what is oxygenation index what is the difference between (2024)

PaO2 is one of the indicators of tissue oxygenation. The ratio between PaO2 and FiO2 {PaO2, FiO2} is the most common index to evaluate oxygenation. PaO2, FiO2 ratio Less than 300 is indicative of Acute Lung Injury {ALI} and Less than 200 of Acute Respiratory Distress Syndrome {ARDS}.

The ratio between the arterial and alveolar oxygen pressure
{PaO2, PAO2} evaluates oxygenation and is age dependent. The normal value is about 0.75 and shows that, of the oxygen available in the alveolus, 75 -95 percent is getting into the pulmonary artery.

Oxygenation index {OI} is a more precise and advanced index to
evaluate oxygenation of patient. OI may be computed as follows {where MAP is mean airway pressure}: OI = {MAP x FiO2 x 100}, PaO2.

Hypoxemia can be also assessed by measuring the alveolar arterial oxygen pressure gradient {P{A-a}O2}. P{A-a}O2 is the difference of alveolar O2 {PAO2} and arterial O2 {PaO2}. The normal P{A-a}O2 at 21 percent FiO2 should be less than 4 mmHg for every 10 years of age. On FiO2 1, every 50 mmHg difference in P{A-a}O2 indicates a 2 percent shunt.

A normal P{A-a}O2 excluded lung parenchymal problems as cause of hypoxemia. A low PaO2 in presence of widened P{A-a}O2 difference is due to V, Q mismatching, shunt and diffusion. In such situations PaCO2 might be lower than normal indicating hyperventilation to compensate for hypoxemia.

Giuseppe Marraro
Italy - Pediatric Intensivist