The neuromuscular response of infants to a continuous infusion of succinylcholine.

The response of the adductor of the thumb to ulnar nerve stimulation (0.1 Hz) was evaluated during continuous infusion of succinylcholine (SCh) in 20 infants anesthetized with halothane (1%) and N2O2/O2. Train-of-four stimulation (2 Hz for 2 s) was used to differentiate between phase I and phase II block. Some infants were very resistant to the neuromuscular effects of SCh. These infants (Group 1) were younger in age, 57 +/- 15 days (mean +/- SE) and required 24.6 +/- 3.3 mg X kg-1h-1 SCh to achieve more than 90% depression of the twitch. Older infants (Group 2), 188 +/- 33 days, required significantly less (P less than 0.001) SCh (8.7 +/- 0.5 mg X kg-1h-1) to achieve the same degree of twitch suppression. Group 1 infants recovered from the effects of SCh very rapidly. After 10 mg/kg SCh, the train-of-four ratio in Group 1 infants recovered to 75% in 4.7 +/- 0.6 min, whereas it took 34 +/- 10 min in Group 2 infants (P less than 0.01). Tachyphylaxis developed in infants after 3.6 +/- 0.3 mg/kg (mean +/- SE) and phase II block after 5.3 +/- 0.7 mg/kg (P less than 0.05) SCh. Combining the data of infants with that of children from a previous study conducted in a similar fashion resulted in significant correlation (P less than 0.001) between the log age and SCh requirement. The rate of administration of a continuous infusion of SCh in infants should be based upon the response of infants and not on a fixed dose (mg X kg-1 X h-1).