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

To determine the influence of circulatory factors on onset of neuromus cular block, we have measured twitch height in an arm with a tournique t inflated during onset and compared this with data from a control arm in 20 patients under fentanyl-thiopentone-nitrous oxide-isoflurane an aesthesia. Patients were allocated randomly to receive either vecuroni um 0.1 mg kg(-1) (n = 10) or suxamethonium 1 mg kg(-1) (n = 10). The E MG response of the first dorsal interosseous to single twitch stimulat ion of the ulnar nerve every 10 s was recorded in both arms. When neur omuscular block was 20% (i.e. twitch height was 80% of control), the t ourniquet was inflated to a pressure of 250 mm Hg. It was deflated 5 m in 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 sux amethonium group, the presence of a tourniquet decreased the rate of o nset by 66%. Maximum block was only 74 (20)% in the tourniquet arm com pared with 97 (5)% in the perfused arm (P < 0.05). The difference in m aximum neuromuscular block between arms was 4 (3)% in the vecuronium g roup and 22 (17)% in the suxamethonium group (P < 0.01). We conclude t hat during onset, neuro muscular block continued to increase in spite of interruption of blood flow, and this increase was greater with vecu ronium than with suxamethonium. These results suggest that redistribut ion of free molecules of drug from extra-junctional to junctional area s is one of the factors governing onset of action of neuromuscular blo cking drugs.