Behavioural effects of norpethidine, a metabolite of pethidine, in rats

Interaction between dextromethorphan and norpethidine in rats

With increasing interest in the application of dextromethorphan in pain control, it is probable that patients will receive this drug in combination with analgesics such as opioids, giving rise to the potential for previously unobserved drug interactions. The interaction between dextromethorphan, and its pharmacologically active metabolite dextrorphan, and norpethidine, a toxic metabolite of pethidine, was examined in rats. Rats were assigned to receive dextromethorphan (0, 20 or 40 mg/kg) or dextrorphan (0, 15 or 30 mg/kg) combined with norpethidine (0, 28 or 42 mg/kg). The occurrence of seizures, myoclonic jerks and shivering was recorded for 60 min after drug administration. Norpethidine produced dose-related increases in the incidence of seizures, myoclonic jerks and shivering. Dextromethorphan, but not dextrorphan, increased the incidence of these behaviours. It is recommended that extreme caution be exercised if dextromethorphan and pethidine are to be used together.

Suspected acute meperidine toxicity in a dog

OBSERVATIONS

A 22-month-old male neutered Coton De Tulear dog was presented for upper gastrointestinal endoscopy under general anesthesia. The anesthetic plan included premedication with intramuscular meperidine (4 mg kg(-1)) but meperidine was inadvertently administered at ten-fold this dose. Within 5 minutes, the dog was unresponsive to external stimulation, and by 10 minutes post-injection developed generalized signs of central nervous system (CNS) excitement. Initial therapy included inspired oxygen supplementation, and single intravenous (IV) doses of diazepam (0.68 mg kg(-1)) and naloxone (0.03 mg kg(-1)) to no effect. A second dose of diazepam (0.46 mg kg(-1), IV) abolished most of the signs of CNS excitement. General anesthesia was induced and the endoscopy performed. Time to extubation was initially prolonged, but administering naloxone (final dose 0.1 mg kg(-1), IV) to effect enabled extubation. After naloxone, the dog became agitated, noise sensitive, and had leg and trunk muscle twitches. Diazepam (0.30 mg kg(-1), IV) abolished these signs and the dog became heavily sedated and laterally recumbent. Naloxone administration was continued as a constant rate infusion (0.02 mg kg(-1) hour(-1), IV) until approximately 280 minutes post-meperidine injection, at which time the dog suddenly sat up. Occasional twitches of the leg and trunk muscles were observed during the night. The dog was discharged the next day appearing clinically normal.

CONCLUSIONS

Given that the CNS excitatory effects of normeperidine are not a mu opioid receptor effect, the use of naloxone should be considered carefully when normeperidine excitotoxicity is suspected. Benzodiazepines may be beneficial in ameliorating clinical signs of normeperidine excitotoxicity.

Meperidine and skin surface warming additively reduce the shivering threshold: a volunteer study

Introduction

Mild therapeutic hypothermia has been shown to improve outcome for patients after cardiac arrest and may be beneficial for ischaemic stroke and myocardial ischaemia patients. However, in the awake patient, even a small decrease of core temperature provokes vigorous autonomic reactions–vasoconstriction and shivering–which both inhibit efficient core cooling. Meperidine and skin warming each linearly lower vasoconstriction and shivering thresholds. We tested whether a combination of skin warming and a medium dose of meperidine additively would reduce the shivering threshold to below 34°C without producing significant sedation or respiratory depression.

Methods

Eight healthy volunteers participated on four study days: (1) control, (2) skin warming (with forced air and warming mattress), (3) meperidine (target plasma level: 0.9 μg/ml), and (4) skin warming plus meperidine (target plasma level: 0.9 μg/ml). Volunteers were cooled with 4°C cold Ringer lactate infused over a central venous catheter (rate ≈ 2.4°C/hour core temperature drop). Shivering threshold was identified by an increase of oxygen consumption (+20% of baseline). Sedation was assessed with the Observer's Assessment of Alertness/Sedation scale.

Results

Control shivering threshold was 35.5°C ± 0.2°C. Skin warming reduced the shivering threshold to 34.9°C ± 0.5°C (p = 0.01). Meperidine reduced the shivering threshold to 34.2°C ± 0.3°C (p < 0.01). The combination of meperidine and skin warming reduced the shivering threshold to 33.8°C ± 0.2°C (p < 0.01). There were no synergistic or antagonistic effects of meperidine and skin warming (p = 0.59). Only very mild sedation occurred on meperidine days.

Conclusion

A combination of meperidine and skin surface warming reduced the shivering threshold to 33.8°C ± 0.2°C via an additive interaction and produced only very mild sedation and no respiratory toxicity.

Trends in the pharmacology of opioids: implications for the pharmacotherapy of pain

Some 25 years ago, it was popular to write a figurative equation for drug treatment: Patient + drug --> effect. Clearly, this was too simplistic for the reliable pharmacotherapy of pain. The 'patient' with pain is a complex individual and the 'drug' is not the same in effect to every patient, regardless of being an opioid analgesic. Besides, the 'effect' needs specification: it really means an acceptable balance between therapeutic and side-effect--to that patient. These days, neural plasticity and neuromodulation associated with nociception are well-known. Psychological involvement in the physical domain of nociception is similarly well-known. Given these complexities, it is surprising that the pharmacological control of pain through the application of relatively simple analgesics can be so effective. Opioid analgesics have been administered by every possible route. Because the degree of invasion of the patient differs between routes, the balance between simplicity, aesthetic appeal, and efficacy of the various routes needs to be considered. The effectiveness of opioid analgesics depends in large measure on the application of the right agent, in the right dose, by the right route, at the right time for that patient. To assist in this task, researchers have produced patient-controlled analgesia and 'pharmacologically engineered' analgetic molecules to achieve receptor selectivity and pharmacokinetic predictability. Also, much relevant data concerning the time courses of analgetic drug (+/- metabolite) concentrations in the body and in the ways in which these properties are modified by different normal and pathophysiological variations have been gathered, and the philosophy of 'opioid rotation' has arisen to maintain therapeutic benefits when tolerance or metabolite-induced side-effects prevail. This essay discusses some of the trends in opioid pharmacotherapy over the past 25 years.

Pain management in the critically ill child

Children frequently received no treatment, or inadequate treatment, for pain and for painful procedures. The newborn and critically ill children are especially vulnerable to no treatment or under-treatment. Nerve pathways essential for the transmission and perception of pain are present and functioning by 24 weeks of gestation. The failure to provide analgesia for pain results in rewiring the nerve pathways responsible for pain transmission in the dorsal horn of the spinal cord and results in increased pain perception for future painful results. Many children would withdraw or deny their pain in an attempt to avoid yet another terrifying and painful experiences, such as the intramuscular injections. Societal fears of opioid addiction and lack of advocacy are also causal factors in the under-treatment of pediatric pain. False beliefs about addictions and proper use of acetaminophen and other analgesics resulted in the failure to provide analgesia to children. All children even the newborn and critically ill require analgesia for pain and painful procedures. Unbelieved pain interferes with sleep, leads to fatigue and a sense of helplessness, and may result in increased morbidity or mortality.

The management of pain in sickle cell disease

The pain of vaso-occlusive crisis in patients with sickle cell disease is excruciating, incapacitating, and sometimes refractory to even the most advanced analgesic treatments. A comprehensive, multimodal approach to therapy that includes education, cognitive therapies, anti-inflammatory drugs, opioids, and psychostimulant adjuvant drugs has been presented. Until a cure for the underlying disease is found, these are the best approaches available. The authors hope that future research will find even better modalities of analgesic care.