Effect of surface electrode positioning on the compound action potential evoked by ulnar nerve stimulation in anaesthetized infants and children.

The effect of surface electrode positioning on the evoked compound action potential (ECAP) was studied during inhalation anaesthesia without neuromuscular blockade. The ECAP of five full-term infants (aged 2-20 weeks) and five children (age 1-10 yr) was recorded with a neuromuscular relaxation monitor (Relaxograph, Datex) after supramaximal ulnar nerve stimulation. The five recording electrode positions compared were: thenar (adductor pollicis) v. second finger (TD2); thenar v. second dorsal metacarpal interspace (TM2); hypothenar v. fifth finger (HD5); hypothenar v. fourth dorsal metacarpal interspace (HM4); thenar v. hypothenar (TH). The ECAP was steady at positions TD2 and HD5, but marked variation and baseline drift was found at TM2, HM4 and TH. The ECAP peak-to-peak amplitudes were twice as great in children compared with infants. The hypothenar ECAP was liable to stimulus artefact interference because of the short onset latency at HD5 (2.2(SD 0.4)ms), whereas the adductor pollicis (3.0(SD 0.5)ms) position (TD2) yielded reliable results.