Introduction

Pulse Oximetry is a way of measuring the oxygen level in the blood.

It is a measure of oxygenation but not a measure of ventilation

  • Oxygenation – the ability f the body (particularly lungs) to maintain high oxygen concentration in the blood
  • Ventilation – the ability of the lungs to adequately provide oxygenation and remove carbon dioxide from the blood

Therefore, pulse oximetry provides on indication of the carbon dioxide levels, or of the adequacy of ventilation. To get a better estimation of this, an arterial blood gas is required.

Normal Pulse Oximetry Values

  • A normal healthy individual usually has a value in the high 90s%
  • Textbook definition of normal is >93%
  • ‘Normal’ for an individual with COPD or other chronic lung disease can be 88-92%

Estimating pO2 from oxygen saturations

Looking at the curve below, you can see how the curve starts to dive sharply downwards below 90%. This is why the ‘normal’ saturation cut-off is so important.

Oxygen Dissociation Curve for Pulse Oximetry

Oxygen Dissociation Curve

How it Works

Pulse oximetry measures the ratio of red to infrared light absorbed by the tissues. This ratio changes because the colour of haemoglobin changes depending on how saturated with oxygen it is.

The method was developed in the 1980s. Prior to this, the only way to assess oxygen saturations was with an arterial blood gas sample.

Limitations

  • Depends on the adequacy of peripheral circulation. If the peripheries are cool – for example if the patent is shut down because they are very sick, then a peripheral pulse oximeter will not give a reliable reading. This problem can be alleviated by using a more centrally placed oximeter – such as a sticker on the forehead.
  • Not reliable in carbon monoxide poisoning. Carbon Monoxide strongly binds to haemoglobin and can give a falsely elevated oxygen saturation. To properly assess oxygenation in CO poisoning, an arterial blood gas is required, which can give the carboxyhemoglobin concentration.
  • Not reliable at very low saturations – below 70% the readings are not clinically reliable.

 

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