Accuracy and dose dependency of the train-of-four count

The estimation of a nondepolarizing neuromuscular block using the train-of- four (TOF) count shows wide differences compared to the mechanomyographic m easurement. The purpose of this study was to evaluate the clinical signific ance of these differences. Methods: 89 patients (ASA I-II) in 6 groups received general anesthesia wit h fentanyl, propofol and a single dose atracurium (150, 200, 250, 300, 450 and 600 mu g/kg). Neuromuscular transmission was monitored by stimulation o f the ulnar nerve at the wrist with supramaximal TOF stimuli repeated every 15 s using a peripheral nerve stimulator. The isometric force contraction of the m. adductor pollicis was recorded. The height of T1 at reappearance of the second (T2) and fourth (T4) twitch was noted. Also noted was the tim e difference between the first reappearance of T4 and the 25% recovery of T 1. Statistical significance of the results was calculated by the h-test of Kruskal and Wallis. Testing the reliability of the TOF count, a 95% interva l of confidence was calculated. Results: There were no significant differences between the mean ages, heigh ts and weights of the six groups. T2 and T4 reappeared at 11+/-2% and 24+/- 6% recovery of Tl,respectively. Again, there were no significant difference s between the six dose groups (Fig.1). The time difference between the reap pearance of T4 and the 25% recovery was -1.0+/-2 (range:-5-3) minutes. The calculation of a 95% interval of confidence indicated a recovery between 14 % and 33% at reappearance of T4, 25% recovery can be expected 5 min before to 3 min after reappearance of T4, respectively. Conclusions: At reappearance of T4, a recovery of neuromuscular block of 25 % is missed only by 3 to 5 min during relaxation with atracurium. We consid er this margin of error as unimportant for clinical use. More-over we were able to show that the TOF-count is not dose dependent.