Low oxygen saturations on the ward
Sit the patient up, give oxygen to the right target, and find out whether it's the chest, fluid, a clot or the airway.
"Bed 5's sats are 86%." She is 74, admitted with a left middle cerebral artery stroke 3 days ago, on NG feeds. RR 28, pulse 112, temperature 38.3 °C, BP 138/80. Coarse crackles at the right base. The feed was running while she lay flat during a bed change an hour ago. She has had 3 litres of IV fluid in the last 24 hours.
What do you do in the first 5 minutes, and what are the two or three likely causes?
- Go now and sit them upUnless spinal injury. Lying flat makes oxygenation worse.
- Oxygen to target94–98%, or 88–92% if at risk of CO₂ retention. Prescribe the target.
- Check the probeGood waveform, warm finger, no nail polish, not on the cuff arm.
- A–EAirway, RR, effort, chest, JVP, fluid balance, calves, drug chart.
- Gas, chest X-ray, ECGType 1 or 2 failure, and the cause.
- Treat the causeNot just the number. Aspiration, pneumonia, oedema, PE, atelectasis, opioids, pneumothorax.
- Escalate earlyRising oxygen needs, tiring, drowsy, acidotic, or not reaching target.
- SpO₂ below target on 15 L/min
- Needs more than 40% oxygen
- RR above 25–30, tiring or drowsy
- pH below 7.35 with high CO₂
- Low BP, chest pain or new arrhythmia
- Suspected PE, pneumothorax or pulmonary oedema
- Aspiration
- Pulmonary embolism
- Pulmonary oedema from IV fluids
- Pneumothorax after a line or pleural tap
- Opioid-induced hypoventilation
- Mucus plugging and lobar collapse
- Sepsis from another source
| Target, most | SpO₂ 94–98% |
| Target, CO₂ retainers | 88–92% (COPD, obesity hypoventilation, neuromuscular disease) |
| Nasal cannula | 1–4 L/min, about 24–40% |
| Simple face mask | 5–10 L/min, about 35–60% |
| Reservoir mask | 15 L/min, about 60–90%, for the critically ill |
| Type 1 failure | PaO₂ below 60 mmHg (8 kPa), normal or low CO₂ |
| Type 2 failure | PaCO₂ above 45 mmHg (6 kPa) |
| Escalation | Needing over 40% oxygen, or SpO₂ below target on a reservoir mask |
Check every dose against a current drug reference and your hospital protocol.
On this page
Why it matters
Falling oxygen saturation is a sign, not a diagnosis. Turning the oxygen up treats the number but not the cause. A rising oxygen requirement is one of the commonest warnings in the hours before a ward cardiac arrest. Keep looking until you know why.
On the phone
- What is the SpO₂, and on how much oxygen? What is the prescribed target?
- RR, pulse, BP, temperature, consciousness? NEWS2?
- Why was the patient admitted? COPD, heart failure, recent surgery, stroke, recent procedure?
- Is the probe reading properly with a good trace?
- Ask the nurse to: sit the patient up, give oxygen to target, check the probe on a warm finger, and stop any NG feed.
When you arrive
- AirwaySecretions, vomit, snoring? Suction, reposition.
- BreathingRR, effort, expansion, percussion, auscultation. Crackles, wheeze, silent areas.
- CirculationPulse, BP, JVP, peripheral oedema, fluid balance chart.
- DisabilityGCS, pupils (pinpoint: opioids), sugar.
- ExposureCalves, wounds, drains, NG tube, lines, recent procedures.
- Investigate and treatGas, chest X-ray, ECG, then treat the cause and reassess in 15–30 minutes.
What is the cause?
- Aspiration or pneumonia
- Sit up, stop feed and check tube, antibiotics
- Pulmonary oedema
- Sit up, stop fluids, furosemide
- Pulmonary embolism
- Wells, anticoagulate, CTPA
- Opioids or sedatives
- Naloxone, review doses
- Atelectasis
- Physio, sit up, analgesia, deep breathing
- Pneumothorax
- Chest X-ray, decompress if tension
Differential diagnosis
| Cause | Clues | First step |
|---|---|---|
| Aspiration | Stroke, drowsy, after a feed or vomit, right lower zone crackles | Stop feed, suction, sit up, antibiotics |
| Pneumonia | Fever, cough, focal crackles, high CRP | Antibiotics per policy |
| Pulmonary oedema | Lots of IV fluid, heart or kidney failure, bibasal crackles, frothy sputum | Sit up, stop fluids, furosemide, GTN if BP allows |
| Pulmonary embolism | Post-op or bed-bound, sudden, clear chest, fast pulse | Anticoagulate if no contraindication, CTPA |
| COPD or asthma flare | Wheeze, history | Nebulisers, steroids |
| Pneumothorax | After a line, tap or ventilation, absent breath sounds | X-ray; decompress if tension |
| Opioid toxicity | Slow breathing, pinpoint pupils, kidney injury | Naloxone |
| Atelectasis or mucus plug | Post-op, weak cough, reduced breath sounds | Physio, suction, sit up |
| Pleural effusion | Stony dullness | Ultrasound, tap |
| Sepsis from elsewhere | Fever, fast breathing, low BP | Sepsis pathway |
Aspiration on the ward
- High risk: stroke, reduced consciousness, NG feeding, vomiting, frail elderly, after sedation.
- Usually the right lower lobe (lying down: posterior segments).
- Stop the feed, check the NG tube position, suction, sit up 30–45°.
- Antibiotics only if pneumonia develops (fever, new crackles, X-ray change), per local policy. Chemical pneumonitis in the first hours doesn't always need them.
- Ask speech and language therapy or the senior to review swallowing and feeding.
Oxygen devices
| Device | Flow | Roughly delivers | Use |
|---|---|---|---|
| Nasal cannula | 1–4 L/min | 24–40% | Mild hypoxia, comfort |
| Simple face mask | 5–10 L/min | 35–60% | Moderate hypoxia |
| Venturi mask | Set by colour | 24, 28, 31, 35, 40, 60% | CO₂ retainers, precise control |
| Reservoir (non-rebreather) | 15 L/min | 60–90% | Critically ill |
| High-flow nasal oxygen | Specialist | Up to 100% | Escalation, HDU |
| CPAP or NIV | Specialist | Up to 100% | Pulmonary oedema (CPAP), type 2 failure (NIV) |
Reading the gas
| Pattern | Meaning | Example |
|---|---|---|
| Low O₂, low or normal CO₂ | Type 1 failure | Pneumonia, oedema, PE |
| Low O₂, high CO₂, normal pH | Chronic type 2 | Stable COPD |
| Low O₂, high CO₂, low pH | Acute type 2: dangerous | COPD flare, opioids, tiring |
| Normal O₂ on high oxygen | Still check the A–a gradient | Hidden shunt |
Always write down how much oxygen the patient was on when the gas was taken.
Investigations
| Test | Why |
|---|---|
| Blood gas (arterial if possible) | Type 1 or 2, pH, lactate |
| Chest X-ray | Consolidation, oedema, pneumothorax, collapse, effusion |
| ECG | PE strain, ischaemia, AF |
| CBC, CRP, urea, electrolytes | Infection, kidney function |
| BNP if available | Heart failure |
| Blood and sputum cultures | Pneumonia |
| CTPA | Suspected PE |
| NG tube position | Before restarting feeds |
Treatment
Common mistakes
- Turning the oxygen up and leaving.
- Lying the patient flat for a procedure or X-ray while hypoxic.
- Withholding oxygen from a hypoxic COPD patient.
- Giving more IV fluid to a patient who is already overloaded.
- Restarting NG feeds without confirming tube position.
- Not telling the senior when oxygen needs keep rising.
Document
SpO₂ and oxygen before and after, the likely cause, investigations, treatment, target SpO₂ prescribed, and review time.
Frequently asked questions
What should I do first when a patient's saturations drop?
Go and see them, sit them up, check the probe, give oxygen to target and assess A–E. Then get a gas, chest X-ray and ECG while you look for the cause.
What is a safe oxygen target in COPD?
88–92% for patients at risk of CO₂ retention, using a Venturi mask, with a blood gas to check CO₂.
How do I know if low saturations are from fluid overload?
A positive fluid balance, crackles at both bases, raised JVP, peripheral oedema and pulmonary oedema on the chest X-ray.
What are the signs of aspiration on the ward?
Sudden coughing, breathlessness, falling saturations, fever and new crackles (often right lower zone) in a patient with a poor swallow, NG feeding or recent vomiting.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A stroke patient on NG feeds develops fever, SpO₂ 86% and crackles at the right base. What is likely?
Answer: Aspiration pneumonia. Stroke, dysphagia and NG feeding raise aspiration risk; the right lower lobe is commonly affected.
- Pneumothorax: No.
- Asthma: No.
- Heart failure: Less likely with fever and one-sided crackles.
- Pulmonary embolism: Crackles and fever suggest aspiration.
2. What is the target oxygen saturation for most acutely ill adults?
Answer: 94–98%. BTS target is 94–98%, or 88–92% for those at risk of hypercapnic failure.
- 85–90%: Too low.
- 88–92%: For CO₂ retainers.
- 100%: Unnecessary and can harm.
- Above 90% for all: Too low for most.
3. A patient's SpO₂ is stable at 95% but oxygen has gone from 2 L to 10 L/min over 4 hours. How should this be interpreted?
Answer: Worsening respiratory failure. Rising oxygen requirement is a warning sign even when the saturation looks fine.
- Stable, as SpO₂ is normal: Rising oxygen needs show deterioration.
- Probe error: Unlikely.
- Expected recovery: No.
- Anxiety: No.
4. An elderly stroke patient with dysphagia develops fever and right lower lobe consolidation 3 days after admission. What is the most appropriate management?
Answer: Antibiotics + swallow assessment. Aspiration pneumonia commonly affects dependent segments (right lower lobe — straighter right main bronchus; posterior upper lobe if supine). Risks: stroke/dysphagia, reduced consciousness, alcohol, seizures, GORD. Treat with antibiotics (co-amoxiclav; hospital-acquired — broader Gram-negative cover per local policy), formal swallow assessment (speech and language therapy), modified diet, positioning, oral hygiene; NG feeding if unsafe swallow.
- Antifungals: This is not fungal.
- Continue oral feeding unchanged: Continued oral feeding risks recurrent aspiration.
- High-dose steroids: Steroids are not indicated.
5. A 72-year-old with known COPD and previous hypercapnia is brought in breathless with SpO₂ 82%. What oxygen target is appropriate?
Answer: SpO₂ 88–92% via Venturi mask. In patients at risk of hypercapnic respiratory failure (COPD, obesity hypoventilation, neuromuscular disease, chest wall deformity), give controlled oxygen via Venturi mask (24–28%) targeting 88–92% and check ABG. Excess oxygen worsens V/Q mismatch and Haldane effect, raising CO₂. Most other acutely ill patients target 94–98%.
- SpO₂ 98–100% via non-rebreather: High-flow oxygen risks hypercapnia in COPD.
- No oxygen until ABG: Hypoxia kills; give controlled oxygen while awaiting ABG.
- SpO₂ 80–85%: 80–85% is too low a target.
6. Which cells lie within the interalveolar septa and alveoli and remove inhaled particles?
Answer: Alveolar macrophages. Alveolar macrophages, derived from monocytes, patrol the alveoli and septa and engulf dust, bacteria and debris. In heart failure, they contain haemosiderin and are called heart failure cells.
- Type I pneumocytes: Thin cells forming most of the gas-exchange surface.
- Type II pneumocytes: Make surfactant and replace type I cells.
- Club cells: Found in bronchioles; secrete protective proteins.
- Goblet cells: Found in the airways, not the alveoli.
7. What is the Bohr effect?
Answer: CO₂ and H⁺ lower O₂ affinity. Bohr effect: increased PCO₂, H⁺ (low pH), temperature and 2,3-BPG shift the oxygen dissociation curve right, releasing more O₂ in active tissues. Haldane effect is the reciprocal: deoxygenated Hb carries more CO₂ and H⁺, aiding CO₂ loading in tissues and unloading in lungs.
- Deoxy-Hb carries more CO₂: Haldane effect.
- Hb binds O₂ cooperatively: Sigmoid curve explanation.
- HbF binds O₂ more tightly: HbF lacks 2,3-BPG binding.
- CO binds Hb 240 times more: Carbon monoxide affinity.
References and dose sources
- Furosemide dosing: Drugs.com: Furosemide dosage (see also our furosemide dose page)
- Salbutamol dosing: Drugs.com: Albuterol (salbutamol) dosage (see also our salbutamol dose page)
- Co-amoxiclav (amoxicillin with clavulanic acid) dosing: Drugs.com: Amoxicillin/clavulanate dosage (see also our co-amoxiclav (amoxicillin with clavulanic acid) dose page)
- Naloxone dosing: Drugs.com: Naloxone dosage (see also our naloxone dose page)
- Enoxaparin dosing: Drugs.com: Enoxaparin dosage (see also our enoxaparin dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.