LAUSR

In 2017, Naguib and colleagues composed an elegant and informative overview of the practical and technical nuances of neuromuscular monitoring, NMM [1]. Despite having the capacity to reduce postoperative residual curarization, undereducation on potential NMM pitfalls have been reported to contribute to their suboptimal perioperative use [2, 3]. During the testing and refinement of a neuromuscular monitoring smartphone application, originally published in this journal, we have frequently encountered neuromuscular monitoring artefacts due to aberrant transcutaneous nerve stimulation [4]. With the present case report, we intend to draw the reader’s attention to a non-reported artefactual confounder of neuromuscular monitoring: stimulating electrode fatigue—the loss of electrode conductive capacity with time. Electrodes used for NMM stimulation purposes are within the extraneural electrode category, aiming to trigger a transcutaneous nerve depolarisation and so elicit contraction of (a) muscle(s) innervated by that nerve [5]. The most frequently used electrodes for this purpose are the silver/silver-chloride coated polymer (the typical electrocardiogram) subtype. These are an example of ideally non-polarizable electrodes (aka “faradaic”), having a negligible capacitive component [5]. These electrodes generally have some sort of stability/adhesive support (mostly foam or cloth), with a hydrogel layer serving as the interface between the metal electrode and the patient’s skin. This layer offers a compensated-for source of impedance (around 200–220 Ohm for standard hydrogel adult/paediatric electrodes) [6]. Since modern neuromuscular monitors/stimulators are of the variable-voltage–constant-intensity type, the voltage needed to arrive at the required supra-maximal current, and as derived from Ohm’s law [Voltage (V) = Intensity (A) × Resistance (Ohms)], is directly proportional to the encountered resistance. The latter is mostly due to the electrode itself and the patient’s skin. Commonly used anaesthesia devices can deliver up to 300 V for a single discharge [7]. Temperature spikes associated with such electrical currents are theoretically supraphysiologic and above the recommended operating temperatures of these electrodes (10–30 °C as per most fabricant specifications) [6]. Intrinsic electrode and hydrogel deterioration, especially with repetitive discharges, is thus possible. It is for this reason that correct skin preparation and other impedance-reducing actions such as skin normothermia maintenance are called for by experts [8]. Illustrative of the limitations of common electrodes, our research group observed and recorded a case of stimulating electrode fatigue. Its present reporting was writtenly consented by the patient. A 72-year-old male undergoing a supratentorial meningioma resection under general anaesthesia with TIVA (total intravenous anaesthesia) was intermittently paralysed with rocuronium boli. Continuous electromyographic Train of four (TOF) monitoring (Datex-Ohmeda E-NMT module with ElectroSensor; TOF monitoring only, no tetanic/posttetanic stimulation) was employed throughout the procedure. Train-of-four quantifications were done at a variable 1to 5-min interval, except for the intubation period, where these were done at 20-s intervals. The total anaesthesia time was 7 h 10 min. A pre-curarization calibrated supramaximal stimulus intensity of 35 mA was retrogradely delivered over the left ulnar nerve, with EMG potentials collected over the ipsilateral thenar eminence and thumb. The respective arm was placed in a padded arm support alongside the body, with the hand resting in a supinated neutral position. Involving and non-restrictive padding was used to fixate the hand. * Hugo Carvalho [email protected]