Neuromuscular blockade studies often use the thumb adductor twitch response to ulna nerve stimulation. Two factors that may influence the results are the resting muscle tension (initial fiber length) and the pattern of the stimulus waveform. This study was undertaken to improve understanding of the effect of these factors and lead to better-controlled study conditions and more consistent data. During nitrous oxide barbiturate-narcotic anesthesia in 10 normal adult patients, as the resting thumb tension was increased from 50 to 200 g, the evoked thumb adductor twitch response (Grass stimulator, 0.25 Hz) was augmented by 28%. There was only a 2.5% increase in the evoked (developed) tension when the resting tension was further increased from 200 to 300 g. Developed tension at 50 g was significantly (p < 0.001) less than at the other resting tensions. The developed tension at 100 g was also significantly (p < 0.05) less than at resting tensions of 200 or 300 g.
Cumulative dose-response curves for gall amine in nine patients were not significantly altered by increasing resting tension from 50 to 200 g. Biphasic (Block-Aide monitor) or single square wave (Grass stimulator) stimuli wave forms in nine normal adult patients yielded gall amine dose-response curves that were not statistically different. The muscle response to biphasic stimulation during a non-depolarizing blockade was not affected by the average muscle refractory period.
Because of the significantly lower developed tension at resting tension settings of 50 to 100 g, a practical consideration during neuromuscular function studies would be to have the resting thumb tension adjusted and rechecked for each patient and kept within the 200–300-g range to ensure maximum uniform developed tension. The type of stimulus wave form selected will not affect results as long as it is used consistently throughout the study.