Tetanic fade and post-tetanic tension in the absence of neuromuscular blocking agents in anesthetized man

Frequency and time dependent changes in neuromuscular transmission were examined in 30 patients undergoing elective minor surgical procedures not requiring the use of muscle relaxants. Anesthesia was induced with sodium thiopenthal and maintained with N2O-O2 and fractional does of meperidine or fentanyl. Neuromuscular function was measured by recording the force of thumb adduction evoked by supramaximal stimulation of the ulnar nerve at the wrist. Single stimuli were applied every 2.5 seconds as square pulses of 0.1-millisecond duration. Tetanic trains of 10-second duration ranging from 10 Hz to 400 Hz were used. From analysis of present data, criteria for normal responses to 10-second tetanic trains of varying frequencies were established. At a frequency of 30 Hz, the tetanic response is fully maintained and followed by post-tetanic potentiation; at a frequency of 50 Hz, both tetanic and post-tetanic responses are maintained; at a frequency of 100 Hz, there is tetanic fade, followed by a post-tetanic depression of the single indirect twitch responses. It is concluded that frequency and duration of indirect stimulation are the most important factors in using tetanic maintenance and post-tetanic events in assessment of recovery from neuromuscular block.