Shunt Vascularity

Shunt vascularity refers to increased number and size of sharply defined pulmonary vessels (in contrast to blurred vessels seen in pulmonary edema) in response to diversion of blood to the lungs. There is no or minimal tapering of these enlarged vessels, especially in the upper lungs (in contrast to the pruning seen in pulmonary hypertension).

The most common causes of shunt vascularity are:

• Left-to-right: Acyanotic.
  • Atrial septal defect: The left atrium is not enlarged. No significant cardiomegaly unless a complicated atrial septal defect exists.
  • Partial anomalous pulmonary venous return: Some pulmonary veins drain into the right atrium or systemic veins instead of the left atrium. The left atrium is not enlarged, but cardiomegaly may occur if the left-to-right shunt is significant.
  • Ventricular septal defect: Cardiomegaly with an enlarged left atrium and main pulmonary artery.
  • Patent ductus arteriosus: Enlarged left atrium. May see a prominent aortic knob.
  • Endocardial cushion defect: Strong association with Trisomy 21. The heart may be enlarged.
  
  Simplistic view for boards purposes:
  
• Mixed: Cyanotic
  • Total anomalous pulmonary venous return: Normal-sized heart.
  • Complete transposition of the great vessels: Egg-on-a-string. Classically described as thymic atrophy and an enlarged heart with an oval-shape and an upturned apex. The position of the aorta and pulmonary trunk results in a narrow superior mediastinum. Together, the upturned heart and narrow mediastinum result in the egg-on-a-string appearance.
  • Persistent truncus arteriosus: A single-vessel carries outflow from the heart, overrides the ventricular septum, and supplies the systemic, pulmonary, and coronary circulation. The heart is enlarged and there is frequently a right-sided aortic arch.
  • Hypoplastic left heart: Oxygenated blood passes to the right side via an atrial septal defect, and mixed blood travels to the aorta via a patent ductus arteriosus. Typically will see an enlarged heart with a prominent right atrium.
  • Single ventricle: Double outlet right ventricle and double outlet left ventricle are conceptually similar.
• High-flow state:
  • Arteriovenous malformation: Vein of Galen malformation, for example.
  • Anemia:
  • Thyrotoxicosis:

References

• Bardo DM, Frankel DG, Applegate KE, Murphy DJ, Saneto RP. Hypoplastic left heart syndrome. Radiographics. 2001 May-Jun;21(3):705-17.
• Bardo DM, Frankel DG, Applegate KE, Murphy DJ, Saneto RP. Hypoplastic left heart syndrome. Radiographics. 2001 May-Jun;21(3):705-17.
• Daffner RH. Clinical radiology: the essentials. Lippincott Williams & Wilkins, 2007.