RESUMEN

BACKGROUND: Infants usually respond differently to a neuromuscular relaxant compared to children or adults. Isoflurane is commonly used as an anesthetic gas in infants. In an RCT design, we investigated whether a dose of mivacurium 250 &mgr;g/kg results in faster onset of action than 200 &mgr;g/kg in infants under isoflurane anesthesia. Spontaneous recovery times and cardiovascular response were also evaluated. METHODS: Twenty-four low surgical risk children, aged 6-24 months, undergoing an elective surgery and requiring tracheal intubation were selected. After anesthetic induction, patients randomly received an iv bolus dose of mivacurium 200 or 250 &mgr;g/kg. After maximal relaxation, the patient was intubated. Isoflurane was administered to maintain anesthetic level during the surgical procedure. Neuromuscular function was monitored by accelerometry (TOF-Guard) at the adductor pollicies. The first twitch (T) of the TOF and the T4/T1 were measured. The time-course of heart rate and systolic and diastolic blood pressure were analysed by transforming them into their respective areas under the curve. RESULTS: Mivacurium 250 &mgr;g/kg produced a maximal T block faster than 200 &mgr;g/kg, i.e. 2.4 +/- 1.1 vs. 3.5 +/- 1.4 min (p < 0.05). Spontaneous recovery times were similar in both groups. Heart rate was similar between doses while systolic and diastolic blood pressures were lower with the higher dose (p < 0.05). Flushing was observed in two cases, one in each group. CONCLUSIONS: The maximal effect of mivacurium 250 &mgr;g/kg, in infants under isoflurane anesthesia, was present one minute faster than 200 &mgr;g/kg. However, it produced a significant cardiovascular response.