Oxygen (Super) saturation?

Sir,

We recently noticed a pulse oximeter (L and T, Star 50) showing saturation in a range of 195 to 199% [Figure 1]. Such a false high value has not been reported before.

Figure 1.

Monitor showing false high saturation

Pulse oximetry is one of the most commonly employed monitoring modalities in the critical care setting. Numerous studies have found that pulse oximeters have a reasonable degree of accuracy. This degree of accuracy, coupled with the ease of operation of most instruments, has led to the widespread use of pulse oximetry for continuous and noninvasive assessment of arterial oxygen saturation in ICU patients, and it has also been called the “fifth vital sign”.[1] In addition to the digital display of oxygen saturation, most pulse oximeters also display a plethysmographic waveform which can help clinicians distinguish an artefact from the true signal.

Pulse oximeters are calibrated during manufacture and the data for calibration comes from human volunteer studies. They are less accurate for values under 70% because it is unethical to allow the saturations to fall below 70% in volunteers. Therefore, below 70% saturation, the readings are extrapolated. Between 70 to 100%, the accuracy is in the range of ±2%.[2]

The accuracy of the pulse oximeter may be affected by improper probe placement resulting in light from only one of the two light-emitting diodes passing through the tissue, causing either a falsely elevated or depressed reading.[3] Placement of the sensor on the same extremity as a blood pressure cuff or arterial line can cause erroneous readings and should be avoided. Motion artefacts like shivering, seizure activity, pressure on the sensor, or transport of the patient are significant source of error and false alarms. Inaccurate readings from an oximeter may also result from intravenous dyes such as methylene blue, indocyanine green, and indigo carmine, nail polish, and low perfusion states, such as low cardiac output, vasoconstriction, and hypothermia. Carboxyhaemoglobin (CoHb) is registered as 90% oxygenated haemoglobin and 10% desaturated haemoglobin – therefore the oximeter will overestimate the saturation.[4]

All these potential causes of error were ruled out in the reported case. The plethysmographic tracing was normal, and the rest of the parameters of the patient were also displayed normally.

This false high range of values continued for about half an hour, after which the module was opened and cleaned by a service engineer. Subsequently, the monitor functioned normally. No satisfactory explanation for this error was given by the manufacturing firm.

This is probably the first reported incidence of false high saturation value due to a technical problem in the monitor.