CAUSES OF HIGH AaDO₂ (continued)

C. Non-uniform Ventilation/Perfusion Ratio (V'_A/Q mismatch)

1. Define: high alveolar ventilation relative to pulmonary capillary perfusion in some lung areas and low alveolar ventilation relative to pulmonary perfusion in other lung regions, or mismatch of V'_A and Q

Examples: uneven lung tissue compliance, pulmonary embolism, or unilateral pulmonary artery stenosis

2. Result: reduced P_aO2 and high AaΔO₂, because the under ventilated regions do not adequately saturate the blood passing through them (local hypoventilation) while the overly ventilated regions do not appreciably increase the oxygen content (above normal) of the blood passing through them, due to the shape of the Hb-O₂ dissociation curve