Table 3.

Mechanisms and features of hypoxemia

Mechanism	PaO2	PaCO2	DA-aO2	Comments
Disorders in oxygen diffusion	↓	↓	↑	Decreased surface area or short time for hematosis (e.g., hydrostatic edema, interstitial pneumonia, drug-associated interstitial lung disease, high-degree metastasized lungs)
Ventilation/ perfusion mismatch	↓	↑	↑	(1) Decreased ventilation in normally perfused lung regions (e.g., pulmonary infiltrates, pneumonia, atelectasis); and (2) Declined perfusion in normally ventilated lung areas (e.g., pulmonary embolism)
Increased intrapulmonary shunt	↓	↓	↑↑	Pulmonary venous blood bypasses ventilated alveoli without be oxygenated (e.g., acute respiratory distress syndrome)
Hypoventilation	↓	↑↑	N	Hypoventilation
Decrease in pressure of inspired oxygen	↓	↓	N	Decreased pressure of inspired oxygen