Pulse Oximetry

Introduction

Pulse oximetry measures peripheral arterial oxygen saturation (SpO2) as a surrogate marker for tissue oxygenation.

  • It has become the standard for continuous, non-invasive assessment of oxygenation.
  • Pulse oximetry uses spectrophotometry to determine the proportion of haemoglobin that is saturated with oxygen (i.e. oxyhaemoglobin) in peripheral arterial blood.



How to Use a Pulse Oximeter

NOTES:

  • Ensure the finger you are going to use (index finger or middle finger) does not have any nail varnish, polish or a false nail on it, as these block the light sensor.
  • The best reading is achieved when your hand is warm, relaxed and held below the level of your heart.
  • Skin pigment significantly impacts the accuracy of pulse oximetry. Extensive recent data shows that in patients with darker skin tones, pulse oximetry often overestimates oxygen saturation, leading to clinically important hypoxaemia remaining undetected and untreated.
  • Motion can induce considerable error into pulse oximetry accuracy, resulting in loss of data, inaccurate readings, and false alarms.
  • If you smoke, the reading on your oximeter may be higher than your actual oxygen saturation. This is because smoking increases carbon monoxide levels in your blood, and the oximeter cannot tell the difference between the gas carbon monoxide from oxygen.



Optimal Peripheral Oxygen Saturation

There is no single optimal level of oxygen saturation below which tissue hypoxia uniformly occurs because of the large number of variables that contribute to hypoxia at the cellular level (temperature, pH, tissue blood flow).

  • The desired SpO2 must be individualized for each patient, taking into account their baseline and underlying disease process.
  • A resting SpO2 of 96% could be highly worrisome if a patient previously had a documented resting baseline of 99%.
  • A target SpO2 between 88-92% may be sufficient in a patient with an acute exacerbation of COPD who is chronically hypercapnic. In contrast, a higher target of SpO2 of >95% is generally considered optimal in a pregnant female with acute respiratory distress syndrome to ensure adequate foetal oxygenation.



Summary

While pulse oximetry is a rapid, non-invasive tool that provides continuous monitoring of oxygenation, it is relatively insensitive to detecting hyperoxemia and can miss clinically significant hypoxemia in specific demographics and physiological states.

  • It should always be paired with a physical clinical assessment.



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