Updated: 4/3/2018

Shunts

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Overview
  • A shunt refers to a portion of cardiac output or blood flow that is diverted or rerouted 
  • Physiologic shunt
    • approximately 2% of cardiac output normally bypasses alveoli
      • e.g., bronchial blood flow (bronchial veins empty into left atrium)
        • physiologic right-to-left shunt
  • Right-to-left shunt 
    • shunting of blood from right heart to left heart
      • e.g., interventricular septal defect (long-standing with Eisenmeiger syndrome)
      • pulmonary arteriovenous malformation
    • hypoxemia results because oxygen-poor blood mixes with oxygen-rich blood
      • admixture of venous blood and arterial blood in left heart
        • left heart normally receives high O2 (arterial) blood
        • low O2 shunted (venous) blood dilutes high O2 (arterial) blood ↓ PaO2
      • degree of hypoxemia depends on location of shunt and amount of shunted blood flow
      • cannot be corrected by O2 treatment because shunted blood never traverses pulmonary capillary to exchange gas with alveolus
  • Left-to-right shunt
    • shunting of blood from left heart to right heart
      • more common as pressures are higher in left heart
    • e.g., VSD in newborn, patent ductus arteriosus, and traumatic injury
    • does not cause hypoxemia
      • admixture of venous blood and arterial blood in right heart
        • right heart normally receives low O2 (venous) blood
        • high O2 shunted (arterial) blood adds to low O2 (venous ) blood → ↑ PaO2
        • "Step-up" in oxygen on right side on right heart catheterization

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