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Acute respiratory distress syndrome, or ARDS, is an acute form of respiratory failure in which widespread lung injury disrupts normal gas exchange. Fluid accumulates in the alveoli, and altered pulmonary surfactant function and tissue injury can impair expansion and reduce lung compliance. Hypoxemia describes the resulting deficiency of oxygen in arterial blood; it is an observation, not a complete explanation of its cause.^[ref]

The alveolar-capillary barrier normally separates the air-containing region from circulating blood while permitting gas transfer. Injury changes both that interface and the mechanical behavior of the lung. A pressure required to inflate a damaged lung and an oxygen measurement therefore describe different parts of the problem.

One quantitative descriptor is the oxygenation ratio,where the numerator is arterial oxygen partial pressure and the denominator is inspired oxygen fraction, written as a fraction rather than a percentage. For stipulated $P_{\mathrm{a}{O}_2}=80\mathrm{mmHg}$, an inspired fraction $0.40$ gives $200\mathrm{mmHg}$, whereas $0.80$ gives $100\mathrm{mmHg}$. The same arterial measurement can therefore reflect different gas-exchange conditions. These examples do not establish a diagnosis or severity class without the other required clinical observations and measurement conditions.

A landmark primary trial tested how mechanical ventilation strategy affected outcomes in acute lung injury and ARDS. The ARDS Network randomized 861 patients to lower- versus traditional-volume strategies, also differing in plateau-pressure limits. Reported mortality was $31.0\%$ versus $39.8\%$. The result showed that the mechanical conditions of supportive ventilation could affect outcomes, rather than treating delivered breath size as a neutral background variable.^[ref] These are historical trial observations, not a prescription for individual ventilator settings.

Mechanistic explanations should keep oxygen transfer, compliance and patient outcomes distinct. A measurement that improves one physiological quantity does not by itself prove improvement in every other endpoint.