ONSET OF NEUROMUSCULAR BLOCK AFTER TOURNIQUET INFLATION - COMPARISON OF SUXAMETHONIUM WITH VECURONIUM

To determine the influence of circulatory factors on onset of neuromuscular block, we have measured twitch height in an arm with a tourniquet inflated during onset and compared this with data from a control arm in 20 patients under fentanyl-thiopentone-nitrous oxide-isoflurane anaesthesia. Patients were allocated randomly to receive either vecuronium 0.1 mg kg(-1) (n = 10) or suxamethonium 1 mg kg(-1) (n = 10). The EMG response of the first dorsal interosseous to single twitch stimulation of the ulnar nerve every 10 s was recorded in both arms. When neuromuscular block was 20% (i.e. twitch height was 80% of control), the tourniquet was inflated to a pressure of 250 mm Hg. It was deflated 5 min later. In the vecuronium group, the rate of onset did not differ in both arms and mean maximum block was 95 (SD 4)% in the tourniquet arm, which was not different from 99 (2)% in the perfused arm. In the suxamethonium group, the presence of a tourniquet decreased the rate of onset by 66%. Maximum block was only 74 (20)% in the tourniquet arm compared with 97 (5)% in the perfused arm (P < 0.05). The difference in maximum neuromuscular block between arms was 4 (3)% in the vecuronium group and 22 (17)% in the suxamethonium group (P < 0.01). We conclude that during onset, neuro muscular block continued to increase in spite of interruption of blood flow, and this increase was greater with vecuronium than with suxamethonium. These results suggest that redistribution of free molecules of drug from extra-junctional to junctional areas is one of the factors governing onset of action of neuromuscular blocking drugs.