Progressive arterial hypoxemia, despite an increased fraction of inspired oxygen (FIO2) by mask, cannual, or endotracheal tube, is a hallmark of ARDS. This development is called refractory hypoxemia.

• 使用面罩、导管或气管内插管增加吸入氧气分数（FIO2）后仍出现进行性动脉血氧低，此为ARDS标志性证据，为难治性低氧血症期。