Postoperative Pharmacokinetics of Methadone in Patients With Chronic Kidney Disease

This study will be carried out at the Pontifical Catholic University of Chile Hospital with prior approval by the ethics committee. Study staff will assess the inclusion criteria, obtain informed consent, withdraw the blood samples, and record and manage the data.

Patients will be recruited before surgery. At the time the patient agrees to participate in the study, the functioning and adequate use of an intravenous patient-controlled analgesia (IV-PCA) pump will be explained by one of the members of the research team. The anesthetic, surgical, or postoperative care will not be affected by this research study.

Once the patient is in the theatre, standard intraoperative monitoring will be applied, including pulse oximetry, electrocardiography, automatic blood pressure cuff, capnography, and bispectral index monitoring (BIS system). The induction of anesthesia will be performed by the administration of remifentanil in a target-controlled infusion (TCI) using the MINTO model, propofol 1-2mg/kg and atracurium 0.5mg/kg or rocuronium 0.6mg/kg.

After induction, a second intravenous line will be installed in the contralateral arm exclusively to obtain blood samples. Subsequently, based on the ideal body weight, patients will receive a bolus of 0.1mg/kg of methadone at the beginning of surgery with a maximum dose of 20mg.

The maintenance of general anesthesia will be through the administration of sevoflurane to achieve BIS values between 40-60. Atracurium or rocuronium in doses of 5-10mg will be administered according to the neuromuscular block monitor and the infusion of remifentanil will be at the discretion of the anesthesiologist in charge to achieve mean arterial pressure within 20% of the baseline measure (previous induction of anesthesia). Episodes of hypotension will be treated with phenylephrine 50-100 mcg, ephedrine 6-12mg, or fluid bolus, as indicated.