What is the principle by which a pulse oximeter measures arterial oxygen saturation (SpO₂)?

Pulse Oximeter Operating Principle

Standard pulse oximeters measure arterial oxygen saturation (SpO₂) by transilluminating tissue with two wavelengths of light—red (660 nm) and infrared (990 nm)—and analyzing the differential absorption between oxygenated and deoxygenated hemoglobin in the pulsatile arterial blood component. 1

Core Measurement Mechanism

The fundamental principle relies on the distinct light absorption properties of different hemoglobin species:

• Oxygenated hemoglobin (O₂Hb) absorbs more infrared light (990 nm) while allowing red light to pass through more readily 2
• Deoxygenated hemoglobin absorbs more red light (660 nm) and transmits infrared light more effectively 2, 3
• The device calculates the ratio of these differential absorbances to determine oxygen saturation 3

The pulse oximeter isolates arterial blood by detecting only the pulsatile component of blood flow, which corresponds to the surge of arterial blood with each heartbeat, effectively filtering out venous blood, tissue, and bone 2

Critical Technical Limitations

Two-Wavelength Constraint

Standard pulse oximeters using only two wavelengths (660 and 990 nm) cannot differentiate carboxyhemoglobin (COHb) from oxyhemoglobin because these species have similar extinction coefficients at 660 nm 1

• COHb is measured similarly to O₂Hb, causing falsely elevated SpO₂ readings in carbon monoxide poisoning 1
• In CO-poisoned patients with COHb ≥25%, pulse oximeters showed SpO₂ >90% in all cases despite significant carboxyhemoglobin levels 1
• The device cannot distinguish more than two hemoglobin species, so methemoglobin (MetHb) also causes measurement errors 3

Empirical Calibration Requirement

The relationship between measured light absorbances and actual oxygen saturation was developed empirically through calibration studies and is programmed into the oximeter software 3

• This empirical calibration introduces an inherent accuracy limitation of approximately ±4-5% even under optimal conditions 2, 3
• Newer calibration-free approaches using two nearby infrared wavelengths (780-808 nm) are under investigation to eliminate this error source 4

Perfusion-Dependent Accuracy

Adequate pulsatile arterial flow is essential for reliable measurements:

• Poor peripheral perfusion yields falsely low readings because the device requires sufficient pulsatile signal amplitude to distinguish arterial from venous blood 2
• Conditions causing poor perfusion include hypothermia, hypotension, vasoconstriction, and peripheral vascular disease 2, 5
• Verify signal quality by confirming the displayed heart rate matches the ECG or palpated pulse—mismatched rates indicate unreliable readings 2

Oxygen Dissociation Curve Limitations

Pulse oximetry is insensitive to changes in PaO₂ when oxygen tension is high:

• When PaO₂ exceeds 100 mmHg, saturation measurements will not detect further increases because the hemoglobin is already fully saturated on the flat portion of the dissociation curve 3
• Even with PaO₂ falling to 70 mmHg, SpO₂ typically remains above 93% due to the curve's shape 2
• Accuracy is less reliable at saturations below 88%, particularly in patients with darker skin pigmentation 2

Systematic Measurement Biases

Dark skin pigmentation systematically interferes with signal detection and causes pulse oximeters to overestimate oxygen saturation 2

• This racial bias is clinically significant and can mask hypoxemia in Black patients 2
• Accuracy deteriorates further at lower saturations (<88%) in individuals with darker skin 2

Movement artifact during measurement causes significant errors and data dropout, requiring the measurement site to remain still 2

Intravenously administered dyes cause errors due to their light absorption properties, particularly methylene blue and indocyanine green 3

Clinical Application Principles

Pulse oximeters excel at monitoring trending phenomena but are not reliable for determining absolute magnitude of change 2

• The device provides continuous, noninvasive assessment of oxygenation status 3
• When clinical assessment suggests respiratory compromise, never rely solely on pulse oximetry—obtain arterial blood gas analysis for definitive evaluation 2
• Alternative measurement sites (ear lobe, forehead) may provide more reliable readings when peripheral perfusion is compromised 2, 5