= Acute respiratory distress syndrome
{title2=ARDS}
= ARDS
{c}
{synonym}
\b[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.<Acute Respiratory Distress Syndrome by NHLBI>[What Is ARDS?: alveolar fluid, surfactant, oxygen transfer and stiffening]{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>,
$$
\frac{P_{\mathrm a O_2}}{F_{\mathrm I O_2}},
$$
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.<Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome by the Acute Respiratory Distress Syndrome Network>[Abstract, Methods and Results, 2000]{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.
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