Can final EMG baseline be used as a reference to calculate neuromuscular recovery?

Background: Investigations recording recovery times of muscle relaxants have used initial or final baseline of a neuromuscular trace, or both, as a reference for data analysis. We evaluated the use of final baseline of EMG traces as a reliable reference to calculate recovery times. Methods: We analyzed EMG traces from 82 children who had full spontaneous neuromuscular recovery following a single dose of mivacurium. Times from administration of mivacurium to 25, 50, 75, and 90% EMG recoveries were measured using both initial and final baselines as a reference. EMG traces with final baseline of 100 ± 10% of the initial baseline were regarded as optimal. Recovery times from all other traces were compared to the times obtained from these optimal traces. Poor final baseline was defined as that of <80% of initial baseline. Inter-group comparisons were made using Kruskal-Wallis test followed by Mann-Whitney U tests. Results: EMG recovery times were similar for optimal traces whether the reference was the initial or the final baseline of the EMG trace. If the final baseline was used as the reference, then traces with poor final EMG baseline also showed similar neuromuscular recovery times. If the initial baseline was used as the reference for EMG traces with poor final baseline, then neuromuscular recovery times became 24-55% longer (P < 0.05). Conclusions: We conclude that the final baseline of an EMG trace can be used as a reference for calculations of neuromuscular recovery times were following a bolus injection of mivacurium.