Onset of neuromuscular block after tourniquet inflation: comparison of suxamethonium with vecuronium

To determine the influence of circulatory factors on onset of neuromuscularblock, we have measured twitch height in an arm with a tourniquet inflatedduring onset and compared this with data from a control arm in 20 patientsunder fentanyl-thiopentone-nitrous oxide- isoflurane anaesthesia. Patientswere 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 dorsalinterosseous to single twitch stimulation of the ulnar nerve every 10 s wasrecorded in both arms. When neuromuscular block was 20% (i.e. twitch heightwas 80% of control), the tourniquet was inflated to a pressure of 250 mmHg. It was deflated 5 min later. In the vecuronium group, the rate of onsetdid not differ in both arms and mean maximum block was 95 (SD 4)% in thetourniquet arm, which was not different from 99 (2)% in the perfused arm.In the suxamethonium group, the presence of a tourniquet decreased the rateof onset by 66%. Maximum block was only 74 (20)% in the tourniquet armcompared with 97 (5)% in the perfused arm (P < 0.05). The difference inmaximum neuromuscular block between arms was 4 (3)% in the vecuronium groupand 22 (17)% in the suxamethonium group (P < 0.01). We conclude thatduring onset, neuromuscular block continued to increase in spite ofinterruption of blood flow, and this increase was greater with vecuroniumthan with suxamethonium. These results suggest that redistribution of freemolecules of drug from extra-junctional to junctional areas is one of thefactors governing onset of action of neuromuscular blocking drugs.