Abstract
The relation between plasma concentration and the effects of atracurium was studied in seven patients anaesthetised with thiopentone, fentanyl and nitrous oxide - oxygen. The responses to train-of-four stimulation at ten-second intervals with tetanic stimuli applied every five minutes were recorded. The first sign of transmission returning after complete blockade was usually the post-tetanic facilitated twitch, which was noted when the mean atracurium concentration was 1.15 mg 1−1 (SD 0.77). The most sensitive parameter was the train-of-four ratio, which recovered to 0.5 when the concentration was 0.217 mg 1−1 (SD 0.56), compared with a concentration of 0.271 (SD 0.85) at 50% recovery for twitch height, 0.221 (SD 0.029) for tetanic peak and 0.231 (SD 0.079) for tetanic fade. The value for the train-of-four ratio differed significantly (P < 0.05) from that for the twitch height, but other differences were not significant. Once recovery commenced, these four parameters recovered at similar rates, with recovery indices (25 to 75% responses) of 14.8 (SD 1.7), 14.0 (SD 1.0), 14.3 (SD 1.8) and 13.7 (SD 2.1) minutes respectively. Post-tetanic facilitation was most marked during severe but incomplete blockade and tetanic stimulation temporarily reversed the atracurium-induced decrease in train-of-four ratio. Clinically, the use of tetanic stimulation did not improve the sensitivity of neuromuscular monitoring, but post-tetanic count may be useful where monitoring of profound relaxation is required.