Modulation of Remifentanil-Induced Postinfusion Hyperalgesia by Propofol

Antihyperalgesic effects of dexmedetomidine on high-dose remifentanil-induced hyperalgesia

Background

Dexmedetomidine is a highly selective α2 adrenergic agonist that has been shown to decrease the intensity of opioid-induced hyperalgesia (OIH). We aimed to investigate the antihyperalgesic effects of dexmedetomidine on high-dose remifentanil-induced hyperalgesia.

Methods

Ninety American Society of Anesthesiologists physical status I-II patients undergoing laparoscopically assisted vaginal hysterectomy (LAVH) were randomly assigned to one of the following three groups, each of which received either dexmedetomidine (an initial dose of 1.0 µg/kg for 10 min, followed by a continuous infusion of 0.7 µg/kg/hr) or placebo saline 15 min before the induction of anesthesia and intraoperative remifentanil infusion: group C received a placebo and 0.05 µg/kg/min remifentanil; group RH received a placebo and 0.3 µg/kg/min remifentanil; and group DRH received dexmedetomidine and 0.3 µg/kg/min remifentanil.

Results

The mechanical hyperalgesia threshold 24 hr after surgery was significantly lower in group RH than in the other two groups. Postoperative pain intensity using visual analog scale (VAS) and cumulative volume of a patient-controlled analgesia (PCA) containing morphine over 24 hr were significantly greater in group RH than in group DRH. The time to the first postoperative analgesic requirement was significantly shorter in group RH than in the other two groups. The desflurane requirement was significantly greater in group C than in the other groups. The frequency of hypotension and bradycardia was significantly higher, but shivering and postoperative nausea and vomiting were significantly lower in group DRH than in the other two groups.

Conclusions

High-doses of remifentanil induced hyperalgesia, which presented a decreased mechanical hyperalgesia threshold, enhanced pain intensity, a shorter time to first postoperative analgesic requirement, and greater morphine consumption, but dexmedetomidine efficiently alleviated those symptoms. Dexmedetomidine may be a novel and effective treatment option for preventing or attenuating OIH.

Phosphorylated CaMKII levels increase in rat central nervous system after large-dose intravenous remifentanil

Background

Postoperative remifentanil-induced pain sensitization is common, but its molecular mechanism remains unclear. Calcium/calmodulin-dependent protein kinase II (CaMKII) has been shown to have a critical role in morphine-induced hyperalgesia. This study was designed to determine how CaMKII phosphorylation and protein expression levels change in the central nervous system of rats with remifentanil-induced hyperalgesia.

Material/Methods

Male Sprague-Dawley^® rats were exposed to large-dose (bolus of 6.0 μg/kg and 2.5 μg/kg/min for 2 hours) intravenous remifentanil to induce post-transfusion hyperalgesia. Levels of phosphorylated CaMKII (P-CaMKII) and total protein of CaMKII (T-CaMKII) were determined at different post-transfusion times by Western blot and immunostaining and were compared with controls.

Results

P-CaMKII increased significantly (P<0.05) at 0, 0.5, and 2 hours. However, P-CaMKII at 5 to 24 hours and T-CaMKII at 0 to 24 hours post-transfusion did not change significantly in rats’ spinal dorsal horn, hippocampus, or primary somatosensory (S1) cortex (n=6 per group). Similarly, immunostaining showed stronger P-CaMKII immunoreactants (P<0.05) and more P-CaMKII- positive cells (P<0.05) in the spinal dorsal horn, CA1 region of the hippocampus, and S1 cortex of rats 0.5 hours post-transfusion compared with the control group treated with 0.9% sodium chloride (n=3 per group).

Conclusions

These results suggest that a temporary rise in the P-CaMKII level in the central nervous system may correlate with remifentanil-induced pain sensitization in the postoperative period.

Effect of oral pregabalin on opioid-induced hyperalgesia in patients undergoing laparo-endoscopic single-site urologic surgery

Background

Pregabalin is an antiepileptic drug that is effective for treating postoperative pain, neuropathic pain, anxiety, and hemodynamic instability. The aim of this study was to investigate the effect of a single preoperative dose of pregabalin in patients with opioid-induced hyperalgesia (OIH).

Methods

Ninety ASA I-II patients undergoing laparoendoscopic single-site urologic surgery were randomly assigned to one of the following three groups that received either pregabalin or placebo 1 h before anesthesia and an intraoperative remifentanil infusion. Group plL received placebo and 0.05 µg/kg/min remifentanil, group plH received placebo and 0.3 µg/kg/min remifentanil, and group prH received 300 mg pregabalin plus 0.3 µg/kg/min remifentanil. The primary endpoint was pain intensity upon movement 1, 6, 12, and 24 h after surgery. Secondary endpoints were the area of hyperalgesia and mechanical hyperalgesia threshold 24 h after surgery, time to first postoperative analgesic requirement, and cumulative postoperative volume of morphine administered via a patient-controlled analgesia (PCA) pump over 24 h.

Results

The time to first postoperative analgesic requirement in group plH was significantly shorter than that in group plL. The injected PCA volume was significantly greater in group plH than that in the other two groups. Postoperative pain intensity in group plH was significantly greater than that in the other two groups at 6, 12, and 24 h after surgery. The mechanical hyperalgesia threshold and the area of hyperalgesia around the surgical incision 24 h after surgery in group plH differed significantly from those in the other two groups, which were not significantly different. Adverse effects were comparable among groups.

Conclusions

High-dose remifentanil induced hyperalgesia, including increased pain intensity, increased area of hyperalgesia, and decreased mechanical hyperalgesia threshold. These effects were attenuated by oral administration of a single preoperative dose of pregabalin (300 mg) in patients undergoing laparo-endoscopic single-site urologic surgery.

Effects of repeated oxycodone administration on its analgesic and subjective effects in normal, healthy volunteers

Tolerance to the analgesic effects of opioids has been demonstrated in laboratory animals after repeated drug administration; yet, this effect has been studied less frequently under controlled laboratory conditions in humans. This within-subject, double-blind, placebo-controlled study was designed to determine whether tolerance developed to the analgesic, subjective, and physiological effects of the commonly prescribed opioid oxycodone when it was administered daily for 5 days. The effects of oxycodone (0, 5, and 20 mg/70 kg, orally) were compared, using a within-session cumulative dosing procedure, on the first and fifth days of the 'daily' dosing phase to assess for tolerance; active oxycodone was administered on the second and fourth days of the daily dosing phase. Changes in the effects of oxycodone were also compared when the medication was only administered on the first and the fifth day of a 5-day 'intermittent' dosing phase; placebo medication was administered on the second and fourth days of the intermittent dosing phase. A 9-day 'washout' period occurred between phases during which no medication was administered. Healthy volunteers (N=10) with no history of drug dependence or current drug use participated in this outpatient study. Analgesia was assessed using the cold pressor test, pain and drug effects were measured using a variety of questionnaires, and pupil diameter was monitored as an index of physiological effects. When administered daily, no differences were observed in oxycodone-induced analgesia between the first and the fifth days, but tolerance did develop to some of the positive subjective effects of oxycodone. In contrast, oxycodone-induced analgesia and participant ratings of some positive subjective drug effects were greater on the fifth compared with the first day of the intermittent dosing phase. No differences in the miotic effects of oxycodone between the first and the fifth days of either dosing phase were detected. Although obtained under limited experimental conditions, these findings suggest that tolerance may not develop to the analgesic effects of therapeutic doses of oxycodone under short-term daily dosing conditions, even though some of its subjective effects may decrease. These data also suggest that intermittent administration may enhance the analgesic effects of oxycodone, while also increasing some of the drug's positive subjective effects related to abuse liability.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

Propofol reduces early post-operative pain after gynecological laparoscopy

BACKGROUND

There is some evidence that propofol may reduce post-operative pain. However, the results on the analgesic effects of propofol are inconsistent. Thus, we hypothesized that propofol reduces acute pain if confounding factors like opioids are avoided.

METHODS

In this prospective, randomized, subject- and assessor-blind, parallel-group, head-to-head comparative study, 90 American Society of Anesthesiologists I or II females underwent elective laparoscopies were randomized to receive either propofol-based (PR), or sevoflurane-based (SR), or sevoflurane-propofol-based anesthesia (SPR). Pain score at rest assessed by a numerical rating scale at 0.5 h after surgery was the primary outcome. The secondary outcomes included pain score at 1 and 24 h post-operatively, duration of post-anesthesia care units stay (PACU), incidence of post-operative nausea and vomiting, incidence of shivering, and post-operative quality of recovery score (QoR-40) within the first 24 h post-operatively.

RESULTS

No patients received rescue analgesia. The pain score at 0.5 h post-operatively was less in group PR when compared with group SR (0.7 ± 1.4 vs. 2.1 ± 1.8; P = 0.010) or group SPR (0.7 ± 1.4 vs. 2.1 ± 2.2; P = 0.008). Group PR was also associated with shorter PACU stay than group SR (21.8 ± 5.7 vs. 26.2 ± 6.9; P = 0.050) or group SPR (21.8 ± 5.7 vs. 27.8 ± 8.9; P = 0.005). Intraoperative bispectral index values, hemodynamic values and post-operative QoR-40 scores did not differ among the three groups.

CONCLUSIONS

Propofol anesthesia was associated with significantly less pain at 0.5 and 1 h after surgery in patients undergoing gynecological laparoscopies with planned opioid-free post-operative analgesia.

[Clonidine for remifentanil-induced hyperalgesia: a double-blind randomized, placebo-controlled study of clonidine under intra-operative use of remifentanil in elective surgery of the shoulder]

BACKGROUND

In the postoperative period, α2-adrenergic agonists have an opioid sparing effect. In a previous, experimental study, it was also shown that clonidine attenuates remifentanil-induced hyperalgesia. In this study, we examined under clinical conditions whether early administration of a single dose of clonidine can inhibit remifentanil-induced hyperalgesia in patients undergoing elective surgery of the shoulder and with continuous intraoperative use of remifentanil.

PATIENTS AND METHODS

In this study 40 patients received double-blind and randomized either 150 µg clonidine or placebo intravenously before skin incision. Anaesthesia was maintained with propofol and remifentanil (0.23 ± 0.09 µg/kg body weight/min) and morphine (0.1 mg/kg body weight) was administered 20 min before incision closure. Postoperatively, the patients were given a patient-controlled analgesia pump (PCA) with morphine.

RESULTS

Overall morphine consumption as well as overall assessment of pain with the visual analogue scale in the first 24 h postoperatively did not differ significantly between the groups. Isolated pain scores at 12 h and 24 h were significantly enhanced in the clonidine group (p<0.05).

CONCLUSION

An early single dose of 150 µg of clonidine did not reduce the postoperative morphine consumption and pain scores in patients undergoing elective surgery of the shoulder with remifentanil/propofol-based anaesthesia. After the effect of clonidine has presumably subsided the pain can even increase, therefore further studies with repetitive doses of clonidine should be carried out.

Opioid-induced hyperalgesia (OIH)

BACKGROUND AND OBJECTIVES

Opioids are commonly used for pain control; however, they can cause hyperalgesia. The reason why this can happen is not known. The objective of this review was to describe the mechanisms, factors implicated, and drug modulation.

CONTENTS

The factors implicated in the development of opioid-induced hyperalgesia (OIH), such as duration of use, dose, and type of opioids are described. Mechanisms involved include the glutamatergic system and N-methyl-D-aspartate receptors (NMDA), spinal cyclooxygenase (COX) activation, excitatory amino acids, dynorphin, cytokines and chemokines; prostaglandins, and descending facilitation. Modulation of hyperalgesia could be done through: NMDA receptor antagonists, alpha2-adrenergic agonists, and COX inhibitors.

CONCLUSIONS

This is a very complex subject, which involves a series of pathophysiological mechanisms that could contribute for OIH and patient discomfort, bringing disastrous consequences.

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.

Long-term potentiation in spinal nociceptive pathways as a novel target for pain therapy

Long-term potentiation (LTP) in nociceptive spinal pathways shares several features with hyperalgesia and has been proposed to be a cellular mechanism of pain amplification in acute and chronic pain states. Spinal LTP is typically induced by noxious input and has therefore been hypothesized to contribute to acute postoperative pain and to forms of chronic pain that develop from an initial painful event, peripheral inflammation or neuropathy. Under this assumption, preventing LTP induction may help to prevent the development of exaggerated postoperative pain and reversing established LTP may help to treat patients who have an LTP component to their chronic pain. Spinal LTP is also induced by abrupt opioid withdrawal, making it a possible mechanism of some forms of opioid-induced hyperalgesia. Here, we give an overview of targets for preventing LTP induction and modifying established LTP as identified in animal studies. We discuss which of the various symptoms of human experimental and clinical pain may be manifestations of spinal LTP, review the pharmacology of these possible human LTP manifestations and compare it to the pharmacology of spinal LTP in rodents.

The anxiolytic effect of pregabalin in outpatients undergoing minor orthopaedic surgery

Preoperative anxiety can increase postoperative pain and is therefore important to avoid. Different approaches have already been tested for preoperative anxiolysis. Gabapentinoids might be a useful alternative to benzodiazepines. Pregabalin is used for treating generalized anxiety disorders and shows a favourable pharmacokinetic profile after oral administration; however, its anxiolytic effect preoperatively in healthy outpatients is still unclear. In this randomised, double-blind, placebo-controlled trial the anxiolytic effect of pregabalin in 40 outpatients undergoing standardised general anaesthesia and postoperative pain therapy for minor orthopaedic surgery was analysed. Patients received preoperatively either 300 mg pregabalin or placebo orally. The primary outcome was anxiety before anaesthesia induction, the secondary outcome the postoperative pain, both assessed using a visual analogue scale from 0 to 100. Without any side effects pregabalin reduced preoperative anxiety compared with the control group (23 ± 10 vs. 38 ± 17; p = 0.003). Pain scores did not differ between groups; however, need of piritramide in the postanaesthesia care unit was reduced to half by pregabalin compared with the control group. A single preoperative dose of 300 mg pregabalin reduces anxiety in patients undergoing minor orthopaedic surgery without any side effects like dizziness or persisting sedation resulting in a prolonged stay in the postanaesthesia care unit.

Review of the efficacy and safety of remifentanil for the prevention and treatment of pain during and after procedures and surgery

Remifentanil is an ultrashort-acting synthetic opioid. It is metabolized by nonspecific tissue and plasma esterases. Remifentanil's metabolism is responsible for its unique pharmacokinetic profile and flat, context-sensitive half-time. Since its introduction into clinical practice, remifentanil has been used for a variety of anesthetic and analgesic applications; however, concerns regarding a potential for rapid induction of tolerance and/or induced hyperalgesia, coupled with an ultrarapid offset of effect, make the drug less than optimal for use in the pharmacologic management of pain.

Gabapentin improves cold-pressor pain responses in methadone-maintained patients

Individuals on methadone maintenance for the treatment of addiction (MM) are demonstrated to be hyperalgesic to cold-pressor pain in comparison to matched controls and ex-opioid addicts, a finding described as clinical evidence of opioid-induced hyperalgesia (OIH). Interestingly, opioids induce hyperalgesia via many of the same neuro-inflammatory and central sensitization processes that occur with the development of neuropathic pain. Evaluated in this study was the efficacy of a key pharmacotherapy for neuropathic pain, gabapentin (GPN), to reverse OIH in MM patients. Utilizing a clinical trial design and double blind conditions, changes in cold-pressor pain threshold and tolerance following a 5-week trial of GPN (titrated to 2400mg/day) were evaluated at peak and trough methadone plasma levels in a well-characterized MM sample. Drug abstinence was encouraged via an escalating payment schedule, and compliance monitored via pill counts and GPN plasma levels; entered into the analyses were only those subjects compliant and abstinent throughout the study (approximately 45%). Utilizing change scores from baseline, significant improvements in cold-pressor pain threshold and pain tolerance were observed at both peak and trough methadone levels (p<0.05). Notably, drop-out rates due to medication side effects were low (2%) and the medication was well-tolerated. These results support that GPN, as prescribed for the treatment of neuropathic pain, is effective in decreasing OIH in patients who are abstinent and stable in methadone treatment.

Validation, reproducibility and safety of trans dermal electrical stimulation in chronic pain patients and healthy volunteers

BACKGROUND

Surrogate pain models have been extensively tested in Normal Human Volunteers (NHV). There are few studies that examined pain models in chronic pain patients. Patients are likely to have altered pain mechanisms. It is of interest to test patient pain responses to selective pain stimuli under controlled laboratory conditions.

METHODS

The Institutional Ethic Committee approved the study. 16 patients with chronic neuropathic radiculopathy and 16 healthy volunteers were enrolled to the study after obtaining informed consent. During electrical stimulation (150 minutes for volunteers and 75 minutes for patients) the following parameters were measured every 10 minutes: Ongoing pain: Visual Analogue Scale (VAS) and Numeric Rate Scale (NRS)Allodynia (soft foam brush)Hyperalgesia (von Frey monofilament 20 g)Flare. For each endpoint, the area under the curve (AUC) was estimated from the start of stimulation to the end of stimulation by the trapezoidal rule. The individual AUC values for both periods were plotted to show the inter- and intra-subject variability. For each endpoint a mixed effect model was fitted with random effect subject and fixed effect visit. The estimate of intra-subject variance and the mean value were then used to estimate the sample size of a crossover study required to have a probability of 0.80 to detect a 25% change in the mean value. Analysis was done using GenStat 8th edition.

RESULTS

Each endpoint achieved very good reproducibility for patients and NHV. Comparison between groups revealed trends towards: Faster habituation to painful stimuli in patients. Bigger areas of hyperalgesia in patients. Similar area of allodynia and flare (no statistical significance)

CONCLUSION

The differences demonstrated between patients and NHVs suggest that the electrical stimulation device used here may stimulate pathways that are affected in the pathological state.

Assessing analgesic actions of opioids by experimental pain models in healthy volunteers - an updated review

AIM

Experimental pain models may help to evaluate the mechanisms of action of analgesics and target the clinical indications for their use. This review addresses how the efficacy of opioids can be assessed in human volunteers using experimental pain models. The drawback with the different study designs is also discussed.

METHOD

A literature search was completed for randomized controlled studies which included human experimental pain models, healthy volunteers and opioids.

RESULTS

Opioids with a strong affinity for the micro-opioid receptor decreased the sensation in a variety of experimental pain modalities, but strong tonic pain was attenuated more than short lasting pain and non-painful sensations. The effects of opioids with weaker affinity for the micro-opioid receptor were detected by a more narrow range of pain models, and the assessment methods needed to be more sensitive.

CONCLUSION

The way the pain is induced, assessed and summarized is very important for the sensitivity of the pain models. This review gives an overview of how different opioids perform in experimental pain models. Generally experimental pain models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. This knowledge can aid the decisions needed to be taken when designing experimental pain studies for compounds entering phase 1 clinical trials.

Chronic morphine administration enhances nociceptive sensitivity and local cytokine production after incision

BACKGROUND

The chronic use of opioids prior to surgery leads to lowered pain thresholds and exaggerated pain levels after these procedures. Several mechanisms have been proposed to explain this heightened sensitivity commonly termed opioid-induced hyperalgesia (OIH). Most of these proposed mechanisms involve plastic events in the central or peripheral nervous systems. Alterations in the abundance of peripheral mediators of nociception have not previously been explored.

RESULTS

In these experiments mice were treated with saline (control) or ascending daily doses of morphine to generate a state of OIH followed by hind paw incision. In other experiments morphine treatment was initiated at the time of incision. Both mechanical allodynia and peri-incisional skin cytokine levels were measured. Myeloperoxidase (MPO) assays were used to determine neutrophil activity near the wounds. The cytokine production inhibitor pentoxifylline was used to determine the functional significance of the excess cytokines in previously morphine treated animals. Mice treated chronically treated with morphine prior to incision were found to have enhanced skin levels of IL-1beta, IL-6, G-CSF, KC and TNFalpha after incision at one or more time points compared to saline pretreated controls. The time courses of individual cytokines followed different patterns. There was no discernable effect of chronic morphine treatment on wound area neutrophil infiltration. Pentoxifylline reduced cytokine levels and reversed the excess mechanical sensitization caused by chronic morphine administration prior to incision. Morphine treatment initiated at the time of incision did not lead to a generalized enhancement of cytokine production or nociceptive sensitization in excess of the levels observed after incision alone.

CONCLUSION

The enhanced level of nociceptive sensitization seen after incision in animals chronically exposed to morphine is associated with elevated levels of several cytokines previously reported to be relevant to this incisional pain model. The cytokines may be functional in supporting nociceptive sensitization because pentoxifylline reverses both peri-incisional skin cytokine levels and OIH. Opioid administration beginning at the time of incision does not seem to have the same cytokine enhancing effect. Approaches to postoperative pain control involving a reduction of cytokines may be an effective way to control excessive pain in patients chronically using opioids prior to surgical procedures.