Propofol clearance and distribution volume increase in patients with hyperthyroidism.

Abstract

We investigated propofol pharmacokinetics in seven hyperthyroid (Group H) and eight euthyroid (Group E) patients undergoing elective subtotal thyroidectomy. Anesthesia was induced with an i.v. injection of 2 mg/kg propofol and maintained with a continuous propofol infusion while ventilation was controlled with 60% nitrous oxide in oxygen. The propofol infusion rate was adjusted in the range of 4-10 mg.kg-1.h-1 based on physiological signs such as heart rate and blood pressure. Arterial blood was sampled to measure the propofol concentration. The mean propofol infusion rates were higher in hyperthyroid than in patients with euthyroidism (median values Group H 10.0 mg.kg-1.h-1, Group E 6.5 mg.kg-1.h-1; P < 0.05), although the reverse was true for average propofol concentrations (Group H 1.8 micrograms/mL, Group E 3.3 micrograms/mL; P < 0.05). Group H also had higher values for propofol clearance (5.1 L/min versus 2.5 L/min; P < 0.05) and distribution volume at steady state (10.0 L/kg versus 2.8 L/kg; P < 0.05). Because distribution volume and clearance in patients with hyperthyroidism were increased, propofol concentrations could not reach anesthetic levels. Propofol decreases heart rate and blood pressure, which are desirable properties for anesthesia in patients with hyperthyroidism. However, because clearance and distribution volume of propofol are increased, propofol infusion rates had to be increased to reach anesthetic blood concentrations.