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At this level there is a marked fall in PaO2 representing serious
hypoxia.
In the following situations the pulse oximeter readings may not be
accurate:
- A reduction in peripheral pulsatile blood flow produced by peripheral
vasoconstriction (hypovolaemia, severe hypotension, cold, cardiac failure,
some cardiac arrhythmias) or peripheral vascular disease. These result in
an inadequate signal for analysis.
- Venous congestion, particularly when caused by tricuspid regurgitation,
may produce venous pulsations which may produce low readings with ear
probes. Venous congestion of the limb may affect readings as can a badly
positioned probe. When readings are lower than expected it is worth
repositioning the probe. In general, however, if the waveform on the flow
trace is good, then the reading will be accurate.
- Bright overhead lights in theatre may cause the oximeter to be
inaccurate, and the signal may be interrupted by surgical diathermy.
Shivering may cause difficulties in picking up an adequate signal.
- Pulse oximetry cannot distinguish between different forms of
haemoglobin. Carbo-xyhaemoglobin (haemoglobin combined with carbon
monoxide) is registered as 90% oxygenated haemoglobin and 10% desaturated
haemoglobin - therefore the oximeter will overestimate the saturation. The
presence of methaemoglobin will prevent the oximeter working accurately and
the readings will tend towards 85%, regardless of the true saturation.
- When methylene blue is used in surgery to the parathyroids or to treat
methaemoglobinaemia a shortlived reduction in saturation estimations is
registered.
- Nail varnish may cause falsely low readings. However the units are not
affected by jaundice, dark skin or anaemia
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