles in left side: indicates intracardiac shunt (such as PFO, etc.)
-Delayed appearance of bubble in left side (after at least 4 cardiac cycles): indicates intrapulmonary shunt

Brain MRI: indicated to screen for brain AVM’s (even in the absence of pulmonary AVM’s)
CBC: polycythemia

Clinical Manifestations

General Comments

Pulmonary Manifestations

Neurologic Manifestations

Other Manifestations

Prognosis

Treatment

Embolization of Arteriovenous Malformation

Resection of Arteriovenous Malformation