How are oxygen saturation calculations done using pulse oximetry (pulse oximeter)?

How Oxygen Saturation Calculations are Done Using Pulse Oximetry

Pulse oximetry calculates oxygen saturation by measuring the differential absorption of two wavelengths of light (typically red and infrared) by oxygenated and deoxygenated hemoglobin in arterial blood. This non-invasive technique provides continuous monitoring of arterial oxygen saturation (SpO2) without requiring blood samples.

Principles of Pulse Oximetry Measurement

Physical Basis

• Pulse oximeters use spectrophotometric principles to measure oxygen saturation
• The device emits two wavelengths of light:
  • Red light (typically 660 nm)
  • Infrared light (typically 940 nm)
• Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through
• Deoxygenated hemoglobin absorbs more red light and allows more infrared light to pass through

Calculation Method

1. The pulse oximeter detects pulsatile blood flow by identifying changes in light absorption that occur with each heartbeat 1
2. It measures the ratio of absorption between the two wavelengths during the pulsatile component (AC) and the non-pulsatile component (DC)
3. The ratio (R) is calculated as: R = (AC660/DC660)/(AC940/DC940)
4. This ratio is then converted to SpO2 using a calibration curve derived from empirical data 2

Technical Aspects of Measurement

Signal Processing

• Modern pulse oximeters use sophisticated algorithms to:
  • Filter out ambient light and motion artifacts
  • Identify the arterial pulsation component
  • Calculate the ratio of red to infrared light absorption
  • Convert this ratio to SpO2 using calibration equations 3

Calibration

• Pulse oximeters are calibrated using data from healthy volunteers whose arterial blood samples were analyzed at various oxygen saturation levels
• Most devices are calibrated for accuracy between 70-100% saturation
• Below 70% saturation, accuracy decreases significantly 4

Accuracy and Limitations

Normal Values

• Normal SpO2 ranges:
  • 18-24 years: 96.9% (±0.40%)
  • 25-34 years: 96.7% (±0.7%)
  • 35-44 years: 96.7% (±0.6%)
  • 45-54 years: 96.5% (±1%)
  • 55-64 years: 95.1% (±0.7%)

  • 64 years: 95.5% (±1.4%) 1

Accuracy Considerations

• Pulse oximetry has high accuracy (>95%) when SpO2 is above 90%
• Agreement rate between SpO2 and directly measured arterial oxygen saturation (SaO2) is approximately 83.2% 2
• The mean difference between SpO2 and SaO2 is typically +5.5% (±4.2%) over the range of 22-100% 4

Important Limitations

• Pulse oximetry cannot distinguish between different types of hemoglobin:
  • Cannot detect carboxyhemoglobin (falsely elevated readings in carbon monoxide poisoning)
  • Cannot detect methemoglobin (readings tend toward 85% regardless of actual saturation) 3
• Other factors affecting accuracy:
  • Poor peripheral perfusion
  • Motion artifacts
  • Ambient light interference
  • Dark skin pigmentation
  • Nail polish or artificial nails 1

Advanced Applications: Co-oximetry

• Co-oximetry is an advanced technique that overcomes some limitations of standard pulse oximetry
• Uses multiple wavelengths of light (typically 4 or more) to distinguish between:
  • Oxyhemoglobin
  • Deoxyhemoglobin
  • Carboxyhemoglobin
  • Methemoglobin 3
• Essential for diagnosing conditions like carbon monoxide poisoning and methemoglobinemia
• Modern blood gas analyzers with co-oximetry can directly measure concentrations of different hemoglobin species 3

Clinical Applications

• Continuous monitoring of oxygenation status during:
  • Surgery and anesthesia
  • Critical illness
  • Exercise testing
  • Sleep studies
• Guiding oxygen therapy titration
• Detecting early hypoxemia before clinical signs appear

Pulse oximetry has revolutionized clinical monitoring by providing continuous, non-invasive assessment of oxygenation status. Understanding its principles, limitations, and proper use is essential for accurate interpretation of results and appropriate clinical decision-making.