Pharmacodynamic behaviour of vecuronium in primary hyperparathyroidism

This study evaluated the potency and time course of action of vecuronium in patients with primary hyperparathyroidism (HPT) and marked hypercalcaemia during nitrous oxide-opioid anaesthesia. Twenty ASA physical status I and II patients were studied by measuring the force of contraction of the adductor pollicis in response to stimulation of the ulnar nerve: ten control patients and ten patients with HPT and ionized calcium concentration over 2.80 mEq.L-1. After induction of anaesthesia with thiopentone and maintenance with N2O/O2 and fentanyl, vecuronium was administered to determine cumulative dose-response curves. When maximum block had been obtained, twitch height was maintained at 10% of baseline value over 20 min by adjusting the infusion rate of a syringe-pump containing vecuronium and vecuronium plasma concentration (EC90ss) was determined. During spontaneous recovery, after termination of infusion, the recovery index, the time from 25 to 75% recovery, was measured. The dose to produce 90% block was greater in the HPT than in control group: 69 (24) vs 54 (18) micrograms.kg-1 (P < 0.02). The calculated ED50 was also greater in HPT: 42 (4) vs 31 (5) micrograms.kg-1 in controls (P < 0.001). (Values are given as mean and coefficient of variation). The slope of the dose-response curve, the dose necessary to maintain 90% block, and the EC90ss did not differ. The RI25-75 was slower in the HPT group although the difference did not reach statistical significance. It is concluded that hyperparathyroidism with hypercalcaemia increases vecuronium requirement; only during the onset of neuromuscular blockade.