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

The effects of an acute administration of phenytoin on the magnitude of the rocuronium-induced neuromuscular block were evaluated.

Twenty patients (classified as American Society of Anesthesiologists physical status I or II) scheduled for craniotomy were studied: 15 received phenytoin during operation (10 mg/kg), and the others served as controls. Anesthesia was induced with thiopental and fentanyl and maintained with nitrous oxide (65%) in oxygen and end-tidal isoflurane (1%). The ulnar nerve was stimulated supramaximally and the evoked electromyography was recorded using a neuromuscular transmission monitor. Continuous infusion of rocuronium maintained the neuromuscular block with first twitch (T1) between 10 and 15% for 45 min before the start of an infusion of either phenytoin or NaCl 0.9%. Twitch recordings continued for 60 min thereafter. Arterial blood samples were collected at the predefined time points (four measurements before and four after the start of the infusion) to determine the concentrations of phenytoin and rocuronium and the percentage of rocuronium bound to plasma proteins.

The first twitch produced by an infusion of rocuronium remained constant during the 15 min before and the 60 min after the start of the saline infusion. After the phenytoin infusion, the twitch decreased progressively, but the plasma concentrations and the protein-bound fraction of rocuronium did not change.

Phenytoin acutely augments the neuromuscular block produced by rocuronium without altering its plasma concentration or its binding to plasma proteins.