Tachyphylaxis and phase II block development during infusion of succinylcholine in children.

Contraction of the adductor of the thumb in response to ulnar nerve stimulation during continuous infusion of succinylcholine (SCh) was evaluated in 22 children anesthetized with halothane (1% to 1.5%) and N2O/O2. Twitch rates of 0.1 Hz were used to evaluate the force of contraction. Train-of-four stimulation (2 Hz for 2 seconds) was used to differentiate between phase I and phase II block. Tachyphylaxis to the effects of SCh occurred in 23 +/- 2 minutes in all children at a mean (+/- SE) dose of 3.0 +/- 0.4 mg/kg of SCh. In 21 of the 22 children, phase II block, as defined by a train-of-four ratio less than 50%, occurred in 29 +/- 5 minutes after 4.1 +/- 0.6 mg/kg of SCh. A mean dose of 10.3 +/- 0.6 mg/kg of SCh administered in a span of 71 +/- 5 minutes produced a mean train-of-four ratio of 12 +/- 5%. The recovery from phase II block after 10.3 +/- 0.6 mg/kg of SCh to train-of-four ratios of 50% and 75% was 16 +/- 3 minutes and 41 +/- 5 minutes, respectively. Caution should be used when more than 3 mg/kg of SCh is infused intravenously; adequate neuromuscular monitoring should be available and ample time should be given for spontaneous recovery.