No evidence for the development of acute tolerance to analgesic, respiratory depressant and sedative opioid effects in humans

Association between intraoperative remifentanil use and postoperative hyperalgesia in adolescent idiopathic scoliosis surgery: a retrospective study

BACKGROUND

The liberal use of remifentanil in spine surgery has been associated with an increased incidence of postoperative hyperalgesia. Nevertheless, controversies remain as the existing evidence is inconclusive to determine the relationship between remifentanil use and the development of opioid-induced hyperalgesia. We hypothesized that intraoperative infusion of higher dose remifentanil during scoliosis surgery is associated with postoperative hyperalgesia, manifesting clinically as greater postoperative morphine consumption and pain scores.

METHODS

Ninety-seven patients with adolescent idiopathic scoliosis (AIS) who underwent posterior spinal fusion surgery at a single tertiary institution from March 2019 until June 2020 were enrolled in this retrospective study. Anesthesia was maintained using a target-controlled infusion of remifentanil combined with volatile anesthetic desflurane in 92 patients, while five patients received it as part of total intravenous anesthesia. Intravenous ketamine, paracetamol, and fentanyl were administered as multimodal analgesia. All patients received patient-controlled analgesia (PCA) morphine postoperatively. Pain scores at rest and on movement, assessed using the numerical rating scale, and the cumulative PCA morphine consumption were collected at a six-hourly interval for up to 48 h. According to the median intraoperative remifentanil dose usage of 0.215 µg/kg/min, patients were divided into two groups: low dose and high dose group.

RESULTS

There were no significant differences in the pain score and cumulative PCA morphine consumption between the low and high dose remifentanil group. The mean duration of remifentanil infusion was 134.9 ± 22.0 and 123.4 ± 23.7 min, respectively.

CONCLUSION

Intraoperative use of remifentanil as an adjuvant in AIS patients undergoing posterior spinal fusion surgery was not associated with postoperative hyperalgesia.

A Prospective Randomized Comparison of Postoperative Pain and Complications after Thyroidectomy under Different Anesthetic Techniques: Volatile Anesthesia versus Total Intravenous Anesthesia

While the postoperative outcome is favorable, post-thyroidectomy pain is considerable. Reducing the postoperative acute pain, therefore, is considered important. This study investigated whether the pain intensity and need for rescue analgesics during the immediate postoperative period after thyroidectomy differ according to the methods of anesthesia. Seventy-two patients undergoing total thyroidectomy under general anesthesia were examined. Patients were randomly assigned to undergo either total intravenous anesthesia with remifentanil and propofol (TIVA, n = 35) or propofol induction and maintenance with desflurane and nitrous oxide (volatile anesthesia [VA], n = 37). The mean administered dose of remifentanil was 1977.7 ± 722.5 μg in the TIVA group, which was approximately 0.268 ± 0.118 μg/min/kg during surgery. Pain scores based on a numeric rating scale (NRS) and the need for rescue analgesics were compared between groups at the postoperative anesthetic care unit (PACU). The immediate postoperative NRS values of the TIVA and VA groups were 5.7 ± 1.7 and 4.7 ± 2.3, respectively (P = 0.034). Postoperative morphine equianalgesic doses in the PACU were higher in the TIVA group than in the VA group (16.7 ± 3.8 mg vs. 14.1 ± 5.9 mg, P = 0.027). The incidence of immediate postanesthetic complications did not differ significantly between groups. In conclusion, more rescue analgesics were required in the TIVA group than in the VA group to adequately manage postoperative pain while staying in the PACU after thyroidectomy.

Postoperative Pain Management in Pediatric Spinal Fusion Surgery for Idiopathic Scoliosis

This article reviews and summarizes current evidence and knowledge gaps regarding postoperative analgesia after pediatric posterior spine fusion for adolescent idiopathic scoliosis, a common procedure that results in severe acute postoperative pain. Inadequate analgesia may delay recovery, cause patient dissatisfaction, and increase chronic pain risk. Despite significant adverse effects, opioids are the analgesic mainstay after scoliosis surgery. However, growing emphasis on opioid minimization and enhanced recovery has increased adoption of multimodal analgesia (MMA) regimens. While opioid adverse effects remain a concern, MMA protocols must also consider risks and benefits of adjunct medications. We discuss use of opioids via different administration routes and elaborate on the effect of MMA components on opioid/pain and recovery outcomes including upcoming regional analgesia. We also discuss risk for prolonged opioid use after surgery and chronic post-surgical pain risk in this population. Evidence supports use of neuraxial opioids at safe doses, low-dose ketorolac, and methadone for postoperative analgesia. There may be a role for low-dose ketamine in those who are opioid-tolerant or have chronic pain, but the evidence for preoperative gabapentinoids and intravenous lidocaine is currently insufficient. There is a need for further studies to evaluate pediatric-specific optimal MMA dosing regimens after scoliosis surgery. Questions remain regarding how best to prevent acute opioid tolerance, opioid-induced hyperalgesia, and chronic postsurgical pain. We anticipate that this timely update will enable clinicians to develop efficient pain regimens and provide impetus for future research to optimize recovery outcomes after spine fusion.

Determination of acute tolerance and hyperalgesia to remifentanil constant rate infusion in dogs undergoing sevoflurane anaesthesia

OBJECTIVE

To determine if acute opioid tolerance (AOT) or opioid-induced hyperalgesia (OIH) could develop and limit the remifentanil-induced reduction in the sevoflurane minimum alveolar concentration (MAC). The response to mechanical nociceptive threshold (MNT) was evaluated and related to OIH.

STUDY DESIGN

A crossover, randomized, experimental animal study.

ANIMALS

A total of nine Beagle dogs.

METHODS

The dogs were anaesthetized with sevoflurane in 50% oxygen. Baseline sevoflurane MAC was measured (MACb1). Remifentanil (0.3 μg kg^-1 minute^-1) or 0.9% saline constant rate infusion (CRI) was administered intravenously (IV). Sevoflurane MAC was determined 20 minutes after CRI was initiated (MACpostdrug1), 30 minutes after MACpostdrug1 determination (MACpostdrug2) and after 1 week (MACb2). The MNT was determined at baseline (before anaesthesia), 3 and 7 days after anaesthesia. An increase of MACpostdrug2 ≥0.25% compared to MACpostdrug1 was considered evidence of AOT. A decrease in MNT at 3 and 7 days or an increase in MACb2 or both with respect to MACb1 were considered evidence of OIH.

RESULTS

Remifentanil CRI reduced sevoflurane MACpostdrug1 by 43.7% with respect to MACb1. MACpostdrug2 was no different from MACpostdrug1 with the saline (p = 0.62) or remifentanil (p = 0.78) treatments. No significant differences were observed in the saline (p = 0.99) or remifentanil (p = 0.99) treatments between MACb1 and MACb2, or for MNT values between baseline, 3 and 7 days.

CONCLUSION AND CLINICAL RELEVANCE

In dogs, under the study conditions, remifentanil efficacy in reducing sevoflurane MAC did not diminish in the short term, suggesting remifentanil did not induce AOT. Hyperalgesia was not detected 3 or 7 days after the administration of remifentanil. Contrary to data from humans and rodents, development of AOT or OIH in dogs is not supported by the findings of this study.

The Pharmacokinetics and Pharmacodynamics of Carfentanil After Recreational Exposure: A Case Report

Carfentanil and related fentanyl analogs have been linked to a number of overdose deaths from drug users in several cities across North America. Diagnosis of carfentanil exposure requires a very high index of clinical suspicion, especially because available laboratory narcotic screens do not detect this agent. We describe a 34-year-old man admitted with depressed level of consciousness and in respiratory failure after recreational exposure to a white powder later inferred to contain carfentanil. Urine and whole blood samples were obtained for conventional preliminary drug screen immunoassays for unknown exposures, in addition to utilizing a high-pressure liquid chromatography-tandem mass spectrometry assay for quantification of carfentanil and its metabolite. The patient was intubated and required mechanically assisted ventilation for 31 hours until he was able to breathe safely on his own. Pharmacokinetic modeling of three timed blood samples identified the elimination half-life as 5.7 hours for carfentanil and 11.8 hours for the norcarfentanil metabolite. Awakening and breathing spontaneously corresponded to an interpolated blood carfentanil concentration of 0.52 ng/ml. This is the first pharmacokinetic and pharmacodynamic case report on the recreational use of carfentanil. Critical care clinicians should anticipate long periods of ventilatory support in the care of patients exposed to carfentanil.

The Pharmacology and Toxicology of the 'Holy Trinity'

Combining opioids with benzodiazepines and skeletal muscle relaxants ('The Holy Trinity') has been reported to potentiate the 'high'. Through unique interactions with colocalized μ-opioid and GABA_A receptors, the combined use of these agents induces a synergistic increase in dopamine in the nucleus accumbens (NAc) and depression of respiration. The inhibition of GABA release mediated by μ₁ -opioid receptor activation results in a subsequent increase in dopamine in the NAc. Benzodiazepines activate the GABA_A R to suppress respiration in the medullary respiratory centres. The skeletal muscle relaxant, carisoprodol, appears to bind to a unique binding domain within the GABA_A R to further enhance the respiratory depressant effects of the benzodiazepines. Therefore, the opioids, the benzodiazepines and carisoprodol alone or in combination are capable of inducing respiratory depression. Current guidelines for opioid prescribing recommend against the concomitant use of benzodiazepines but do not recognize the potential risk associated with the addition of skeletal muscle relaxants.

Remifentanil-acute opioid tolerance and opioid-induced hyperalgesia: a systematic review

The use of opioids may seem to be a double-edged sword; they provide straight analgesic and antihyperalgesic effects initially, but subsequently are associated with the expression of acute opioid tolerance (AOT) and opioid-induced hyperalgesia (OIH) that have been reported in experimental studies and clinical observations. It has been suggested that opioids can induce an acute tolerance and hyperalgesia in dose- and/or time-dependent manners even when used within the clinically accepted doses. Recently, remifentanil has been used for pain management in clinical anesthesia and in the intensive care units because of its rapid onset and offset. We reviewed articles analyzing AOT and/or OIH by remifentanil and focused on the following issues: (1) evidence of remifentanil inducing AOT and/or OIH and (2) importance of AOT and/or OIH in considering the reduction of remifentanil dosage or adopting preventive modulations. Twenty-four experimental and clinical studies were identified using electronic searches of MEDLINE (PubMed, Ovid, Springer, and Elsevier). However, the development of AOT and OIH by remifentanil administration remains controversial. There is no sufficient evidence to support or refute the existence of OIH in humans.

Does Fentanyl Lead to Opioid-induced Hyperalgesia in Healthy Volunteers?

Background

Although opioids in general and remifentanil in particular have been shown to induce hyperalgesia, data regarding fentanyl are scarce. Thus, the authors investigated the effect of fentanyl dosing on pain perception and central sensitization in healthy volunteers using established pain models.

Methods

Twenty-one healthy, male volunteers were included in this randomized, double-blind, crossover study and received either intravenous low-dose (1 μg/kg) or high-dose (10 μg/kg) fentanyl. Pain intensities and hyperalgesia were assessed by intracutaneous electrical stimulation, and cold pressor pain was used as an additional measure of acute pain. The primary outcome was hyperalgesia from 4.5 to 6.5 h after fentanyl administration.

Results

A higher dose of fentanyl led to significantly decreased pain scores as measured by the numeric rating scale (0.83 units lower [95% CI, 0.63 to 1.02]; P < 0.001) but increased areas of hyperalgesia (+30.5% [95% CI, 16.6 to 44.4%]; P < 0.001) from 4.5 to 6.5 h after fentanyl administration. Allodynia did not differ between groups (+4.0% [95% CI, −15.4 to 23.5%]; P = 0.682).The high dose also led to both increased cold pressor pain threshold (+43.0% [95% CI, 29.7 to 56.3%]; P < 0.001) and tolerance (+32.5% [95% CI, 21.7 to 43.4%]; P < 0.001) at 4.5 to 6.5h. In the high-dose group, 19 volunteers (90%) required reminders to breathe, 8 (38%) required supplemental oxygen, and 12 (57%) experienced nausea.

Conclusions

A higher dose of fentanyl increased hyperalgesia from 4.5 to 6.5 h in healthy volunteers while simultaneously decreasing pain scores.

Direct Effect of Remifentanil and Glycine Contained in Ultiva® on Nociceptive Transmission in the Spinal Cord: In Vivo and Slice Patch Clamp Analyses

Background

Ultiva® is commonly administered intravenously for analgesia during general anaesthesia and its main constituent remifentanil is an ultra-short-acting μ-opioid receptor agonist. Ultiva® is not approved for epidural or intrathecal use in clinical practice. Previous studies have reported that Ultiva® provokes opioid-induced hyperalgesia by interacting with spinal dorsal horn neurons. Ultiva® contains glycine, an inhibitory neurotransmitter but also an N-methyl-D-aspartate receptor co-activator. The presence of glycine in the formulation of Ultiva® potentially complicates its effects. We examined how Ultiva® directly affects nociceptive transmission in the spinal cord.

Methods

We made patch-clamp recordings from substantia gelatinosa (SG) neurons in the adult rat spinal dorsal horn in vivo and in spinal cord slices. We perfused Ultiva® onto the SG neurons and analysed its effects on the membrane potentials and synaptic responses activated by noxious mechanical stimuli.

Results

Bath application of Ultiva® hyperpolarized membrane potentials under current-clamp conditions and produced an outward current under voltage-clamp conditions. A barrage of excitatory postsynaptic currents (EPSCs) evoked by the stimuli was suppressed by Ultiva®. Miniature EPSCs (mEPSCs) were depressed in frequency but not amplitude. Ultiva®-induced outward currents and suppression of mEPSCs were not inhibited by the μ-opioid receptor antagonist naloxone, but were inhibited by the glycine receptor antagonist strychnine. The Ultiva®-induced currents demonstrated a specific equilibrium potential similar to glycine.

Conclusions

We found that intrathecal administration of Ultiva® to SG neurons hyperpolarized membrane potentials and depressed presynaptic glutamate release predominantly through the activation of glycine receptors. No Ultiva®-induced excitatory effects were observed in SG neurons. Our results suggest different analgesic mechanisms of Ultiva® between intrathecal and intravenous administrations.

The effect of two different doses of remifentanil on postoperative pain and opioid consumption after cardiac surgery--a randomized controlled trial

BACKGROUND

Remifentanil, an ultra-short-acting opioid, provides intensive analgesia without prolonged respiratory depression and is widely used in cardiac surgery. Diminished dosing may also offer stable hemodynamics, even during sternotomy and sternal retraction. However, increased postoperative pain and induced opioid tolerance after remifentanil dosing during abdominal surgery was reported. We tested whether remifentanil 0.3 μg/kg/min infusion increased postoperative opioid consumption and pain compared to 0.1 μg/kg/min dosing.

METHODS

Ninety coronary artery bypass grafting or heart valve surgery patients were randomized to remifentanil 0.1 μg/kg/min or 0.3 μg/kg/min infusions during surgery. All patients received oxycodone bolus 0.15 μg/kg postoperatively, and patient-controlled analgesia (PCA) with oxycodone thereafter. Postoperative pain was estimated thrice daily by visual analogue scale, and 48-h opioid consumption was recorded from the PCA-device.

RESULTS

Total remifentanil dosing was 64 μg/kg in the higher and 22 μg/kg in the lower dosing group during the 3-h cardiac operations. Mean postoperative opioid consumption was 107 (SD 36) mg in the lower and 104 (SD 33) mg in the higher dose remifentanil groups. Postoperative pain did not differ between groups, at rest or during deep breathing, at any time (P = 0.110 and 0.941, respectively).

CONCLUSIONS

Remifentanil 0.3 μg/kg/min infusion did not increase postoperative pain or opioid consumption after cardiac surgery compared to the 0.1 μg/kg/min infusion. Remifentanil infusion 0.1-0.3 μg/kg/min during cardiac surgery was safe, with no exaggerated postoperative pain or opioid consumption.

Postoperative Pain in Children After Dentistry Under General Anesthesia

The objective of this study was to determine the prevalence, severity, and duration of postoperative pain in children undergoing general anesthesia for dentistry. This prospective cross-sectional study included 33 American Society of Anesthesiology (ASA) Class I and II children 4-6 years old requiring multiple dental procedures, including at least 1 extraction, and/or pulpectomy, and/or pulpotomy of the primary dentition. Exclusion criteria were children who were developmentally delayed, cognitively impaired, born prematurely, taking psychotropic medications, or recorded baseline pain or analgesic use. The primary outcome of pain was measured by parents using the validated Faces Pain Scale-Revised (FPS-R) and Parents' Postoperative Pain Measure (PPPM) during the first 72 hours at home. The results showed that moderate-to-severe postoperative pain, defined as FPS-R ≥ 6, was reported in 48.5% of children. The prevalence of moderate-to-severe pain was 29.0% by FPS-R and 40.0% by PPPM at 2 hours after discharge. Pain subsided over 3 days. Postoperative pain scores increased significantly from baseline (P < .001, Wilcoxon matched pairs signed rank test). Moderately good correlation between the 2 pain measures existed 2 and 12 hours from discharge (Spearman rhos correlation coefficients of 0.604 and 0.603, P < .005). In conclusion, children do experience moderate-to-severe pain postoperatively. Although parents successfully used pain scales, they infrequently administered analgesics.

Reporting of sample size calculations in analgesic clinical trials: ACTTION systematic review

Sample size calculations determine the number of participants required to have sufficiently high power to detect a given treatment effect. In this review, we examined the reporting quality of sample size calculations in 172 publications of double-blind randomized controlled trials of noninvasive pharmacologic or interventional (ie, invasive) pain treatments published in European Journal of Pain, Journal of Pain, and Pain from January 2006 through June 2013. Sixty-five percent of publications reported a sample size calculation but only 38% provided all elements required to replicate the calculated sample size. In publications reporting at least 1 element, 54% provided a justification for the treatment effect used to calculate sample size, and 24% of studies with continuous outcome variables justified the variability estimate. Publications of clinical pain condition trials reported a sample size calculation more frequently than experimental pain model trials (77% vs 33%, P < .001) but did not differ in the frequency of reporting all required elements. No significant differences in reporting of any or all elements were detected between publications of trials with industry and nonindustry sponsorship. Twenty-eight percent included a discrepancy between the reported number of planned and randomized participants. This study suggests that sample size calculation reporting in analgesic trial publications is usually incomplete. Investigators should provide detailed accounts of sample size calculations in publications of clinical trials of pain treatments, which is necessary for reporting transparency and communication of pre-trial design decisions.

PERSPECTIVE

In this systematic review of analgesic clinical trials, sample size calculations and the required elements (eg, treatment effect to be detected; power level) were incompletely reported. A lack of transparency regarding sample size calculations may raise questions about the appropriateness of the calculated sample size.

Intraoperative use of remifentanil and opioid induced hyperalgesia/acute opioid tolerance: systematic review

INTRODUCTION

The use of opioids has been increasing in operating room and intensive care unit to provide perioperative analgesia as well as stable hemodynamics. However, many authors have suggested that the use of opioids is associated with the expression of acute opioid tolerance (AOT) and opioid-induced hyperalgesia (OIH) in experimental studies and clinical observations in dose and/or time dependent exposure even when used within the clinically accepted doses. Recently, remifentanil has been used for pain management during anesthesia as well as in the intensive care units because of its rapid onset and offset.

OBJECTIVES

Search of the available literature to assess remifentanil AOT and OIH based on available published data.

METHODS

We reviewed articles analyzing remifentanil AOT and OIH, and focused our literature search on evidence based information. Experimental and clinical studies were identified using electronic searches of Medline (PubMed, Ovid, Springer, and Elsevier, ClinicalKey).

RESULTS

Our results showed that the development of remifentanil AOT and OIH is a clinically significant phenomenon requiring further research.

DISCUSSIONS AND CONCLUSIONS

AOT - defined as an increase in the required opioid dose to maintain adequate analgesia, and OIH - defined as decreased pain threshold after chronic opioid treatment, should be suspected with any unexplained pain report unassociated with the disease progression. The clinical significance of these findings was evaluated taking into account multiple methodological issues including the dose and duration of opioids administration, the different infusion mode, the co-administrated anesthetic drug's effect, method assessing pain sensitivity, and the repetitive and potentially tissue damaging nature of the stimuli used to determine the threshold during opioid infusion. Future studies need to investigate the contribution of remifentanil induced hyperalgesia to chronic pain and the role of pharmacological modulation to reverse this process.

Update on prescription extended-release opioids and appropriate patient selection

Chronic pain is largely underdiagnosed, often undertreated, and expected to increase as the American population ages. Many patients with chronic pain require long-term treatment with analgesic medications, and pain management may involve use of prescription opioids for patients whose pain is inadequately controlled through other therapies. Yet because of the potential for abuse and addiction, many clinicians hesitate to treat their patients with pain with potentially beneficial agents. Finding the right opioid for the right patient is the first - often complicated - step. Ensuring that patients continue to properly use the medication while achieving therapeutic analgesic effects is the long-term goal. Combined with careful patient selection and ongoing monitoring, new formulations using extended-release technologies incorporating tamper-resistant features may help combat the growing risk of abuse or misuse, which will hopefully reduce individual suffering and the societal burden of chronic pain. The objective of this manuscript is to provide an update on extended-release opioids and to provide clinicians with a greater understanding of which patients might benefit from these new opioid formulations and how to integrate the recommended monitoring for abuse potential into clinical practice.

Aversive and reinforcing opioid effects: a pharmacogenomic twin study

BACKGROUND

The clinical utility of opioids is limited by adverse drug effects including respiratory depression, sedation, nausea, and pruritus. In addition, abuse of prescription opioids is problematic. Gaining a better understanding of the genetic and environmental mechanisms contributing to an individual's susceptibility to adverse opioid effects is essential to identify patients at risk.

METHODS

A classic twin study paradigm provided estimates for the genetic and familial (genetic and/or shared environment) contribution to acute adverse and affective opioid responses, all secondary outcomes of a larger dataset. One hundred twenty-one twin pairs were recruited in a single occasion, randomized, double-blind, and placebo-controlled study. The μ-opioid receptor agonist alfentanil and saline placebo were administered as target-controlled infusions under carefully monitored laboratory conditions. Measured outcomes included respiratory depression, sedation, nausea, pruritus, drug liking, and drug disliking. Demographic information was collected, and aspects of mood and sleep were evaluated.

RESULTS

Significant heritability was detected for respiratory depression (30%), nausea (59%), and drug disliking (36%). Significant familial effects were detected for sedation (29%), pruritus (38%), dizziness (32%), and drug liking (26%). Significant covariates included age, sex, race, ethnicity, education, mood, and depression. Covariates affected sedation, pruritus, drug liking and disliking, and dizziness.

CONCLUSIONS

This study demonstrates that large-scale efforts to collect quantitative and well-defined opioid response data are not only feasible but also produce data that are suitable for genetic analysis. Genetic, environmental, and demographic factors work together to control adverse and reinforcing opioid responses, but contribute differently to specific responses.

Postoperative pain following hospital discharge after knee replacement surgery: a patient survey

SUMMARY Aim: To determine patients’ pain experience and potential barriers to effective pain relief after discharge. Materials & methods: Cross-sectional survey at weeks 2 and 4 to consecutive patients after discharge following total knee arthroplasty on pain severity; use of pain medication and nonpharmacological strategies; side effects and perceptions of pain medication; adequacy of information; and patient satisfaction. Results: We recruited 105 participants (response rate: 94%). During the first 2 weeks at home, 40% of the participants experienced frequent severe–extreme pain and 20% of the participants reported that this was the most painful period. There was no/inadequate information on pain medication for 30% of the participants and nonpharmacological strategies for pain relief for 60% of the participants. Many participants had misconceptions about pain medications. More no to mild pain participants walked or exercised their knees longer daily, or were satisfied with pain relief since returning home, compared with moderate–severe pain participants. Conclusion: Following discharge for total knee replacement, there was suboptimal use of pain medication and nonpharmacological strategies, probably leading to unnecessary pain, reduced mobility, limited therapeutic exercise and patient dissatisfaction.

No evidence for the development of acute analgesic tolerance during and hyperalgesia after prolonged remifentanil administration in mice

Background

Acute opioid tolerance (AOT) and opioid-induced hyperalgesia (OIH) are undesirable effects of opioids that have been reported in both animals and humans. However, the development of AOT and OIH in cases of potent, short-acting μ-opioid receptor agonist remifentanil administration remains controversial. It has been suggested that the emergence of AOT and OIH by remifentanil could be dose and infusion duration dependent, i.e., low dose and short infusions may lead to negative results. In this study, we determined whether AOT and OIH could be elicited by prolonged, continuous administration of remifentanil at maximally tolerable doses in C57BL/6 mice.

Results

The analgesic effects of continuously administered remifentanil [by short (1 h) and prolonged (4 h) intraperitoneal infusions] were studied. These experiments involved repeated measurements of thermal thresholds during remifentanil administration. Therefore, particular attention was paid to prevent cumulative tissue injury, which could mimic pronociceptive effects of remifentanil. To exclude the possibility of pseudoAOT during infusion, we used brief cooling of all ipsilateral hindpaws that exhibited analgesic response. Thermal thresholds remained steadily elevated over a 1-h period during continuous administration at infusion rates of 120, 180, and 240 mg/kg/h, which indicated no AOT development. To exclude the possibility of pseudoOIH after infusion, intact contralateral hindpaws were used for all postinfusion threshold measurements. Thermal thresholds at each infusion rate returned to the baseline values within 15 min after the termination of the administration. They did not decrease below the baseline values during 1 h following infusion, which indicated no OIH development. Similar threshold dynamics were also observed for thermal and mechanical testing modalities in animals infused at 120 mg/kg/h for 4 h as well as in animals with rapidly attained and maintained maximum analgesia for 3 h.

Conclusions

These results suggest that neither intra-infusion AOT nor postinfusion OIH develops in mice receiving continuous remifentanil when the possibility of cumulative tissue injury mimicking AOT or OIH is carefully avoided.

Pain sensitivity and opioid analgesia: a pharmacogenomic twin study

Opioids are the cornerstone medication for the management of moderate to severe pain. Unfortunately, vast inter-individual differences in dose requirements complicate their effective and safe clinical use. Mechanisms underlying such differences are incompletely understood, are likely multifactorial, and include genetic and environmental contributions. While accumulating evidence suggests that variants of several genes account for some of the observed response variance, the relative contribution of these factors remains unknown. This study used a twin paradigm to provide a global estimate of the genetic and environmental contributions to inter-individual differences in pain sensitivity and analgesic opioid effects. Eighty one monozygotic and 31 dizygotic twin pairs successfully underwent a computer-controlled infusion with the μ-opioid agonist alfentanil in a single occasion, randomized, double-blind and placebo-controlled study design. Pain sensitivity and analgesic effects were assessed with experimental heat and cold pressor pain models along with important covariates including demographic factors, depression, anxiety, and sleep quality. Significant heritability was detected for cold pressor pain tolerance and opioid-mediated elevations in heat and cold pressor pain thresholds. Genetic effects accounted for 12-60% of the observed response variance. Significant familial effects accounting for 24-32% of observed variance were detected for heat and cold pressor pain thresholds and opioid-mediated elevation in cold pressor pain tolerance. Significant covariates included age, gender, race, education, and anxiety. Results provide a strong rationale for more detailed molecular genetic studies to elucidate mechanisms underlying inter-individual differences in pain sensitivity and analgesic opioid responses. Such studies will require careful consideration of the studied pain phenotype.

Opioid Pharmacogenomics Using a Twin Study Paradigm: Methods and Procedures for Determining Familial Aggregation and Heritability

Opioids are the cornerstone medication for the treatment of moderate to severe pain. However, analgesic opioid requirements and the propensity to suffer from aversive opioid effects, including fatal respiratory depression and addiction, vary widely among patients. The factors underlying the substantial response variance remain largely unknown and need clarification for using opioids more effectively in appropriately selected patients. This ongoing study takes advantage of the twin paradigm to estimate the genetic and environmental contributions to inter-individual differences in opioid responses. Evidence of significant heritability will justify more detailed and extensive genomic studies. The enrollment target is 80 monozygotic and 45 dizygotic twin pairs who undergo a target-controlled infusion of the opioid alfentanil and saline placebo in sequential but randomized order. In a laboratory-type setting, well-defined pharmacodynamic endpoints are measured to quantify pain sensitivity, analgesic opioid effects, and aversive opioid effects including respiratory depression, sedation and reinforcing affective responses. First results obtained in 159 participants provide evidence for the feasibility and utility of this interventional study paradigm to estimate familial aggregation and heritability components of relevant drug effects. Areas highlighted in this report include recruitment strategies, required infrastructure and personnel, selection of relevant outcome measures, drug infusion algorithm minimizing pharmacokinetic variability, and considerations for optimizing data quality and quantity without hampering feasibility. Applying the twin paradigm to complex and potentially harmful studies comprehensively characterizing pharmacological response profiles is without much precedent. Methods and first results including heritability estimates for heat and cold pain sensitivity should be of interest to investigators considering similar studies.

Acute Pain Management in Opioid-Tolerant Patients: A Growing Challenge

In Australia and New Zealand, in parallel with other developed countries, the number of patients prescribed opioids on a long-term basis has grown rapidly over the last decade. The burden of chronic pain is more widely recognised and there has been an increase in the use of opioids for both cancer and non-cancer indications. While the prevalence of illicit opioid use has remained relatively stable, the diversion and abuse of prescription opioids has escalated, as has the number of individuals receiving methadone or buprenorphine pharmacotherapy for opioid addiction. As a result, the proportion of opioid-tolerant patients requiring acute pain management has increased, often presenting clinicians with greater challenges than those faced when treating the opioid-naïve.

Treatment aims include effective relief of acute pain, prevention of drug withdrawal, assistance with any related social, psychiatric and behavioural issues, and ensuring continuity of long-term care. Pharmacological approaches incorporate the continuation of usual medications (or equivalent), short-term use of sometimes much higher than average doses of additional opioid, and prescription of non-opioid and adjuvant drugs, aiming to improve pain relief and attenuate opioid tolerance and/or opioid-induced hyperalgesia. Discharge planning should commence at an early stage and may involve the use of a ‘Reverse Pain Ladder’ aiming to limit duration of additional opioid use. Legislative requirements may restrict which drugs can be prescribed at the time of hospital discharge. At all stages, there should be appropriate and regular consultation and liaison with the patient, other treating teams and specialist services.

The effect of treatment expectation on drug efficacy: imaging the analgesic benefit of the opioid remifentanil

Evidence from behavioral and self-reported data suggests that the patients' beliefs and expectations can shape both therapeutic and adverse effects of any given drug. We investigated how divergent expectancies alter the analgesic efficacy of a potent opioid in healthy volunteers by using brain imaging. The effect of a fixed concentration of the μ-opioid agonist remifentanil on constant heat pain was assessed under three experimental conditions using a within-subject design: with no expectation of analgesia, with expectancy of a positive analgesic effect, and with negative expectancy of analgesia (that is, expectation of hyperalgesia or exacerbation of pain). We used functional magnetic resonance imaging to record brain activity to corroborate the effects of expectations on the analgesic efficacy of the opioid and to elucidate the underlying neural mechanisms. Positive treatment expectancy substantially enhanced (doubled) the analgesic benefit of remifentanil. In contrast, negative treatment expectancy abolished remifentanil analgesia. These subjective effects were substantiated by significant changes in the neural activity in brain regions involved with the coding of pain intensity. The positive expectancy effects were associated with activity in the endogenous pain modulatory system, and the negative expectancy effects with activity in the hippocampus. On the basis of subjective and objective evidence, we contend that an individual's expectation of a drug's effect critically influences its therapeutic efficacy and that regulatory brain mechanisms differ as a function of expectancy. We propose that it may be necessary to integrate patients' beliefs and expectations into drug treatment regimes alongside traditional considerations in order to optimize treatment outcomes.

Timing of intravenous patient-controlled remifentanil bolus during early labour

BACKGROUND

Remifentanil labour analgesia is superior to nitrous oxide but less potent than epidural analgesia. The short onset and offset times of effect suggest that the timing of the bolus in the contraction cycle could have importance. We hypothesised that administering a remifentanil bolus during contraction pause would improve analgesia in early labour.

METHODS

With permission from the ethical committee and the National Authority of Medicines, 50 parturients with uncomplicated singleton pregnancies and informed consent participated in a double blind cross-over study. Intravenous doses of 0.4 μg/kg remifentanil with 1-min infusion times were used during two study periods lasting six to eight contractions. Remifentanil and saline syringes were attached to two patient-controlled devices, one of which administered the bolus immediately after a trigger and the other targeted to start 140 s before the next contraction. The parturients assessed contraction pain, pain relief, sedation and nausea. Oxygen saturation (SaO(2)) pulse and blood pressure were recorded. SaO(2)<95% was the indication for oxygen supplement.

RESULTS

Forty-one parturients were included in the analyses. Because of the period effect, pain and pain relief scores were analysed separately for each of the study periods. The mean pain and pain relief scores were similar during the two different dosing regimens. Side effects, the need for supplemental oxygen, SaO(2) and haemodynamics were similar. In a subgroup with long and regular contractions, however, delayed boluses were associated with lower pain scores.

CONCLUSIONS

Administering a remifentanil bolus during the uterine contraction pause does not improve pain relief.

Chronic inflammatory pain does not attenuate the development of tolerance to chronic morphine in adult male rats

The overall impact of chronic pain on the response to opioids is ambiguous in the literature, and comparisons between human and animal studies are complicated by vast differences between the manner and dosage of opioids given to humans treated for pain in comparison to rodents as well as a lack of healthy participant studies examining the impact of chronic opioids. The purpose of this study was to evaluate the impact of chronic pain on the development of tolerance to morphine and to assess how the concentration of drug affects this process. Twenty-four hours after the injection of CFA or normal saline in the left hind paw, the level of mechanical hypersensitivity was assessed and animals were randomly assigned to a morphine dose (1, 3 or 8 mg/kg or saline). Morphine was administered by subcutaneous injection twice a day for 5 days. On Day 6, animals were challenged with a single dose of 3 mg/kg morphine prior to formalin testing. Evidence of tolerance was mixed, and the results varied widely among the conditions. Analysis of mean paw withdrawal thresholds indicated that the analgesic efficacy of subcutaneous morphine diminished following repeated dosing. The presence of the chronic inflammatory pain condition during the morphine dosing period produced an increase in formalin pain behaviors compared to saline controls, such that animals given any dose of morphine during the 5-day dosing period showed higher responding to formalin following the 3 mg/kg dose than animals that had received saline injections. These results indicate that chronic pain does influence the development of opioid tolerance, but it does not prevent this phenomenon from occurring as suggested by some researchers.

Remifentanil, a different opioid: potential clinical applications and safety aspects

IMPORTANCE OF THE FIELD

Opioids play an important role in every aspect of modern anesthetic practice. Remifentanil is an ultra-short-acting opioid featuring a unique pharmacokinetic profile allowing clinical versatility and improved control of its action. In this review, we assess the pharmacology of remifentanil, its clinical uses as well as safety issues on its action on the major organ systems and in particular clinical settings.

AREAS COVERED IN THIS REVIEW

A synthesis of evidence from a MEDLINE search for articles from 1993 to 2009 for available up-to-date information on remifentanil and its current applications and safety profile.

WHAT THE READER WILL GAIN

A synopsis of the unique pharmacokinetic properties of remifentanil and its action on major organ systems will provide insight on the safe and effective use of the drug in a variety of clinical settings.

TAKE HOME MESSAGE

Remifentanil is a valuable opioid in the armamentarium of the clinician, providing great clinical flexibility and safety but vigilance is required to avoid pitfalls.