The onset of alcuronium and tubocurarine: alone and in combination.

The rates of onset of neuromuscular blockade have been measured following 0.25 mg.kg-1 alcuronium (ED95), 0.51 mg.kg-1 tubocurarine (ED95), a combination of 50% of the ED95 of each and a combination of 33% of the ED95 of each. Train-of-four stimulation of the ulnar nerve was used with recording of the amplitude of the evoked compound electromyogram from the thenar prominence. The rate of increase in blockade in patients receiving the 50% combination was significantly greater than for either agent given alone, or than for the 33% combination. The mean time to 75% block of the first contraction of the train (T1) with the 50% mixture was 90 s, significantly faster than alcuronium alone (132 s), tubocurarine alone (174 s) or the 33% mixture (127 s). These latter three groups did not differ significantly. In conclusion, the rate of onset of the 50% combination resulted in a more rapid onset of neuromuscular blockade, whereas the rate of onset of the 33% combination was no different to that of either drug alone. This small degree of acceleration with agents which are known to be markedly synergistic makes it unlikely that this technique will prove to be of clinical importance.