Reference — Oxygen Therapy
Hypoxemia Severity & Oxygenation Indices
How to grade hypoxemia from a PaO₂, read the SpO₂ that goes with it, and put a number on the efficiency of gas exchange — the P/F ratio, A–a gradient, S/F ratio, and oxygenation index, with the thresholds that matter at the bedside.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Hypoxemia Severity by PaO₂
Severity grades below assume a room-air arterial sample. On supplemental oxygen the raw PaO₂ understates the problem — a “normal” PaO₂ on a high FiO₂ still represents impaired gas exchange, which is why the indices further down are needed.
| Severity | PaO₂ (room air) |
|---|---|
| Normal | 80 – 100 mmHg |
| Mild | 60 – 79 mmHg |
| Moderate | 40 – 59 mmHg |
| Severe | < 40 mmHg |
SpO₂ & PaO₂ Correlation
| SpO₂ | Approx PaO₂ |
|---|---|
| 90% | ≈ 60 mmHg |
| 80% | ≈ 48 mmHg |
| 75% | ≈ 40 mmHg |
These pairings track the oxyhemoglobin dissociation curve and hold only when the curve sits in its normal position. A right shift (acidosis, ↑CO₂, ↑temperature, ↑2,3-DPG) lowers hemoglobin’s affinity and releases O₂ to the tissues, so a given SpO₂ corresponds to a higher PaO₂. A left shift (alkalosis, ↓CO₂, ↓temperature, ↓2,3-DPG) does the opposite — hemoglobin holds oxygen more tightly, and the same SpO₂ reflects a lower PaO₂.
Oxygenation Indices
P/F ratio
P/F = PaO₂ ÷ FiO₂
Normal is roughly 400 – 500. The Berlin definition grades ARDS (with PEEP ≥ 5 cm H₂O): mild 200 – 300, moderate 100 – 200, severe ≤ 100. The single most-used bedside oxygenation index. Open the P/F ratio calculator →
A–a gradient
A–a = PAO₂ − PaO₂
Expected gradient ≈ (age ÷ 4) + 4. An elevated gradient points to V/Q mismatch, shunt, or a diffusion defect; a normal gradient with hypoxemia points instead to hypoventilation or a low inspired oxygen. Open the A–a gradient calculator →
S/F ratio
S/F = SpO₂ ÷ FiO₂
A noninvasive surrogate for the P/F ratio when no arterial sample is available: S/F ≈ 235 corresponds to a P/F of 200, and S/F ≈ 315 to a P/F of 300. Most reliable while SpO₂ is ≤ 97%, before the dissociation curve flattens.
Oxygenation index (OI)
OI = (FiO₂ × MAP × 100) ÷ PaO₂
Incorporates mean airway pressure (MAP), so it reflects how much support is buying the oxygenation. Used to grade severe and pediatric respiratory failure — a higher OI means worse disease.
Clinical Notes
- Indices need a consistent, known FiO₂. Every ratio that contains FiO₂ assumes the delivered fraction is stable and accurate. On a low-flow device whose true FiO₂ swings with the breathing pattern, a P/F or S/F ratio is an estimate, not a measurement.
- Respect pulse oximeter limitations. SpO₂ readings degrade with poor perfusion, patient motion, and carboxyhemoglobin (which the device misreads as oxyhemoglobin). Dark nail polish can lower readings, and skin-tone-related bias in pulse oximetry — occult hypoxemia missed more often in patients with darker skin — is documented in the literature. Confirm with an ABG when the number doesn’t fit the patient.
- The number grades, the patient decides. An index tells you how severe the gas-exchange defect is; work of breathing, trend, and trajectory tell you what to do about it. A borderline P/F that is falling hour over hour is more concerning than a worse one that is stable on the same support.
Related Resources
Sources
- 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.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Gas exchange and oxygenation.
- West JB, Luks AM. West's Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2020.