Pulse Oximetry: How It Works and What It Measures

Pulse oximetry noninvasively measures arterial oxygen saturation (SpO₂).

Principle

Oxygenated hemoglobin absorbs more infrared light; deoxygenated hemoglobin absorbs more red light. A probe emits both wavelengths through a fingertip or earlobe. A photodetector measures transmitted light, and a microprocessor calculates the ratio to determine SpO₂.

Key Points

• Painless, rapid, continuous monitoring capability.
• Used in hospitals, clinics, and home settings (e.g., COPD, sleep apnea).
• Limitations: poor perfusion, motion artifact, nail polish, carbon monoxide poisoning (falsely normal SpO₂), methemoglobinemia.