Prescribe oxygen like a drug: SpO2 94-98% for most acutely ill patients (92-96% is a reasonable ICU range), 88-92% if at risk of hypercapnia. Avoid both hypoxaemia and hyperoxia; choose the right device.
At the bedside
Timed steps- 1
Assess
0 minSpO2, RR, work of breathing, mental state; ABG if SpO2 < 94% on air, at risk of hypercapnia, or critically ill.
- 2
Critical illness
0 minShock, sepsis, cardiac arrest, major trauma, anaphylaxis: start reservoir mask 15 L/min, then titrate down once stable.
- 3
Pick a target
0-5 min94-98% for most patients; 88-92% for COPD with previous hypercapnia, obesity hypoventilation, neuromuscular or chest wall disease, cystic fibrosis.
- 4
Device by need
TitrateNasal cannula 1-4 L/min (FiO2 ~24-40%); simple face mask 5-10 L/min; Venturi 24-60% (fixed FiO2, ideal in hypercapnic risk); reservoir mask 15 L/min (~60-80%); HFNC if more is needed.
- 5
Recheck gas
ReassessRepeat ABG/VBG within 30-60 min in hypercapnic-risk patients after any change in FiO2.
- 6
Ventilated patients
ICUAvoid hyperoxia (SpO2 persistently 100%, PaO2 > 100-150 mmHg). A PaO2 target of ~60 mmHg was not better than ~90 mmHg (HOT-ICU); many ICUs target SpO2 92-96%.
Oxygen devices and approximate FiO2
| Device | Flow (L/min) | Approximate FiO2 | Use |
|---|---|---|---|
| Nasal cannula | 1-4 | 0.24-0.40 | Mild hypoxaemia, stable patients |
| Simple face mask | 5-10 | 0.35-0.55 | Moderate hypoxaemia (not in hypercapnic risk) |
| Venturi mask | As marked on valve (2-15) | 0.24, 0.28, 0.31, 0.35, 0.40, 0.60 | Precise FiO2; COPD and hypercapnic risk |
| Reservoir (non-rebreather) mask | 15 | 0.60-0.80 | Critical illness, initial resuscitation |
| HFNC | 30-60 | 0.21-1.0 (set) | Hypoxaemic respiratory failure |
Calculators for this topic
SpO₂/FiO₂ ratio
Non-invasive oxygenation ratio; valid only when SpO₂ ≤ 97%. Global ARDS definition uses S/F ≤ 315.
PaO₂/FiO₂ ratio
Oxygenation ratio with ARDS severity bands (Berlin; PEEP ≥ 5 cmH₂O).
Alveolar-arterial O₂ gradient
PAO₂ = FiO₂ × (Patm − 47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂.
Oxygen cylinder duration
Minutes of oxygen left at a given flow.
References
- 1
- 2
Oxygen prescription
Step 1 of ~5
Decision: Critical illness (shock, arrest, major trauma, anaphylaxis)?
Based on O'Driscoll BR, Howard LS, Earis J, Mak V. Educational use; follow local protocol.
Pathophysiology
Hypoxaemia mechanisms: low inspired O2, hypoventilation, V/Q mismatch, shunt, and diffusion limitation. Shunt responds poorly to FiO2 and needs PEEP or recruitment. In COPD, excess oxygen raises PaCO2 mainly through worsened V/Q matching (loss of hypoxic vasoconstriction) and the Haldane effect, not only by reduced drive.
Evidence
- BTS 2017: 94-98% for most acutely ill adults; 88-92% for hypercapnic risk; prescribe a target range; reservoir mask for critical illness.
- HOT-ICU (2021): in 2928 ICU patients with acute hypoxaemic respiratory failure, PaO2 target 60 vs 90 mmHg gave similar 90-day mortality.
Controversies
- Exact optimum targets in ventilated patients (conservative vs liberal trials show mixed results).
🇮🇳 India notes
- Many Indian wards rely on cylinder oxygen and simple masks; Venturi masks are cheap and should be stocked for COPD.
- The COVID-19 oxygen crisis of 2021 showed the need for oxygen audits: avoid wasted flow when SpO2 is above target.
References
- 1
- 2
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Educational reference for trained clinicians. Check doses against your hospital protocol and the patient in front of you.