ABG in ARDS typically shows which pattern?

• A. Metabolic acidosis with high CO2
• B. Refractory hypoxemia
• C. Respiratory alkalosis
• D. Normal ABG

Answer: (B)

In ARDS the gas exchange problem is driven by alveolar flooding and capillary damage, which creates a significant intrapulmonary shunt. The result is marked hypoxemia that does not improve with higher levels of inspired oxygen. On an arterial blood gas you’ll see a low PaO2 with a reduced PaO2/FiO2 ratio, and that hypoxemia tends to be refractory to increasing FiO2—meaning even with high oxygen concentrations the oxygen level in the blood remains poor. This persistent hypoxemia is the hallmark pattern linked to ARDS.

Early in ARDS, patient breathing may be rapid, so PaCO2 can be normal or low (leading to a respiratory alkalosis), but as the condition progresses and work of breathing increases or fatigue sets in, CO2 retention can occur. Still, the defining ABG feature for ARDS is that severe hypoxemia is not readily corrected by supplemental oxygen.