Oxygenation Index & OSI Calculator
Calculate Oxygenation Index when an ABG PaO₂ is available, or Oxygen Saturation Index when only SpO₂ is available. These values connect oxygenation severity to the amount of ventilator support being used.
Calculator
Calculate OI or OSI
Use OI when PaO₂ is available. Use OSI as a noninvasive estimate when SpO₂ is available instead.
Oxygenation Index
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OI
Enter FiO₂, MAP, and PaO₂ or SpO₂ to calculate.
Formula
OI and OSI Formulas
OI = (FiO₂ × MAP × 100) ÷ PaO₂
OSI = (FiO₂ × MAP × 100) ÷ SpO₂
OSI = (FiO₂ × MAP × 100) ÷ SpO₂
FiO₂ should be used as a decimal in the formula. This calculator accepts either decimal form, such as 0.80, or percent form, such as 80.
Worked Example
Oxygenation Index Example
FiO₂ = 0.80
MAP = 15 cmH₂O
PaO₂ = 60 mmHg
OI = (0.80 × 15 × 100) ÷ 60
OI = 20
MAP = 15 cmH₂O
PaO₂ = 60 mmHg
OI = (0.80 × 15 × 100) ÷ 60
OI = 20
RT Memory Trick
OI Gets Worse When Support Goes Up and PaO₂ Stays Low
Oxygenation Index “punishes” the patient for needing more support. If FiO₂ goes up, MAP goes up, or PaO₂ goes down, the OI becomes higher.
Remember:
High FiO₂ + high MAP + low PaO₂ = worse oxygenation index.
High FiO₂ + high MAP + low PaO₂ = worse oxygenation index.
Interpretation
How to Interpret OI and OSI
| Value | General Interpretation | Clinical Context |
|---|---|---|
| < 5 | Mild oxygenation impairment | Less severe support-adjusted oxygenation problem. |
| 5–15 | Moderate impairment | Requires closer trend monitoring and clinical context. |
| 15–25 | Significant impairment | Reflects substantial oxygenation difficulty and ventilator support. |
| > 25 | Severe oxygenation impairment | High severity; interpret with diagnosis, age group, MAP, FiO₂, PaO₂/SpO₂, and care pathway. |
| > 40 | Very severe range | Historically associated with ECMO consideration in some neonatal/pediatric contexts, depending on protocol and patient factors. |
Compare Oxygenation Metrics
How OI Compares to P/F Ratio and A-a Gradient
| Metric | Uses | What Makes It Different |
|---|---|---|
| P/F Ratio | Quick oxygenation severity assessment | Uses PaO₂ and FiO₂ but not ventilator pressure support. |
| A-a Gradient | Cause of hypoxemia reasoning | Helps separate hypoventilation from oxygen transfer problems. |
| OI | Oxygenation severity plus support level | Includes MAP, so it reflects how much ventilator pressure support is needed. |
| OSI | Noninvasive OI surrogate | Uses SpO₂ instead of PaO₂ when ABG is not available. |
Clinical Pearl
Same PaO₂, Very Different Severity
Patient A
PaO₂ 80
FiO₂ 0.40
MAP 10
OI = 5
PaO₂ 80
FiO₂ 0.40
MAP 10
OI = 5
Patient B
PaO₂ 80
FiO₂ 1.00
MAP 20
OI = 25
PaO₂ 80
FiO₂ 1.00
MAP 20
OI = 25
Same PaO₂
Both have PaO₂ of 80.
Both have PaO₂ of 80.
Different support
Patient B needs much more FiO₂ and MAP to achieve that PaO₂.
Patient B needs much more FiO₂ and MAP to achieve that PaO₂.
Common Student Mistakes
Avoid These Errors
Using FiO₂ incorrectly
Use 0.80 in the formula, not 80. This calculator converts percent entries.
Use 0.80 in the formula, not 80. This calculator converts percent entries.
Forgetting MAP
OI includes mean airway pressure, unlike the P/F ratio.
OI includes mean airway pressure, unlike the P/F ratio.
Mixing OI and OSI
OI uses PaO₂. OSI uses SpO₂.
OI uses PaO₂. OSI uses SpO₂.
Using SpO₂ when saturation is unreliable
Poor waveform, poor perfusion, dyshemoglobins, or motion can make SpO₂ misleading.
Poor waveform, poor perfusion, dyshemoglobins, or motion can make SpO₂ misleading.
Continue Learning
Connect OI to MAP and P/F Ratio
Use OI/OSI with mean airway pressure, P/F ratio, oxygenation trends, and the full clinical picture.