Background: Waiting five to six minutes before measuring a train-of-four (TOF) after a 50-Hz tetanic stimulation or post-tetanic count (PTC) in order to allow the facilitation of transmission to subside is commonly recommended but is based on limited evidence. The purpose of this study was to measure the TOF responses after PTC in one hand and to compare the responses with those in the contralateral (control) hand.
Methods: Twenty-two adult patients undergoing elective surgery under opioid-desflurane anesthesia were fitted with sensors to measure displacement of their thumbs in response to ulnar nerve stimulation. Rocuronium 0.6 mg·kg(-1) was administered and TOF stimulation was applied to both sides. One side was randomized to PTC (50-Hz tetanus followed by a three-second pause and 15 1-Hz stimuli) when the opposite (control) side recovered to 10% first twitch (T1) height. Train-of-four stimulation was continued bilaterally every 20 sec until 30 min after PTC. Comparisons of the T1 and TOF ratio (T4/T1) were made at two, five, ten, 20, and 30 min.
Results: The mean (standard deviation [SD]) T1 value in the PTC arm was 11.8 (7.1)% just before PTC. The T1 values were significantly greater in the PTC arm at two, five, and ten minutes, with mean (SD) differences of 6.3 (6.7)%, 9.4 (6.8)%, and 7.4 (3.9)%, respectively (P = 0.008). There were no significant differences in T1 values between groups at 20 and 30 min, and no statistically significant differences in T4/T1 values at any time.
Conclusion: A small but clinically insignificant increase in T1 is seen for at least ten minutes after PTC without any detectable change in T4/T1 values. The TOF responses are reliable as early as one minute after PTC.