Acute opiate tolerance in man

Relevance of Mu-Opioid Receptor Splice Variants and Plasticity of Their Signaling Sequelae to Opioid Analgesic Tolerance

Opioid dose escalation to effectively control pain is often linked to the current prescription opioid abuse epidemic. This creates social as well as medical imperatives to better understand the mechanistic underpinnings of opioid tolerance to develop interventions that minimize it, thereby maximizing the analgesic effectiveness of opioids. Profound opioid analgesic tolerance can be observed in the absence of mu-opioid receptor (MOR) downregulation, aggregate MOR G protein uncoupling, and MOR desensitization, in the absence of impaired G protein coupled receptor kinase phosphorylation, arrestin binding, or endocytosis. Thus, we have explored alternative biochemical sequelae that might better account for opioid analgesic tolerance. Our findings indicate that substantial plasticity among upstream and downstream components of opioid receptor signaling and the emergence of alternative signaling pathways are major contributors to opioid analgesic tolerance. An exemplar of this plasticity is our findings that chronic morphine upregulates the MOR variants MOR-1B2 and MOR-1C1 and phosphorylation of their C-terminal sites not present in MOR-1, events causally associated with the chronic morphine-induced shift in MOR G protein coupling from predominantly Gi/Go inhibitory to Gs-stimulatory adenylyl cyclase signaling. The unique feature(s) of these variants that underlies their susceptibility to adapting to chronic morphine by altering the nature of their G protein coupling reveals the richness and pliability of MOR signaling that is enabled by generating a wide diversity of MOR variants. Furthermore, given differential anatomical expression patterns of MOR variants, MOR splice variant-dependent adaptations to chronic morphine could enable mechanistic underpinnings of tolerance and dependence that are CNS region- and cell-specific.

Associating high intensity and modulated frequency of TENS delays analgesic tolerance in rats

BACKGROUND

Transcutaneous electrical nerve stimulation (TENS) is a non-invasive analgesic resource extensively used in painful conditions. However, preclinical studies suggest that the prolonged use of TENS results in the development of tolerance to its analgesic effect. The present study investigated the analgesic effect and development of tolerance to TENS with four different stimulation protocols.

METHODS

Male Wistar rats induced with joint inflammation were divided into four groups: sensory intensity, low motor intensity, high motor intensity and sham groups. TENS was applied daily for 20 min with alternating frequency between 4 and 100 Hz until tolerance development was evidenced. Mechanical hyperalgesia was measured before and after each TENS daily application.

RESULTS

After TENS, tolerance was evidenced There was a significant reduction in the mechanical withdrawal threshold in all groups 24 h after induction of inflammation (p  <  0.01). We observed a loss of analgesic efficacy of TENS around the 12th, 19th and 19th days in the groups treated with sensory intensity, low motor intensity and high motor intensity, respectively (p < 0.02) when analysed using paired measurements and compared with the control.

CONCLUSIONS

The association between frequency variation and intensity at motor level promotes a delay in the development of analgesic tolerance to TENS, optimizing and extending its therapeutic effectiveness.

Strategies for preventing side effects of systemic opioid in postoperative pediatric patients

BACKGROUND AND OBJECTIVES

Opioid is the gold standard for treating moderate-to-severe pain in pediatric patients. However, its undesirable side effects lead to unsatisfied postoperative pain management outcome (Pediatr Anesth, 17, 2007, 756). The most commonly reported opioid-related side effects are vomiting (40%), pruritus (20-60%) (Anesthesiology, 77, 1992, 162; Drugs, 67, 2007, 2323), and constipation (15-90%) (Int J Clin Pract, 61, 2007, 1181). The potential life-threatening adverse event, respiratory depression, is less common (0.0013%) (Pediatr Anesth, 20, 2010, 119). The aim of this review was to evaluate prevention strategies that have been shown to decrease opioid side effects in pediatric patients during the postoperative period.

METHODS

Literature searches were conducted from 1984 to February 2013. Meta-analysis, systematic review, and randomized, placebo-controlled studies were obtained from PubMed and the Cochrane Library. The medical subject heading (MeSH) terms were opioid analgesics, adverse effects, pediatrics, children, side effects, and postoperative pain.

RESULTS AND CONCLUSION

Data from 62 studies were reviewed. The strategies that could effectively prevent and reduce opioid side effects in pediatric patients during the postoperative period included minimizing the amount of opioid consumption by a multimodal approach, opioid titration, using local anesthetic techniques and providing the specific prophylaxis for each side effect.

Acquired pharmaco-dynamic opioid tolerance: a concept analysis

AIM

To report an analysis of the concept of acquired pharmaco-dynamic opioid tolerance.

BACKGROUND

Acquired pharmaco-dynamic opioid tolerance is a complex and poorly understood phenomenon associated with strong opioid therapy for managing pain. Critical review of the concept provides greater clarification of the attributes, assisting healthcare professionals in addressing pain and functional management of patients, particularly those with non-malignant pain.

DESIGN

Concept analysis.

DATA SOURCES

A systematic literature search was undertaken using electronic data bases: CINAHL, British Nursing Index, EMBase, Medline, Pubmed and AMED. All literature reviewed was in English and published between 1976 and 2012. The key search terms were 'chronic non-malignant pain', 'strong opioid therapy' and 'development of acquired pharmaco-dynamic opioid tolerance'; all possible variant terms were also searched.

METHOD

The Walker and Avant approach was used to guide the concept analysis.

RESULTS

The concept analysis revealed four empirical referents: plasticity, drug administration, reduced analgesic efficacy and increased drug dosing. Tachyphylexia was identified as a borderline case, opioid induced hyperalgesia as a related case and pseudo-tolerance as a contrary case. The antecedent is administration of an opioid analgesic drug and the consequences, increasing opioid drug dose to maintain analgesic effect.

CONCLUSION

Untangling the antecedents, empirical referents and consequences of tolerance help healthcare professionals understand its complexities. Improved knowledge may ultimately influence patient outcomes through the construction of better monitoring systems. This concept analysis may also provide insights for policy change and give empirical direction for future research.

Tolerance and addiction; the patient, the parent or the clinician?

Tolerance has been recognized for some time where chronic exposure to certain drugs, particularly benzodiazepines and opioids, is associated with apparent tachyphylaxis. When these drugs are stopped or progressively reduced as in 'tapering', withdrawal symptoms may result. Tolerance and the flip side of the coin, withdrawal, are the determinants of addiction. It is increasingly apparent that tolerance can occur acutely, even within the time span of a single anesthetic for a surgical procedure. Addiction is caused by agents, foreign to the body, that provoke adaptation by homeostatic biological processes. When these agents are withdrawn, the adaptive mechanisms, devoid of substrate, take time to diminish and produce symptoms recognizable under the term of 'withdrawal'. Children may be exposed to these agents in different ways; in utero, as a result of substances that the mother ingests by enteral, parenteral or inhalational means that are transmitted to the infant via the placenta; as a result of an anesthetic for surgery; or as a result of sedation and analgesia administered to offset the stresses and trauma inherent from intensive care treatment in the neonatal intensive care unit or pediatric intensive care unit. Additionally, anesthetic and intensive care staff are exposed to powerful and addictive drugs as part of everyday practice, not simply by overt access, but also by subliminal environmental exposure.

Negligible analgesic tolerance seen with extended release oxymorphone: a post hoc analysis of open-label longitudinal data

OBJECTIVE

To examine the development of analgesic tolerance in patients on oxymorphone extended-release (OxymER).

DESIGN

Post hoc analysis of data from a previously conducted prospective 1 year multi-center open-label extension study in which patients were able to titrate as needed.

PATIENTS

Sample of 153 hip and knee osteoarthritis (OA) subjects on OxymER. Primary analyses were limited to study completers (n = 62) due to the large amount of missing data for the noncompleters (n = 91).

OUTCOME MEASURES

Main outcome measures included OxymER doses (pill counts) and pain intensity ratings using a visual analog scale at monthly visits.

RESULTS

There were significant dose increases from weeks 1 to 2 and 2 to 6 (P < 0.05). Doses stabilized around week 6, suggesting the completion of what we defined as "titration." Both doses and pain ratings were stable when this titration phase was excluded from the analysis (P = 0.751; P = 0.056, respectively). Only 28% of the patients had any dose changes following this titration. While there was a significantly greater dose at week 52 compared with week 10 (P = 0.010), the increase in dose became insignificant after excluding four subjects who required two dose increases (P = 0.103).

CONCLUSIONS

The results showed that most of the titration/dose stabilization changes occurred within the first 10 weeks. A minority (28%) of subjects required dosage increases after this (defined) titration period. Pain reports stabilized statistically after 2 weeks. The findings of this post hoc analysis suggest a lack of opioid tolerance in the majority (72%) of these OA patients who completed this study following a defined titration period on OxymER.

SUMMARY

This post hoc analysis of oxymorphone ER consumption in osteoarthritis pain vs pain report showed that most dose changes occurred during an initial "titration period" as defined. Following this titration few subjects increased dose and analgesia remained stable. These findings suggest a lack of longitudinal opioid tolerance in the majority of those OA subjects who completed the trial.

Single dose of morphine produced a prolonged effect on dopamine neuron activities

Background

Clinical observation and experimental studies have indicated that a single exposure to morphine could induce tolerance and dependence. It has become a concern in clinical antinociceptive practice. However, the underling mechanism remains unknown. This study was designed to explore the changes of dopamine (DA) neuron activities in the ventral tegmental area (VTA) by employing a spectral analysis followed single morphine treatment.

Results

Acute morphine treatment significantly increased not only the firing rate and firing population but also the power of slow oscillation of DA neurons in naïve rats. These changes lasted at least for 3 days following the morphine treatment. During this period of time, responses of the DA neurons to subsequent morphine challenge were diminished. We further demonstrated a transient desensitization of opiate receptors as monitored by GTPγS binding to G-proteins. The present study provided first direct evidence for the temporal changes in the VTA DA neuron activities and opiate receptors desensitization.

Conclusion

Prolonged VTA DA neuron activation and opiate receptors desensitization followed single morphine exposure may underlie the development of dependence and tolerance that may associate with the acute analgesic tolerance and acute addiction of morphine.

Modulation between high- and low-frequency transcutaneous electric nerve stimulation delays the development of analgesic tolerance in arthritic rats

OBJECTIVE

To investigate whether repeated administration of modulating frequency transcutaneous electric nerve stimulation (TENS) prevents development of analgesic tolerance.

DESIGN

Knee joint inflammation (3% carrageenan and kaolin) was induced in rats. Either mixed or alternating frequency was administered daily (20min) for 2 weeks to the inflamed knee under light halothane anesthesia (1%-2%).

SETTING

Laboratory.

ANIMALS

Adult male Sprague-Dawley rats (N=36).

INTERVENTION

Mixed- (4Hz and 100Hz) or alternating- (4Hz on 1 day; 100Hz on the next day) frequency TENS at sensory intensity and 100micros pulse duration.

MAIN OUTCOME MEASURES

Paw and joint withdrawal thresholds to mechanical stimuli were assessed before induction of inflammation, and before and after daily application of TENS.

RESULTS

The reduced paw and joint withdrawal thresholds that occur 24 hours after the induction of inflammation were significantly reversed by the first administration of TENS when compared with sham treatment or to the condition before TENS treatment, which was observed through day 9. By the tenth day, repeated daily administration of either mixed- or alternating-frequency TENS did not reverse the decreased paw and joint withdrawal thresholds.

CONCLUSIONS

These data suggest that repeated administration of modulating frequency TENS leads to a development of opioid tolerance. However, this tolerance effect is delayed by approximately 5 days compared with administration of low- or high-frequency TENS independently. Clinically, we can infer that a treatment schedule of repeated daily TENS administration will result in a tolerance effect. Moreover, modulating low and high frequency TENS seems to produce a better analgesic effect and tolerance is slower to develop.

Changes in morphine analgesia and side effects during daily subcutaneous administration in healthy volunteers

Tolerance to the anti-nociceptive effects of opioids develops rapidly in animals. In contrast, humans with chronic pain show little or no loss of pain relief in prospective opioid trials of 4-8 weeks duration. Employing the Brief Thermal Sensitization model to induce transient cutaneous secondary hyperalgesia, we tested the hypothesis that opioid analgesic tolerance would develop rapidly. In this outpatient randomized placebo-controlled study, subjects in the MMMMP group received two injections of subcutaneous morphine 6 mg (150 min apart) on Monday-Thursday (total 48 mg over 4 days) and matching saline placebo on Friday. Subjects in the PPPPM group received placebo on Monday-Thursday and morphine (total 12 mg) on Friday. Sixty-one healthy volunteers were enrolled; morphine side effects accounted for all nine non-completions. Compared to the first placebo day, the reduction in the area of secondary hyperalgesia on the first morphine day was significant and robust in both groups. Morphine suppression of the painfulness of skin heating and elevation of the heat pain detection threshold were also significant. During 4 days of twice-daily injections, the decline in anti-hyperalgesic effects of morphine did not reach statistical significance (p=0.06) compared to placebo. Morphine side effects did not correlate with anti-hyperalgesic effects and withdrawal symptoms did not emerge. As 4 days is the threshold for demonstrating analgesic tolerance to twice-daily morphine in animal models, a longer period of opioid exposure in healthy volunteers might be needed to detect analgesic tolerance.

Adding ketamine to morphine for patient-controlled analgesia after thoracic surgery: influence on morphine consumption, respiratory function, and nocturnal desaturation

BACKGROUND

I.V. patient-controlled analgesia (PCA) with morphine is often used for postoperative analgesia after thoracic surgery, but the required doses may increase postoperative respiratory disorders. Adjunction of ketamine could reduce both doses and related respiratory side-effects.

METHODS

The main objective of this prospective, randomized double-blinded study was to evaluate the influence of adding ketamine to PCA on morphine consumption and postoperative respiratory disorders. Consecutive patients undergoing lobectomy (n = 50) were randomly assigned to receive, during the postoperative period, either i.v. morphine 1 mg ml(-1) or morphine with ketamine 1 mg ml(-1) for each. Morphine consumption was evaluated by cumulative doses every 12 h for the three postoperative days. Postoperative respiratory disorders were assessed by spirometric evaluation and recording of nocturnal desaturation.

RESULTS

The adjunction of ketamine resulted in a significant reduction in cumulative morphine consumption as early as the 36th postoperative hour [43 (SD 18) vs 32 (14) mg, P = 0.03] with a similar visual analogue scale. In the morphine group, the percentage of time with desaturation < 90% was higher during the three nights [1.80 (0.21-6.37) vs 0.02 (0-0.13), P < 0.001; 2.15 (0.35-8.65) vs 0.50 (0.01-1.30), P = 0.02; 2.46 (0.57-5.51) vs 0.55 (0.21-1.00), P = 0.02]. The decrease in forced expiratory volume in 1 s was less marked in the ketamine group at the first postoperative day [1.04 (0.68-1.22) litre vs 1.21 (1.10-0.70) litre, P = 0.039].

CONCLUSIONS

Adding small doses of ketamine to morphine in PCA devices decreases the morphine consumption and may improve respiratory disorders after thoracic surgery.

Methadone in the treatment of chronic nonmalignant pain: a 2-year follow-up

OBJECTIVE

To examine the longitudinal use of methadone in a pain clinic.

DESIGN

Follow-up study of 40 patients initially treated with methadone and re-evaluated 2 years later, comparing those maintained on methadone with those who were switched to other opioids.

SETTING

Pain clinic at a university hospital.

RESULTS

The 14 patients (35%) who stayed on methadone for the duration of the study, had higher employment rates (P <.05) and higher functional ratings (P <.02) than those on other opioids. Side effects were the most common reason (33.4%) for discontinuation of methadone. Dose escalation occurred in 11 of 14 patients (78.6%).

CONCLUSIONS

Chronic pain patients may be safely and effectively treated with methadone. Those not responding or tolerating methadone may be benefited by treatment with other opioids.

Development of opioid tolerance with repeated transcutaneous electrical nerve stimulation administration

The analgesia produced by low and high frequency transcutaneous electrical nerve stimulation (TENS) is mediated by the release of mu- or delta-opioids, respectively in the central nervous system. Repeated administration of either mu- or delta-opioid agonists induce opioid analgesic tolerance. Thus, we tested if repeated administration of TENS (either low or high frequency) in rats leads to a development of tolerance to its antihyperalgesic effects with a corresponding cross-tolerance to mu- and delta-opioid agonists. Unilateral knee joint inflammation (3% carrageenan) was induced in adult Sprague-Dawley rats. Either low (4 Hz) or high frequency (100 Hz) TENS was administered for 6 days (20 min daily) to the inflamed knee joint under halothane anesthesia. The no TENS controls were administered anesthesia only for the same period. Withdrawal threshold to mechanical stimuli was measured before and after administration of TENS on each day and also on the sixth day. A separate group of animals was tested for tolerance to either the mu-opioid agonist, morphine (1.32, 3.95, 13.2 nmol/10 ml, intrathecal (i.t.)) or the delta-opioid agonist, SNC-80 (6, 20, 60, 120 nmol/10 ml, i.t.) 30 min after i.t. administration. The reduced mechanical withdrawal threshold following the induction of inflammation was reversed by the application of TENS. However, repeatedly administering either low or high frequency TENS for 6 days, lead to a diminution in its effectiveness in reversing the ipsilateral secondary mechanical hyperalgesia by the fourth day. The effects of morphine in the low and SNC-80 in the high frequency TENS groups were significantly less than the group that did not receive TENS. On the other hand, morphine and SNC-80 were similar to the no TENS control in the high and low frequency TENS groups, respectively. Thus, repeated administration of low and high frequency TENS leads to a development of opioid tolerance with a corresponding cross-tolerance to i.t. administered mu- and delta-opioid agonists, respectively. Clinically, it can be inferred that a treatment schedule of repeated daily TENS administration should be avoided to possibly obviate the induction of tolerance.

The magnitude of acute tolerance to morphine analgesia: concentration-dependent or time-dependent?

We evaluated the relationship of either the infusion time or the plasma morphine concentrations on the magnitude of acute tolerance to morphine analgesia. Male New Zealand White rabbits were randomly allocated to one of four groups. Group 1 received an IV bolus of morphine 40 mg followed by an infusion at 20 mg/h for 8 h. Group 2 received a 20-mg morphine bolus followed by an infusion at 10 mg/h. Group 3 received a 10-mg morphine bolus followed by an infusion at 5 mg/h. Group 4 received a saline bolus and infusion. Analgesia was determined by the paw-pressure test, and the plasma concentrations of morphine were measured by high-performance liquid chromatography. We found that the plasma concentrations of morphine were maintained at a steady-state between 2 and 8 h after the morphine administration. However, from 2 to 8 h after the morphine infusion, the longer the infusion time was, the less the analgesic effect remained. Furthermore, the magnitude of acute tolerance was significantly correlated to the duration of morphine infusion (r = 0.93; P < 0.01) but not the different steady-state plasma morphine concentrations. We conclude that the magnitude of morphine tolerance is significantly correlated to the duration of infusion but not the different steady-state plasma morphine concentrations.

IMPLICATIONS

We evaluated the relationship of either the infusion time or the plasma morphine concentrations on the magnitude of acute tolerance to morphine analgesia in rabbits. We found that the magnitude of morphine tolerance is significantly correlated to the duration of infusion but not to the different steady-state plasma morphine concentrations.

Attenuation of morphine tolerance after antisense oligonucleotide knock-down of spinal mGluR1

1. Chronic systemic treatment of rats with morphine leads to the development of opioid tolerance. This study was designed to examine the effects of intrathecal (i.t.) infusion of a metabotropic glutamate receptor 1 (mGluR1) antisense oligonucleotide, concomitant with chronic morphine treatment, on the development of tolerance to morphine's antinociceptive effects. 2. All rats received chronic (6 day) s.c. administration of morphine to induce opioid tolerance. Additionally, rats were treated with either mGluR1 antisense (AS), missense (MIS) or artificial cerebrospinal fluid (ACSF) by i.t. infusion via chronically implanted i.t. catheters connected to osmotic mini-pumps. The effects of acute i.t. or s.c. morphine on tail-flick latencies were assessed prior to and following chronic s.c. morphine treatment for all chronic i.t. infusion groups. mGluR1 protein level in the spinal cord was determined by Western blot analysis for all treatments, assessing the efficiency of knock-down with AS treatment. 3. Acute i.t. morphine dose-dependently produced antinociception in the tail-flick test in naïve rats. Systemic morphine-treated rats administered i.t. ACSF or MIS developed tolerance to i.t. morphine. Chronic i.t. infusion with mGluR1 AS significantly reduced the development of tolerance to i.t. morphine. 4. In contrast to i.t. morphine, tolerance developed to the antinociceptive effects of s.c. morphine, in all i.t. infusion groups, including the mGluR1 AS group. 5. The spinal mGluR1 protein level was dramatically decreased after mGluR1 AS infusion when compared to control animals (naïve and ACSF-treated animals). 6. These findings suggest that the spinal mGluR1 is involved in the development of tolerance to the antinociceptive effects of morphine. Selective blockade of mGluR1 may be beneficial in preventing the development of opioid analgesic tolerance.

Can inflammatory pain prevent the development of acute tolerance to alfentanil?

Constant pain could, in principle, counteract mobilization of antianalgesia systems and prevent the development of acute tolerance to the analgesic effects of opioids. We sought to determine whether a tonic nociceptive input caused by inflammation inhibits the development of acute tolerance to alfentanil. The inflammation was induced by injection of carrageenan into the rat hind paw. A threshold of motor response to increasing pressure on the paw was used to determine analgesia. Alfentanil was administered IV with an infusion algorithm designed to maintain a constant plasma level of opioid for 4 h. The degree of acute tolerance was determined on the basis of decline in the level of analgesia. The continuous decline of the analgesic effect from its peak at 30 min to the end of the 4-h infusion period was profound, despite the constant-rate infusion of alfentanil. The degrees of decline were very similar in rats with and without carrageenan-induced inflammation (from 242 +/- 31 to 154 +/- 20 g, P < 0.0001; and from 242 +/- 33 to 148 +/- 14 g, P < 0.0001, respectively). The results suggest that inflammatory nociceptive input does not prevent the development of acute tolerance to opioid-induced analgesia measured as an increased reaction threshold to painful pressure. We conclude that acute tolerance to the analgesic effect of opioids is profound and develops very rapidly, even in the presence of constant nociceptive input.

IMPLICATIONS

We examined whether inflammatory pain can prevent the rapid decline in analgesic effectiveness (acute tolerance) of alfentanil during its IV infusion. We found that acute tolerance to the analgesic effect of alfentanil, in the presence of constant pain caused by inflammation, develops as rapidly as without it.

Intraoperative high dose fentanyl induces postoperative fentanyl tolerance

PURPOSE

In a randomized, double-blind clinical trial, we compared the postoperative analgesic effect and dose consumption of fentanyl after intraoperative high dose and low dose fentanyl administration.

METHODS

Sixty ASA class I to II female patients undergoing total abdominal hysterectomy (TAH), were randomly allocated to receive either 1 microg x kg(-1) (low dose group, n = 30) or 15 microg x kg(-1) (high dose group, n = 30) fentanyl during induction of anesthesia. Anesthesia depth was maintained with inhalation of halothane in the low dose group, or combined with 100 microg x hr(-1) fentanyl i.v. in the high dose group. Postoperative pain was treated with an intravenous patient-controlled analgesia system and was assessed with a visual analog pain score at rest.

RESULTS

Patients in the high dose group had higher pain intensity at four and eight hours postoperatively, more fentanyl consumption and a greater incidence of emesis in the postoperative period of 16 hr than those in the low dose group (P < 0.05). Heart rate, blood pressure, and respiratory rate were similar between the two groups.

CONCLUSION

Our results suggest that acute fentanyl tolerance develops after administration of high dose fentanyl during surgery and, consequently, results in a higher postoperative pain intensity and greater fentanyl consumption.

Rapid development of tolerance to analgesia during remifentanil infusion in humans

Studies in experimental animals have demonstrated a rapidly developing acute tolerance to the analgesic effect of opioids administered by continuous i.v. infusion. The aim of the present study was to determine whether acute tolerance plays an important role in the analgesic effect of remifentanil provided by i.v. infusion to humans. The analgesic effect of remifentanil, infused at a constant rate of 0.1 microg x kg(-1) x min(-1) for 4 h, was evaluated by measuring pain tolerance with thermal (2 degrees C water) and mechanical (pressure) noxious stimulations in 13 paid volunteers. The constant-rate infusion of remifentanil resulted in a threefold increase in pain tolerance with both tests. After reaching its maximum in 60-90 min, the analgesic effect of remifentanil began to decline despite the constant-rate infusion, and after 3 h of infusion, it was only one fourth of the peak value. A comparative rate in the development of acute tolerance measured in terms of time to 50% recovery during infusion was 129 +/- 27 min (mean +/- SD) with the cold water test and 138 +/- 39 min with the pressure test. We conclude that the development of tolerance should be included in the calculations for target-controlled infusions.

IMPLICATIONS

Our study shows that tolerance to analgesia during remifentanil infusion is profound and develops very rapidly. The administration of opioids during anesthesia based on target-controlled infusions should include corrections for the development of tolerance.

Acute tolerance associated with a single opiate administration: involvement of N-methyl-D-aspartate-dependent pain facilitatory systems

Mechanisms underlying the development of acute tolerance to the analgesic effect of opiates were investigated. In the rat tail-flick test, administration of naloxone (1 mg/kg, s.c.) 40 min after heroin (1 mg/kg, s.c.) was shown to induce hyperalgesia, indicative of a short-onset, opiate-activated pain facilitatory systems masking the opiate analgesia. Pretreatment with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate blocked, in a dose-dependent manner, the naloxone-induced hyperalgesia and potentiated the heroin-induced analgesia. Using a schedule of two successive injections of 1 mg/kg heroin, acute tolerance was indicated by a marked reduction (-52%) in analgesia induced by the second dose. After pretreatment with dizocilpine maleate, the acute tolerance was abolished and the analgesic effects of both injections of heroin were strongly potentiated. These observations indicate that acute tolerance appears after the first exposure to opiates and stems from opiate activation of N-methyl-D-aspartate-dependent pain facilitatory systems.

Opioid and benzodiazepine tolerance and dependence: application of theory to critical care practice

Critical care clinicians frequently manage patient pain and agitation and promote ventilator stability through use of opioids and benzodiazepines. Often, doses of these drugs must be increased considerably over time as they lose their effectiveness-an indication of drug tolerance. Furthermore, patients can experience negative physiologic responses to withdrawal of these drugs-an indication of drug dependence. Withdrawal symptoms due to abrupt discontinuation of drug therapy can be profound and dangerous. It is important that clinicians understand the mechanisms of drug therapies and their potential negative sequelae. The purpose of this article is to present physiologic theories of opioid and benzodiazepine actions, as well as drug tolerance and dependence, as a basis of knowledge for clinical practice. A clinical scenario of an intensive care unit patient is presented, and a care plan is offered, to provide guidance to practitioners who care for patients experiencing the consequences of long-term opioid and benzodiazepine use.

Opioid therapy for chronic nonmalignant pain: a review of the critical issues

The controversy surrounding the long-term use of opioid drugs in patients with nonmalignant pain has intensified in recent years. This debate is driven by a new willingness to consider the potential benefits of an approach that has been traditionally rejected as invariably ineffective and unsafe. The published literature continues to be very limited, but a growing clinical experience, combined with a critical reevaluation of issues related to efficacy, safety, and addiction or abuse, suggests that there is a subpopulation of patients with chronic pain that can achieve sustained partial analgesia from opioid therapy without the occurrence of intolerable side effects or the development of aberrant drug-related behaviors. Future research must confirm this impression through controlled clinical trials and clarify those factors that may predict therapeutic success or failure. For the present, the clinician who contemplates this approach must have a clear grasp of the relevant issues and an understanding of the guidelines for treatment and monitoring that have proved useful in practice.

The efficacy of intrathecal injection of sufentanil using a microspinal catheter for labor analgesia

Intrathecal sufentanil can provide labor analgesia. We investigated the efficacy of multiple injections and the maternal and neonatal effects of intrathecal sufentanil during labor. Seventeen healthy women in active labor received multiple injections of intrathecal sufentanil of 5 micrograms each through microspinal catheters. Overall maternal satisfaction of analgesia was quantified using 10 cm visual analogue scales and side effects were evaluated. Neonatal outcome was also determined. Onset of analgesia was less than 5 min after the first injection and lasted approximately 148 min. Tolerance developed for the successive injections. The mean onset times were 12.9 and 20.1 min and the durations were 76.6 and 33.9 min for the second and third injections, respectively (P < 0.05). Failure to obtain analgesia developed in all patients after the forth injection. No motor blockade was observed in any of the patients. Mild or moderate pruritus developed in 88% of the patients. Mean systolic blood pressure decreased by a maximum of 11.3% at 30 min and up to 90 min (P < 0.05) after the first injection; three patients required ephedrine treatment. No significant hemodynamic changes were observed after subsequent injections. Five patients experienced transient decrease in sensation. Neonatal status, as evaluated by Apgar scores, Neurological Adaptive Capacity Scores (NACS), fetal heart rate (FHR), and umbilical cord acid-base status, were within normal limits. Results from our study suggest that multiple small doses of sufentanil administered intrathecally provided satisfactory analgesia for parturients with short duration of labor since acute tolerance developed with multiple injections. High incidence of mild or moderate pruritus was observed during the study. Close attention should be given to hemodynamically unstable patients when this technique is applied.

Pharmacokinetics and pharmacodynamics in the critically ill patient

1. Until recently, when drugs were used in critically ill patients they were expected to behave in the same way as in less seriously ill patients. Now the unpredictability of even the most reliable drugs has been recognized. With this there is an awareness of the adverse effects drugs may have on organs other than the ones the drug was intended to act on. In patients with multiorgan dysfunction, poly-pharmacy is usually needed. The drugs may not only interfere with the action of each other at the receptor and enzyme level, but may also change protein binding and elimination. All these effects may be unimportant in less seriously ill patients, but may affect outcome in the critically ill. A high degree of awareness and suspicion of unknown drug-induced adverse reaction is needed by clinicians and pharmacologists alike.

[Patient-controlled analgesia: effect of adding continuous infusion of morphine]

This double blind study aimed to assess the effects of a continuous intravenous (i.v.) infusion of morphine added to an intermittent bolus patient controlled analgesia on morphine demand and related side-effects. Patients scheduled for abdominal and thoracic surgery (ASA 2 or 3) were randomly allocated postoperatively to three groups (n = 10 each): group 1 were given i.v. boluses of 2 mg of morphine (lockout interval = 15 min); the other two groups were given the same boluses as well as a continuous i.v. infusion of either 1 mg.kg-1 of morphine (group 2) or 2 mg.kg-1 (group 3). Pain was assessed with a visual analog scale before starting analgesia, and after 1, 2, 3, 4, 8, 16, 24 and 36 h. Total and bolus morphine doses were recorded at the same time. Breathing rate and the level of sedation were measured every hour and blood gases every time 40 mg of morphine had been consumed. Morphine administration was stopped if breathing rate decreased to less than 10 c.min-1, the patient became too sedated, or PaCO2 rose to more than 45 mmHg. Pain scores were similar in the three groups. Total amounts of morphine were higher in groups 2 (56.8 +/- 23.8 mg) and 3 (116.2 +/- 41.8 mg) compared with group 1 (38.2 +/- 17.8 mg) (p < 0.05). Morphine administration was stopped in 5 patients in group 3 and in 1 in group 2 because PaCO2 had risen to more than 45 mmHg. Therefore, a continuous i.v. infusion is not required in patients receiving PCA, all the more so as this has deleterious respiratory effects.

[Sedation with propofol and fentanyl in patients under intensive care]

This study investigated the efficacy of a constant rate infusion of propofol and fentanyl in thirty patients requiring artificial ventilation for more than 24 h. A loading dose, which differed according to the patient's age, was administered over a 30 min period: 2.5 mg.kg-1 for patients less than 50 (G1) (n = 9), 2 mg.kg-1 for patients between 50 and 60 years old (G2) (n = 9), and 1.5 mg.kg-1 for patients over 60 (G3) (n = 12). This was followed by an infusion of 3 mg.kg-1.h-1 in G1 and G2, and 2 mg.kg-1.h-1 in G3. A 1 microgram.kg-1.h-1 infusion of fentanyl was also given. The degree of sedation was assessed with the Ramsay scale before starting, after induction, and every four hours thereafter. When this proved to be insufficient, the dose of propofol was increased by 0.5 mg.kg-1.h-1 as well as that of fentanyl by 0.5 microgram.kg-1.h-1. Heart rate, mean arterial blood pressure, blood propofol, creatinine, transaminase and lipid levels, and urine output were measured before, during, and after the infusion. The blood propofol level increased during the infusion, being correlated to the doses given (r = 0.64, p less than 0.001). Sedation lasted 91.7 +/- 57.7 h. After stopping the infusion of propofol, mean recovery times were 7.5 +/- 5.9 min (G1), 11.4 +/- 11.4 min, and 14.4 +/- 13.5 min (G3) (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

The efficacy of intravenous indomethacin in prevention of postoperative pain

Since intravenous prophylactic anti-inflammatory agents have been suggested to reduce or even replace opiates in postoperative pain therapy, we studied the demand for morphine in 45 patients recovering from abdominal surgery who had received a baseline infusion of either indomethacin, morphine or saline placebo. When extubated after inhalational anaesthesia, each patient received an i.v. bolus of either 0.5 mg.kg-1 indomethacin, 0.07 mg.kg-1 morphine or saline placebo. Thereafter a 20-h infusion of the same test analgesic was started, either 0.1 mg.kg-1.h-1 indomethacin, 0.03 mg.kg-1.h-1 morphine or saline placebo. For additional analgesia, a patient-controlled analgesia device (PCA) delivering 5-mg boluses of morphine was used. For the first 5 postoperative hours, significantly more (P less than 0.05) PCA morphine was needed in the indomethacin group (35 mg) than in the morphine group (24 mg), while the placebo group demanded mean 30 mg. For equal analgesia (measured by VAS and VRS) between 5-20 h, similar amounts (mean 23 and 19 mg) of PCA morphine were required in the indomethacin and morphine groups, in contrast to the placebo group (mean 40 mg) (P less than 0.001). Morphine infusion increased the total consumption of morphine by 25% as compared to placebo. We conclude that, following abdominal surgery, the analgesic effect of indomethacin infusion became apparent after the first 5 postoperative hours, thereafter reducing the demand for PCA morphine by about 40%. Continuous morphine infusion diminishes the postoperative demand for PCA morphine, but also increases the total morphine consumption.

Postoperative patient-controlled analgesia with alfentanil: analgesic efficacy and minimum effective concentrations

Forty ASA I-III patients recovering from major abdominal or orthopedic operations were investigated in an open clinical study to evaluate analgesic efficacy and threshold plasma concentrations of alfentanil during the early postoperative period using patient-controlled analgesia (PCA) by means of the On-Demand Analgesia Computer. Alfentanil demand dose was 212 micrograms, continuous infusion rate 25 micrograms/hr, hourly maximum dose 1.5 mg/hr; the lockout time was set to 1 min. The duration of PCA was 18.1 +/- 5.2 hr (mean, SD) during which time 23.8 +/- 14.2 demands per patient were recorded, resulting in an average alfentanil consumption of 4.99 +/- 3.03 micrograms/kg/hr. Patient acceptance of PCA was high. Side effects were only of minor intensity and never gave rise to concern. Based on our own earlier PCA experience with other opiate analgesics, alfentanil proved to be about 1/15th as potent an analgesic as fentanyl and about 6-7 times more potent than morphine if both intensity and duration of effect were considered. Minimum effective alfentanil plasma concentration (MEC) varied greatly and could be best described by a lognormal distribution (range 0.6-99.2 ng/mL, median 14.9 ng/mL). Intraindividual MEC variability was consistently lower than intersubject variability (37.0% vs 65.2%).

Hyperalgesia during naloxone-precipitated withdrawal from morphine is associated with increased on-cell activity in the rostral ventromedial medulla

Hyperresponsiveness to noxious stimulation (hyperalgesia) is observed with naloxone-precipitated morphine withdrawal in several experimental models, and may be due to changes in central nervous system neurons. Previous studies have demonstrated that certain neurons in the rostral ventromedial medulla (on-cells) discharge just prior to nocifensive withdrawal reflexes and are inhibited by morphine. Because the tail flick latency (TFL) is shorter when on-cells are active, it has been proposed that on-cells facilitate nocifensive reflexes. The present study examined the hypothesis that the hyperalgesia observed following naloxone-precipitated withdrawal from morphine is caused by increased on-cell discharge. Rats were maintained in a lightly anesthetized state with chloral hydrate. Administration of saline (1.25 cc, i.v.) or morphine sulfate (1.25 mg/kg, i.v.) was followed by naloxone (1.0 mg/kg, i.v.). On- and off-cell activity was continuously recorded and was correlated with TFL and paw withdrawal threshold (PWT). As previously reported, morphine increased off-cell activity, blocked on-cell activity, and suppressed the tail flick and paw withdrawal reflexes. When naloxone was given after morphine, TFL and PWT were reduced to values significantly below baseline (hyperalgesia). Both spontaneous and reflex-related on-cell activity increased to levels greater than the premorphine baseline. Spontaneous off-cell activity decreased abruptly to near zero when morphine was followed by naloxone. Linear regression analysis during the hyperresponsive state revealed a significant correlation between increased on-cell activity and reduced TFL, but not between decreased off-cell activity and TFL. These findings are consistent with the hypothesis that on-cells facilitate spinal nocifensive reflexes, and that the naloxone-precipitated hyperalgesia is at least in part accounted for by increased on-cell activity. A neural model of opiate dependence, tolerance, and withdrawal is proposed.

Variables of patient-controlled analgesia. 2. Concurrent infusion

The effectiveness of morphine after surgery by patient-controlled analgesia alone or with a concurrent infusion was studied. The infusion did not reduce the dose of self-administered analgesic and patients treated in this way received twice as much drug as those who used patient-controlled analgesia alone. Pain control was similar in both groups. The practice of patient-controlled analgesia plus infusion requires critical review.

Alfentanil infusions in patients requiring intensive care

Alfentanil is a short acting opioid that has an established place in anaesthesia. Its predictable pharmacokinetics and pharmacodynamics, particularly its rapid termination of effect and haemodynamic stability, have led to its use by continuous intravenous infusion both during anaesthesia and more recently in critically ill patients. Fine control of a potent analgesic that has respiratory depressant and antitussive properties would be particularly advantageous in this group, offering patients an improvement in comfort without increasing the risk of oversedation. Pharmacokinetic studies of alfentanil have demonstrated wide interindividual variations. This may be due to a wide variety of factors including age, obesity, hepatic dysfunction, changes in regional haemodynamics, sex, and alterations in plasma protein binding ability and concentration. The importance of pharmacogenetic differences and tolerance to alfentanil remains to be elucidated. Renal disease does not appear to significantly alter the pharmacokinetics of this agent, which may make it particularly useful in this situation. Since alfentanil does not depress conscious level or produce anxiolysis, additional agents such as a benzodiazepine will be necessary to provide adequate sedation. The difficulties in accurately predicting the response of an individual critically ill patient necessitate careful and continuous dose titration of alfentanil according to the clinical response.

Early extubation after high-dose fentanyl anaesthesia for aortocoronary bypass surgery: reversal of respiratory depression with low-dose nalbuphine

To investigate the possibility of selective reversal of narcotic-induced respiratory depression following fentanyl anaesthesia, we studied 20 patients after aortocoronary bypass surgery. All patients were anaesthetized with fentanyl, 40 micrograms . kg-1 and oxygen, with isoflurane as indicated. In a random double blind fashion either incremental doses of nalbuphine, or normal saline were administered approximately four hours after cardiopulmonary bypass. Respiratory depression was evaluated using blood gas and end tidal CO2 (PETCO2) measurement, and in addition, a ventilatory response to CO2 was obtained preoperatively and at selected intervals postoperatively. Despite randomization, patients with more respiratory depression were assigned to nalbuphine. There appeared to be a reversal of respiratory depression with nalbuphine, indicated by a fall in the resting PETCO2 value. This apparent reversal of respiratory depression was associated with a significant increase in pain, requiring treatment in three patients. We conclude that low-dose nalbuphine is not an acceptable method of antagonism of respiratory depression in this group of patients. Many patients who did not receive nalbuphine were able to breathe adequately at an earlier stage than was previously suspected. Close monitoring of the respiratory system may permit earlier extubation without the requirement of a narcotic antagonist after this dose of fentanyl.

A novel form of dependency of hepatic extraction ratio of opioids in vivo upon the portal vein concentration of drug: comparison of morphine, diamorphine, fentanyl, methadone and buprenorphine in the chronically cannulated cow

In the chronically cannulated cow, the hepatic extraction ratio for intravenous boluses of morphine, diamorphine, fentanyl, methadone and buprenorphine increased towards a plateau value as portal vein drug concentration increased. An extraction ratio close to zero for morphine was observed at a portal vein plasma drug concentration of about 200 nanomol per litre, which is within the range for significant pharmacodynamic effects. The similar concentrations extrapolated for the other narcotics would be of less pharmacodynamic importance. The phenomenon did not depend with morphine on the history of drug delivery to the liver; measurement of hepatic blood flow showed the effect was not an artifact of unrepresentative blood sampling, and was not related to any action of the narcotics on hepatic blood flow. The existence of this novel type of concentration dependent hepatic extraction ratio in vivo can explain a number of anomalous observations on narcotic pharmacokinetics, especially for morphine. Furthermore, similar behaviour may be expected for non-opioid drugs having similar pharmacokinetic properties.