Table 1:

Mechanisms of hypoxemia

Mechanism of Hypoxemia	Clinical Characteristics	Examples
V/Q mismatch	Widened A-a gradient Improves with O2 Limited impact on PaCO2 unless >30% dead space fraction	Alveolar filling (pneumonia, pulmonary edema, pulmonary hemorrhage, malignancy, etc.) Atelectasis
Shunt	Widened A-a gradient Does not improve significantly with O2 When shunt fraction >50%, PaO2 is independent of FiO2	Intracardiac shunt (ASD, PFO) Intrapulmonary shunt (pulmonary AVM, hepatopulmonary syndrome) Massive PEAtelectasis, pneumonia, pulmonary edema, ARDS Central airway occlusion Severe small airways disease/asthma exacerbation
Diffusion limitation	Widened A-a gradient Improves with O2	Interstitial lung disease Emphysema Pulmonary vascular disease
Hypoventilation	Normal A-a gradient Improves with O2	Neuromuscular disease Chest wall disorders Central neurologic disorders Sedating medications Obstructive lung disease
Low inspired FiO2	Normal A-a gradient Improves with O2	Altitude Air travel
Oxygen delivery-consumption imbalance	Widened A-a gradient	Anemia Hypermetabolic states Low cardiac output