Opioid Tolerance in Critical Illness

Acute pain is protective and a cardinal feature of inflammation. Chronic pain after arthritis, nerve injury, cancer, and chemotherapy is associated with chronic neuroinflammation, a local inflammation in the peripheral or central nervous system. Accumulating evidence suggests that non-neuronal cells such as immune cells, glial cells, keratinocytes, cancer cells, and stem cells play active roles in the pathogenesis and resolution of pain. We review how non-neuronal cells interact with nociceptive neurons by secreting neuroactive signaling molecules that modulate pain. Recent studies also suggest that bacterial infections regulate pain through direct actions on sensory neurons, and specific receptors are present in nociceptors to detect danger signals from infections. We also discuss new therapeutic strategies to control neuroinflammation for the prevention and treatment of chronic pain.

Standard treatment of critically ill patients undergoing mechanical ventilation is continuous sedation. Daily interruption of sedation has a beneficial effect, and in the general intesive care unit of Odense University Hospital, Denmark, standard practice is a protocol of no sedation. We aimed to establish whether duration of mechanical ventilation could be reduced with a protocol of no sedation versus daily interruption of sedation. Of 428 patients assessed for eligibility, we enrolled 140 critically ill adult patients who were undergoing mechanical ventilation and were expected to need ventilation for more than 24 h. Patients were randomly assigned in a 1:1 ratio (unblinded) to receive: no sedation (n=70 patients); or sedation (20 mg/mL propofol for 48 h, 1 mg/mL midazolam thereafter) with daily interruption until awake (n=70, control group). Both groups were treated with bolus doses of morphine (2.5 or 5 mg). The primary outcome was the number of days without mechanical ventilation in a 28-day period, and we also recorded the length of stay in the intensive care unit (from admission to 28 days) and in hospital (from admission to 90 days). Analysis was by intention to treat. This study is registered with ClinicalTrials.gov, number NCT00466492. 27 patients died or were successfully extubated within 48 h, and, as per our study design, were excluded from the study and statistical analysis. Patients receiving no sedation had significantly more days without ventilation (n=55; mean 13.8 days, SD 11.0) than did those receiving interrupted sedation (n=58; mean 9.6 days, SD 10.0; mean difference 4.2 days, 95% CI 0.3-8.1; p=0.0191). No sedation was also associated with a shorter stay in the intensive care unit (HR 1.86, 95% CI 1.05-3.23; p=0.0316), and, for the first 30 days studied, in hospital (3.57, 1.52-9.09; p=0.0039), than was interrupted sedation. No difference was recorded in the occurrences of accidental extubations, the need for CT or MRI brain scans, or ventilator-associated pneumonia. Agitated delirium was more frequent in the intervention group than in the control group (n=11, 20%vs n=4, 7%; p=0.0400). No sedation of critically ill patients receiving mechanical ventilation is associated with an increase in days without ventilation. A multicentre study is needed to establish whether this effect can be reproduced in other facilities. Danish Society of Anesthesiology and Intensive Care Medicine, the Fund of Danielsen, the Fund of Kirsten Jensa la Cour, and the Fund of Holger og Ruth Hess. Copyright 2010 Elsevier Ltd. All rights reserved.

For more than 50 years, ketamine has proven to be a safe anesthetic drug with potent analgesic properties. The active enantiomer is S(+)-ketamine. Ketamine is mostly metabolized in norketamine, an active metabolite. During "dissociative anesthesia", sensory inputs may reach cortical receiving areas, but fail to be perceived in some association areas. Ketamine also enhances the descending inhibiting serotoninergic pathway and exerts antidepressive effects. Analgesic effects persist for plasma concentrations ten times lower than hypnotic concentrations. Activation of the (N-Methyl-D-Aspartate [NMDA]) receptor plays a fundamental role in long-term potentiation but also in hyperalgesia and opioid-induced hyperalgesia. The antagonism of NMDA receptor is responsible for ketamine's more specific properties. Ketamine decreases the "wind up" phenomenon, and the antagonism is more important if the NMDA channel has been previously opened by the glutamate binding ("use dependence"). Experimentally, ketamine may promote neuronal apoptotic lesions but, in usual clinical practice, it does not induce neurotoxicity. The consequences of high doses, repeatedly administered, are not known. Cognitive disturbances are frequent in chronic users of ketamine, as well as frontal white matter abnormalities. Animal studies suggest that neurodegeneration is a potential long-term risk of anesthetics in neonatal and young pediatric patients. © 2013 John Wiley & Sons Ltd.