eTable 25-1.

Causes of Hypoxemia: The Effect on Alveolar-Arterial Po₂ Differences and Arterial Pco₂

Cause	Effect on Alveolar Po2	Effect on (A–a)Po2	Effect on Arterial Pco2
Normal lungs/inadequate oxygenation
Deficiency of oxygen in atmosphere	↓	↔	↓
Hypoventilation (neuromuscular disorder)	↓	↔	↑
Pulmonary disease
Hypoventilation (airway/parenchymal disorder)	↓	↔	↑
Diffusion abnormality	↓*†	↑*†	↓
Ventilation-perfusion imbalance	↓†	↑†	↓ ↔, or ↑
Right-to-left shunts	↓	↑	↓, ↔, or ↑
Inadequate transport/delivery of oxygen
Anemia	↔	↔	↔
General/localized circulatory insufficiency	↔		↔
Inadequate tissue oxygenation
Abnormal tissue demand/poisoned enzymes/edema	↔	↔	↔

↑, increased; ↔, no change; ↓, decreased.

^*Infrequently observed at rest but more likely during exercise.

^†Unless patient is hyperventilating.

Adapted from Comroe JH Jr, Forster RE II, DuBois AB, et al: Arterial blood oxygen, carbon dioxide and pH. In Comroe JH Jr, Forster RE II, DuBois AB, et al: The lung: clinical physiology and pulmonary function tests, ed 2, Chicago, 1962, Year Book, pp 140–161.