Reliability of pulse oximetry

[Russell Coupland]

Today, we would never be without our SPO₂ monitor. We are taught they sometimes read false low due to artefacts, but never read false high.

We had a case a few days ago where the pulse oximeter was reading a steady 99% with a heart rate of 76beats/min. We then noticed that the EKG was obviously faster than 76 - probably in the low 100's. We checked the monitor - the heart rate was set to read from the oximeter (- I often do this as a default as I often do not apply EKG to young healthy patients). We then checked the finger probe - it was STILL CLIPPED TO THE STAINLESS STEEL ANESTHETIC MACHINE TROLLEY! yet it gave a perfect waveform, a rate of 76 and a saturation of 99%.

We placed the probe on the patient and instantly the saturation reading changed to the actual value of about 96% and the heart rate showed about 100/min.

How could a non-applied probe give such a falsely reassuring reading? I have my suspicions that it was an incompatibility of the probe (Nelcor) with the monitor (Datex-Ohmeda). Any other ideas?