Effect of isometric thumb preload on the evoked compound muscle action potential

The effect of thumb preload on adductor pollicis muscle compound action potential (ECAP) was assessed after supramaximal ulnar nerve stimulus during steady isoflurane anaesthesia in 20 ASA I-II patients without neuromuscular block. During thumb preload, the peak-to-peak amplitude of the ECAP increased by 0.9 mV, to 9.5 (SD 2.2) mV (P < 0.01), because of the increased height of the negative half-wave. The positive half-wave amplitude did not change, but its duration was prolonged by 0.7 ms, to 6.4 (0.7) ms (P < 0.05). Consequently, there was a 15.9% increase in the area of the response curve (P < 0.01). Without preload, the first dorsal interosseus muscle peak-to-peak amplitude was 13.0 (4.3) mV. Thus thumb preload may not be the solution to improved monitoring of neuromuscular block by ECAP.