Evoked electromyographic and mechanical responses of the adductor pollicis compared during the onset of neuromuscular blockade by atracurium or alcuronium, and during antagonism by neostigmine.

The effects of atracurium and alcuronium on the evoked mechanical and electromyographic responses of the adductor pollicis were investigated in 30 adult patients. The ulnar nerve was stimulated with trains of four supramaximal pulses of 0.2 ms duration and a frequency of 2 Hz at intervals of 20 s. The mechanical response was measured using a strain gauge force transducer and the evoked compound action potential (ECAP) was recorded simultaneously using a Medelec MS91 electromyography system. Depression of the electromyographic and mechanical responses, caused by atracurium, was similar during onset and antagonism of blockade. During onset of blockade by alcuronium, the mechanical response was depressed to a significantly greater extent compared with the ECAP. This difference was not observed during antagonism with neostigmine. The rates of onset of atracurium and alcuronium were generally similar, but the rate of onset of mechanical fade was greater in the alcuronium group. There was no significant quantitative difference between the rates of antagonism of alcuronium and atracurium when assessed by the mechanical first response ratio. Electromyographic first response and electromyographic and mechanical train-of-four ratios recovered more rapidly in the atracurium group. These findings suggest that the differences between mechanical and electromyographic measurements of neuromuscular blockade are drug-specific and are more pronounced during the onset of blockade than during its antagonism by neostigmine.