Oxygen Desaturation
ABCs
- ABCs, full vitals, cardiac monitor
- confirm → probe position, waveform, perfusion; new vs baseline; repeat on another finger / device if doubtful; never delay oxygen in an unwell patient
- give oxygen by severity, not an NRBM for everyone
- target 94–98% for most acutely ill adults
- target 88–92% if risk of hypercapnic respiratory failure (assess blood gases); not every COPD patient
- follow local disease-specific targets
- critically unwell / peri-arrest → reservoir mask 15 L/min, titrate once stable
escalate early if
- rising O₂ requirements or persistent hypoxemia
- severe respiratory distress or exhaustion
- altered mental status
- hemodynamic instability
if unstable or unresponsive
- call for help and ICU assessment
- intubated on a ventilator → involve RT; check circuit, suction for mucus plugging
- tracheostomy or airway-device problem → call airway / ICU / ENT team immediately; follow local tracheostomy-emergency algorithm
oxygen devices & 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 reservoir bag before applying NRBM; keep inflated during inspiration
- low-flow FiO₂ is approximate: varies with mask seal, respiratory rate, minute ventilation
- Venturi masks give controlled FiO₂: useful when precise delivery matters
S/F and P/F ratios (reference)
- S/F ratio · MDCalc ↗︎ = SpO₂ ÷ FiO₂ · P/F ratio · MDCalc ↗︎ = PaO₂ ÷ FiO₂ · enter FiO₂ as a decimal (40% = 0.40)
- S/F: non-intubated patients, when no ABG; for ARDS assessment use only when SpO₂ ≤ 97%
- S/F ≤ 315 approximates P/F ≤ 300; low-flow FiO₂ estimates make either ratio less precise
- P/F: needs an ABG, not mechanical ventilation
- ARDS cannot be diagnosed from either ratio alone: timing, bilateral imaging abnormalities, hydrostatic-edema exclusion, and respiratory-support criteria still apply → Berlin ARDS criteria · MDCalc ↗︎
- global ARDS definition includes non-intubated patients on HFNO ≥ 30 L/min or NIV/CPAP who otherwise meet criteria
HPI & examination
important causes of acute desaturation; direct workup by onset, history, exam, and bedside findings, not a routine panel:
- measurement error or poor peripheral perfusion
- pneumonia or aspiration; mucus plugging or atelectasis
- asthma or COPD exacerbation
- pulmonary edema
- pneumothorax
- pulmonary embolism
- ARDS
- airway-device, tracheostomy, or ventilator problem
- history → baseline oxygen requirement, sudden vs gradual onset, chest pain, fever, sputum, hemoptysis, calf swelling, aspiration risk, PMH (asthma, COPD, ILD, IHD, CHF, VTE)
- examination
- lungs: wheeze, crackles, absent or unequal breath sounds
- heart: new murmurs, JVP
- volume status: peripheral edema, mottling
- extremities: signs of DVT
workup
- blood gases
- VBG assesses pH and gives an initial CO₂ estimate
- VBG cannot assess PaO₂ or quantify hypoxemia
- obtain ABG when accurate PaO₂, PaCO₂, A–a gradient, or P/F ratio is needed
- ECG and CXR in most significant desaturations
- source-directed, not routine: CBC, CRP, troponin, BNP, D-dimer, CTPA only when clinical picture points there
- bedside lung ultrasound if skilled operator available
management
- sit upright; oxygen to target (ABCs above)
- treat underlying cause → asthma: see below. COPD exacerbation: bronchodilators + steroids as below, plus antibiotics if infective and NIV per protocol → full pathway on dyspnea
acute asthma
- salbutamol (albuterol; Ventolin) 2.5–5 mg nebulized, repeat every 20 min for three doses, then reassess
- ipratropium bromide (Atrovent) 500 mcg nebulized with salbutamol in severe exacerbations
- continuous nebulization only in monitored severe / life-threatening cases
- systemic corticosteroids early: oral prednisolone if swallowing; IV methylprednisolone if oral not possible or critical illness
- magnesium sulphate 2 g IV over 20 min in life-threatening exacerbation or poor response
- titrate oxygen to local asthma target, not automatic NRBM
call ICU / anesthesia / resuscitation immediately if life-threatening or impending respiratory arrest
- silent chest or markedly reduced air entry
- exhaustion or poor respiratory effort; inability to speak
- altered consciousness, agitation or confusion
- cyanosis or persistent / worsening hypoxemia
- hypotension, bradycardia or arrhythmia
- normal or rising PaCO₂ despite severe distress
- PEF < 33% of predicted or personal best, if safely measurable
- deterioration despite initial treatment
- rising PaCO₂, worsening drowsiness, or reduced respiratory effort may indicate near-fatal asthma: prepare for intubation by experienced clinician
pneumothorax, stable
- oxygen if hypoxemic
- bedside lung ultrasound or urgent CXR
- respiratory / thoracic review; manage by symptoms, size, cause, and local protocol
tension pneumothorax with hemodynamic or respiratory instability
do not wait for imaging
- call resuscitation / ICU immediately; decompress by trained clinician using local equipment and protocol
- common lateral site: 4th or 5th intercostal space, anterior to mid-axillary line, above upper border of rib
- 2nd intercostal space, mid-clavicular line remains an alternative in some protocols
- finger thoracostomy may be preferred when trained clinician and equipment are immediately available
- emergency decompression is temporary: follow with definitive chest drainage in safe triangle
pulmonary embolism, stable
- assess clinical probability → Wells score for PE · MDCalc ↗︎
- arrange CTPA when indicated
- high probability with delayed imaging → consider interim anticoagulation after assessing bleeding risk; obtain CBC/platelets, renal function, LFTs, coagulation
- do not use D-dimer routinely in high-probability patients: limited inpatient specificity
- initial LMWH example: enoxaparin 1 mg/kg SC q12h, actual body weight (20/40/60/80 mg prefilled syringes); severe renal impairment → follow MOH / local renal-adjustment protocol
pulmonary embolism, hemodynamically unstable (high-risk)
- call ICU / PERT / senior team; do not send an unstable patient to CTPA before stabilization
- bedside echocardiography if it does not delay treatment
- assess immediately for reperfusion therapy, weighing clinical context and contraindications
- IV unfractionated heparin generally preferred when thrombolysis or urgent procedure may be needed; bolus and infusion per local nomogram
fluid overload / pulmonary edema
- CHF history, orthopnea, CXR congestion
- IV furosemide 20–40 mg IV if diuretic-naïve; on oral diuretics → 1–2.5× total daily PO dose IV; no response → increase the dose, do not repeat it; ceiling 80–200 mg per dose, 600 mg/day maximum
evidence
| source | type |
|---|---|
| BTS emergency oxygen guidance | international guideline |
| 2023 global ARDS definition | international guideline |
| GINA 2026 | international guideline |
| NICE NG158 | international guideline |
Verify indication, dose, allergies, interactions, renal/hepatic function and local protocols before prescribing.
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Last clinically reviewed · 17 August 2026 · Last updated · September 2026