Tolerance and withdrawal from prolonged opioid use in critically ill children

Remifentanil-induced pronociceptive effect and its prevention with pregabalin

Background

Experimental and clinical studies have suggested that remifentanil probably causes acute tolerance or postinfusion hyperalgesia. This study was designed to confirm whether remifentanil given during propofol anesthesia induced postoperative pain sensitization, and we wanted to investigate whether pregabalin could prevent this pronociceptive effect.

Methods

Sixty patients who were scheduled for total abdominal hysterectomy were randomly allocated to receive (1) a placebo as premedication and an intraoperative saline infusion (control group), (2) a placebo as premedication and an intraoperative infusion of remifentanil at a rate of 3-4 ng/ml (remifentanil group), or (3) pregabalin 150 mg as premedication and an intraoperative infusion of remifentanil at a rate of 3-4 ng/ml (pregabalin-remifentanil group). Postoperative pain was controlled by titration of fentanyl in the postanesthetic care unit (PACU), followed by patient-controlled analgesia (PCA) with fentanyl. The patients were evaluated using the visual analogue scale (VAS) for pain scores at rest and after cough, consumption of fentanyl, sedation score and any side effects that were noted over the 48 h postoperative period.

Results

The fentanyl titration dose given in the PACU was significantly larger in the remifentanil group as compared with those of the other two groups. At rest, the VAS pain score in the remifentanil group at 2 h after arrival in the PACU was significantly higher than those in the other two groups.

Conclusions

The results of this study show that remifentanil added to propofol anesthesia causes pain sensitization in the immediate postoperative period. Pretreatment with pregabalin prevents this pronociceptive effect and so this may be useful for the management of acute postoperative pain when remifentanil and propofol are used as anesthetics.

Increased clearance of morphine in sickle cell disease: implications for pain management

Acute vaso-occlusive painful episodes associated with sickle cell disease (SCD) are frequently treated with morphine. Many SCD individuals require relatively higher doses of morphine to achieve optimal analgesia. We studied pharmacokinetics of morphine in SCD to explore if altered disposition could be a factor contributing to increased requirement of morphine in this population. The study subjects were in steady state of health to avoid the effect of hemodynamic changes associated with vaso-occlusion on morphine disposition. The plasma concentrations of morphine and its major metabolites were measured at timed intervals in 21 SCD subjects after they received a single .1 mg/Kg infusion of morphine sulfate. USCPACK software was used to fit candidate pharmacokinetic models. Noncompartmental pharmacokinetic parameters for morphine were calculated. Morphine clearance was 2.4-3.6 L/h, half-life was .3-.7 hours, AUC(0-∞) was 27.7-42.5 ng∗h/mL, and volume of distribution was .96-3.38 L/kg. Clearance of morphine in the study population was 3-10 folds higher than published estimates in the non-SCD population, with correspondingly lower AUC and half-life. Volume of distribution was similar. This observation suggests that due to increased clearance SCD individuals may require higher dose and frequency of morphine to achieve comparable plasma levels.

PERSPECTIVE

Accelerated clearance of morphine likely related to increased hepatic and renal blood flow may be responsible for increased requirement of morphine in SCD. Although SCD individuals may require higher and more frequent doses of morphine, inter-individual variability of morphine disposition highlights the importance of individualization of the therapy.

Changing mechanisms of opiate tolerance and withdrawal during early development: animal models of the human experience

Human infants may be exposed to opiates through placental transfer from an opiate-using mother or through the direct administration of such drugs to relieve pain (e.g., due to illness or neonatal surgery). Infants of many species show physical dependence and tolerance to opiates. The magnitude of tolerance and the nature of withdrawal differ from those of the adult. Moreover, the mechanisms that contribute to the chronic effects of opiates are not well understood in the infant but include biological processes that are both common to and distinct from those of the adult. We review the animal research literature on the effects of chronic and acute opiate exposure in infants and identify mechanisms of withdrawal and tolerance that are similar to and different from those understood in adults. These mechanisms include opioid pharmacology, underlying neural substrates, and the involvement of other neurotransmitter systems. It appears that brain circuitry and opioid receptor types are similar but that NMDA receptor function is immature in the infant. Intracellular signaling cascades may differ but data are complicated by differences between the effects of chronic versus acute morphine treatment. Given the limited treatment options for the dependent infant patient, further study of the biological functions that are altered by chronic opiate treatment is necessary to guide evidenced-based treatment modalities.