CAUSES OF HIGH AaDO₂ (continued)

B. Shunt

1. Define: Right-to-left shunt is systemic venous blood that does not exchange with alveolar gas but which contributes to systemic arterial blood; it represents a functional bypass of the lungs

Systemic arterial blood = Pulmonary capillary blood + Shunt blood
                                         [equilibrated, normal] [low P_O2 & high P_CO2]

2. Shunt is sometimes termed "venous admixture"

3. Level of the shunt  (see figure =>)

a. lungs (e.g., atelectasis with maintained circulation)
b. heart (e.g., intracardiac septal defect)
c. peripheral circulation (e.g., congenital anomaly of the circulation or patent ductus arteriosis)

4. Only right-to-left shunts cause hypoxia (but left-to-right shunts reduce systemic perfusion and increase cardiac work)

5. "Normal" value ≈ 1-4%