Horovitz Quotient (P/F Ratio)

Formula

P/F Ratio = PaO₂ (mmHg) / FiO₂ (fraction, 0.21–1.0)

Example: PaO₂ = 80 mmHg, FiO₂ = 0.4 (40 %)
P/F Ratio = 80 / 0.4 = 200 mmHg  →  Moderate ARDS

Interpretation

P/F Ratio (mmHg)	Classification
≥ 400	Normal oxygenation
300–399	Mild impairment
200–299	Mild ARDS (Berlin 2012)
100–199	Moderate ARDS (Berlin 2012)
< 100	Severe ARDS (Berlin 2012)

ARDS Berlin Definition (2012)

For ARDS diagnosis, all four criteria must be met:

• Timing: Acute onset within 1 week of a known clinical insult or new / worsening respiratory symptoms.
• Chest imaging: Bilateral opacities not fully explained by effusions, lobar/lung collapse, or nodules (CXR or CT).
• Origin of oedema: Respiratory failure not fully explained by cardiac failure or fluid overload (objective assessment, e.g. echocardiography, required if no risk factor).
• Oxygenation impairment: P/F ratio < 300 mmHg on PEEP or CPAP ≥ 5 cmH₂O.

Clinical Notes

• PaO₂ must be measured from an arterial blood gas — venous or capillary values are not valid.
• Verify actual delivered FiO₂: high-flow systems may differ from set FiO₂; room air = 0.21.
• Serial P/F measurements during the first 24 hours after PEEP optimisation better reflect true ARDS severity than a single early value.
• The SpO₂/FiO₂ (SF) ratio is a non-invasive surrogate when ABG is unavailable; SF ≈ 235 corresponds to PF ≈ 200 mmHg.

Literature

Horovitz JH, Carrico CJ, Shires GT. Pulmonary response to major injury. Arch Surg. 1974;108(3):349–355.

ARDS Definition Task Force; Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526–2533.