Mivacurium in infants and children

Mivacurium is the only available short-acting nondepolarizing muscle relaxant in clinical use. It is a bis-quaternary benzylisoquinolinium ester hydrolysed by plasma-cholinesterase into inactive compounds. The ED₅₀ and ED₉₅ in children are about 50 μg·kg^?1 and 90 μg·kg^?1 respectively. In infants, they have a tendency to be lower. A standard intubating dose of 0.25 mg·kg^?1 causes complete neuromuscular depression in 1.5–2 min, recovery to 5% in 6–10 min, and complete recovery in 15–20 min. The recent tendency is to use 0.3 mg·kg^?1 to obtain better intubating conditions with slight prolongation of effect. Since the recovery profile of mivacurium is independent of the dose and duration, it is most suitable for administration by continuous infusion. The infusion requirement in children is 10–16 μg·kg^?1 min^?1, which is about twice that of adults. Cutaneous flushes from histamine release are commonly seen with the larger doses of mivacurium; however, the associated hypotensive effects are minimal and counteracted by the tracheal intubation. The duration of action of mivacurium is prolonged in patients with cholinesterase deficiency. Mivacurium's neuromuscular effects can be satisfactorily antagonized by edrophonium or neostigmine.     

Evaluation of a device for the measurement of the evoked tensions of the rectus abdominis muscle

A device was developed to measure the evoked tensions of the rectus abdominis muscle which consisted of a fluid-filled reservoir wedged between the rectus abdominis muscle and a self-retaining retractor. The evoked contractions of the rectus muscle were compared with that of the tibialis anterior muscle in twelve dogs anaesthetised with pentobarbitone. Significantly greater amounts of tubocurarine were required to depress the response to train-of-four stimuli and the twitch tensions of the rectus muscle than the tibialis. The tibialis recovered faster, spontaneously or after neostigmine, than the rectus in eight of the animals; the opposite occurred in the other four. The present device can be useful during surgery for the evaluation of abdominal muscle tension.     

RECOVERY FROM PANCURONIUM BLOCKADE IN THE NEONATAL INTENSIVE CARE UNIT

Neuromuscular blockade with pancuronium and its antagonism wasevaluated in 33 critically ill infants. The evoked contractionof the adductor pollicis from indirect stimulation of the ulnarnerve was measured. The neuromuscular blockade recovered spontaneouslyfrom pancuronium in seven infants, 23 required one or more dosesof atropine 0.02 ng kg^–1 and neostigmine 0.06mg kg^–1.In three infants the blockade railed to reverse. Immature infantsless than 32 weeks did not show any significant difference intheir requirement for pancuronium compared with mature infants.Age and birth weight of the infant, dose of pancuronium andduration of its administration did not affect the requirementsfor reversal. Train-of-four and tetanus:twitch ratios were lower(P<0.05) in infants less than 32 weeks of developmental agereflecting immaturity of neuromuscular transmission.     

Plasma cholinesterase activity and dibucaine number in infants and children

Summary
Reference values for plasma cholinesterase in adults are available, but data for paediatric patients are scarce. In this study plasma cholinesterase activity and dibucaine number were measured in 111 healthy infants and children two months to 12 years. There were no significant differences in cholinesterase values between the infants and the older children. The cholinesterase activity of the paediatric population was comparable to that of adults. One infant had markedly decreased cholinesterase activity and was presumed to have an atypical variant of the enzyme. Further analysis of this child and the family, identified that this child had most probably the atypical allele (E₁^a E₁^a), whereas his two siblings, his mother and maternal uncle were probably heterozygous. The implications of cholinesterase activity in the paediatric population are discussed.     

Histamine release from the administration of suxamethonium and atracurium in adolescents

Cutaneous reactions and plasma histamine levels were evaluated in 30 adolescents subsequent to the administration of suxamethonium 1.5 mg kg⁻¹ or atracurium 0.6 mg kg⁻¹. Plasma histamine levels were measured by the radio-enzymatic assay using histamine N -methyl transferase. In the atracurium group, the change in plasma histamine level was significant at the 2 min mark, but was not significant in the suxamethonium group. About 60% of the patients in each group developed a cutaneous reaction. Erythematous cutaneous reactions following atracurium were frequently associated with a mild increase in plasma histamine levels, whereas the rashes induced by suxamethonium in adolescents were not associated with changes in plasma histamine.     

End-tidal CO2 monitoring

Two cases of malignant hyperthermia are described where the earliest sign was a rise in the end-tidal CO2 concentration. This led to nearly immediate detection and adequate treatment with sodium dantrolene. These cases demonstrate the efficacy of monitoring end-expired CO2 concentrations in patients at risk from malignant hyperthermia, as well as a means for following the adequacy of treatment.     

HISTAMINE RELEASE DURING THE ADMINISTRATION OF ATRACURIUM OR VECURONIUM IN CHILDREN

The histamine releasing potential of equivalent bolus dosesof atracurium 0.6 mg kg^–1 or vecuronium 0.12 mg kg^–1was evaluated in 20 children anaesthetized with halothane. Bloodsamples were obtained before, and at 2 and 5 min after the administrationof the neuromuscular blocker. The twitch response to 0.15Hzwas also evaluated. None of the 10 patients receiving vecuroniumhad a significant increase in plasma histamine concentration.In two of the 10 children receiving atracurium, the plasma histamineconcentration increased markedly, but without any apparent clinicalmanifestations. Recovery of neuromuscular function (to 95% twitchheight) after vecuronium 0.12 mg kg^–1 was faster thanafter atracurium 0.6 mg kg^–1 (P < 0.02). ^*Present address: Department of Anesthesia and Critical Care,University of Chicago, Chicago, II, U.S.A.