Neuromuscular blockade in infants following intramuscular succinylcholine in two or five per cent concentration

This study determined the characteristics of the neuromuscular block which followed intramuscular succinylcholine 4 mg . kg-1 in 20 infants during halothane anaesthesia. The infants were divided into two groups of ten; the first received succinylcholine in two percent solution and the second in five per cent solution. The mean maximum depression of the first twitch of the train-of-four (T1) was 89.7 +/- 5.0 per cent in 4.0 +/- 0.6 min, and the mean full recovery of T1 occurred in 15.6 +/- 0.9 min after injection. The maximum block achieved and the onset and recovery times were not affected by the concentration used. Depolarizing block, with equal depression of all twitches of the train-of-four was observed during the onset of neuromuscular blockade. During recovery, phase II block, as determined by a train-of-four ratio (T4/T1) of 0.5 or less, occurred frequently at T1 recovery of 25-50 per cent, but was not associated with prolonged paralysis. It is concluded that the onset time of 4 min for intramuscular succinylcholine 4 mg . kg-1 may be too long for emergency use in infants, and no improvement is obtained by increasing the concentration of injected succinylcholine from two to five per cent.