Oxygenation Index
Calculate the oxygenation index (OI) using FiO₂, MAP, and PaO₂ to assess respiratory function in ICU. Includes interpretation, P/F ratio, and ARDS staging.
About This Calculator
The oxygenation index (OI) is a critical clinical metric used in intensive care units to evaluate the severity of respiratory failure in mechanically ventilated patients. It combines three key physiological parameters — the fraction of inspired oxygen (FiO₂), mean airway pressure (MAP), and partial pressure of arterial oxygen (PaO₂) — into a single numerical value that reflects the lung's ability to oxygenate blood relative to the ventilatory support being provided.
The OI is calculated using the formula: OI = (FiO₂ × MAP × 100) / PaO₂. In this formula, FiO₂ is the concentration of oxygen in the inspired air (expressed as a decimal, e.g., 0.40 for 40%), MAP is the mean pressure delivered by the mechanical ventilator measured in cmH₂O, and PaO₂ is the partial pressure of oxygen in arterial blood measured in mmHg. A higher OI indicates greater respiratory impairment and the need for more aggressive therapeutic interventions.
The OI is widely used in neonatal and pediatric intensive care, where it was originally developed, and has since been validated in adult populations as a predictor of mortality and a guide for ECMO (extracorporeal membrane oxygenation) candidacy. An OI exceeding 25 signals increased mortality risk, while values above 40 are a common threshold for considering ECMO support. The PaO₂/FiO₂ (P/F) ratio, which can also be derived from the same inputs, is used for staging acute respiratory distress syndrome (ARDS) according to the Berlin Definition.
Frequently Asked Questions
What is the oxygenation index (OI)?
The oxygenation index (OI) is a clinical metric used in intensive care to assess the severity of respiratory failure. It combines three parameters: the fraction of inspired oxygen (FiO₂), mean airway pressure (MAP), and partial pressure of arterial oxygen (PaO₂). The formula is OI = (FiO₂ × MAP × 100) / PaO₂, where FiO₂ is expressed as a decimal.
How is the oxygenation index calculated?
The oxygenation index is calculated using the formula OI = (FiO₂ × MAP × 100) / PaO₂. You need three values: FiO₂ entered as a percentage (e.g., 40 for 40% oxygen), mean airway pressure in cmH₂O measured from the ventilator, and PaO₂ in mmHg from an arterial blood gas test. Higher OI values indicate worse lung function.
What do the oxygenation index results mean?
OI values are interpreted as follows: OI under 5 indicates healthy lung function, OI 5-25 suggests mild to moderate lung disease, OI 25-40 indicates increased mortality risk with severe impairment, and OI over 40 is critical and may warrant ECMO (extracorporeal membrane oxygenation) therapy.
What is a normal oxygenation index?
A normal oxygenation index is less than 5, which corresponds to healthy lung function. Values between 5 and 25 indicate some degree of lung disease or impairment. The OI is primarily used in intubated and mechanically ventilated patients, so a truly normal value is typically seen in patients with healthy lungs.
What is the PaO₂/FiO₂ ratio?
The PaO₂/FiO₂ (P/F) ratio is another measure of lung function calculated by dividing PaO₂ by FiO₂ (expressed as a decimal). It is used to assess acute respiratory distress syndrome (ARDS) severity: P/F over 300 is normal, 200-300 indicates mild ARDS, 100-200 indicates moderate ARDS, and under 100 indicates severe ARDS.
How is OI used in ARDS assessment?
The oxygenation index helps assess the severity of acute respiratory distress syndrome (ARDS) and guides treatment decisions. It is particularly useful in evaluating the need for advanced therapies like ECMO. The P/F ratio, calculated alongside OI, is used for ARDS staging according to the Berlin Definition.
What is ECMO and when is it considered?
ECMO (extracorporeal membrane oxygenation) is a life support technique that temporarily replaces heart and lung function by circulating blood through an artificial lung outside the body. It is considered when the oxygenation index exceeds 40, indicating critical respiratory failure that cannot be managed with conventional mechanical ventilation alone.
Is this calculator a substitute for professional medical advice?
No. This calculator is for educational and reference purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional for interpretation of clinical measurements and treatment decisions.