Causes of hypoxaemia in ARDS, response to oxygen supplementation and differentiation from other diseases

Cause of hypoxaemiaPA–aO2PaO2 response to increased FIO2Typical pathological condition
Global pulmonary limitations
 Diffusion limitation (decreased exchange area, increased diffusion distance)IncreasedImprovedInterstitial lung diseases; aggravation during exercise (low PvO2 and short erythrocyte transit time)
 Global hypoventilationNormalImprovedMuscular diseases, ventilatory failure
 Decreased PIO2NormalImprovedHigh altitude
Local pulmonary limitations
 Low V/QIncreasedImprovedCOPD, ARDS, perfusion redistribution from areas with high V/Q (e.g. pulmonary embolism)
 Shunt (V/Q=0)IncreasedMinimal improvementAtelectasis; aggravation by low PvO2 (e.g. low CO)

PA–aO2: alveolar–arterial O2 tension difference; PaO2: arterial partial pressure of O2; FIO2: inspiratory O2 fraction; PvO2: mixed venous partial pressure of O2; PIO2: partial pressure of O2 in inspired gas; V/Q: ratio of alveolar ventilation to perfusion; COPD: chronic obstructive pulmonary disease; ARDS: acute respiratory distress syndrome; CO: cardiac output. Adapted from [7].