About

The Horovitz Index, commonly referred to as the P/F Ratio, is a cornerstone metric in intensive care medicine for evaluating the severity of hypoxemia in patients with Acute Respiratory Distress Syndrome (ARDS) or acute lung injury. By relating the partial pressure of oxygen in arterial blood (PaO2) to the fraction of inspired oxygen (FiO2), this ratio standardizes the patient's oxygenation status regardless of the supplemental oxygen support they are receiving.

This digital tool facilitates rapid bedside calculation adhering to the Berlin Definition of ARDS. Accuracy in this calculation is vital; a ratio below 300 mmHg indicates abnormality, while values dropping below 100 mmHg signify severe ARDS requiring immediate and aggressive intervention (e.g., prone positioning, ECMO consideration). The interface restricts FiO2 inputs to physiological limits to prevent manual entry errors during high-stress clinical scenarios.

Formulas

The index is a simple ratio of arterial oxygen tension to inspired oxygen fraction.

P/F Ratio = PaO₂FiO₂

Note: FiO₂ must be expressed as a decimal (e.g., 50% oxygen = 0.5) for the calculation, though clinical monitors often display percentages.

Reference Data

Classification	P/F Ratio (mmHg)	P/F Ratio (kPa)	Clinical Severity
Normal	> 400 − 500	> 53 − 66	Healthy Lung Function
Mild ARDS	200 − 300	26.6 − 40	Acute Lung Injury
Moderate ARDS	100 − 200	13.3 − 26.6	Significant Hypoxemia
Severe ARDS	< 100	< 13.3	Critical Failure

Frequently Asked Questions

Yes. The Berlin Definition requires a minimum PEEP (Positive End-Expiratory Pressure) of 5 cmH2O for the diagnosis of ARDS using the P/F ratio.

Not for the strict Horovitz Index. PaO2 requires an arterial blood gas (ABG) sample. However, the "SF Ratio" (SpO2/FiO2) acts as a non-invasive surrogate when ABG is unavailable, though with lower precision.

1 kPa is approximately 7.5 mmHg. This tool handles the unit conversion automatically if you select kPa input.

FiO2 represents the Fraction of Inspired Oxygen. It is impossible to breathe air with more than 100% oxygen, so the maximum mathematical value is 1.0.