Stimulation with Submaximal Current for Train-of-Four Monitoring

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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 < 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 post-operative 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.

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