Apparent arterial desaturation due a nerve stimulator.

Abstract

At our institution, we commonly monitor the onset of neuromuscular blockade following the administration of non-depolarising muscle relaxants. We would like to report a case where a false low pulse oximetry reading resulted from the concomitant use of a peripheral nerve stimulator. A 54 year-old woman was anaesthetised for routine surgery with fentanyl, propofol, and vecuronium. The lungs were easily ventilated with oxygen. Peripheral nerve stimulator electrodes were placed over the ulnar nerve to monitor the onset of the neuromuscular blockade, using a frequency of one-stimulus per second. Within 10 s of turning the nerve stimulator on, the pulse oximetry (SpO2) reading fell from 100% to 80% despite on-going bag-mask ventilation. After confirming adequate ventilation as evidenced by the end-tidal CO2 trace and good chest movement, we noticed that the SpO2 tracing was abnormal with a double-peak appearance. The pulse oximeter probe was present on the left index finger and the peripheral nerve stimulator was placed over the left ulnar nerve. We surmised the nerve stimulator and the resultant muscle twitch was causing an artifact in the SpO2 reading. When the nerve stimulator was turned off, the SpO2 tracing returned to a single-peak pulse waveform, and the saturation reading immediately returned to 100%. To confirm the SpO2 artifact from the peripheral nerve stimulator, it was turned back on. The SpO2 returned to around 80% within 10 heart beats. As the neuromuscular blockade took effect, the muscle twitches from the peripheral nerve stimulator diminished and the SpO2 returned to 100%. Though previously reported [1,2], it is not common knowledge that peripheral nerve stimulation can cause pulse oximetry artifact and artificially low SpO2 readings. The muscle twitch can create an overwhelming pulsatile component as measured by the pulse oximeter. This can result in artificial oxygen saturation readings ranging from 75% to 95%. The effect of the nerve stimulator on the SpO2 tracing is related to the device output and frequency, causing the greatest interference when the patient's heart rate is close to 60 bpm and the stimulator is set on a repetitive, 1 Hz pulse (60 cycles.min−1), with high output settings (>5 milliamp) [1]. The problem can be simply avoided by placing the nerve stimulator electrodes and the pulse oximeter probe on different arms.