What a Pulse Oximeter Measures
A pulse oximeter measures arterial oxygen saturation (SpO2) and heart rate by detecting light absorption through pulsating vascular tissue. 1
Primary Parameters Measured
The pulse oximeter fundamentally measures:
- SpO2 (peripheral oxygen saturation): The percentage of hemoglobin saturated with oxygen in arterial blood, expressed as a percentage 1
- Heart rate/pulse rate: Derived from the pulsatile waveform detected by the device 2
The device works by shining light at two wavelengths (typically red and infrared) through tissue and measuring the light absorbance of pulsating vascular tissue. The relationship between measured light absorbances and saturation was developed empirically and is built into the oximeter software 2, 3.
Important Distinction from Invasive Measurement
SpO2 is NOT the same as SaO2 (arterial oxygen saturation measured directly from arterial blood gas analysis). The "p" in SpO2 specifically denotes that it is measured by pulse oximetry, not invasively 1. Manufacturers typically claim accuracy within 2% standard deviation, but this translates to expected errors of 4% or more in 5% of examinations 3.
Additional Parameters in Advanced Devices
Newer multi-wavelength pulse oximeters can measure beyond basic SpO2 4:
- Carboxyhemoglobin (SpCO)
- Methemoglobin (SpMet)
- Total hemoglobin (SpHb)
- Perfusion Index (PI): The ratio of pulsatile to non-pulsatile infrared signal, expressed as a percentage 5
- Pleth Variability Index (PVI): Dynamic changes in PI during the respiratory cycle 5
- Respiratory rate: Can be derived from waveform analysis 4
However, these advanced measurements have limitations in accuracy and precision, particularly SpHb for transfusion decisions 4.
Normal Values by Age
According to BTS guidelines, normal SpO2 values vary by age 1:
- Ages 18-24: Mean 98.0% (range 97-99%)
- Ages 25-34: Mean 97.6% (range 97-99%)
- Ages 45-54: Mean 96.8% (range 96-98%)
- Ages ≥65: Mean 95.8% (range 95-98%)
Critical Limitations to Recognize
Standard two-wavelength pulse oximeters cannot distinguish more than two hemoglobin species 2. This means:
- Carbon monoxide poisoning (COHb) causes falsely elevated SpO2 readings
- Methemoglobinemia (MetHb) causes inaccurate readings
- Intravenous dyes (especially methylene blue and indocyanine green) cause significant errors 2
The device may fail to detect adequate signal during poor perfusion states, hypotension, or hypothermia 2, 3. Additionally, SpO2 is insensitive to changes in PaO2 when oxygen levels are above 100 mmHg due to the flat portion of the oxyhemoglobin dissociation curve 2.