Influence and relative sensitivities of 50-Hz and 100-Hz tetanic stimuli on subsequent tetanic fade ratios in patients receiving vecuronium.

We studied the possible effects of repetitive (1-min interval) 50- and 100-Hz tetanic stimuli on 50-Hz and 100-Hz tetanic fade ratios (RF50HZ and RF100HZ). We also evaluated the sensitivity of the recorded responses to these two tests to assess residual neuromuscular block (isometric adductor pollicis mechanical activity), either during spontaneous recovery, or 15 min after neostigmine administration, in 22 adult anesthetized (thiopental, fentanyl, N2O/O2) patients receiving vecuronium. Two 50-Hz and two 100-Hz, 5-s duration, tetanic stimulations were randomly assessed at 1-min intervals: in a spontaneous (SPO) group (n = 11), when train-of-four (TOF) ratio spontaneously regained 0.7, and in a neostigmine (NEO) group (n = 11), 15 min after 40 micrograms/kg neostigmine was given intravenously at 25% return of control twitch tension. In the SPO group, when TOF ratio was 0.7, RF50HZ was 0.92 +/- 0.01 before and after subsequent tetanic stimulation, while RF100HZ was 0.48 +/- 0.05 and 0.47 +/- 0.05, respectively (not significant [NS]). In the NEO group, when TOF ratio was approximately 0.9, RF50HZ was 0.93 +/- 0.01 before and after subsequent tetanic stimulation, while RF100HZ was 0.80 +/- 0.02 and 0.78 +/- 0.02, respectively (NS). From patient to patient, both RF50HZ and RF100HZ were also identical. In conclusion, in patients receiving vecuronium, 1) 5-s, 50- and 100-Hz tetanic stimuli may be repeated without changes at 1-min intervals and, 2) in contrast to RF50HZ, recorded RF100HZ enables one to determine residual neuromuscular block during spontaneous recovery (P < 0.001) such as after neostigmine reversal (P < 0.05).