Stimulation with submaximal current for train-of-four monitoring.

The present study evaluated responses to train-of-four (TOF) stimulation at a range of stimulating currents. Traditionally TOF has been applied with a supramaximal stimulus but this may be quite uncomfortable for the awake patient. In the first part of this study, 12 healthy volunteers quantified (by 10-cm visual analog scale) the discomfort associated with TOF stimulation at 20, 30, and 50 mA. The median VAS scores were 2, 3, and 6, respectively (P less than 0.05 for differences between each group). In the second part, single twitch and TOF responses were compared at 20, 30, and 50 mA in 64 postoperative and in 19 intraoperative patients who had ratios of the fourth to the first twitch (T4/T1) ranging from 0.15-1.03. In all patients, neuromuscular responses to nerve stimulation were recorded by a mechanogram, and the T4/T1 ratios were calculated. Although single twitch heights increased significantly as amperage was increased, there was no statistical difference in the T4/T1 ratios at the three different currents. The mean +/- SD T4/T1 ratios at 20, 30, and 50 mA were 0.795 +/- 0.247, 0.798 +/- 0.237, and 0.802 +/- 0.233, respectively (P = ns). It is concluded that TOF monitoring using a submaximal stimulus is more comfortable for the awake patient who is suspected of residual weakness, and that T4/T1 testing can be reliably accomplished intraoperatively as well as postoperatively using submaximal stimuli.