oxygenation

PaO₂/FiO₂ Ratio (Horowitz Index)

P/F = PaO₂ ÷ FiO₂

Normal: > 400 mmHg (normal); 200–400 mild ARDS; 100–200 moderate ARDS; < 100 severe ARDS

What it measures

The P/F ratio provides a standardized measure of oxygenation efficiency that accounts for the FiO₂ being administered. It allows comparison of oxygenation across different oxygen concentrations. A ratio of 400 means the patient has a normal PaO₂ of 80 on room air (80/0.21 ≈ 381) or a normal response to any oxygen level.

When to use

Primary tool for diagnosing and categorizing ARDS severity (Berlin Definition). Used to track oxygenation trends over time and guide ventilator management and escalation decisions (prone positioning, ECMO candidacy).

Calculator

PaO₂

mmHg

FiO₂

decimal

Worked examples

Normal patient on room air

→ 452 mmHg

Normal oxygenation

Moderate ARDS, FiO₂ 0.60

→ 140 mmHg

Moderate-severe ARDS — consider prone positioning

Severe ARDS, FiO₂ 1.0

→ 65 mmHg

Severe ARDS — VV-ECMO candidacy evaluation

Clinical significance

P/F < 300 on PEEP ≥ 5 cmH2O = ARDS. P/F 200–300 = mild ARDS; P/F 100–200 = moderate ARDS; P/F < 100 = severe ARDS. P/F < 150 triggers consideration of prone positioning and neuromuscular blockade. P/F < 80 despite optimal conventional ventilation is an ECMO consideration.

Variables

P/F

PaO₂/FiO₂ ratio(mmHg)

Normal: > 400 mmHg

PaO₂

Arterial partial pressure of oxygen(mmHg)

Normal: 80–100 mmHg

FiO₂

Fraction of inspired oxygen(decimal)

Normal: 0.21–1.0

NBRC pearls

Berlin ARDS criteria: P/F < 300 on PEEP ≥ 5, acute onset within 1 week, bilateral infiltrates, not fully explained by cardiac failure
P/F ratio should be measured on the SAME PEEP/CPAP level (≥ 5 cmH2O for ARDS classification)
Prone positioning indicated when P/F < 150 — reduces ARDS mortality by 16% (PROSEVA trial)
P/F < 80 on optimal settings = consider VV-ECMO referral

Memory tips

P/F: 'PaO₂ For every FiO₂ — how many mmHg per percent oxygen?' Normal should be ~400+
Cutoffs: 400-300-200-100 — each step down = worse ARDS category
P/F < 150: Prone! Proning improves oxygenation by opening dorsal (collapsed) alveoli

All Formulas

What it measures

NBRC pearls

Berlin ARDS criteria: P/F < 300 on PEEP ≥ 5, acute onset within 1 week, bilateral infiltrates, not fully explained by cardiac failure

P/F ratio should be measured on the SAME PEEP/CPAP level (≥ 5 cmH2O for ARDS classification)

Prone positioning indicated when P/F < 150 — reduces ARDS mortality by 16% (PROSEVA trial)

P/F < 80 on optimal settings = consider VV-ECMO referral