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

Two-Phase Inpatient Withdrawal Programme for Long-Term Opioid Use in Non-Cancer Pain

BACKGROUND

High-dose long-term opioid treatment for chronic non-cancer pain (CNCP) has become an increasing burden in industrialised countries. Opioid tapering and withdrawal in patients with CNCP remain challenging. This study evaluated a two-phase inpatient opioid withdrawal (OW) programme aimed at safely discontinuing opioid use in CNCP patients.

METHODS

This prospective observational study was conducted from 2018 to 2023 at a Swiss tertiary care centre, involving CNCP patients on long-term opioid therapy (≥ 6 months, ≥ 100 mg morphine equivalent daily dose) who had failed outpatient withdrawal attempts. The programme consisted of a withdrawal phase (Phase 1) followed by multimodal pain rehabilitation (Phase 2). Outcomes included the proportion of patients opioid-free after Phase 2 (primary) and at 3 months, pain intensity changes, and adverse events (secondary).

RESULTS

Among the 38 enrolled patients (58% female, median age 54 years [IQR 49, 62]), 34 (89%) completed both phases, and 32 (84%) were opioid-free at the end of Phase 2. At 3 months, 23 patients (61%) remained opioid-free, while 4 (11%) resumed opioids, and 11 (29%) were lost to follow-up. Median pain intensity remained stable after discharge. One patient died by suicide 10 days post-withdrawal.

CONCLUSIONS

This two-phase inpatient withdrawal and rehabilitation programme enabled most CNCP patients to discontinue opioids without increased pain intensity, with a majority remaining opioid-free at 3 months. These findings highlight the importance of ongoing psychological support and careful patient selection in OW management.

SIGNIFICANCE STATEMENT

This study introduces a structured inpatient opioid withdrawal model tailored for chronic non-cancer pain patients on high-dose opioid therapy, demonstrating that high cessation rates can be achieved without worsening pain intensity. By addressing the gap in care for patients who fail outpatient tapering, this research provides clinical insights into optimising withdrawal protocols and highlights the need for targeted resource allocation for intensive, multidisciplinary pain management. These findings support evidence-based decision-making in designing more effective opioid tapering strategies.

Preferential C-nociceptor stimulation facilitates peripheral axon reflex flare, but not secondary mechanical hyperalgesia

"Silent" C-nociceptors are crucial for inducing the axon reflex erythema in humans and may also contribute to spinal sensitization such as secondary hyperalgesia. Electrical slow depolarizing stimulation paradigms activate unmyelinated C-fibers [25 ms half-sine (HS) profile] whereas A-fibers are stimulated by 500 µs rectangular (R) pulses. We therefore expect to provoke larger areas of axon-reflex flare (silent nociceptor activation) and secondary hyperalgesia to HS stimuli. We compared axon-reflex erythema and secondary mechanical hyperalgesia areas induced by intracutaneous electrical HS and R stimuli using stimulation intensities that induced pain ratings of 3 and 6 on a numeric rating scale (NRS 0-10) in 24 healthy volunteers. Slowly depolarizing C-fiber stimulation was linked to lower current intensities required to induce pain (NRS 6: HS 3.6 vs. R 9.2 mA, p = 0.001) and resulted in larger axon reflex erythema for high stimulus intensities (AUCFlare: NRS 6, 320.7 vs. 234.1 cm2⋅min, p = 0.015; NRS 3, 79.1 vs. 51.0 cm2⋅min; p = 0.114). Preferential C-fiber stimulation indicated a correlation of axon-reflex erythema with the areas of secondary mechanical hyperalgesia (NRS 6: r = 0.21, p = 0.036; NRS 3: r = 0.48, p = 0.0016). In contrast, the mean area of secondary mechanical hyperalgesia did not differ between HS and R [AUCHyper: NRS 6, 1,555 (HS) vs. 1,585 cm2⋅min (R), p = 0.893; NRS 3, 590 (HS) vs. 449 cm2⋅min (R), p = 0.212] albeit it developed faster during HS. Our data confirm that silent nociceptors provoke the axon reflex erythema, but their role in secondary hyperalgesia appears to be less crucial. Clinical trial number: NCT0544026.

Mechanisms of chronic postsurgical pain

Chronic pain after surgery, also known as chronic postsurgical pain (CPSP), is recognized as a significant public health issue with serious medical and economic consequences. Current research on CPSP underscores the significant roles of both peripheral and central sensitization in pain development and maintenance. Peripheral sensitization occurs at the site of injury, through the hyperexcitability of nerve fibers due to surgical damage and the release of inflammatory mediators. This leads to increased expression of pronociceptive ion channels and receptors, such as transient receptor potential and acid-sensing ion channels (ASIC), enhancing pain signal transmission. Central sensitization involves long-term changes in the central nervous system, particularly in the spinal cord. In this context, sensitized spinal neurons become more responsive to pain signals, driven by continuous nociceptive input from the periphery, which results in an enhanced pain response characterized by hyperalgesia and/or allodynia. Key players in this process include N-methyl-D-aspartate receptor and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, along with proinflammatory cytokines and chemokines released by activated glia. These glial cells release substances that further increase neuronal excitability, maintaining the sensitized state and contributing to persistent pain. The activation of antinociceptive systems is required for the resolution of pain after surgery, and default in these systems may also be considered as an important component of CPSP. In this review, we will examine the clinical factors underlying CPSP in patients and the mechanisms previously established in preclinical models of CPSP that may explain how acute postoperative pain may transform into chronic pain in patients.

Opioid sparing anesthesia in patients with liver cirrhosis undergoing liver resection: a controlled randomized double-blind study

OBJECTIVE

Opioid metabolism and pharmacodynamics may be affected in hepatic patients. Ketamine and dexmedetomidine are conventional anesthetics used in our daily practice. The opioid-sparing effects of this combination have not been evaluated in patients with liver cirrhosis undergoing liver resection. We aimed to investigate the potential peri-operative opioid-sparing effects of intra-operative dexmedetomidine and ketamine infusions in patients with Child A liver cirrhosis undergoing liver resection.

METHODS

This study was a randomized controlled double-blind trial. 92 adult patients of both sex with Child class (A) liver cirrhosis aged 18 to 65 years entering and completing the study. We excluded patients with renal or cardiac dysfunction or contraindications from study medications.46 patients in the opioid-sparing group (OS) receiving ketamine and dexmedetomidine infusions and 46 patients in the opioid-based (OB) group as controls. The main outcome measures: were intra-operative fentanyl requirements, postoperative fentanyl requirements, visual analogue pain scores, postoperative nausea, vomiting, ileus, desaturation, intra-operative hemodynamic events, and ICU stay were recorded.

RESULTS

The total intra-operative fentanyl consumption was significantly lower in the OS group compared with the OB group, 183.2 ± 35.61 µg and 313.5 ± 75.06 µg, respectively, P < 0.001. The postoperative 1st 48 h fentanyl consumption was significantly lower in the OS group compared with the OB group, 354.5 ± 112.62 µg and 779.1 ± 294.97 ± µg, respectively, P < 0.001. Visual analogue scores were significantly better in the OS group at the early 2-hour assessment point postoperatively. The postoperative adverse events were significantly more frequent in the opioid-based group. ICU stay was significantly shorter in the OS group.

CONCLUSIONS

Administering dexmedetomidine and ketamine infusions intra-operatively to patients with Child A liver cirrhosis undergoing liver resection resulted in notable opioid-sparing effects, with reductions of approximately 40% intra-operatively and 55% postoperatively. The opioid-sparing group exhibited improved postoperative outcomes, including reduced pain, decreased incidence of opioid-related side effects and shorter ICU stays.

The μ-opioid receptor differentiates two distinct human nociceptive populations relevant to clinical pain

The shortfall in new analgesic agents is a major impediment to reducing reliance on opioid medications for control of severe pain. In both animals and man, attenuating nociceptive transmission from primary afferent neurons with a μ-opioid receptor agonist yields highly effective analgesia. Consequently, deeper molecular characterization of human nociceptive afferents expressing OPRM1, the μ-opioid receptor gene, is a key component for advancing analgesic drug discovery and understanding clinical pain control. A co-expression matrix for the μ-opioid receptor and a variety of nociceptive channels as well as δ- and κ-opioid receptors is established by multiplex in situ hybridization. Our results indicate an OPRM1-positive population with strong molecular resemblance to rodent peptidergic C-nociceptors associated with tissue damage pain and an OPRM1-negative population sharing molecular characteristics of murine non-peptidergic C-nociceptors. The empirical identification of two distinct human nociceptive populations that differ profoundly in their presumed responsiveness to opioids provides an actionable translational framework for human pain control.

Parenteral, Non-Intravenous Analgesia in Acute Traumatic Pain-A Narrative Review Based on a Systematic Literature Search

Traumatic pain is frequently encountered in emergency care and requires immediate analgesia. Unfortunately, most trauma patients report sustained pain upon arrival at and discharge from the Emergency Department. Obtaining intravenous access to administer analgesics can be time-consuming, leading to treatment delay. This review provides an overview of analgesics with both fast onset and parenteral, non-intravenous routes of administration, and also indicates areas where more research is required.

Pain Management in Burn Patients: Pharmacologic Management of Acute and Chronic Pain

Burn-related pain can contribute to decreased quality of life and long-term morbidity, limiting functional recovery. Burn-related pain should be assessed first by chronicity (acute or chronic), followed by type (nociceptive, neuropathic, nociplastic), to guide multimodal pharmacologic management in a stepwise algorithm approach. Combination therapies increase the efficacy and reduce toxicity by offering a multimodal approach that targets different receptors in the peripheral nervous system and central nervous system. When multimodal pharmacologic management is ineffective, etiologies of burn-related pain amenable to surgical interventions must be considered. It is important to know when to refer a patient to pain management.

Boosting open-label placebo effects in acute induced pain in healthy adults (BOLPAP-study): study protocol of a randomized controlled trial

Introduction

Pain is a highly prevalent symptom in the hospital setting, but treatment options remain limited. Harnessing the placebo effect in an ethical manner could provide a new possibility to reduce pain in clinical practice. So called open-label placebos (OLP) have been shown to elicit significant effects in reducing acute pain. But, before implementation, more knowledge concerning the properties of OLPs is needed. This study aims to assess the duration of analgesic effects from OLP and to determine the possibility of boosting such effects.

Methods and analysis

This is the protocol of an ongoing (first patient enrolled in March 2023) single-site randomized trial investigating OLPs in two parts (i.e., substudies). In both parts, pain will be induced in healthy adults using an intradermal electrical stimulation model. Participants in Part 1 will have two study visits: An interventional visit with one OLP injection accompanied by an evidence-based treatment rationale and a control visit with no treatment. For Part 2, participants will be randomized into three groups: (1) A fixed-time "Booster" group including one single repetition of the OLP injection at a fixed time point, (2) an on-demand "Booster" group including one single repetition of the OLP injection on-demand, and (3) a control group who will receive just one OLP injection. Differences in pain ratings over time (using the Numeric Rating Scale) will be analyzed with several two-sample t-tests. The time point for a fixed-time "Booster" in Part 2 will be derived from Part 1 with additional statistical tools such as a broken-stick mixed-effect model.

Discussion

This study aims to further characterize the analgesic effects of OLPs. In doing so, it will provide valuable information needed for later implementation of OLPs in clinical practice, where they could play a role in multimodal analgesic concepts.

Ethics and dissemination

The "Ethikkommission Nordwest- und Zentralschweiz" (BASEC 2023-00296) approved the study protocol. Results of the analysis will be submitted for publication in a peer-reviewed journal.

Clinical Trial Registration

This study is registered at ClinicalTrials.gov (NCT05819476) and is listed in the Swiss National Registry at kofam.ch (SNCTP000005470).

Cold nociception as a measure of hyperalgesia during spontaneous heroin withdrawal in mice

Opioids are powerful analgesic drugs that are used clinically to treat pain. However, chronic opioid use causes compensatory neuroadaptations that result in greater pain sensitivity during withdrawal, known as opioid withdrawal-induced hyperalgesia (OWIH). Cold nociception tests are commonly used in humans, but preclinical studies often use mechanical and heat stimuli to measure OWIH. Thus, further characterization of cold nociception stimuli is needed in preclinical models. We assessed three cold nociception tests-thermal gradient ring (5-30 °C, 5-50 °C, 15-40 °C, and 25-50 °C), dynamic cold plate (4 °C to -1 °C at -1 °C/min, -1 °C to 4 °C at +1 °C/min), and stable cold plate (10 °C, 6 °C, and 2 °C)-to measure hyperalgesia in a mouse protocol of heroin dependence. On the thermal gradient ring, mice in the heroin withdrawal group preferred warmer temperatures, and the results depended on the ring's temperature range. On the dynamic cold plate, heroin withdrawal increased the number of nociceptive responses, with a temperature ramp from 4 °C to -1 °C yielding the largest response. On the stable cold plate, heroin withdrawal increased the number of nociceptive responses, and a plate temperature of 2 °C yielded the most significant increase in responses. Among the three tests, the stable cold plate elicited the most robust change in behavior between heroin-dependent and nondependent mice and had the highest throughput. To pharmacologically characterize the stable cold plate test, we used μ-opioid and non-opioid receptor-targeting drugs that have been previously shown to reverse OWIH in mechanical and heat nociception assays. The full μ-opioid receptor agonist methadone and μ-opioid receptor partial agonist buprenorphine decreased OWIH, whereas the preferential μ-opioid receptor antagonist naltrexone increased OWIH. Two N-methyl-d-aspartate receptor antagonists (ketamine, MK-801), a corticotropin-releasing factor 1 receptor antagonist (R121919), a β2-adrenergic receptor antagonist (butoxamine), an α2-adrenergic receptor agonist (lofexidine), and a 5-hydroxytryptamine-3 receptor antagonist (ondansetron) had no effect on OWIH. These data demonstrate that the stable cold plate at 2 °C yields a robust, reliable, and concise measure of OWIH that is sensitive to opioid agonists.

The nuts and bolts of multimodal anaesthesia in the 21st century: a primer for clinicians

PURPOSE OF REVIEW

This review article explores the application of multimodal anaesthesia in general anaesthesia, particularly in conjunction with locoregional anaesthesia, specifically focusing on the importance of EEG monitoring. We provide an evidence-based guide for implementing multimodal anaesthesia, encompassing drug combinations, dosages, and EEG monitoring techniques, to ensure reliable intraoperative anaesthesia while minimizing adverse effects and improving patient outcomes.

RECENT FINDINGS

Opioid-free and multimodal general anaesthesia have significantly reduced opioid addiction and chronic postoperative pain. However, the evidence supporting the effectiveness of these approaches is limited. This review attempts to integrate research from broader neuroscientific fields to generate new clinical hypotheses. It discusses the correlation between high-dose intraoperative opioids and increased postoperative opioid consumption and their impact on pain indices and readmission rates. Additionally, it explores the relationship between multimodal anaesthesia and pain processing models and investigates the potential effects of nonpharmacological interventions on preoperative anxiety and postoperative pain.

SUMMARY

The integration of EEG monitoring is crucial for guiding adequate multimodal anaesthesia and preventing excessive anaesthesia dosing. Furthermore, the review investigates the impact of combining regional and opioid-sparing general anaesthesia on perioperative EEG readings and anaesthetic depth. The findings have significant implications for clinical practice in optimizing multimodal anaesthesia techniques (Supplementary Digital Content 1: Video Abstract, http://links.lww.com/COAN/A96 ).

Opioid infusions at different times of the day produce varying degrees of opioid-induced hyperalgesia

BACKGROUND

Opioids are metabolised by enzymes the activities of which vary with the circadian rhythm. We examined whether opioid infusions administered at different times of the day produce varying degrees of opioid-induced hyperalgesia (OIH) in animal experiments and clinical studies.

METHODS

Male Sprague-Dawley rats received remifentanil infusions (1 μg kg-1·min-1 for 1 h) at Zeitgeber times (ZT) 0, 4, 8, 12, 16, or 20 h. Rhythmicity of mechanical hypersensitivity was assayed after the infusion. Mechanical hypersensitivity, drug concentration, and metabolic enzyme activity of Wistar rats that received sufentanil (10 μg kg-1; four consecutive i.p. injections at 15-min intervals) or remifentanil infusion at ZT0 or ZT8 were assayed. Sixty patients who underwent abdominal laparoscopic surgery under general anaesthesia received remifentanil infusion (0.15 μg kg-1 min-1) and sufentanil injection (0.2 μg kg-1) at induction and skin incision, respectively. Postoperative pressure pain sensitivity, pain Numeric Rating Scale (NRS), drug concentrations, and nonspecific esterase activity were assessed.

RESULTS

Sprague-Dawley rats that received remifentanil infusion exhibited a robust rhythmic paw withdrawal threshold (JTK_CYCLE: P=0.001, Q=0.001, Phase=26). Wistar rats infused with remifentanil or sufentanil at ZT8 exhibited greater OIH (P<0.001) than those infused at ZT0, with higher blood concentrations (P<0.001) and lower metabolic enzyme activities (P=0.026 and P=0.028, respectively). Patients in the afternoon group exhibited higher pressure pain sensitivity at forearm (P=0.002), higher NRS (P<0.05), higher drug concentrations (sufentanil: P=0.037, remifentanil: P=0.005), and lower nonspecific esterase activity (P=0.024) than the morning group.

CONCLUSIONS

Opioid infusions administered at different times of day produced varying degrees of OIH, possibly related to circadian rhythms of metabolic enzyme activities.

CLINICAL TRIAL REGISTRATION

NCT05234697.

Trajectory of pain threshold and its association with acute pain after thoracic surgery: a prospective observational study

BACKGROUND

Postoperative analgesic management is an ongoing challenge. The pain threshold (PT) is an objective index that reflects the body's sensitivity to pain and can be used for quantitative pain assessment. We hypothesized that the PT is correlated with postoperative pain and can thus be used to guide postoperative pain management.

METHODS

This study involved 93 patients who underwent thoracoscopic surgery from December 2019 to February 2020. The PT was measured with transcutaneous electrical stimulation before surgery (T0) and at 1 h (T1), 6 h (T6), and 24 h (T24) after surgery. The visual analogue scale (VAS) score was used to evaluate the severity of postoperative pain at the same time. The PT variation (PTV) after surgery was calculated as the ratio of the postoperative PT to preoperative PT.

RESULTS

The postoperative PT was higher than the preoperative PT and showed a downward trend within 24 h after surgery; the PTV also showed a downward trend within 24 h after surgery. PT-T1 was negatively correlated with VAS-T1 at rest and during motion (rest: VAS-T1r = - 0.274, P = 0.008; motion: VAS-T1r = - 0.298, P = 0.004). PTV-T1 was negatively correlated with VAS-T1 during motion (r = - 0.213, P = 0.04). Lower VAS-T1 scores (< 4) at rest and during motion were associated with higher PT-T1 (rest: t = 2.452, P = 0.016; motion: t = 2.138, P = 0.035). The intraoperative sufentanil dose was associated with a postoperative increase in PTV-T1. Increased rescue analgesic administration was associated with PTV elevation. However, the incidence of dizziness in patients with moderate PTV-T24 was lower than that in patients with high or low PTV-T24 (χ2 = 8.297, P = 0.015).

CONCLUSIONS

The postoperative PT was higher than the preoperative PT and showed a downward trend within 24 h after surgery; PTV also showed a downward trend within 24 h after surgery. The PT and PTV were negatively correlated with the pain intensity at rest and during motion and were associated with perioperative analgesic consumption and the incidence of adverse events.

Opioid stewardship and perioperative management of pediatric tympanoplasty

OBJECTIVE

To provide insight into the intraoperative management, admission course, pain management, and graft success of microscope- and endoscope-assisted tympanoplasty.

STUDY DESIGN

Retrospective Chart Review.

METHODS

This study included children 18 years and younger who underwent ambulatory tympanoplasty at a tertiary pediatric hospital between January 2018 and December 2020. Medical records were reviewed and information about intraoperative factors, surgical approach, laterality, complications, and post-operative perforation closure success rates was collected. Multivariate analysis was performed to compare and contrast the two surgical approaches.

RESULTS

The review included 321 pediatric patients who underwent a tympanoplasty. Endoscopic tympanoplasty accounted for 17.4%, while microscopic tympanoplasty accounted for 82.6%. In both approaches, the rate of intraoperative complications, postoperative complications, audiological improvements, and perforation closure success rates were statistically similar. However, patients who underwent endoscopic tympanoplasty were 3.96 times less likely to require opioids in the post-anesthesia care unit (PACU) and had a shorter post-operative admission length. This pattern emerged regardless of the type of graft used. Obtaining an autograft was not associated with a higher opioid requirement in the PACU.

CONCLUSION

While both approaches are viable, our findings demonstrate the reduced need for opioids with similar success rates following an endoscopic tympanoplasty. Ultimately, the trade-off for the minimally invasive endoscopic approach appears to be a less painful experience for the child while promoting clinically appropriate opioid stewardship in the perioperative setting.

A single dose of ketamine relieves fentanyl-induced-hyperalgesia by reducing inflammation initiated by the TLR4/NF-κB pathway in rat spinal cord neurons

A large amount of clinical evidence has revealed that ketamine can relieve fentanyl-induced hyperalgesia. However, the underlying mechanism is still unclear. In the current study, a single dose of ketamine (5 mg/kg or 10 mg/kg), TAK-242 (3 mg/kg), or saline was intraperitoneally injected into rats 15 min before four subcutaneous injections of fentanyl. Results revealed that pre-administration of ketamine alleviated fentanyl-induced hyperalgesia according to hind paw-pressure and paw-withdrawal tests. High-dose ketamine can reverse the expression of toll-like receptor-dimer (d-TLR4), phospho- nuclear factor kappa-B (p-NF-κB, p-p65), cyclooxygenase-2 (COX-2), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α) 1 d after fentanyl injection in the spinal cord. Moreover, fentany-linduced-hyperalgesia and changes in the expression of the aforementioned proteins can be attenuated by TAK-242, an inhibitor of TLR4, as well as ketamine. Importantly, TLR4, p-p65, COX-2, and IL-1β were expressed in neurons but not in glial cells in the spinal cord 1 d after fentanyl injection. In conclusion, results suggested that a single dose of ketamine can relieve fentanyl-induced-hyperalgesia via the TLR4/NF-κB pathway in spinal cord neurons.

HCN-Channel-Dependent Hyperexcitability of the Layer V Pyramidal Neurons in IL-mPFC Contributes to Fentanyl-Induced Hyperalgesia in Male Rats

Opioids are often first-line analgesics in pain therapy. However, prolonged use of opioids causes paradoxical pain, termed "opioid-induced hyperalgesia (OIH)." The infralimbic medial prefrontal cortex (IL-mPFC) has been suggested to be critical in inflammatory and neuropathic pain processing through its dynamic output from layer V pyramidal neurons. Whether OIH condition induces excitability changes of these output neurons and what mechanisms underlie these changes remains elusive. Here, with combination of patch-clamp recording, immunohistochemistry, as well as optogenetics, we revealed that IL-mPFC layer V pyramidal neurons exhibited hyperexcitability together with higher input resistance. In line with this, optogenetic and chemogenetic activation of these neurons aggravates behavioral hyperalgesia in male OIH rats. Inhibition of these neurons alleviates hyperalgesia in male OIH rats but exerts an opposite effect in male control rats. Electrophysiological analysis of hyperpolarization-activated cation current (Ih) demonstrated that decreased Ih is a prerequisite for the hyperexcitability of IL-mPFC output neurons. This decreased Ih was accompanied by a decrease in HCN1, but not HCN2, immunolabeling, in these neurons. In contrast, the application of HCN channel blocker increased the hyperalgesia threshold of male OIH rats. Consequently, we identified an HCN-channel-dependent hyperexcitability of IL-mPFC output neurons, which governs the development and maintenance of OIH in male rats.

Preclinical study in a postoperative pain model to investigate the action of ketamine, lidocaine, and ascorbic acid in reversing fentanyl-induced, non-glutamate-dependent hyperalgesia

Introduction

Opioid-induced hyperalgesia (OIH) is a paradoxical phenomenon in which exposure to opioids can increase sensitivity to painful stimuli. Currently, several drugs have been used in an attempt to prevent OIH. We design this study to address the effect of preemptive treatment with ketamine, lidocaine, and ascorbic acid in a rat preclinical model of perioperative opioid-induced hyperalgesia.

Methods

To reproduce OIH in a model of postoperative pain, rats received successive doses of fentanyl subcutaneously and underwent an incision in the paw. In an attempt to prevent OIH, ketamine, lidocaine, and ascorbic acid were administered before treatment with fentanyl. The von Frey test and the hot-plate test were used to evaluate mechanical allodynia and thermal hyperalgesia, respectively, with a follow-up period from 1 hour up to 7 days after surgery. Spinal cord nerve terminals (synaptosomes) were used to assess glutamate release under our experimental conditions.

Results

Consecutive fentanyl injections increased the postoperative pain as indicated by increased thermal hyperalgesia and allodynia 48 hours after incision. Ketamine, lidocaine, and the combination of ketamine + lidocaine were able to prevent thermal hyperalgesia but not mechanical allodynia. Ascorbic acid did not prevent the hyperalgesia induced by fentanyl. We found no correlation between spinal glutamate release and the pharmacological treatments.

Conclusion

Fentanyl induced a hyperalgesic effect that last few days in a postoperative model of pain. Hyperalgesic effect was not totally inhibited by ketamine and lidocaine in rats. Increased glutamate release was not the main molecular mechanism of fentanyl-induced hyperalgesia.

Improved postoperative recovery profile in pediatric oral rehabilitation with low-dose dexmedetomidine as an opioid substitute for general anesthesia: a randomized double-blind clinical trial

Background

Low-dose dexmedetomidine may be a suitable alternative to opioids for pediatric ambulatory procedures under general anesthesia (GA). However, the recovery profile remains unclear. Herein, we aimed to evaluate the effects of low-dose dexmedetomidine on the recovery profile of children.

Methods

Seventy-two children undergoing ambulatory oral rehabilitation under GA were randomly and equally distributed into two groups (D and F). Group D received an infusion of dexmedetomidine 0.25 µg/kg for 4 min for induction, followed by maintenance of 0.4 µg/kg/h. Group F received an infusion of fentanyl 1 µg/kg over 4 min for induction, followed by maintenance at 1 µg/kg/h. The primary outcome was the extubation time. The secondary outcomes were awakening time, end-tidal sevoflurane (ET-Sevo) requirement, change in hemodynamic parameters, Richmond Agitation-Sedation Scale (RASS), Children's Hospital of Eastern Ontario pain scale (CHEOPS) score, length of PACU stay, and incidence of adverse events.

Results

Statistically significant differences were observed in the recovery profile between the groups: the median time for extubation was 3.65 (3.44-6.2) vs. 6.25 (4.21-7) minutes in groups D vs. F (P = 0.001), respectively, while the corresponding awakening times were 19 (18.75-21) and 22.5 (22-24) minutes, respectively (P < 0.001). The mean ET-Sevo was low in group D (1.1 vs. 1.2; P < 0.001). The heart rate was significantly low across all time points in group D, without resulting in bradycardia. The median RASS and CHEOPS scores were also significantly lower in group D. No significant differences were observed in the mean arterial pressure, incidence of adverse events, or length of PACU stay.

Conclusion

Low-dose dexmedetomidine was more effective than fentanyl as an opioid substitute at providing a better recovery profile in pediatric ambulatory oral rehabilitation under GA. Dexmedetomidine also significantly reduced sevoflurane consumption without causing adverse events or prolonging hospital stay.

Pain response to cannabidiol in opioid-induced hyperalgesia, acute nociceptive pain, and allodynia using a model mimicking acute pain in healthy adults in a randomized trial (CANAB II)

ABSTRACT

Opioids in general and remifentanil in particular can induce hyperalgesia. Preclinical data suggest that cannabidiol might have the capacity to reduce opioid-induced hyperalgesia (OIH). Thus, we investigated the effect of oral cannabidiol on OIH in healthy volunteers using an established pain model. Twenty-four healthy participants were included in this randomized, double-blinded, crossover study and received either a 1600-mg single-dose oral cannabidiol or placebo. Hyperalgesia, allodynia, and pain were induced by intracutaneous electrical stimulation. To provoke OIH, participants recieved an infusion of 0.1 µg/kg/min remifentanil over a time frame of 30 minutes, starting 100 minutes after oral cannabidiol ingestion. The primary outcome was the area of hyperalgesia (in square centimetres) up to 60 minutes after remifentanil administration. The area of allodynia (in square centimetres) and pain (numeric rating scale) were also assessed.Cannabidiol had no significant effect on hyperalgesia, allodynia, or pain at any time point of measurement compared with placebo. The area of hyperalgesia after remifentanil administration significantly increased compared with baseline (17.0 cm 2 [8.1-28.7] vs 25.3 cm 2 [15.1-39.6]; P = 0.013). Mean cannabidiol blood levels were 4.1 ± 3.0 µg/L (mean ± SD) at 130 minutes after ingestion and were 8.2 μg/L ± 6.9 µg/L (mean ± SD) at 200 minutes. Cannabidiol was well tolerated. We conclude that a high single-oral dose of 1600-mg cannabidiol is not effective in reducing OIH. Before excluding an effect of cannabidiol on OIH, research should focus on drug formulations enabling higher cannabidiol concentrations.

Effects of Intraoperative Opioid Administration on Postoperative Pain and Pain Threshold: A Randomized Controlled Study

Fentanyl and short-acting remifentanil are often used in combination. We evaluated the effect of intraoperative opioid administration on postoperative pain and pain thresholds when the two drugs were used. Patients who underwent gynecological laparoscopic surgery were randomly assigned into two groups (15 patients each) to receive either sufficient (group A) or minimum (group B) fentanyl (maximum estimated effect site concentration: A: 7.86 ng/mL, B: 1.5 ng/mL). The estimated effect site concentration at the end of surgery was adjusted to the same level (1 ng/mL). Patients in both groups also received continuous intravenous remifentanil during surgery. The primary outcome was the pressure pain threshold, as evaluated by a pressure algometer 3 h postoperatively. The pressure pain threshold at 3 h postoperatively was 51.1% (95% CI: [44.4–57.8]) in group A and 56.6% [49.5–63.6] in group B, assuming a preoperative value of 100% (p = 0.298). There were no significant differences in pressure pain threshold and numeric rating scale scores between the groups after surgery. The pain threshold decreased significantly in both groups at 3 h postoperatively compared to preoperative values, and recovered at 24 h. Co-administration of both opioids caused hyperalgesia regardless of fentanyl dose.

Chemogenetic and Optogenetic Manipulations of Microglia in Chronic Pain

Chronic pain relief remains an unmet medical need. Current research points to a substantial contribution of glia-neuron interaction in its pathogenesis. Particularly, microglia play a crucial role in the development of chronic pain. To better understand the microglial contribution to chronic pain, specific regional and temporal manipulations of microglia are necessary. Recently, two new approaches have emerged that meet these demands. Chemogenetic tools allow the expression of designer receptors exclusively activated by designer drugs (DREADDs) specifically in microglia. Similarly, optogenetic tools allow for microglial manipulation via the activation of artificially expressed, light-sensitive proteins. Chemo- and optogenetic manipulations of microglia in vivo are powerful in interrogating microglial function in chronic pain. This review summarizes these emerging tools in studying the role of microglia in chronic pain and highlights their potential applications in microglia-related neurological disorders.

Suspected opioid-induced hyperalgesia in an infant following surgery: A case report

WHAT IS KNOWN AND OBJECTIVE

Opioids are commonly administered to infants having surgery. Opioid induced hyperalgesia (OIH) is a known adverse effect of opioids in adults but can be difficult to identify in infants.

CASE SUMMARY

A 3-month-old received high-dose fentanyl during anorectoplasty for imperforate anus. He had signs and symptoms of OIH immediately after surgery. His pain and agitation were difficult to manage but improved after he received ketamine.

WHAT IS NEW AND CONCLUSION

OIH should be considered in infants postoperatively when pain worsens despite administration of escalating doses of opioids. Ketamine can be an effective therapeutic for OIH.

New Insights Into the Pharmacological Management of Postoperative Pain: A Narrative Review

Postoperative pain is prevalent and often undertreated. There is a risk that untreated or suboptimally treated postoperative pain may transition into chronic postoperative pain, which can be challenging to treat. Clinical guidelines recommend the use of multimodal analgesia, including non-steroidal anti-inflammatory drugs (NSAIDs), acetaminophen, and, in some cases, opioids. NSAIDs are a broad class of drugs with different attributes such as cyclo-oxygenase (COX)-1 or COX-2 selectivity, onset of action, and analgesic potency. NSAIDs are associated with gastrointestinal and cardiovascular side effects and should be administered at the lowest effective dose for the shortest effective duration but can be effective in postoperative pain. The role of opioids in postoperative analgesia is long-standing but has recently come under scrutiny. Opioids are often used in multimodal analgesic combinations in such a way as to minimize the total consumption of opioids without sacrificing analgesic benefit. Special clinical considerations are required for surgical patients already on opioid regimens or with opioid use disorder. A particularly useful fixed-dose combination product for postoperative analgesia is dexketoprofen-tramadol, which confers safe and effective postoperative pain control and reduces the risk of persistent postoperative pain.

Pain response to cannabidiol in induced acute nociceptive pain, allodynia, and hyperalgesia by using a model mimicking acute pain in healthy adults in a randomized trial (CANAB I)

ABSTRACT

Preclinical studies have demonstrated the analgesic potential of cannabidiol (CBD). Those suggesting an effect on pain-processing receptors have brought CBD back into focus. This study assessed the effect of CBD on acute pain, hyperalgesia, and allodynia compared with placebo. Twenty healthy volunteers were included in this randomized, placebo-controlled, double-blinded, crossover study assessing pain intensities (using numeric rating scale), secondary hyperalgesia (von Frey filament), and allodynia (dry cotton swab) in a well-established acute pain model with intradermal electrical stimulation. The authors compared the effect of 800-mg orally administered CBD on pain compared with placebo. They further examined the effect on hyperalgesia and allodynia. Cannabidiol whole blood levels were also measured. Pain ratings (mean ± SD) did not differ significantly after CBD application compared with placebo (5.2 ± 0.7 vs 5.3 ± 0.7, P-value 0.928), neither did the areas of hyperalgesia and allodynia differ significantly after CBD application compared with placebo (hyperalgesia 23.9 ± 19.2 cm2 vs 27.4 ± 17.0 cm2, P-value 0.597; allodynia 16.6 ± 13.1 cm2 vs 17.3 ± 14.1 cm2, P-value 0.884). The CBD whole blood level (median, first to third quartile) was 2.0 µg/L (1.5-5.1) 60 minutes and 5.0 µg/L (4.0-10.4) 130 minutes after CBD application. Although the oral application of 800-mg CBD failed to show a significant effect, it is important to focus future research on different dosing, routes of administration, and CBD as a part of multimodal treatment strategies before negating its effects on acute pain.

Low-dose fentanyl reduces pain perception, muscle sympathetic nerve activity responses, and blood pressure responses during the cold pressor test

Our knowledge about how low-dose (analgesic) fentanyl affects autonomic cardiovascular regulation is primarily limited to animal experiments. Notably, it is unknown if low-dose fentanyl influences human autonomic cardiovascular responses during painful stimuli in humans. Therefore, we tested the hypothesis that low-dose fentanyl reduces perceived pain and subsequent sympathetic and cardiovascular responses in humans during an experimental noxious stimulus. Twenty-three adults (10 females/13 males; 27 ± 7 yr; 26 ± 3 kg·m^-2, means ± SD) completed this randomized, crossover, placebo-controlled trial during two laboratory visits. During each visit, participants completed a cold pressor test (CPT; hand in ∼0.4°C ice bath for 2 min) before and 5 min after drug/placebo administration (75 μg fentanyl or saline). We compared pain perception (100-mm visual analog scale), muscle sympathetic nerve activity (MSNA; microneurography, 11 paired recordings), and beat-to-beat blood pressure (BP; photoplethysmography) between trials (at both pre- and postdrug/placebo timepoints) using paired, two-tailed t tests. Before drug/placebo administration, perceived pain (P = 0.8287), ΔMSNA burst frequency (P = 0.7587), and Δmean BP (P = 0.8649) during the CPT were not different between trials. After the drug/placebo administration, fentanyl attenuated perceived pain (36 vs. 66 mm, P < 0.0001), ΔMSNA burst frequency (9 vs. 17 bursts/min, P = 0.0054), and Δmean BP (7 vs. 13 mmHg, P = 0.0174) during the CPT compared with placebo. Fentanyl-induced reductions in pain perception and Δmean BP were moderately related (r = 0.40, P = 0.0641). These data provide valuable information regarding how low-dose fentanyl reduces autonomic cardiovascular responses during an experimental painful stimulus.

Association between fentanyl treatment for acute pain in the emergency department and opioid use two weeks after discharge

BACKGROUND

Analgesia with fentanyl can be associated with hyperalgesia (higher sensitivity to pain) and can contribute to escalating opioid use. Our objective was to assess the relationship between emergency department (ED) acute pain management with fentanyl compared to other opioids, and the quantity of opioids consumed two-week after discharge. We hypothesized that the quantity of opioids consumed would be higher for patients treated with fentanyl compared to those treated with other opioids.

METHODS

Patients were selected from two prospective cohorts assessing opioids consumed after ED discharge. Patients ≥18 years treated with an opioid in the ED for an acute pain condition (≤2 weeks) and discharged with an opioid prescription were included. Patients completed a 14-day paper or electronic diary of pain medication use. Quantity of 5 mg morphine equivalent tablets consumed during a 14-day follow-up by patients treated with fentanyl compared to those treated with other opioids during their ED stay were analyzed using a multiple linear regression and propensity scores.

RESULTS

We included 707 patients (mean age ± SD: 50 ± 15 years, 47% women) in this study. During follow-up, patients treated with fentanyl (N = 91) during their ED stay consumed a median (IQR) of 5.8 (14) 5 mg morphine equivalent pills compared to 7.0 (14) for those treated with other opioids (p = 0.05). Results were similar using propensity score sensitivity analysis. However, after adjusting for confounding variables, ED fentanyl treatment showed a trend, but not a statistically significant association with a decreased opioid consumption during the 14-day follow-up (B = -2.4; 95%CI = -5.3 to 0.4; p = 0.09).

CONCLUSIONS

Patients treated with fentanyl during ED stay did not consume more opioids after ED discharge, compared to those treated with other opioids. If fentanyl does cause more hyperalgesia compared to other opioids, it does not seem to have a significant impact on opioid consumption after ED discharge.

An Exploratory Pilot Study of Changes in Global DNA Methylation in Patients Undergoing Major Breast Surgery Under Opioid-Based General Anesthesia

This study aimed to investigate DNA methylation levels in patients undergoing major breast surgery under opioid-based general anesthesia. Blood samples were collected from eleven enrolled patients, before, during and after anesthesia. PBMC were isolated and global DNA methylation levels as well as DNA methyltransferase (DNMT) and cytokine gene expression were assessed. DNA methylation levels significantly declined by 26%, reversing the direction after the end of surgery. Likewise, DNMT1a mRNA expression was significantly reduced at all time points, with lowest level of −68%. DNMT3a and DNMT3b decreased by 65 and 71%, respectively. Inflammatory cytokines IL6 and TNFα mRNA levels showed a trend for increased expression at early time-points to end with a significant decrease at 48 h after surgery. This exploratory study revealed for the first time intraoperative global DNA hypomethylation in patients undergoing major breast surgery under general anesthesia with fentanyl. The alterations of global DNA methylation here observed seem to be in agreement with DNMTs gene expression changes. Furthermore, based on perioperative variations of IL6 and TNFα gene expression, we hypothesize that DNA hypomethylation may occur as a response to surgical stress rather than to opiate exposure.

The Utilization of an Opioid-Free Anesthetic for Pediatric Circumcision in an Ambulatory Surgery Center

Circumcision is one of the most common urologic procedures performed at pediatric ambulatory centers. Emerging data on the short- and long-term effects of perioperative opioid administration has highlighted the importance of an opioid-free anesthetic regimen. We sought to evaluate the effectiveness of an opioid-free anesthetic in pediatric circumcision and its correlation with ambulatory surgery center efficiency. Patients, 3 years of age and younger, who underwent circumcision or circumcision revision by two surgeons pre and post introduction of an opioid-free anesthetic fast-track regimen at an outpatient surgical center were included. There were 100 patients included in this analysis, with 50 patients in each cohort. On univariate analysis, fast-tracking was associated with a decrease in median combined in-room and post-anesthesia care unit times (102.5 vs. 129.0 min, p-value < 0.001). This difference continued after multivariable analysis with an adjusted median combined in-room and post-anesthesia care unit time difference of -15.6 min (95% CI -34.2 to -12.7 min, p-value 0.018). In addition, the fast-track cohort received less intraoperative morphine equivalents without an increase in post-operative analgesic administration or change in postoperative questionnaire score. This demonstrates that opioid-free anesthesia may be used effectively in pediatric circumcision while also allowing for significant time savings for surgical centers.

In Pursuit of an Opioid-Free Pediatric Ambulatory Surgery Center: A Quality Improvement Initiative

BACKGROUND

Opioids have been a central component of routine adult and pediatric anesthesia for decades. However, the long-term effects of perioperative opioids are concerning. Recent studies show a 4.8%-6.5% incidence of persistent opioid use after surgery in older children and adults. This means that >2 million of the 50 million patients undergoing elective surgeries in the United States each year are likely to develop persistent opioid use. With this in mind, anesthesiologists at Bellevue Clinic and Surgery Center assembled an interdisciplinary quality improvement team focused on 2 goals: (1) develop effective anesthesia protocols that minimize perioperative opioids and (2) add value to clinical services by maintaining or improving perioperative outcomes while reducing costs. This article describes our project and findings but does not attempt to make inferences or generalizations about populations outside our facility.

METHODS

We performed a large-scale implementation of opioid-sparing protocols at our standalone pediatric clinic and ambulatory surgery facility, based in part on the prior success of our previously published tonsillectomy and adenoidectomy protocol. Multiple Plan-Do-Study-Act cycles were performed using data captured from the electronic medical record. The percentage of surgical patients receiving intraoperative opioids and postoperative morphine preintervention and postintervention were compared. The following measures were evaluated using statistical process control charts: maximum postoperative pain score, postoperative morphine rescue rate, total postanesthesia care unit minutes, total anesthesia minutes, and postoperative nausea and vomiting rescue rate. Intraoperative analgesic costs were calculated.

RESULTS

Between January 2017 and June 2019, 10,948 surgeries were performed at Bellevue, with 10,733 cases included in the analyses. Between December 2017 and June 2019, intraoperative opioid administration at our institution decreased from 84% to 8%, and postoperative morphine administration declined from 11% to 6% using analgesics such as dexmedetomidine, nonsteroidal anti-inflammatory drugs, and regional anesthesia. Postoperative nausea and vomiting rescue rate decreased, while maximum postoperative pain scores, total anesthesia minutes, and total postanesthesia care unit minutes remained stable per control chart analyses. Costs improved.

CONCLUSIONS

By utilizing dexmedetomidine, nonsteroidal anti-inflammatory drugs, and regional anesthesia for pediatric ambulatory surgeries at our facility, perioperative opioids were minimized without compromising patient outcomes or value.

Pain Response to Open Label Placebo in Induced Acute Pain in Healthy Adult Males

Background

Open label placebos with patient education are effective in reducing chronic pain, and recent studies on their effect on pain have established interest in this field. Nevertheless, data on their effect on acute pain are scarce, and on hyperalgesia and allodynia, absent. This study assessed the effect of open label placebos on acute pain in healthy adult males and the influence of placebo education.

Methods

Thirty-two healthy males were included in this prospective, randomized, assessor-blinded crossover, single-center study assessing pain intensities (via numeric rating scale), area of hyperalgesia (von Frey filament), and allodynia (dry cotton swab) in a pain model utilizing intracutaneous electrical stimulation. The authors compared the effect of intravenous open label placebo on pain compared to no treatment. The authors further examined the effect of placebo on hyperalgesia and allodynia, and the influence of education (short vs. detailed) before placebo application. Saliva cortisol concentrations were also measured.

Results

Pain ratings (median, first to third quartile) were 21% lower during placebo treatment compared to no treatment, 4.0 (3.2 to 4.9) versus 5.1 (4.7 to 5.4), respectively (P = 0.001). The areas of hyperalgesia and allodynia were lower during placebo treatment compared to no treatment (hyperalgesia, 30 cm2 [17 to 47] vs. 55 cm2 [42 to 68], P = 0.003; allodynia, 24 cm2 [11 to 39] vs. 45 cm2 [31 to 62], P = 0.007). This corresponds to reductions of 47%. The extent of placebo education had no effect on pain. Saliva cortisol decreased significantly over time and was under the limit of detectability in the majority of participants in postbaseline measurements in both treatment branches. Baseline cortisol was not associated with the placebo effect or strength applied of current to reach defined pain ratings.

Conclusions

Open label placebos might play a role in multimodal analgesic concepts.

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Low- versus high-dose intraoperative opioids: A systematic review with meta-analyses and trial sequential analyses

BACKGROUND

Opioid-induced hyperalgesia is a state of nociceptive sensitisation secondary to opioid administration. The objective of this meta-analysis was to test the hypothesis that high-dose intraoperative opioids contribute to increased post-operative pain and hyperalgesia when compared with a low-dose regimen in patients under general anaesthesia.

METHODS

We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement guidelines and rated the certainty of evidence with the Grading of Recommendations, Assessment, Development and Evaluation system. Only trials investigating pain outcomes and comparing two different dosages of the same intraoperative opioid in patients under general anaesthesia were included. The primary outcome was pain score (analogue scale, 0-10) at 24 post-operative hours. Secondary outcomes included pain score and cumulative intravenous morphine equivalents (mg) consumed at 2 post-operative hours, together with mechanical pain threshold (g·mm^-2 ).

RESULTS

Twenty-seven randomised controlled trials, including 1630 patients, were identified. Pain score at rest at 24 post-operative hours was increased in the high-dose group (mean difference [95% CI]: -0.2 [-0.4, -0.1]; trial sequential analysis-adjusted CI: -0.4, -0.02; low certainty of evidence). Similarly, at 2 post-operative hours, both pain score (mean difference [95% CI]: -0.4 [-0.6, -0.2]; low certainty of evidence) and cumulative intravenous morphine equivalents consumed (mean difference [95% CI]: -1.6 mg [-2.6, -0.7]; low certainty of evidence) were significantly higher in the high-dose group. Finally, the threshold for mechanical pain was significantly lower in the high-dose group (mean difference to pressure [95% CI]: 3.8 g·mm^-2 [1.8, 5.8]; low certainty of evidence).

CONCLUSIONS

There is low certainty of evidence that high-dose intraoperative opioid administration increases pain scores in the post-operative period, when compared with a low-dose regimen.

Opioid-Induced Hyperalgesia in the Nonsurgical Setting: A Systematic Review

BACKGROUND

Opioid-induced hyperalgesia (OIH) is a phenomenon that causes an increased pain sensitization and perception of pain to noxious stimuli secondary to opioid exposure. While this clinical effect has been described in the surgical setting, it is unclear if OIH occurs in the nonsurgical setting.

STUDY QUESTION

To review the available literature which evaluated OIH in nonsurgical settings.

DATA SOURCES

A comprehensive literature search was performed using PubMed (January 1946-July 2017) using a variety of keywords for OIH. This review included randomized controlled trials with objectives to identify OIH in the nonsurgical setting. The clinical outcomes of interest were identification of OIH, adverse events, and impact of OIH on opioid consumption.

RESULTS

The search identified 8 studies that fulfilled the criteria. Six studies enrolled healthy male volunteers, 1 study used chronic low-back patients, and another used heroin-dependent treatment-seeking adults. Studies used various opioids and dosages, including remifentanil, alfentanil, fentanyl, morphine, methadone, and buprenorphine. Three primary experimental pain induction models were used to evaluate for OIH. Measured outcomes included hyperalgesia area, pain threshold, and pain tolerance. All 5 studies that used the electrical stimulation model identified OIH as a significant outcome. However, only 2 of 5 studies using the cold pressor model and 1 of 3 studies using the heat pressor model identified OIH. None of the trials explored clinical outcomes, such as effects on opioid consumption.

CONCLUSIONS

Most included studies identified OIH as a significant outcome within the nonsurgical setting. However, due to conflicting conclusions and various limitations, the clinical impact of OIH could not be assessed. Clinicians should monitor for effects of OIH in the nonoperative setting because there is insufficient evidence from the available literature to conclude that OIH is consistently observed in this setting.

Patterns and predictors of outpatient opioid use after cytoreductive surgery with hyperthermic intraperitoneal chemotherapy

Background: Long-term opioid use is a well-known complication after surgery. In this retrospective study of adults who had undergone cytoreductive surgery with hyperthermic intraperitoneal chemotherapy (CRS-HIPEC), we sought to determine the rates and factors associated with outpatient opioid use within the sixth and twelfth postoperative months. Methods: Records of 288 opioid-naïve patients were included. Logistic regression models were used to determine factors prognostic of outpatient opioid use. Results: The median patient age was 54 years, and 63% were female. Rates of outpatient opioid use within the sixth and twelfth postoperative months were 21 and 13%, respectively. In the multivariate analysis, every doubling in the amount of in-hospital postoperative opioid consumption was associated with a 44% increase in odds of opioid use within the sixth postoperative month (OR 1.44, 95% CI 1.11-1.87, p = .006) and a 70% increase within the twelfth postoperative month (OR 1.70, 95% CI 1.70-2.37, p = .001). Other factors associated with opioid use within the sixth postoperative month included physical status (OR 5.26, 95% CI 1.08-25.55, p = .039) and recent additional surgery (OR 23.02, 95% CI 2.03-261.30, p = .011). Age (OR 4.39, 95% CI 1.77-10.89, p = .001) and tumor grade (OR 3.31, 95% CI 1.31-8.41, p = .012) were associated with opioid use within the twelfth postoperative month. Conclusion: In this study, the amount of in-hospital postoperative opioid consumption was an important contributory factor to outpatient opioid use in the sixth and twelfth postoperative months. Synopsis In this study of adults who had undergone CRS-HIPEC, higher postoperative opioid consumption during hospitalization was associated with higher odds of opioid use within the sixth and twelfth postoperative months.

A Retrospective Analysis of the Safety and Efficacy of Opioid-free Anesthesia versus Opioid Anesthesia for General Cesarean Section

Introduction While uncommon for cesarean delivery, general anesthesia may be patient requested or necessary due to maternal contraindication. Traditionally, opioids are used as a part of the general anesthetic. Because of their associated complications, it is standard to limit opioid use and fetal narcotic exposure during cesarean delivery. We conducted a retrospective study to evaluate the feasibility of multi-modal opioid-free general anesthesia for cesarean delivery. Methods Electronic medical records were obtained for patients receiving general anesthesia for cesarean delivery of live pregnancies through 2017 at our tertiary care facility. Post-operative pain was estimated using a 10-cm visual analogue scale and by calculating postoperative narcotic requirements in milligram morphine equivalents (MME) over three-time periods: during post-anesthesia recovery in the post-anesthesia care unit (PACU), the first 24 hrs after PACU discharge, and 24-48 hrs after PACU discharge. Apgar scores were also obtained to quantify neonatal effects of the general anesthetic. Results Eight of 17 patients (47.06%) received opioid-free anesthesia (OFA), and nine of 17 patients (52.94%) received anesthesia with opioids (OA). No significant difference was found between groups in terms of postoperative mean Visual Analog Scale (VAS) pain score over each time period. Similarly, no significant difference was found between groups in terms of postoperative narcotics requirement at all study points. Apgar scores were not significantly different between the two groups. Conclusion The OFA group displayed equivalent analgesia to the OA group in terms of self-reported VAS pain scores and postoperative MME. A larger prospective study is recommended to fully evaluate OFA for cesarean delivery.

In Vitro Nociceptor Neuroplasticity Associated with In Vivo Opioid-Induced Hyperalgesia

Opioid-induced hyperalgesia (OIH) is a serious adverse event produced by opioid analgesics. Lack of an in vitro model has hindered study of its underlying mechanisms. Recent evidence has implicated a role of nociceptors in OIH. To investigate the cellular and molecular mechanisms of OIH in nociceptors, in vitro, subcutaneous administration of an analgesic dose of fentanyl (30 μg/kg, s.c.) was performed in vivo in male rats. Two days later, when fentanyl was administered intradermally (1 μg, i.d.), in the vicinity of peripheral nociceptor terminals, it produced mechanical hyperalgesia (OIH). Additionally, 2 d after systemic fentanyl, rats had also developed hyperalgesic priming (opioid-primed rats), long-lasting nociceptor neuroplasticity manifested as prolongation of prostaglandin E₂ (PGE₂) hyperalgesia. OIH was reversed, in vivo, by intrathecal administration of cordycepin, a protein translation inhibitor that reverses priming. When fentanyl (0.5 nm) was applied to dorsal root ganglion (DRG) neurons, cultured from opioid-primed rats, it induced a μ-opioid receptor (MOR)-dependent increase in [Ca²⁺]_i in 26% of small-diameter neurons and significantly sensitized (decreased action potential rheobase) weakly IB4⁺ and IB4^- neurons. This sensitizing effect of fentanyl was reversed in weakly IB4⁺ DRG neurons cultured from opioid-primed rats after in vivo treatment with cordycepin, to reverse of OIH. Thus, in vivo administration of fentanyl induces nociceptor neuroplasticity, which persists in culture, providing evidence for the role of nociceptor MOR-mediated calcium signaling and peripheral protein translation, in the weakly IB4-binding population of nociceptors, in OIH.SIGNIFICANCE STATEMENT Clinically used μ-opioid receptor agonists such as fentanyl can produce hyperalgesia and hyperalgesic priming. We report on an in vitro model of nociceptor neuroplasticity mediating this opioid-induced hyperalgesia (OIH) and priming induced by fentanyl. Using this model, we have found qualitative and quantitative differences between cultured nociceptors from opioid-naive and opioid-primed animals, and provide evidence for the important role of nociceptor μ-opioid receptor-mediated calcium signaling and peripheral protein translation in the weakly IB4-binding population of nociceptors in OIH. These findings provide information useful for the design of therapeutic strategies to alleviate OIH, a serious adverse event of opioid analgesics.

Fentanyl versus remifentanil-based TIVA for pediatric scoliosis repair: does it matter?

INTRODUCTION

Opioid-induced hyperalgesia (OIH) and acute opioid tolerance have been demonstrated extensively in patients undergoing adolescent idiopathic scoliosis (AIS) repair. Remifentanil infusion has been strongly linked to both tolerance and OIH in these patients; however, the impact of using an intraoperative fentanyl infusion has not been well studied. This study aims to determine if patients undergoing operative management of AIS have decreased opioid consumption and pain scores when an intraoperative fentanyl infusion is used as compared with a remifentanil infusion.

METHODS

This is a retrospective chart review of patients with AIS who underwent posterior spinal fusion. During the period January 2012-June 2013, patients received remifentanil infusion as part of total intravenous anesthesia. From July 2013 to June 2015, remifentanil was replaced by fentanyl as standard protocol. The remifentanil cohort included 37 patients and the fentanyl cohort included 25 patients. The primary outcome was the total opioid consumption (morphine equivalents) in the first 24 hours postsurgery. Secondary outcomes included mean postoperative pain score in the first 24 hours postsurgery, postoperative opioid consumption 24-48 hours after surgery, time to extubation, time to assisted ambulation, length of stay, and incidence of postoperative nausea and vomiting.

RESULTS

Compared with the remifentanil group, the fentanyl group had significantly higher postoperative opioid usage during the first 48 hours and significantly higher postoperative mean pain score during the first 24 hours. There was no difference between the two groups in mean pain score for 24-48 hours, extubation time, time to assisted ambulation, length of stay, or postoperative nausea and vomiting.

DISCUSSION

Despite concerns for hyperalgesia and acute tolerance, remifentanil is widely used for intraoperative opioid infusions for surgical correction of AIS. This retrospective study examined a practice change from intraoperative remifentanil to intraoperative fentanyl as a potential approach to avoid OIH. Surprisingly, patients receiving fentanyl intraoperatively showed increased postoperative opioid use and pain scores in the first 24 hours postsurgery compared with the prior cohort receiving remifentanil. Substitution of fentanyl for remifentanil during surgical correction of AIS does not appear to solve the problem of OIH or acute tolerance. Prospective studies are needed to confirm this unexpected result.

Role of spinal cyclooxygenase-2 and prostaglandin E2 in fentanyl-induced hyperalgesia in rats

BACKGROUND

Accumulated evidence suggests that spinal cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2) may be implicated in the development of opioid-induced hyperalgesia.

METHODS

Rats received subcutaneous fentanyl injections at different doses (20-80 μg kg^-1), four separate times at 15-min intervals. Some rats only received fentanyl (60 μg kg^-1 × 4 doses) with or without surgical incision. Fentanyl-induced hyperalgesia was evaluated via a tail-pressure or paw-withdrawal test. The concentrations of spinal COX-2, EP-1 receptor (EP-1R) mRNA, and PGE2 were measured. The effects of the COX-2 inhibitor, parecoxib (intraperitoneal 10 mg kg^-1), or the EP-1R antagonist, SC51089 (intraperitoneal 100 μg kg^-1), on hyperalgesia and spinal PGE2 were examined.

RESULTS

Acute repeated injections of fentanyl dose-dependently induced mechanical hyperalgesia, which reached a peak at the 1^st day and persisted for 1-4 days postinjection. This hyperalgesia could be partly or totally prevented by the pretreatment of either parecoxib or SC51089. Consistently, the levels of spinal COX-2 mRNA and PGE2 were also dose-dependently increased, reaching a peak at the first day and persisting for 2 days postinjection. Pretreatment with parecoxib could block the increase in spinal PGE2 and had no effects on spinal COX-2 and EP-1R mRNA. Fentanyl injection enhanced incision-induced mechanical and thermal hyperalgesia.

CONCLUSIONS

Acute repeated fentanyl administration dose-dependently produced mechanical hyperalgesia and augmented surgery induced postoperative hyperalgesia. This behavioural change was paralleled with an increase in spinal COX-2 mRNA and PGE2 after fentanyl administration. Inhibition of COX-2 or blockade of EP-1R can partly or totally prevent hyperalgesia.

Efficacy of preemptive analgesia on acute postoperative pain in children undergoing major orthopedic surgery of the lower extremities

Background

Children undergoing major orthopedic surgery of the lower extremities can experience severe postoperative pain; yet, the ideal postoperative pain management strategy is unknown. Thus, in this patient population, we investigated the effect of intraoperative epidural infusion of local anesthetic on acute postoperative pain and analgesic consumption.

Patients and methods

Patients (N=50, 3-12 years) randomly received either ropivacaine 0.15% (preemptive group) or normal saline (control group) as an initial bolus of 0.2 mL/kg, followed by continuous infusion of 0.15 mL/kg/h throughout surgery. Following surgery, patient-controlled epidural analgesia with ropivacaine 0.1% was provided. The main study outcomes were the revised Face, Legs, Activity, Cry, and Consolability pain scores, epidural ropivacaine consumption, and additional analgesic requirements during the first 48 hours postoperatively.

Results

Forty-seven patients completed the study, 23 in the preemptive group and 24 in the control group, respectively. The revised Face, Legs, Activity, Cry, and Consolability pain scores were significantly lower in the preemptive group only at 30 minutes after postanesthesia care unit arrival and 6 hours after surgery (median difference -1.0, 95% CI -2.0 to -1.0, P=0.001 and median difference -2.0, 95% CI -3.0 to -1.0, P=0.005, respectively). However, they were not significantly different between the groups at 12, 24, and 48 hours postoperatively. Epidural ropivacaine consumption and additional analgesic requirements throughout 48 hours postoperatively were not significantly different between the groups.

Conclusion

Intraoperative epidural infusion of ropivacaine did not demonstrate preemptive analgesic efficacy within 48 hours postoperatively in children undergoing extensive lower limb orthopedic surgery.

Time course of copeptin during a model of experimental pain and hyperalgesia: A randomised volunteer crossover trial

BACKGROUND

A reliable biomarker for quantifying pain or hyperalgesia has yet to be found. A surrogate marker of arginine vasopressin, copeptin, is elevated in a number of states of physiological and psychological stress and may have a role in quantifying pain and/or hyperalgesia.

OBJECTIVES

To evaluate copeptin as a biomarker for pain or hyperalgesia developing after 120 min of sustained electrical stimulation.

DESIGN

Secondary analysis of a randomised, double-blinded, crossover trial.

SETTING

Single, tertiary university hospital from September 2014 to January 2015.

PARTICIPANTS

A total of 16 healthy, opioid-naïve white men with no confounding medication or history of pain.

INTERVENTIONS

Copeptin and cortisol were measured five times during an established model of transdermal electrical stimulation designed to assess pain and hyperalgesia.

MAIN OUTCOME MEASURES

The primary outcome was the change in copeptin concentration after 120 min of sustained electrical stimulation. Secondary outcomes were copeptin and cortisol concentrations after a subsequent period of rest and analyses of copeptin and cortisol concentrations were made in high-dose and low-dose fentanyl groups separately.

RESULTS

Total copeptin concentrations were not significantly elevated after 120 min [9.15 pmol l (interquartile ranges (IQR), 3.45 to 35.45 pmol l); P = 0.150] compared with baseline [6.15 pmol l (IQR, 3.60 to 10.62 pmol l)]. In the high-dose fentanyl group, there was a significant increase in copeptin within individuals [P = 0.001; median, 37.9 pmol l (IQR, 8.1 to 62 pmol l)] after 120 min, and in the low-dose fentanyl group a significant decrease in copeptin concentrations within individuals [P = 0.006; median, 4.7 pmol l (IQR, 3.13 to 9.35 pmol l)]. No correlation between copeptin concentration and either the area under the pain curve or area under the hyperalgesia curve could be found, indicating that the observed differences may be due to other fentanyl-mediated effects.

CONCLUSION

Copeptin concentrations do not appear to be associated directly with pain and hyperalgesia. Instead, some fentanyl-mediated effect or effects appear to have greatly increased copeptin concentrations from baseline to 120 min.

TRIAL REGISTRATION

Clinicaltrials.gov identifier: NCT02252458.

Opioid-induced hyperalgesia in clinical anesthesia practice: what has remained from theoretical concepts and experimental studies?

PURPOSE OF REVIEW

This article reviews the phenomenon of opioid-induced hyperalgesia (OIH) and its implications for clinical anesthesia. The goal of this review is to give an update on perioperative prevention and treatment strategies, based on findings in preclinical and clinical research.

RECENT FINDINGS

Several systems have been suggested to be involved in the pathophysiology of OIH with a focus on the glutaminergic system. Very recently preclinical data revealed that peripheral μ-opioid receptors (MORs) are key players in the development of OIH and acute opioid tolerance (AOT). Peripheral MOR antagonists could, thus, become a new prevention/treatment option of OIH in the perioperative setting. Although the impact of OIH on postoperative pain seems to be moderate, recent evidence suggests that increased hyperalgesia following opioid treatment correlates with the risk of developing persistent pain after surgery. In clinical practice, distinction among OIH, AOT and acute opioid withdrawal remains difficult, especially because a specific quantitative sensory test to diagnose OIH has not been validated yet.

SUMMARY

Since the immediate postoperative period is not ideal to initiate long-term treatment for OIH, the best strategy is to prevent its occurrence. A multimodal approach, including choice of opioid, dose limitations and addition of nonopioid analgesics, is recommended.

Fentanyl Induces Rapid Onset Hyperalgesic Priming: Type I at Peripheral and Type II at Central Nociceptor Terminals

Systemic fentanyl induces hyperalgesic priming, long-lasting neuroplasticity in nociceptor function characterized by prolongation of inflammatory mediator hyperalgesia. To evaluate priming at both nociceptor terminals, we studied, in male Sprague Dawley rats, the effect of local administration of agents that reverse type I (protein translation) or type II [combination of Src and mitogen-activated protein kinase (MAPK)] priming. At the central terminal, priming induced by systemic, intradermal, or intrathecal fentanyl was reversed by the combination of Src and MAPK inhibitors, but at the peripheral terminal, it was reversed by the protein translation inhibitor. Mu-opioid receptor (MOR) antisense prevented fentanyl hyperalgesia and priming. To determine whether type I and II priming occur in the same population of neurons, we used isolectin B4-saporin or [Sar⁹, Met(O₂)¹¹]-substance P-saporin to deplete nonpeptidergic or peptidergic nociceptors, respectively. Following intrathecal fentanyl, central terminal priming was prevented by both saporins, whereas that in peripheral terminal was not attenuated even by their combination. However, after intradermal fentanyl, priming in the peripheral terminal requires both peptidergic and nonpeptidergic nociceptors, whereas that in the central terminal is dependent only on peptidergic nociceptors. Pretreatment with dantrolene at either terminal prevented fentanyl-induced priming in both terminals, suggesting communication between central and peripheral terminals mediated by intracellular Ca²⁺ signaling. In vitro application of fentanyl increased cytoplasmic Ca²⁺ concentration in dorsal root ganglion neurons, which was prevented by pretreatment with dantrolene and naloxone. Therefore, acting at MOR in the nociceptor, fentanyl induces hyperalgesia and priming rapidly at both the central (type II) and peripheral (type I) terminal and this is mediated by Ca²⁺ signaling.SIGNIFICANCE STATEMENT Fentanyl, acting at the μ-opioid receptor (MOR), induces hyperalgesia and hyperalgesic priming at both the central and peripheral terminal of nociceptors and this is mediated by endoplasmic reticulum Ca²⁺ signaling. Priming in the central terminal is type II, whereas that in the peripheral terminal is type I. Our findings may provide useful information for the design of drugs with improved therapeutic profiles, selectively disrupting individual MOR signaling pathways, to maintain an adequate long-lasting control of pain.

Increased Hyperalgesia and Proinflammatory Cytokines in the Spinal Cord and Dorsal Root Ganglion After Surgery and/or Fentanyl Administration in Rats

BACKGROUND

Perioperative fentanyl has been reported to induce hyperalgesia and increase postoperative pain. In this study, we tried to investigate behavioral hyperalgesia, the expression of proinflammatory cytokines, such as interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α), and the activation of microglia in the spinal cord and dorsal root ganglion (DRG) in a rat model of surgical plantar incision with or without perioperative fentanyl.

METHODS

Four groups of rats (n = 32 for each group) were subcutaneously injected with fentanyl at 60 μg/kg or normal saline for 4 times with 15-minute intervals. Plantar incisions were made to rats in 2 groups after the second drug injection. Mechanical and thermal nociceptive thresholds were assessed by the tail pressure test and paw withdrawal test on the day before, at 1, 2, 3, 4 hours, and on the days 1-7 after drug injection. The lumbar spinal cord, bilateral DRG, and cerebrospinal fluid of 4 rats in each group were collected to measure IL-1β, IL-6, and TNF-α on the day before, at the fourth hour, and on the days 1, 3, 5, and 7 after drug injection. The lumbar spinal cord and bilateral DRG were removed to detect the ionized calcium-binding adapter molecule 1 on the day before and on the days 1 and 7 after drug injection.

RESULTS

Rats injected with normal saline only demonstrated no significant mechanical or thermal hyperalgesia or any increases of IL-1β, IL-6, and TNF-α in the spinal cord or DRG. However, injection of fentanyl induced analgesia within as early as 4 hours and a significant delayed tail mechanical and bilateral plantar thermal hyperalgesia after injections lasting for 2 days, while surgical plantar incision induced a significant mechanical and thermal hyperalgesia lasting for 1-4 days. The combination of fentanyl and incision further aggravated the hyperalgesia and prolonged the duration of hyperalgesia. The fentanyl or surgical incision upregulated the expression of IL-1β, IL-6, and TNF-α in the spinal cord and bilateral DRG for more than 7 days and increase of ionized calcium-binding adapter molecule 1 in the spinal cord. The combination of fentanyl and incision resulted in higher increase of IL-1β, IL-6, and TNF-α in the spinal cord and bilateral DRG.

CONCLUSIONS

The surgical plantar incision with or without perioperative fentanyl induced significant mechanical and thermal hyperalgesia, an increased expression of IL-1β, IL-6, TNF-α in the spinal cord and DRG, and activation of microglia in the spinal cord.