The arterial-to-alveolar ratio (a/A ratio)

a    ₌                   PaO₂

A            (713 x FiO₂) – PaCO₂

a = arterial oxygen tension

A = alveolar oxygen tension

713 = atmospheric pressure – water vapor pressure = (760 -47) mmHg

FiO₂ = fraction of inspired oxygen

PaO₂= partial pressure of arterial oxygen

PaCO₂ = partial pressure of arterial carbon dioxide

The a/A ratio decreases with increasing degree of illness.

An a/A ratio of < 0.2 in the setting of RDS in an indication for surfactant treatment.

The oxygenation index (OI)

OI =    FiO₂ x MAP    x 100

     PaO₂

FiO₂ = fraction of inspired oxygen

MAP = mean airway pressure

PaO₂= partial pressure of arterial oxygen

The OI increases with increasing degree of illness.

An OI of >40 despite optimal medical management is an indication for potential ECMO.

These equations can also be found in the 2001 NICU Manual pg. 3-4.