Pulse oximetry is a non-invasive method of measuring the haemoglobin (Hb) oxygen saturation (SpO2) in arterial blood. A multi-centre case control study carried out between 2002 and 2004 looking at peri-anaesthetic mortality in cats showed reduced odds of death if pulse monitoring and pulse oximetry were used as part of patient monitoring (Brodbelt et al. 2007).
How does it work?
The technique utilises a clip placed over a peripheral area of tissue, most commonly the tongue although other areas such as the lip, ear, vulva and scrotum may be used. Other types of probe are available that may be wrapped around tissues or used in the rectum. The clip contains two light emitting diodes (LED) on one side and a photo-detecting crystal on the opposite side. One LED emits red light and the other infra-red (IR) light. The LEDs come on one at a time, followed by a pause with neither on, this allows differentiation between oxy and deoxyhaemoglobin and compensation for any natural light. Absorption of red and IR light differs between the Hb species and pulsatile blood flow is used to distinguish arterial from venous blood. The pulse oximeter using this data and a built-in algorithm calculates SpO2.
More advanced units, most commonly used in medical anaesthesia are available that emit up to 8 wavelengths of light and can differentiate between types of haemoglobin.
SpO2 should be above 97% in the anaesthetised patient breathing 100% oxygen and any persistent value below this should be investigated. The first step that often remedies most problems is to change the probe position – this allows tissue beds to re-perfuse. If SpO2 is low then the HR measured by the monitor should be checked against a manual rate from the patient. If they do not agree the SpO2 is likely to be inaccurate. If the value is definitely low then the patient, anaesthetic machine, gas cylinders and breathing system should be checked and then checked again for a potential fault.
Recent advances in signal extraction technology have improved the accuracy and reliability of pulse oximetry. The Masimo monitor is proven to cope better than others with movement and vasoconstriction. They are now available commercially.
Pulse oximetry is a useful monitoring tool because it requires blood flow and therefore tissue perfusion to display any information. The addition of a displayed HR and in some cases a plethysmograph (pulse waveform) add further information on pulse rate and quality. Errors may be introduced by factors such as movement, peripheral vasoconstriction, ambient light and inadequate or excess tissue depth. Vasoconstriction and thin tissue depth may be a particular problem in the cat but should not discourage its use.