Describe the physical principles of the pulse oximeter. What are the limitations of the technique?

Most anaesthetists believe that continuous measurement of oxygen saturation during anaesthesia is absolutely essential. Equally essential, therefore, is a broad understanding of how the technique works with particular reference to its limitations and potential sources of error.

Introduction

Pulse oximetry has been widely available for only about 12 years but very rapidly has become established as arguably the single most important form of monitoring in anaesthetic practice.

Physical principles

• Oxygenated haemoglobin (HbO2) and deoxygenated haemoglobin (Hb) have differential absorption spectra.

• At a wavelength of 660 nm (red light) HbO2 absorbs less than Hb, hence its red colour.

• At a wavelength of 940 nm (infra-red light) this is reversed and Hb absorbs more than HbO2. At 800 nm – the isobestic point - the absorption coefficients are identical.

• The pulse oximeter uses two light emitting diodes which emit pulses of red (660 nm) and infra-red (980 nm) light every 5–10 μs from one side of the probe. The light is transmitted through tissue to be sensed by a photocell.

• The output is submitted to electronic processing during which the absorption of the blood at the two different wavelengths is converted to a ratio which is compared to an algorithm produced from experimental data.

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