What is the principle behind a pulse oximeter (pulse oxygen meter)?

Principle of Pulse Oximetry

Pulse oximeters detect the variation in transmission of light at two different wavelengths (typically 660 nm red light and 990 nm infrared light) that occur with arterial pulsations in an extremity, exploiting the fact that oxygenated and deoxygenated hemoglobin absorb these wavelengths differently to estimate arterial oxygen saturation. 1

Core Operating Mechanism

Light Absorption Differential

• Oxygenated hemoglobin (O2Hb) absorbs more infrared light (990 nm) while deoxygenated hemoglobin (HHb) absorbs more red light (660 nm), allowing the device to distinguish between the two hemoglobin states 1, 2
• The device measures light absorbance of pulsating vascular tissue at these two wavelengths to calculate oxygen saturation 3

Arterial vs. Venous Blood Distinction

• The pulse oximeter isolates arterial blood by detecting only the pulsatile component of blood flow, which corresponds to arterial pulsations with each heartbeat 1, 4, 5
• This pulsatile signal allows the device to distinguish arterial blood from venous blood and surrounding tissues, which do not pulsate 4, 2
• The calculation is based on the variation in light absorption caused specifically by the pulsatility of arterial blood 5

Calibration and Calculation

• The relationship between measured light absorbances and oxygen saturation was developed empirically through studies in human volunteers and is built into the oximeter software as calibration algorithms 3, 5
• The device computes oxygen saturation (SpO2) using these pre-programmed algorithms that approximate the oxygenated fraction of available hemoglobin 1

Critical Limitations to Understand

Cannot Detect Abnormal Hemoglobins

• Standard two-wavelength pulse oximeters cannot distinguish carboxyhemoglobin (COHb) or methemoglobin (MetHb) because they use only two wavelengths of light 1, 3
• COHb and O2Hb have similar absorbances at 660 nm, causing pulse oximeters to measure COHb similarly to O2Hb, resulting in falsely elevated readings in carbon monoxide poisoning 1
• In CO-poisoned patients, SpO2 can read >90% even when COHb is ≥25% 1

Perfusion-Dependent Accuracy

• Poor perfusion of the extremity yields falsely low readings because adequate pulsatile flow is required for accurate measurement 1, 4
• Cold peripheries compromise pulsatile flow and can produce inaccurate readings 4
• The device requires adequate signal strength from arterial pulsations to function properly 1, 4

Skin Pigmentation Issues

• Dark skin color can interfere with signal detection and pulse oximeters systematically overestimate oxygen saturation in individuals with darker skin pigmentation 1
• Black patients have almost 3 times the frequency of hypoxemia missed by pulse oximetry compared with White patients 1

Other Interfering Factors

• Movement artifacts, stray light, and ambient light can yield spurious readings 1
• Intravenous dyes (particularly methylene blue and indocyanine green) cause errors due to their absorbance properties 3
• Nail polish, nail thickness, and abnormal heart rhythms affect accuracy 1, 5

Clinical Application Principles

Accuracy Parameters

• Pulse oximeters have reasonable accuracy with 95% confidence limits of 4-5% compared with directly measured arterial oxygen saturation, provided a good pulse signal is obtained 1
• Accuracy is thought to be less reliable at saturations below 88%, which is further exacerbated in Black patients 1

Appropriate Use

• Pulse oximeters are good for monitoring trending phenomena but not reliable for determining absolute magnitude of change 1
• Significant desaturation (change in SpO2 ≥5%) should be confirmed with arterial blood gases 1
• Physical examination and history should remain the primary assessment methods, with pulse oximetry used in context of complete assessment 1

Oxygen Dissociation Curve Limitation

• The device measures arterial oxygen saturation rather than PaO2, which is more relevant for assessing lung disease effects on gas exchange 1
• Despite a fall in arterial PaO2 to 70 mmHg, saturation would still remain above 93% because the oxygen dissociation curve at this point is insensitive to changes in PO2 1