Oxygen Desaturation
at the bedside
- full vitals, cardiac monitor; sit the patient up
- confirm → probe position, waveform, perfusion; compare with baseline; another finger or device if in doubt, without delaying oxygen
- oxygen to target
- 90–94% for most; never above 96%
- 88–92% if at risk of hypercapnia (known CO₂ retainer, severe COPD, obesity hypoventilation, neuromuscular disease); check a gas
- MI or stroke with SpO₂ ≥ 90% → no oxygen
- sickle cell crisis or acute chest syndrome → ≥ 95%, or within 3% of the patient’s baseline; gas if ≤ 94% on air (the 96% ceiling does not apply)
- peri-arrest → reservoir mask 15 L/min, titrate down once stable
- tracheostomy or ventilator → airway or ICU team now; check the circuit, suction
oxygen devices and FiO₂
| device | flow | approx FiO₂ |
|---|---|---|
| room air | – | 21% |
| nasal cannula | 1–6 L/min | 24–44% |
| simple face mask | 5–10 L/min | 35–60% |
| non-rebreather / reservoir mask | 10–15 L/min | ~60–95% |
| Venturi mask | adapter-specific | fixed 24–60% |
- never run a simple face mask below 5 L/min: CO₂ rebreathing
- inflate the reservoir bag before applying a non-rebreather
escalate now if
- rising O₂ requirement or persistent hypoxemia → ICU now if
- severe respiratory distress or exhaustion → ICU now
- altered mental status
- hemodynamic instability
- by cause → asthma, tension pneumothorax, PE, pulmonary edema
focused check
- causes
- pneumonia or aspiration; mucus plugging, atelectasis
- asthma or COPD exacerbation
- pulmonary edema
- pulmonary embolism
- pneumothorax
- opioids or sedatives (hypoventilation)
- history → baseline O₂ requirement, sudden or gradual, chest pain, fever, sputum, hemoptysis, calf swelling, aspiration risk; asthma, COPD, ILD, heart failure, VTE
- examine → wheeze, crackles, absent or unequal breath sounds; JVP, new murmur, peripheral edema; signs of DVT
send now
- VBG for pH and CO₂; ABG when you need PaO₂, an exact PaCO₂ or a P/F ratio
- ECG, CXR
- by cause → CBC, CRP, troponin, D-dimer, CTPA; NT-proBNP if heart failure is in question (< 300 pg/mL makes it unlikely)
- bedside lung ultrasound if someone can do it
S/F and P/F ratios
- S/F ratio · MDCalc ↗︎ = SpO₂ ÷ FiO₂ · P/F ratio · MDCalc ↗︎ = PaO₂ ÷ FiO₂ · enter FiO₂ as a decimal (40% = 0.40)
- S/F when there is no ABG, valid only while SpO₂ ≤ 97%; S/F ≤ 315 approximates P/F ≤ 300
- P/F needs an ABG, not a ventilator
- neither ratio alone diagnoses ARDS → Berlin ARDS criteria · MDCalc ↗︎; the global definition includes patients on HFNO ≥ 30 L/min or NIV
treat
- oxygen to target (bedside); then treat the cause
- COPD exacerbation → nebulisers and steroids as for asthma below, antibiotics if infective, NIV per protocol → dyspnea
acute asthma
- oxygen only if SpO₂ < 92%, target 92–95%
- salbutamol (Ventolin) 2.5–5 mg nebulised every 20 min for the first hour, then by response
- moderate or severe → add ipratropium (Atrovent) 0.5 mg nebulised with each of the first three salbutamol doses, not after the first hour
- steroids within 1 h: prednisolone 1 mg/kg PO daily, max 50 mg, for 5–7 days; methylprednisolone IV if not swallowing
- not responding at 1 h, or life-threatening → magnesium sulphate 2 g IV over 20 min
- no IV aminophylline, no sedatives → full pathway on dyspnea
ICU now if
- silent chest, poor effort, cannot speak
- drowsy, confused or agitated
- SpO₂ < 92% despite oxygen, or cyanosis
- normal or rising PaCO₂; hypotension or arrhythmia
- PEF < 33% of predicted or best; not improving after the first hour
- rising PaCO₂, drowsiness or falling effort → near-fatal asthma, ICU for intubation
pneumothorax, stable
- urgent CXR or lung ultrasound
- respiratory or thoracic review; treat by symptoms, size and cause
tension pneumothorax, unstable
- do not wait for imaging: resuscitation or ICU team now
- needle decompression by a trained clinician: 4th or 5th intercostal space anterior to the mid-axillary line, above the rib; 2nd intercostal space mid-clavicular is the alternative
- decompression is temporary: chest drain in the safe triangle follows
pulmonary embolism, stable
- clinical probability → Wells score for PE · MDCalc ↗︎; CTPA when indicated; no D-dimer if probability is high
- high probability and imaging delayed → anticoagulate now if bleeding risk allows: CBC, RFT, LFT, coagulation first
- enoxaparin 1 mg/kg SC q12h, actual body weight (20/40/60/80 mg syringes); severe renal impairment → local renal-adjustment protocol
pulmonary embolism, unstable
- ICU and senior now; no CTPA transfer until stabilised; bedside echo if it does not delay treatment
- thrombolysis is the ICU decision; unfractionated heparin IV per local nomogram while it is considered
fluid overload / pulmonary edema
- furosemide 40 mg IV if diuretic-naïve; on an oral loop diuretic → double the daily oral dose, given IV
- response at 2 h (spot urine Na ≥ 70 mmol/L) or 6 h (urine output ≥ 100 mL/h); not met → double the dose, or add acetazolamide 500 mg IV daily
- sit up; oxygen only if SpO₂ < 90%; NIV or CPAP if RR > 25 with SpO₂ < 90%; GTN if SBP > 110 mmHg → dyspnea
- hand over → O₂ device and flow, SpO₂ target, trend, escalation trigger
evidence
| source | type |
|---|---|
| BMJ 2018 Rapid Recommendation, oxygen therapy for acutely ill medical patients | international guideline |
| BSH 2015 guideline on the management of acute chest syndrome in sickle cell disease (oxygen target) | international guideline |
| 2023 global ARDS definition | international guideline |
| GINA 2026 | international guideline |
| NICE NG158 | international guideline |
| 2026 ESC heart failure guideline (decongestion, Figure 15) | international guideline |
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reviewed 17 Aug 2026updated Oct 2026file vital/oxygen-desaturation