Prolonged mivacurium infusion in young and elderly adults

Purpose: This study was designed to evaluate pharmacodynamically and pharmacokinetically if cis-cis isomer of mivacurium contributed to neuromuscular block during prolonged infusions lasting more than four hours in young adult and elderly (> 60 yr) patients. Methods: The mechanomyogramic neuromuscular-response of the adductor pollicis was recorded in 32 adults 18-59 yr, and 19 elderly (> 60 yr.) patients during N2O:O-2:opioid anaesthesia. The mivacurium infusion rate was adjusted to maintain single twitch depression at 95 +/- 4% of control. Blood samples were taken every 30 min to determine the plasma concentration of cis-cis isomer of mivacurium. At the end of the surgical procedure, patients were allowed to recover spontaneously to at least 25% of control twitch response. Results: The mean mivacurium infusion requirement to maintain 97 +/- 1 (mean +/- SD)% depression of the twitch response was 6.0 +/- 0.4 mu g.kg(-1).min(-1) in young adults, and 4.3 +/- 0.3 mu g.kg(-1).min(-1) in elderly patients (P < 0.001). The infusion requirement in patients with low plasma cholinesterase activity was the lowest 2.4 +/- 1.2 mu g.kg(-1).min(-1). Plasma cis-cis isomer concentrations reached peak levels within one-two hours and remained relatively constant throughout the duration of infusion even in patients with low cholinesterase activity. There was no relationship between duration of infusion, plasma concentrations of cis-cis isomer and the early recovery indices of mivacurium um (up to 25%). Neuromuscular transmission recovered adequately with or without antagonism in ail patients, Conclusion: When the mivacurium infusion was titrated to maintain 95 +/- 4% twitch depression, the plasma concentration of the cis-cis isomer did not increase during prolonged infusions (four hours) and neuromuscular transmission recovers satisfactorily.