The Cardiovascular
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Horowitz index (PaO2/FiO2 ratio)

Oxygenation index for hypoxaemia and ARDS severity.

Horowitz index (PaO2/FiO2 ratio)
PaO2 (arterial)
mmHg
FiO2
%
Complete the fields above to see the result.

When to use

  • Grading hypoxaemia and classifying ARDS severity in ventilated patients.

Formula

P/F ratio = PaO2 (mmHg) / FiO2 (as a fraction). Berlin ARDS thresholds: mild 200-300, moderate 100-200, severe <100, all on PEEP/CPAP >=5.

Pearls and pitfalls

  • Ratio falls with a rising FiO2 even when PaO2 is stable, so interpret alongside ventilator settings.

References

  1. Horovitz JH, Carrico CJ, Shires GT. Arch Surg. 1974;108(3):349-55.
  2. ARDS Definition Task Force. JAMA. 2012;307(23):2526-33.