Oxygenation Index

About the Oxygenation Index

The oxygenation index assesses the intensity of ventilatory support that is needed to maintain adequate oxygenation. The formula is:

Oxygenation index = (FiO2 x PAW) / PaO2

Where:

■ Fraction of inspired oxygen FiO₂ in %;

■ Mean airway pressure P_AW in mmHg;

■ Partial pressure of oxygen PaO₂ in mmHg;

The FiO₂ represents the amount of oxygen present in the air that is being breathed in. In intensive care units, when the patient is receiving oxygen, this can be finely regulated artificially. The oxygenation process greatly depends on the amount of oxygen inhaled and the physical integrity of the lungs.

Oxygenation index interpretation

OI <25 Good outcome

OI 25-40 >40% Mortality

OI >40 Consider ECMO

The lower the OI the better the outcome: as the oxygenation of the patient improves, they can achieve a higher partial pressure of oxygen (PaO₂) at a lower fraction of inspired oxygen (FiO₂).

OI is used in pediatric practice to help determine need for extracorporeal membrane oxygenation (ECMO) in patients with acute lung injury and acute respiratory distress syndrome. The threshold for a neonate to be placed on ECMO is set at OI >40.

ECMO is a means of safely supporting respiration and circulation in newborns with severe respiratory failure and a moribund clinical presentation, with a survival rate of 83%.

The severity of oxygenation failure at any point in time during acute hypoxemic respiratory failure was found to correlate with the duration of mechanical ventilation and mortality. The study by Trachsel et al. found that this was reflected by the OI, which showed a direct correlation to outcome in a time-independent manner.

The OI gives the clinician another means to assess ventilation and predict future outcomes and performance of the pediatric patient.

Other measures of lung function

The PaO₂ / FiO₂ is the ratio between the fraction of inspired oxygen and the partial pressure of the oxygen. It is also known as the Horowitz Index and is another measure of lung function for intubated patients.

The lower the ratio, the greater the degree of respiratory distress. This ratio is often used in the assessment of acute respiratory distress syndrome (ARDS), which is a condition with high morbidity and mortality risk.

References

Original reference

Trachsel D, McCrindle BW, Nakagawa S, Bohn D. Oxygenation index predicts outcome in children with acute hypoxemic respiratory failure. Am J Respir Crit Care Med. 2005; 172(2):206-11. Epub 2005 Apr 7.

Other references

Horovitz JH, Carrico CJ, Shires GT. Pulmonary response to major injury. Arch Surg. 1974; 108(3):349-55.

Mathur M, Bullock D. Oxygenation index as a predictor of outcome in children with acute hypoxemic respiratory failure. Am J Respir Crit Care Med. 2006; 173(6):683; author reply 683.

Hammond BG, Garcia-Filion P, Kang P, Rao MY, Willis BC, Dalton HJ. Identifying an Oxygenation Index Threshold for Increased Mortality in Acute Respiratory Failure.

Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012; 307(23):2526-33.

Brudno DS, Boedy RF, Kanto WP Jr. Compliance, alveolar-arterial oxygen difference, and oxygenation index changes in patients managed with extracorporeal membrane oxygenation. Pediatr Pulmonol. 1990; 9(1):19-23.