What is the P/F ratio formula?

The P/F ratio formula is: P/F Ratio = PaO2 (mmHg) ÷ FiO2 (as a decimal). PaO2 is the partial pressure of arterial oxygen from an arterial blood gas (ABG). FiO2 is the fraction of inspired oxygen as a decimal (e.g., 40% oxygen = FiO2 0.40). Example: PaO2 80 mmHg ÷ FiO2 0.40 = P/F ratio of 200 mmHg. A normal P/F ratio on room air (FiO2 0.21) is approximately 400-500 mmHg (PaO2 ~85-100 mmHg ÷ 0.21).

What P/F ratio indicates ARDS?

Per the Berlin Definition (2012): Mild ARDS = P/F ratio 200-300 mmHg with PEEP or CPAP ≥5 cmH2O; Moderate ARDS = P/F ratio 100-200 mmHg with PEEP ≥5 cmH2O; Severe ARDS = P/F ratio less than 100 mmHg with PEEP ≥5 cmH2O. A P/F ratio above 300 mmHg is not ARDS by Berlin Definition, though it may still indicate hypoxic respiratory failure. The PEEP requirement (≥5 cmH2O) distinguishes ARDS from other causes of hypoxemia.

Can I use SpO2 instead of PaO2 for the P/F ratio?

When an ABG is not available, the S/F ratio (SpO2/FiO2) can be used as a surrogate. The approximate S/F ratio equivalents are: S/F ≥315 corresponds to P/F ≥300 (no ARDS); S/F 235-315 corresponds to P/F 200-300 (Mild ARDS); S/F 148-235 corresponds to P/F 100-200 (Moderate ARDS); S/F <148 corresponds to P/F <100 (Severe ARDS). The S/F ratio is less accurate when SpO2 is greater than 97% due to the flat upper portion of the oxyhemoglobin dissociation curve. Use only when SpO2 is reliable and below 97%.

What ventilator settings are used in ARDS?

ARDS management uses lung-protective ventilation (ARDSNet protocol): Tidal volume 6 mL/kg ideal body weight (IBW) — avoid volumes that cause volutrauma; Plateau pressure ≤30 cmH2O; PEEP titrated to oxygenation target using ARDSNet PEEP-FiO2 table; Target SpO2 88-95% or PaO2 55-80 mmHg. For severe ARDS (P/F <150): prone positioning for 16+ hours per day (PROSEVA trial); neuromuscular blockade consideration (ACURASYS trial); ECMO at experienced centers if P/F <80 despite optimal management.

What is the P/F ratio formula?

The P/F ratio formula is: P/F Ratio = PaO2 (mmHg) ÷ FiO2 (as a decimal). PaO2 is the partial pressure of arterial oxygen from an arterial blood gas (ABG). FiO2 is the fraction of inspired oxygen as a decimal (e.g., 40% oxygen = FiO2 0.40). Example: PaO2 80 mmHg ÷ FiO2 0.40 = P/F ratio of 200 mmHg. A normal P/F ratio on room air (FiO2 0.21) is approximately 400-500 mmHg (PaO2 ~85-100 mmHg ÷ 0.21).

What P/F ratio indicates ARDS?

Per the Berlin Definition (2012): Mild ARDS = P/F ratio 200-300 mmHg with PEEP or CPAP ≥5 cmH2O; Moderate ARDS = P/F ratio 100-200 mmHg with PEEP ≥5 cmH2O; Severe ARDS = P/F ratio less than 100 mmHg with PEEP ≥5 cmH2O. A P/F ratio above 300 mmHg is not ARDS by Berlin Definition, though it may still indicate hypoxic respiratory failure. The PEEP requirement (≥5 cmH2O) distinguishes ARDS from other causes of hypoxemia.

Can I use SpO2 instead of PaO2 for the P/F ratio?

When an ABG is not available, the S/F ratio (SpO2/FiO2) can be used as a surrogate. The approximate S/F ratio equivalents are: S/F ≥315 corresponds to P/F ≥300 (no ARDS); S/F 235-315 corresponds to P/F 200-300 (Mild ARDS); S/F 148-235 corresponds to P/F 100-200 (Moderate ARDS); S/F <148 corresponds to P/F <100 (Severe ARDS). The S/F ratio is less accurate when SpO2 is greater than 97% due to the flat upper portion of the oxyhemoglobin dissociation curve. Use only when SpO2 is reliable and below 97%.

What ventilator settings are used in ARDS?

ARDS management uses lung-protective ventilation (ARDSNet protocol): Tidal volume 6 mL/kg ideal body weight (IBW) — avoid volumes that cause volutrauma; Plateau pressure ≤30 cmH2O; PEEP titrated to oxygenation target using ARDSNet PEEP-FiO2 table; Target SpO2 88-95% or PaO2 55-80 mmHg. For severe ARDS (P/F <150): prone positioning for 16+ hours per day (PROSEVA trial); neuromuscular blockade consideration (ACURASYS trial); ECMO at experienced centers if P/F <80 despite optimal management.

🫁 Berlin Definition 2012 · ARDSNet · Lung-Protective Ventilation · PROSEVA

P/F Ratio Calculator — PaO₂/FiO₂

Calculate PaO₂/FiO₂ ratio for ARDS severity classification using the Berlin Definition. Includes S/F ratio surrogate, oxygenation index, FiO₂ delivery reference, and EMR-ready respiratory documentation.

NM Clinically reviewed byDr. Nikhil Mahajan, PT, MPT · Jan 15, 2025

> 300 mmHg Normal Mortality ~5%

200–300 mmHg Mild ARDS Mortality ~27%

100–200 mmHg Moderate ARDS Mortality ~32%

< 100 mmHg Severe ARDS Mortality ~45%

P/F Formula PaO₂ (mmHg) ÷ FiO₂ (decimal)

Normal (Room Air) 400–500 mmHg

ARDS Threshold ≤ 300 mmHg + PEEP ≥5

Severe ARDS < 100 mmHg

Enter Oxygenation Values

PaO₂ — Arterial Oxygen Partial Pressure

From Arterial Blood Gas (ABG). Normal room air PaO₂ = 85–100 mmHg.

mmHg

Normal: 80–100 mmHg on room air · From ABG sample (arterial, not venous)

FiO₂ — Fraction of Inspired Oxygen

Enter as percentage (21–100). Room air = 21%. Ventilator FiO₂ is exact; delivery device FiO₂ is estimated.

Room air = 21% · Nasal cannula 2L = ~28% · 100% NRB = ~70–80%

SpO₂ — Pulse Oximetry Optional — S/F Ratio

If no ABG available, enter SpO₂ to calculate the S/F ratio surrogate. Most accurate when SpO₂ ≤97%.

Berlin Definition requires PEEP ≥5 cmH₂O for ARDS diagnosis. Ensure patient is on at least 5 cmH₂O PEEP or CPAP when interpreting results for formal ARDS classification.

P/F Ratio — mmHg

Enter PaO₂ and FiO₂ above

Berlin Definition — ARDS Severity Classification

ARDS Level	P/F Ratio	S/F Surrogate	PEEP	Approx. Mortality	Management
Normal	> 300 mmHg	> 315	N/A	~5%	Standard oxygen therapy; investigate cause of hypoxemia
Mild ARDS	200–300 mmHg	235–315	≥5 cmH2O	~27%	NIV/CPAP consideration; lung-protective ventilation; prone if worsening
Moderate ARDS	100–200 mmHg	148–235	≥5 cmH2O	~32%	Intubation + lung-protective ventilation; PEEP optimization; prone positioning
Severe ARDS	< 100 mmHg	< 148	≥5 cmH2O	~45%	Prone 16+ hours/day (PROSEVA); NMB consideration; ECMO referral if P/F <80

Mortality estimates from the ARDS Definition Task Force (JAMA 2012) — Berlin Definition validation cohort of 4,188 patients. Individual outcomes vary significantly based on underlying cause, comorbidities, and institution.

FiO₂ by Oxygen Delivery Device — Reference

Delivery Device	Approximate FiO₂	Notes
Room Air	21%	0.21 — baseline
Nasal Cannula 1L	~24%	~4% per litre
Nasal Cannula 2L	~28%	~4% per litre
Nasal Cannula 4L	~36%	~4% per litre
Nasal Cannula 6L	~44%	Max flow for NC
Simple Face Mask	~35–50%	Flow-dependent
Venturi Mask	24–60%	Colour-coded valves
Non-Rebreather	~60–80%	15 L/min flow
High-Flow NC (HFNC)	21–100%	Titrated precisely
Mechanical Ventilator	21–100%	Set exactly on vent

FiO₂ for non-rebreather masks and nasal cannulas are estimates — actual delivered FiO₂ varies with patient breathing pattern, mask fit, and flow rate. Only mechanical ventilators and HFNC deliver a precisely controlled FiO₂.

P/F Ratio — Clinical Reference for ICU and Respiratory Medicine

The PaO₂/FiO₂ ratio (P/F ratio) is the most widely used index of oxygenation impairment in the ICU. It normalizes the arterial oxygen level for the amount of supplemental oxygen being delivered, allowing direct comparison of oxygenation severity regardless of the FiO₂ being used. On room air (FiO₂ 0.21), a normal PaO₂ of 95 mmHg produces a P/F ratio of 452 — well within the normal range. A patient requiring 80% FiO₂ to maintain the same PaO₂ would have a P/F ratio of 95/0.80 = 119 — classified as Moderate ARDS by the Berlin Definition.

Berlin Definition — Full Diagnostic Criteria for ARDS

The ARDS Definition Task Force (JAMA 2012) defined ARDS using four criteria:

Timing: Acute onset within 1 week of a known clinical insult or new/worsening respiratory symptoms
Imaging: Bilateral opacities on chest X-ray or CT not fully explained by effusions, collapse, or nodules
Origin of edema: Respiratory failure not fully explained by cardiac failure or fluid overload (requires objective assessment if no risk factor present)
Oxygenation (P/F ratio): Measured with PEEP or CPAP ≥5 cmH₂O:
- Mild ARDS: P/F 200–300 mmHg · Mortality ~27% · NIV/CPAP consideration; lung-protective ventilation; prone if worsening
- Moderate ARDS: P/F 100–200 mmHg · Mortality ~32% · Intubation + lung-protective ventilation; PEEP optimization; prone positioning
- Severe ARDS: P/F < 100 mmHg · Mortality ~45% · Prone 16+ hours/day (PROSEVA); NMB consideration; ECMO referral if P/F <80

ARDSNet Lung-Protective Ventilation — Key Parameters

Tidal Volume: 6 mL/kg Ideal Body Weight (IBW) — not actual body weight. IBW = 50 + 0.91×(height in cm − 152.4) for men; 45.5 + 0.91×(height − 152.4) for women.
Plateau Pressure: Keep ≤30 cmH₂O — reduce tidal volume by 1 mL/kg if exceeded
PEEP: Titrate using ARDSNet PEEP-FiO₂ table to achieve target SpO₂ 88–95%
Target oxygenation: PaO₂ 55–80 mmHg or SpO₂ 88–95%
Severe ARDS (P/F <150): Prone positioning ≥16 hours/day (PROSEVA trial — 28-day mortality 16% vs 33%); neuromuscular blockade
Refractory ARDS (P/F <80): ECMO referral at experienced centre

NM Dr. Nikhil Mahajan, PT, MPT · Reviewed January 15, 2025 · View credentials

Frequently Asked Questions

Why does the Berlin Definition require PEEP ≥5 cmH₂O?

The PEEP requirement distinguishes true ARDS (diffuse alveolar damage causing non-cardiogenic pulmonary edema) from hypoxemia caused by atelectasis, which can be rapidly reversed with PEEP. If a patient has a P/F ratio of 180 with PEEP 0 but the ratio improves to 350 with PEEP 8 cmH₂O, they do not have ARDS by Berlin criteria. The PEEP requirement also ensures more consistency across institutions in how the ratio is measured.

What is the Oxygenation Index (OI) and how does it differ from P/F ratio?

The Oxygenation Index (OI) = (FiO₂ × Mean Airway Pressure × 100) / PaO₂. Unlike the P/F ratio, OI incorporates mean airway pressure (MAP), which reflects the total ventilatory burden placed on the lungs. OI is more accurate than P/F ratio for patients on mechanical ventilation because two patients can have the same P/F ratio but very different mean airway pressures — indicating different degrees of lung injury severity. OI >25 generally corresponds to severe ARDS. OI is especially used in pediatric ARDS (PARDS) criteria and ECMO candidacy assessment.

What causes ARDS and which are most common in the ICU?

ARDS causes are divided into direct pulmonary (primary) and indirect (secondary/extrapulmonary) causes. Direct causes: Pneumonia (most common — bacterial, viral, fungal), aspiration of gastric contents, inhalation injury, lung contusion, drowning. Indirect causes: Sepsis (most common indirect cause — up to 30–40% of ARDS), pancreatitis, major trauma/transfusions (TRALI), drug overdose, burns. COVID-19 pneumonia became a major cause of ARDS globally from 2020 onward, with a distinct pathophysiology (type H vs type L phenotypes described by Gattinoni et al.).

When should ECMO be considered in ARDS?

Veno-venous ECMO (VV-ECMO) is considered in severe refractory ARDS when optimal lung-protective ventilation has failed. Standard ECMO candidacy criteria include: P/F ratio <80 mmHg despite FiO₂ 1.0 and PEEP ≥10 cmH₂O; pH <7.25 with PaCO₂ ≥60 mmHg (CO₂ retention); Oxygenation Index >25. The Murray Score ≥3.0 is also used for ECMO candidacy assessment. VV-ECMO should only be performed at experienced centres — the EOLIA trial showed a trend toward benefit but did not reach statistical significance, with significant crossover (28% of control patients were rescued by ECMO).