Stimulus frequency and dose-respone curve to d-tubocurarine in man

The relationship of the frequency of motor-nerve stimulation to the dose--response to d-tubocurarine was studied in 45 adult patients during nitrous oxide--oxygen--morphine--thiopental anesthesia. One of five stimulus frequencies, 0.1, 0.15, 0.25, 0.5, and 1.0 Hz, was employed in each of five groups of nine patients. Cumulative dose-response curves for inhibition of evoked thumb adduction were constructed at each frequency on log probit scales and the ED50 and ED95 values were determined. The apparent potencies of d-tubocurarine at 0.5 and1.0 Hz were significantly different from that at 0.1 Hz; for example, at 0.1 Hz the ED50 and ED95 were 0.25 and 0.52 mg/kg, respectively. The corresponding values at 1.0 Hz were 0.07 and 0.15 mg/kg, respectively, or approximately 3.5 times less. The durations of recoveries of the twitch from 5--25 per cent of control at 1.0 and 0.5 Hz were 13 +/- 2 min (mean +/- SE) and 20 +/- 2 min, respectively. These durations were significantly different from that at 0.1 Hz (30 +/- 2 min). These results emphasize the importance of defining the stimulus frequency for meaningful interpretation of the dose--response relationships for nondepolarizing relaxants in man. Slow stimulus rates (0.1--0.15 Hz) are most useful clinically, since all levels of clinical relaxation can be achieved at these rates without abolishing the evoked twitch response.