PROPOFOL CLEARANCE AND DISTRIBUTION VOLUME INCREASE IN PATIENTS WITH HYPERTHYROIDISM

We investigated propofol pharmacokinetics in seven hyperthyroid (Group H) and eight euthyroid (Group E) patients undergoing elective subtota l thyroidectomy. Anesthesia was induced with an IV injection of 2 mg/k g propofol and maintained with a continuous propofol infusion while ve ntilation was controlled with 60% nitrous oxide in oxygen. The propofo l infusion rate was adjusted in the range of 4-10 mg.kg(-1).h(-1) base d on physiological signs such as heart rate and blood pressure. Arteri al blood was sampled to measure the propofol concentration. The mean p ropofol infusion rates were higher in hyperthyroid than in patients wi th 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 avera ge propofol concentrations (Group H 1.8 mu g/mL, Group E 3.3 mu g/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 st ate (10.0 L/kg versus 2.8 L/kg; P < 0.05). Because distribution volume and clearance in patients with hyperthyroidism were increased, propof ol concentrations could not reach anesthetic levels. Implications: Pro pofol decreases heart rate and blood pressure, which are desirable pro perties for anesthesia in patients with hyperthyroidism. However, beca use clearance and distribution volume of propofol are increased, propo fol infusion rates had to be increased to reach anesthetic blood conce ntrations.