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
- e.g., bronchial blood flow (bronchial veins empty into left atrium)
- approximately 2% of cardiac output normally bypasses alveoli
- 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
- admixture of venous blood and arterial blood in left heart
- shunting of blood from right heart to left heart
- 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
- admixture of venous blood and arterial blood in right heart
- shunting of blood from left heart to right heart