Use of post-tetanic count in assessment of a repetitive vecuronium-induced neuromuscular block.

In order to evaluate the use of the post-tetanic count (PTC) method during repetitive administration of vecuronium, we studied 20 patients allocated randomly to one of two groups: 10 patients received droperidol-fentanyl anaesthesia (control group); 10 other patients were given droperidol-fentanyl anaesthesia modified subsequently by addition of 0.5% isoflurane (isoflurane group). Before tracheal intubation, a bolus dose of vecuronium 0.08 mg kg-1 was given i.v. followed by repeated doses of 0.03 mg kg-1. The twitch response of adductor pollicis was recorded after supramaximal stimulation of the ulnar nerve at the wrist using a Myograph 2000 neuromuscular transmission analyser. In the control group, a close correlation was found between PTC and time to first reaction to train-of-four (TOF) nerve stimulation. This relationship was unchanged when comparing the bolus dose and each of eight consecutive maintenance doses. Further, the degree and the duration of intense block were unchanged after each of the eight maintenance doses. In the isoflurane group, the relationship between PTC and time to first reaction to TOF stimulation remained unchanged after addition of isoflurane. However, isoflurane caused a significant prolongation of the duration of intense block and a corresponding lower PTC in all patients. We conclude that PTC is a reliable method to evaluate intense neuromuscular block caused by vecuronium, even after repetitive administration of the drug and in combination with 0.5% isoflurane.