The authors thank Drs. Fuchs-Buder and Schreiber for their interest in our clinical investigation,1and we welcome the opportunity to respond to the questions raised in their Letter to the Editor. We agree that the degree of spontaneous neuromuscular recovery at the time of neostigmine administration has an important effect on the incidence of residual paresis in the postanesthesia care unit. As demonstrated in the investigation of Kim et al. , the median (range) times required to achieve a train-of-four (TOF) ratio of 0.9 after reversal with neostigmine (0.07 mg/kg) were 22.6 (8.3–57.4) min at a TOF count of 2 and 9.7 (5.1–26.4) min at a TOF count of 4.2The relatively common practice of administering an anticholinesterase agent only a few minutes before tracheal extubation, regardless of the TOF count, likely explains the high incidence of residual neuromuscular blockade reported in numerous clinical investigations.3
However, our data do not support the conclusion by Drs. Fuchs-Buder and Schreiber “that the difference in symptoms of muscle weakness and improved quality of recovery observed in this study could simply be the consequence of the dose or the timing of the administration of neostigmine.” In fact, no differences in management of reversal of neuromuscular blockade (time or dose) were observed between the acceleromyography and control groups. All patients in the study received a standard dose of neostigmine (50 μg/kg) “at the conclusion of the surgical closure.” The study protocol specifically defined the timing of neostigmine administration to standardize management in both groups and to account for the relatively long time required to achieve adequate recovery of neuromuscular function, even at a TOF count of 4 (10 min).3The protocol also stated that neostigmine should be given at a TOF count of 3–4 and this was achieved in the majority of patients because the median TOF count in both groups was 4 at reversal (only 1 patient of 150 had a TOF count of 0 at this time). Nonetheless, a higher percentage of patients in the acceleromyography group achieved spontaneous recovery to a TOF count of 4 at the time of reversal compared with our control group (80.3%vs. 55.4%, respectively). We believe this was due to more careful management of neuromuscular blockade at the end of surgery in the patients monitored with acceleromyography. In both study groups, dosing of neuromuscular blocking agents was carefully standardized; however, administration of additional neuromuscular blocking agents during the times of the operation not requiring neuromuscular blockade was at the discretion of the anesthesia care team. We hypothesized that acceleromyography monitoring would allow for more accurate titration of neuromuscular blocking agents during this important time period, and our findings supported this hypothesis (i.e. , fewer patients in the acceleromyography group received neuromuscular blocking agents during the last 45 min of the surgical procedure). Thus, our findings suggest that improved spontaneous recovery of neuromuscular function at the time of neostigmine administration in the acceleromyography group resulted in fewer patients with residual muscle weakness in the postanesthesia care unit.
It should be noted that the protocol used a simple definition of the TOF count at the time of reversal. As indicated, neostigmine was administered when at least three to four visual responses (“twitches”) to TOF stimulation were observed. Data from the TOF-Watch SX®(Bluestar Enterprises, Omaha, NE) display were not used in the decision process at the time of reversal. We did not attempt to differentiate between a “strong fourth twitch” and a “weak fourth twitch.”
In conclusion, we agree that improved spontaneous recovery at the time of reversal resulted in fewer symptoms of muscle weakness in the postanesthesia care unit. Our data suggest that acceleromyography monitoring, by allowing for more rational and accurate management of neuromuscular blockade, resulted in more patients with a TOF count of 4 at the time of reversal and fewer patients with residual paresis after tracheal extubation. Although additional analysis of our data could reveal that patients with higher TOF counts at the time of reversal have fewer symptoms of muscle weakness and improved quality of recovery, such an analysis is beyond the scope of a Letter to the Editor. Additional studies are needed to identify the mechanisms by which acceleromyography monitoring decreases the incidence of residual neuromuscular blockade.