Download Update 11 - Update in Anaesthesia - WFSA

14Update in Anaesthesia(i)(ii)(iii)(iv)(v)(vi)oxygenation, but no direct information aboutventilation, particularly as in this case, whensupplemental oxygen is being administered.Critically ill patients It may be less effective invery sick patients, because tissue perfusion may bepoor and thus the oximeter probe may not detect apulsatile signal.Waveform presence If there is no waveformvisible on a pulse oximeter, any percentage saturationvalues obtained are meaningless.Inaccuracies Bright overhead lighting, shiveringand motion artefact may give pulsatile waveformsand saturation values when there is no pulse.Abnormal haemoglobins such asmethaemoglobinaemia, for example followingoverdose of prilocaine, cause readings to tendtowards 85%.Carboxyhaemoglobin, caused by carbon monoxidepoisoning, causes saturation values to tend towards100%. A pulse oximeter is extremely misleading incases of carbon monoxide poisoning for this reasonand should not be used. CO-oximetry is the onlyavailable method of estimating the severity of carbonmonoxide poisoning.Dyes and pigments, including nail varnish, may giveartificially low values.Vasoconstriction and hypothermia cause reducedtissue perfusion and failure to register a signal.Rare cardiac valvular defects such as tricuspidregurgitation cause venous pulsation and thereforevenous oxygen saturation is recorded by theoximeter.Oxygen saturation values less than 70% areinaccurate as there are no control values to comparethem to.(vii) Cardiac arrhythmias may interfere with the oximeterpicking up the pulsatile signal properly and withcalculation of the pulse rate.NB. Age, sex, anaemia, jaundice and dark-skin havelittle or no effects on oximeter function.Lag monitor This means that the partial pressureof oxygen can have fallen a great deal before theoxygen saturation starts to fall. If a healthy adultpatient is given 100% oxygen to breathe for a fewminutes and then ventilation ceases for any reason,several minutes may elapse before the oxygensaturation starts to fall. A pulse oximeter in thesecircumstances warns of a potentially fatalcomplication several minutes after it has happened.The pulse oximeter has been described as “a sentrystanding at the edge of the cliff of desaturation.”because of this fact. The explanation of this lies inthe sigmoid shape of the haemoglobin / oxygendissociation curve (figure 1).Response delay due to signal averaging. Thismeans that there is a delay after the actual oxygensaturation starts to drop because the signal isaveraged out over 5 to 20 seconds. Patient safety there have been one or two casereports of skin burns or pressure damage under theprobe because some early probes had a heater unitto ensure adequate skin perfusion. The probe shouldbe correctly sized, and should not exert excessivepressure. Special probes are now available forpaediatric use.The penumbra effect re-emphasises the importanceof correct probe positioning. This effect causesfalsely low readings and occurs when the probe isnot symmetrically placed, such that the pathlengthbetween the two LEDs and the photodetector isunequal, causing one wavelength to be“overloaded”. Repositioning of the probe often leadsto sudden improvement in saturation readings. Thepenumbra effect may be compounded by thepresence of variable blood flow through cutaneouspulsatile venules. Note that the waveform mayappear normal despite the penumbra effect as itmeasures predominantly one wavelength only.Alternatives to pulse oximetry?Bench CO-oximetry is the gold standard - and isthe classic method by which a pulse oximeter iscalibrated. The CO-oximeter calculates the actualconcentrations of haemoglobin, deoxyhaemoglobin,carboxyhaemoglobin and methaemoglobin in thesample and hence calculates the actual oxygensaturation. CO-oximeters are much more accuratethan pulse oximeters - to within 1%, but they give a‘snapshot’ of oxygen saturation, are bulky, expensiveand require constant maintenance as well as requiringa sample of arterial blood to be taken.