Effect of surface electrode position on the compound action potential evoked by ulnar nerve stimulation during isoflurane anaesthesia.

The effect of surface electrode positioning on the evoked compound action potential was studied during isoflurane anaesthesia without neuromuscular block. In 20 ASA I-II patients (age 36-50 yr), the response after supramaximal ulnar nerve stimulation was analysed with a neuromuscular relaxation monitor (Relaxograph, Datex) and with a memory recorder. Seven pairs of surface recording electrodes were compared: (1) adductor pollicis muscle vs second finger; (2) adductor pollicis m. vs first finger; (3) first dorsal interosseus m. vs second finger; (4) abductor digiti minimi m. vs fifth finger; (5) adductor pollicis m. vs second dorsal metacarpal; (6) abductor digiti minimi m. vs fourth dorsal metacarpal; (7) thenar vs hypothenar. Steady biphasic response was recorded with electrode pairs 1-4. Marked variation and low amplitude existed at pairs 5-7. Peak-to-peak amplitude at pair 3 was the greatest (12.5 (SD 3.7) mV) compared with pair 4 (9.4 (SD 2.0) mV) and pair 1 (8.5 (SD 2.0) mV). A close correlation between the amplitudes and integrated areas was found. The first dorsal interosseus muscle response was optimal and the electrodes were simple to fix; this site may be recommended for clinical monitoring.