Fetal haemoglobin (HbF), SulphHb, Bilirubin (absorption peaks are 460, 560 and 600 nm), dark skin.
Falsely low reading
Methaemoglobin (MetHb). The presence of MetHb will prevent the oximeter from working accurately and the readings will tend towards 85%, regardless of the true saturation.
Methylene blue. When methylene blue is used in surgery (e.g. parathyroidectomy or to treat methaemoglobinaemia), a short-lived reduction in saturation estimations is seen. Readings may fall by 65% at a concentration of 2-5 mg/kg for between 10 and 60 minutes.
Indocyanine green. Use of this dye (e.g. in cardiac output studies) may cause a transient reduction in recorded saturations.
A reduction in peripheral pulsatile blood flow produced by peripheral vasoconstriction results in an inadequate signal for analysis.
Venous congestion, which may be caused by tricuspid regurgitation, high airway pressures and the Valsalva manoeuvre, may produce venous pulsations which can produce low readings.
Venous congestion of the limb may affect readings, as can a badly positioned probe.
External fluorescent light in the operating 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.
Nail varnish may cause falsely low readings.
Falsely high reading
Carboxyhaemoglobin (CoHb). CoHb (haemoglobin combined with carbon monoxide) is registered as 90% oxygenated haemoglobin and 10% desaturated haemoglobin - therefore the oximeter will overestimate the saturation.
Oximeters are calibrated during manufacture and automatically check their internal circuits when they are turned on. They are accurate in the range of oxygen saturations of 70% to 100% (+/-2%), but are less accurate under 70%. Below the saturation of 70%, readings are extrapolated. The data for calibration came from human volunteer studies, hence it was unethical to allow the saturations to fall below 70%. Due to the shape of the oxyhaemoglobin curve, the saturation starts to fall rapidly at 90%.
The oximeter averages its readings every 10-20 seconds. Hence, they cannot detect acute desaturation. The finger probe has a response time of approximately 60 seconds, whereas the ear probe has a response time of 10-15 seconds.
The site of application should be checked at regular intervals, as pressure sores and burns have been reported.
The pulse oximeter only provides information about oxygenation. It does not give any indication of the patient’s carbon dioxide elimination.
[i] Pulse oximetry for perioperative monitoring: systematic review of randomized, controlled trials.
Pedersen T, Moller AM, Pedersen BD.
Anesth Analg. 2003 Feb;96(2):426-31
[ii] Pulse oximetry for perioperative monitoring.
Pedersen T, Pedersen P, Moller AM.
Cochrane Database Syst Rev. 2001;(2):CD002013