Pulse oximetry provides an early warning of hypoxaemia by estimating the proportion of oxyhaemoglobin (HbO₂) compared to deoxyhaemoglobin (Hb).

🔬 Scientific Concepts

🧬 Haemoglobin and Oxygen Transport

Haemoglobin (Hb) in red blood cells carries oxygen from the lungs to tissues. It exists as:

✅ Oxyhaemoglobin (HbO₂) – bound to oxygen
❌ Deoxyhaemoglobin (Hb) – unbound

The proportion of HbO₂ relative to total Hb = oxygen saturation (SpO₂).

💡 Spectrophotometry

Pulse oximeters use light absorption:

🔴 DeoxyHb absorbs more red light (660 nm)
🌐 OxyHb absorbs more infrared light (940 nm)

⚙️ How It Works

Emitter + Detector – sends red + infrared light through a pulsatile arterial site (finger, toe, ear).
Absorption Ratio – compares absorbed light to calculate SpO₂.
Pulsatile Flow – detects arterial pulsation, filters out venous/skin/bone signals.

📉 Factors Affecting Accuracy

💧 Perfusion: Poor circulation → weak signals.
☀️ Light interference: Sunlight/fluorescent light distort readings.
🖤 Skin pigmentation: Darker skin may cause underestimation.
💅 Nail polish: Especially dark colours → false readings.

🏥 Clinical Significance

Rapidly detects hypoxaemia.
Used in anaesthesia, ICU, emergency medicine, and long-term monitoring of lung/cardiac patients.
Limitations must be understood for safe interpretation.

🩺 Common Uses

Monitoring SpO₂ in respiratory/cardiac disease.
Assessing oxygenation during surgery or sedation.
Checking response to oxygen therapy.
Screening in asthma, pneumonia, COPD.

📖 Understanding Readings

✅ Normal: 95–100%
⚠️ Low: <90% → significant hypoxaemia
🎯 Accuracy within ±2–3% of arterial blood gases under ideal conditions.

🚧 Limitations

Pulse oximetry does not measure CO₂ and can miss CO poisoning or abnormal haemoglobins.

⚠️ Key Limitations
Oxygen dissociation curve – SpO₂ doesn’t always reflect PaO₂ changes linearly. Dyshaemoglobins – carboxyHb & methaemoglobin → false results. IV dyes – can temporarily alter absorption. Low perfusion – shock, hypothermia. Skin pigmentation – may underestimate values. Anaemia – SpO₂ can appear normal despite reduced O₂ carrying capacity. Nail polish – blocks light. Motion artefact – false fluctuations. User error – poor placement or misinterpretation.

⚠️ Key Limitations

Oxygen dissociation curve – SpO₂ doesn’t always reflect PaO₂ changes linearly.
Dyshaemoglobins – carboxyHb & methaemoglobin → false results.
IV dyes – can temporarily alter absorption.
Low perfusion – shock, hypothermia.
Skin pigmentation – may underestimate values.
Anaemia – SpO₂ can appear normal despite reduced O₂ carrying capacity.
Nail polish – blocks light.
Motion artefact – false fluctuations.
User error – poor placement or misinterpretation.

📚 References

The content on this website is provided for educational and informational purposes only to support exam preparation (e.g., MLA, MRCP, USMLE) and learning. This is NOT medical advice, diagnosis, treatment, or professional guidance. It does not replace consultation with a qualified healthcare professional, official guidelines (e.g., NICE, GMC, BNF), or supervised clinical practice. Always verify information with current, authoritative sources. Makindo and its contributors accept no liability for any reliance on this content, including errors, omissions, or any resulting harm, loss, or consequences. By using this site, you agree to these terms.

Pulse oximetry

📊 Pulse Oximetry