The staircase phenomenon: implications for monitoring of neuromuscular transmission

Repeated indirect stimulation enhances the evoked mechanical response of muscle (the staircase phenomenon). There are few data that document the magnitude of this effect in man. Inexpensive acceleromyographic monitors of neuromuscular function are now available. If these units are to be used as scientific tools or clinical monitors, additional information regarding how to achieve proper baseline stabilization and calibration is needed.

Anesthesia was induced and maintained with nitrous oxide, propofol, and an opioid. Tracheal intubation or laryngeal mask insertion was accomplished without muscle relaxants. Thirty adult patients classified as American Society of Anesthesiologists physical status I or II were divided into groups of 10. The mechanical response of the thumb to supramaximal ulnar nerve stimulation was recorded continuously with an acceleromyograph. Group 1 had train-of-four stimuli at 15-s intervals for 25 min. Group 2 had single stimuli at 1.0 Hz for 10 min. Group 3 had the same stimuli as group 1 except that a 50-Hz tetanus of 5 seconds' duration immediately preceded instrument calibration.

In group 1, average twitch height (T1) increased rapidly to 148+/-19% (mean +/- SD) of control at 15 min and then more slowly to reach 158+/-26% of control at 25 min. The train-of-four fade ratio did not vary with the duration of stimulation. In group 2, T1 increased to 172+/-19% of control after 400 stimuli (6.7 min) and 180+/-22% of control at 10 min In group 3, average T1 did not decrease below 97+/-5% or increase above 105+/-15% of control at any time.

A 5-s, 50-Hz tetanus administered before initial twitch calibration considerably shortens the time required to achieve baseline stability.