Real time versus slow-motion train-of-four monitoring: a theory to explain the inaccuracy of visual assessment.

The present study was undertaken to determine why visual assessment of thumb adduction in response to train-of-four (TOF) stimulation of the ulnar nerve commonly overestimates the ratio that is obtained mechanographically. In patients undergoing general endotracheal anesthesia plus vecuronium for relaxation, 73 data sets were collected at different depths of neuromuscular block in response to supramaximal TOF stimulation. Each data set consisted of: (i) visual estimation of the TOF ratio by an experienced observer; (ii) mechanographic measurement of the TOF ratio with an adductor pollicis force transducer; and (iii) determination of the TOF ratio by measuring the slow-motion thumb displacement recorded on videotape. The last 23 data sets also included visual assessment and videotape recording of evoked responses at low stimulating current (30 mA). Visual inspection at 60 mA overestimated the mechanographic ratio by 0.20 units (48%). Videotape review provided a ratio that was 0.23 units (56%) greater than that determined mechanographically. However, after the first three twitches (T1-3), the thumb did not return to the same resting position as the (original) baseline prior to the first twitch. When the change in thumb position as a result of T1-3 was taken into account, the measured height of T4 was 40% less than it was when measured from the original baseline, and the T4/T1 ratio was identical to that obtained mechanographically. For the 23 data sets obtained at low current visual assessment overestimated the mechanographic value to a lesser degree than when obtained at high current. Again, correction for the T1-3 baseline shift improved the accuracy of videotape analysis.(ABSTRACT TRUNCATED AT 250 WORDS)