There are four distinct mechanisms by which hypoxaemia occurs, and understanding them is essential because they have different responses to oxygen therapy and different treatment implications.

1. Hypoventilation

When alveolar ventilation falls, CO2 accumulates in the alveoli and displaces oxygen molecules. This relationship is described by the alveolar gas equation: PAO2 = PiO2 − (PaCO2 / R), where PiO2 is the inspired oxygen partial pressure and R is the respiratory quotient (approximately 0.8). Hypoventilation therefore causes hypoxaemia through CO2 accumulation rather than any intrinsic lung abnormality. Crucially, this type of hypoxaemia responds very well to supplemental oxygen — even a modest increase in FiO2 corrects the hypoxaemia because the underlying lung is normal. The CO2 retention, however, requires the underlying ventilatory problem to be addressed.

2. V/Q Mismatch

Ventilation-perfusion mismatch is the commonest cause of hypoxaemia in clinical practice and contributes to virtually all forms of respiratory failure to some degree. It occurs when some lung units receive more blood flow than ventilation (low V/Q — perfusion without adequate ventilation, functionally like shunt) and others receive more ventilation than blood flow (high V/Q — dead space). The net effect is that blood leaving poorly ventilated areas has low oxygen content, dragging down the overall arterial oxygen level. V/Q mismatch responds to supplemental oxygen because even in low V/Q units, increasing the FiO2 raises the alveolar oxygen tension enough to improve oxygenation of the blood flowing through.

3. Shunt

True shunt occurs when blood passes through the pulmonary circulation without coming into contact with ventilated alveoli at all — either through anatomical connections (intracardiac or intrapulmonary) or through completely collapsed or fluid-filled lung units. Because the shunted blood has no contact with oxygen regardless of how high the FiO2 is raised, shunt is characteristically resistant to oxygen therapy. Conditions causing shunt include massive pulmonary consolidation (e.g. severe pneumonia), pulmonary oedema (where alveoli are flooded with fluid), and atelectasis. This is why a patient with severe pneumonia may require FiO2 of 1.0 to maintain an SpO2 of 88%.

4. Diffusion Defect

A diffusion defect occurs when thickening of the alveolar-capillary membrane impairs the transfer of oxygen from alveolar gas into the blood. This is seen in conditions like pulmonary fibrosis and can contribute to hypoxaemia in ARDS. Diffusion defects rarely cause hypoxaemia at rest but typically manifest during exercise when the transit time of blood through the pulmonary capillary is shortened. In ICU practice, diffusion defects usually coexist with other mechanisms rather than acting alone.

CLINICAL PEARL The response to oxygen is diagnostically useful: hypoxaemia that corrects easily with low-flow oxygen suggests hypoventilation or mild V/Q mismatch; hypoxaemia that persists despite high-flow oxygen (FiO2 >0.6) suggests significant shunt and warrants urgent senior review and likely respiratory support.