Propofol EC50: an effect of luteal phase core temperature differences?

Abstract

Editor—We read with interest the article by Fu and colleagues,1Fu F Chen X Feng Y et al.Propofol EC50 for inducing loss of consciousness is lower in the luteal phase of the menstrual cycle.Br J Anaesth. 2014; 112: 506-513Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar in which the authors found that patients during the luteal phase recovered faster from general anaesthesia maintained with propofol and remifentanil than during the follicular phase. The authors provided us with two possible explanations for this phenomenon: one is that patients in the luteal phase had less accumulation of propofol in the body than those in the follicular phase at the end of surgery; the other is that effect-site propofol concentration or effect-site remifentanil concentration in patients in the luteal phase declined more rapidly than those in the follicular phase. We agree with the authors' points of view in general. Meanwhile, we hold the opinion that patients' body temperature should not be ignored. In the luteal phase, core temperature is consistently increased when both progesterone and oestrogen levels are elevated. In the presence of high levels of the two hormones, skin blood flow, sweating, and heart rate are likewise all shifted to higher internal temperatures.2Charkoudian N Johnson JM Female reproductive hormones and thermoregulatory control of skin blood flow.Exerc Sport Sci Rev. 2000; 28: 108-112PubMed Google Scholar One well-validated finding in women is that core temperature is increased by 0.3–0.5°C in the luteal phase compared with that of the follicular phase.3Hessemer V Bruck K Influence of menstrual cycle on shivering, skin blood flow, and sweating responses measured at night.J Appl Physiol. 1985; 59: 1902-1910PubMed Google Scholar It is well known that all general anaesthetics markedly impair normal autonomic themoregulatory control, especially in the first 30 min after induction of anaesthesia. Patients' core temperature usually decreases by 0.5–1.5°C, and the difference between the luteal and follicular phase may be increased. Drug metabolism is markedly decreased by perioperative hypothermia. It has been reported that the blood concentration of propofol in patients with a core temperature of 34°C increased by 28% compared with patients at 37°C using the same dosage.4Leslie K Sessler DI Bjorksten AR et al.Mild hypothermia alters propofol pharmacokinetics and increases the duration of action of atracurium.Anesth Analg. 1995; 80: 1007-1014PubMed Google Scholar Hypothermia also decreases the plasma clearance and prolongs the duration of action of rocuronium.5Caldwell JE Heier T Wright PM et al.Temperature-dependent pharmacokinetics and pharmacodynamics of vecuronium.Anesthesiology. 2000; 92: 84-93Crossref PubMed Scopus (103) Google Scholar Owing to these effects of hypothermia, the duration of postanaesthetic recovery was extended. Several studies have focused on the effects of hypothermia; tissue metabolic rate is reduced by ∼8% per 1°C. In the study by Fu and colleagues,1Fu F Chen X Feng Y et al.Propofol EC50 for inducing loss of consciousness is lower in the luteal phase of the menstrual cycle.Br J Anaesth. 2014; 112: 506-513Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar the effect of the patients' body temperature on drug metabolism was not mentioned. We speculate that the difference in body temperature between the two groups may be one of the factors that contributes to the phenomenon. Further research is needed to confirm this hypothesis. None declared.