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