Horowitz Index for Lung Function (P/F Ratio)

Why Use

A simple calculation that measures the efficiency of gas exchange in the lungs and provides a crude assessment of the A-a gradient. Used in several diagnostic and severity scoring systems, including CPIS , SOFA , SAPS II and SAPS III , SMART-COP , and the Murray Score . Included in the Berlin criteria for ARDS .

When to Use

Most commonly applied to assess lung function and risk-stratify patients with acute respiratory distress syndrome (ARDS).

Formula

Pearls / Pitfalls

Requires arterial blood gas (ABG) analysis. Cannot distinguish between hypoventilation and V/Q mismatch, unlike the A-a gradient. Can only be used to estimate the A-a gradient, assuming PaCO 2 is normal and no shunt is present.

Advice

Lower P/F ratios indicate higher severity of ARDS and higher mortality risk. May guide ventilator management and help determine the need for additional treatments based on disease severity, such as the use of glucocorticoids, neuromuscular blockade, prone positioning, lung recruitment maneuvers, and ECMO. The ratio can be monitored throughout treatment to assess response to therapies and changes in clinical status.

More Information

Interpretation: Horowitz Index (P/F Ratio), mmHg Severity of ARDS Mortality >300 Not ARDS - >200-300 Mild 27% >100-200 Moderate 32% ≤100 Severe 45% From Ranieri 2012 .