THE ONSET OF ROCURONIUM, BUT NOT OF VECURONIUM OR MIVACURIUM, IS MODIFIED BY TOURNIQUET INFLATION

A previous investigation showed that inflation of a tourniquet did not interrupt onset of vecuronium neu romuscular block. To test the hypot hesis that this effect depended on potency, twitch tension was measure d in an arm with a tourniquet inflated during onset and compared with a control arm in 30 patients under fentanyl-thiopental-nitrous oxide-i soflurane anesthesia. Patients were randomly allocated to receive eith er vecuronium 0.1 mg/kg (n = 10), rocuronium 0.6 mg/kg (n = 10), or mi vacurium 0.2 mg/kg (n = 10). The electromyographic response of the fir st dorsal interosseus to single twitch stimulation of the ulnar nerve every 10 s was recorded in both arms. When neuromuscular block was 20% (i.e., twitch tension was 80% of control), the tourniquet was inflate d to a pressure of 300 mm Hg. It was deflated 5 min later. In the vecu ronium and mivacurium groups, the tourniquet did not influence onset o f block. In the rocuronium group, maximum neuromuscular block was (mea n +/- SD) 79% +/- 10% in the tourniquet arm, compared with 96% +/- 4% in the perfused arm (P < 0.05). The maximum rate of onset was half tha t of the perfused arm. The difference in maximum neuromuscular block b etween arms was 17% +/- 7%, 5% +/- 5%, and 0% +/- 2% in the rocuronium , vecuronium, and mivacurium groups (P < 0.05). To explain that onset of block continues in spite of interruption of blood flow, drug molecu les must gain access to the neuromuscular junction via routes other th an the circulation. The results of this investigation are consistent w ith the hypothesis that there is redistribution of drug from extrajunc tional to junctional areas during onset of action of muscle relaxants and this process is more important for the more potent drugs (vecuroni um and mivacurium) than for rocuronium.