PaO2/FiO2 Ratio (Horowitz Index): ARDS Assessment
Calculate the PaO2/FiO2 ratio (Horowitz Index) to assess hypoxemia severity and ARDS (Berlin Criteria). Essential ICU clinical tool.
The PaO2/FiO2 Ratio (Horowitz Index): ARDS Evaluation
The PaO2/FiO2 Ratio, universally known as the Horowitz Index, is a fundamental clinical indicator in intensive care and pulmonology. It precisely quantifies the degree of hypoxemia (oxygenation deficit) and measures the efficiency of alveolar-capillary gas exchange.
How is the Horowitz Index calculated?
The calculation requires two parameters:
- PaO₂ (Arterial Partial Pressure of Oxygen): Measured in mmHg via an arterial blood gas.
- FiO₂ (Fraction of Inspired Oxygen): Expressed as a percentage (e.g., 40%), but converted to a decimal for the formula (e.g., 0.4).
- Calculation formula: P/F Ratio = PaO₂ (mmHg) / FiO₂ (decimal).
The Berlin Criteria (ARDS)
The Horowitz Index is the pillar of the Berlin Definition (2012) for diagnosing and classifying Acute Respiratory Distress Syndrome (ARDS). Note: For an ARDS diagnosis, the patient must be on mechanical ventilation with a PEEP ≥ 5 cm H₂O.
ARDS Classification (Berlin Criteria)
| PaO₂/FiO₂ Ratio | Classification | Estimated Mortality |
|---|---|---|
| > 300 mmHg | Normal / Mild Hypoxemia | - |
| 201 - 300 mmHg | Mild ARDS | ~ 27 % |
| 101 - 200 mmHg | Moderate ARDS | ~ 32 % |
| ≤ 100 mmHg | Severe ARDS | ~ 45 % |
References:
1. ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-33.
[JAMA Network]
2. Horowitz JH, et al. Pulmonary response to major injury. Arch Surg. 1974;108(2):349-55.
[JAMA Surgery]
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