Introduction
• Non-invasive method to measure arterial oxygen saturation (SpO₂).
• Uses the principle of spectrophotometry and plethysmography.
• Detects functional hemoglobin saturation but does not measure PaO₂ (Partial Pressure of Oxygen in Arterial Blood).
Principle of Pulse Oximetry
1. Spectrophotometry
• Uses two wavelengths of light:
• Red Light (660 nm) → Absorbed by deoxygenated hemoglobin (HHb).
• Infrared Light (940 nm) → Absorbed by oxygenated hemoglobin (O₂Hb).
• The ratio of absorbed light determines oxygen saturation (SpO₂).
2. Plethysmography
• Measures pulsatile changes in arterial blood volume.
• Differentiates arterial blood from venous and tissue absorption.
Beer-Lambert’s Law
The amount of light absorbed by a solution is:
Where:
• A = Absorbance
• ε = Molar extinction coefficient
• C = Concentration of solute
• L = Path length
Pulse oximetry follows Beer-Lambert’s law to calculate SpO₂ based on light absorption at two wavelengths.
Components of a Pulse Oximeter
1. Light Source
• Red LED (660 nm) & Infrared LED (940 nm).
2. Photodetector
• Detects transmitted light and calculates SpO₂.
3. Microprocessor
• Converts light signals into oxygen saturation values.
4. Display Unit
• Shows SpO₂, pulse rate, plethysmographic waveform.
Types of Pulse Oximeters
1. Transmittance Oximeter (Common Type)
• Light passes through the tissue (e.g., finger, earlobe).
• Photodetector on opposite side of the light source.
2. Reflectance Oximeter
• Light is reflected from tissues (e.g., forehead, neonatal foot).
• Used in cases where peripheral perfusion is poor.
Normal Oxygen Saturation Values
Condition
SpO₂ (%)
Normal Healthy Adult
95-100
Mild Hypoxia
90-94
Moderate Hypoxia
75-89
Severe Hypoxia
<75
• SpO₂ > 95% is considered normal.
• SpO₂ < 90% indicates significant hypoxia.
Pulse Oximeter Accuracy & Limitations
Factors Affecting Accuracy
1. Motion Artifacts
• Shivering, tremors, or patient movement → false readings.
2. Low Perfusion States
• Hypotension, shock, cold extremities → reduces signal strength.
3. Ambient Light Interference
• Surgical lights, phototherapy can affect readings.
4. Nail Polish / Pigmentation
• Dark nail polish, henna, skin pigmentation → may cause underestimation.
5. Carboxyhemoglobin (COHb) Presence
• COHb absorbs light similar to O₂Hb → falsely high SpO₂.
• Common in carbon monoxide poisoning.
6. Methemoglobinemia
• SpO₂ plateaus at ~85% despite actual oxygenation status.
• Common with nitrate toxicity, local anesthetic overdose.
7. Dyshemoglobinemias
• Abnormal hemoglobins like sulfhemoglobin interfere with readings.
8. Severe Anemia (Hb <5 g/dL)
• Pulse oximeter may still show normal SpO₂, but oxygen delivery is impaired.
9. Venous Pulsation
• Tricuspid regurgitation, high CVP → falsely low SpO₂.
10. High Inspired Oxygen (FiO₂ 100%)
• SpO₂ remains ~100%, even if PaO₂ is dangerously high (>300 mmHg).
Advantages of Pulse Oximetry
✅ Non-invasive & continuous monitoring.
✅ Early detection of hypoxia.
✅ Useful in ICU, anesthesia, emergency settings.
✅ Helps in titrating oxygen therapy.
✅ Portable & easy to use.
Disadvantages of Pulse Oximetry
❌ Does not measure PaO₂ directly.
❌ Cannot detect hyperoxia (PaO₂ > 100 mmHg).
❌ Affected by motion artifacts, hypoperfusion, dyshemoglobinemias.
❌ Delayed response to sudden desaturation.
❌ Inaccurate when SpO₂ < 70%.
Clinical Applications of Pulse Oximetry
1. Perioperative Monitoring
• Continuous monitoring in anesthesia & surgery.
• Detects hypoxia, apnea, and respiratory depression.
2. Intensive Care Unit (ICU)
• Monitors hypoxia in ventilated patients.
• Guides oxygen therapy & weaning.
3. Emergency & Trauma
• Rapid assessment of respiratory distress, shock, CO poisoning.
• Helps decide intubation & oxygen therapy.
4. Neonatology
• Monitors oxygenation in preterm babies.
• Prevents retinopathy of prematurity (ROP) by avoiding hyperoxia.
5. Sleep Medicine
• Used in polysomnography to diagnose sleep apnea.
6. High-Altitude Medicine
• Identifies altitude sickness & hypoxia in climbers, pilots.
Troubleshooting Pulse Oximeter Errors
Problem
Possible Cause
Solution
No reading
Poor perfusion, sensor off
Warm hands, reposition sensor
Low SpO₂ falsely
Dark nail polish, motion artifacts
Remove nail polish, stabilize hand
High SpO₂ falsely
CO poisoning
Check ABG with CO-oximetry
Constant ~85% SpO₂
Methemoglobinemia
Give methylene blue
Unstable SpO₂
Arrhythmias, venous pulsation
Check ECG, relocate sensor
Key Takeaways
• Uses two wavelengths (660 nm & 940 nm) to measure SpO₂.
• Cannot differentiate between normal and dysfunctional hemoglobin (COHb, MetHb).
• Affected by perfusion, motion, and ambient light.
• Essential for ICU, anesthesia, and emergency care.
• Should be used alongside ABG for accurate oxygenation assessment.
