Augmentation of the rocuronium-induced neuromuscular block by the acutely administered phenytoin

Background: The effects of an acute administration of phenytoin on the magn itude of the rocuronium-induced neuromuscular block were evaluated. Methods: Twenty patients (classified as American Society of Anesthesiologis ts physical status I or II) scheduled for craniotomy mere studied: 15 received phenytoin during operation (10 mg/kg), and the others served as controls. Anesthesia was induced with thiopental and fentanyl and maintain ed with nitrous oxide (65%) in oxygen and end-tidal isoflurane (1%). The ul nar nerve was stimulated supramaximally and the evoked electromyography was recorded using a neuromuscular transmission monitor. Continuous Infusion o f rocuronium maintained the neuromuscular block with first twitch (T-1) bet ween 10 and 15% for 45 min before the start of an infusion of either phenyt oin or NaCl, 0.9%. Twitch recordings continued for 60 min thereafter. Arter ial blood samples were collected at the predefined time points (four measur ements before and four after the start of the infusion) to determine the co ncentrations of phenytoin and rocuronium and the percentage of rocuronium b ound to plasma proteins. Results: The first twitch produced by an infusion of rocuronium remained co nstant during the 15 min before and the 60 min after the start of the salin e infusion. After the phenytoin infusion, the twitch decreased progressivel y, but the plasma concentrations and the protein-bound fraction of rocuroni um did not change. Conclusion: Phenytoin acutely augments the neuromuscular block produced by rocuronium without altering its plasma concentration or its binding to plas ma proteins.