Myotonic and neuromuscular blocking effects of increased doses of suxamethonium in infants and children.

The myotonic effects and duration of action of several doses of suxamethonium were determined in 24 infants and 16 children during thiopentone-fentanyl-nitrous oxide anaesthesia. Infants received suxamethonium 2, 3 or 4 mg kg-1; children received 1 or 2 mg kg-1. The increase in muscle tone during onset of neuromuscular block was independent of dose. Onset of block was faster in children who received suxamethonium 2 mg kg-1 compared with those who received 1 mg kg-1, and in infants given 2 mg kg-1 than in children given the same dose. Compared with adults given suxamethonium 1 mg kg-1, infants required 3-4 mg kg-1 and children at least 2 mg kg-1 to produce 6-8 min of neuromuscular block. These results provide a clear indication for increasing the intubating doses of suxamethonium in infants and children, and a explanation for the unduly high rate of "masseter spasm" in some paediatric centres.