Effects of naloxone on nitrous oxide actions in healthy volunteers

The Use of a Fixed 50:50 Mixture of Nitrous Oxide and Oxygen to Reduce Lumbar Puncture-Induced Pain in the Emergency Department: A Randomized Controlled Trial

Lumbar puncture (LP) is stressful and often painful. We evaluated the efficacy of a fixed 50% nitrous oxide−oxygen mixture (50%N2O-O2) versus placebo to reduce immediate procedural pain and anxiety during LP performed in an emergency setting. We conducted a randomized controlled trial involving adults who needed a cerebrospinal fluid analysis in an emergency department. Patients were randomly assigned to inhale either 50%N2O-O2 or medical air. The primary endpoint, assessed using a numerical scale, was the maximum pain felt by the patient during the procedure and the maximum anxiety and satisfaction as secondary outcomes. Eighty-eight patients were randomized and analyzed (ITT). The maximal pain was 5.0 ± 2.9 for patients receiving air and 4.2 ± 3.0 for patients receiving 50%N2O-O2 (effect-size = −0.27 [−0.69; 0.14], p = 0.20). LP-induced anxiety was 4.7 ± 2.8 vs. 3.7 ± 3.7 (p = 0.13), and the proportion of patients with significant anxiety (score ≥ 4/10) was 72.7% vs. 50.0% (p = 0.03). Overall satisfaction was higher among patients receiving 50%N2O-O2 (7.4 ± 2.4 vs. 8.9 ± 1.6, p < 0.001). No serious adverse events were attributable to 50%N2O-O2 inhalation. Although inhalation of 50%N2O-O2 failed to reduce LP-induced pain in an emergency setting, it tended to reduce anxiety and significantly increased patient satisfaction.

Fixed 50:50 mixture of nitrous oxide and oxygen to reduce lumbar-puncture-induced pain: a randomized controlled trial

BACKGROUND AND PURPOSE

Lumbar puncture (LP) has been frequently performed for more than a century. This procedure is still stressful and often painful. The aim of the study was to evaluate the efficacy of a fixed 50% nitrous oxide-oxygen mixture compared to placebo to reduce immediate procedural pain and anxiety during LP.

METHODS

A randomized controlled trial was conducted involving adults who needed a cerebrospinal fluid analysis. Patients were randomly assigned to inhale either a fixed 50% nitrous oxide-oxygen mixture (50% N₂ O-O₂ ) or medical air (22% O₂ -78% N₂ ). Cutaneous application of a eutectic mixture of local anaesthetics was systematically done and all LPs were performed with pencil point 25G needles (20G introducer needle). The primary end-point was the maximal pain level felt by the patient during the procedure, the maximal anxiety level being a secondary outcome, both measured using a numerical rating scale (0-10).

RESULTS

A total of 66 consecutive patients were randomized. The analysis was intention to treat. The maximal pain was 4.9 ± 2.7 for the 33 patients receiving air and 2.7 ± 2.7 for the 33 receiving 50% N₂ O-O₂ (P = 0.002). Similarly, the maximal LP-induced anxiety was 4.5 ± 3.1 vs. 2.6 ± 2.6 (P = 0.009), respectively. The number needed to treat to avoid one patient undergoing significant pain (pain score ≥ 4/10) was 2.75. Body mass index >25 kg/m² was significantly associated with higher pain intensity (P = 0.03). No serious adverse events were attributable to 50% N₂ O-O₂ inhalation.

CONCLUSIONS

Inhalation of a fixed 50% N₂ O-O₂ mixture is efficient to reduce LP-induced pain and anxiety.

N-methyl-D-aspartate receptor channel blocker-like discriminative stimulus effects of nitrous oxide gas

Nitrous oxide (N2O) gas is a widely used anesthetic adjunct in dentistry and medicine that is also commonly abused. Studies have shown that N2O alters the function of the N-methyl-d-aspartate (NMDA), GABAA, opioid, and serotonin receptors among others. However, the receptors systems underlying the abuse-related central nervous system effects of N2O are unclear. The present study explores the receptor systems responsible for producing the discriminative stimulus effects of N2O. B6SJLF1/J male mice trained to discriminate 10 minutes of exposure to 60% N2O + 40% oxygen versus 100% oxygen served as subjects. Both the high-affinity NMDA receptor channel blocker (+)-MK-801 maleate [(5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate] and the low-affinity blocker memantine partially mimicked the stimulus effects of N2O. Neither the competitive NMDA antagonist, CGS-19755 (cis-4-[phosphomethyl]-piperidine-2-carboxylic acid), nor the NMDA glycine-site antagonist, L701-324 [7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2(1H)-quinolinone], produced N2O-like stimulus effects. A range of GABAA agonists and positive modulators, including midazolam, pentobarbital, muscimol, and gaboxadol (4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-3-ol), all failed to produce N2O-like stimulus effects. The μ-, κ-, and δ-opioid agonists, as well as 5-hydroxytryptamine (serotonin) 1B/2C (5-HT1B/2C) and 5-HT1A agonists, also failed to produce N2O-like stimulus effects. Ethanol partially substituted for N2O. Both (+)-MK-801 and ethanol but not midazolam pretreatment also significantly enhanced the discriminative stimulus effects of N2O. Our results support the hypothesis that the discriminative stimulus effects of N2O are at least partially mediated by NMDA antagonist effects similar to those produced by channel blockers. However, as none of the drugs tested fully mimicked the stimulus effects of N2O, other mechanisms may also be involved.

Lack of sex differences to the subjective effects of nitrous oxide in healthy volunteers

BACKGROUND

Although numerous studies have assessed subjective effects of nitrous oxide, few studies have analyzed for sex differences. Since sex differences have been reported in subjective effects of several drugs such as opioids, nicotine and alcohol, we sought to determine if sex modulates the subjective effects of the inhalant, nitrous oxide, in healthy volunteers.

METHODS

Thirty-eight females and seventy-two males from nine studies that were conducted in our laboratory were included in this retrospective analysis. All experimental studies utilized randomized, placebo-controlled, repeated measures designs in which subjects inhaled 30% nitrous oxide in oxygen and 100% oxygen (placebo). Dependent measures in this analysis were subjective effects measured at baseline and 15 min into the inhalation period.

RESULTS

Nitrous oxide produced a number of subjective effects, including those that could be considered abuse liability-related ("elated," "having pleasant thoughts," drug liking), but sex did not modulate these effects.

CONCLUSIONS

Females and males showed similar subjective responses to 30% nitrous oxide. Future prospective studies might assess other concentrations, other measures (choice, analgesic response), and other inhaled general anesthetics to more comprehensively characterize the role of sex in response to inhalants.

Effect of heterotopic noxious conditioning stimulation on electrical and pressure pain thresholds in two different anatomical regions

OBJECTIVES

The aims of the study were to investigate the influence of heterotopic noxious conditioning stimulation (HNCS) on pain thresholds in the orofacial and spinal regions and to find out whether there are gender differences in this respect.

MATERIAL AND METHODS

Thirty healthy subjects (15 of each sex) with a mean (SD) age of 25.1 (4.4) years participated. Pain thresholds to electrical (EPT) and pressure stimuli (PPT) were recorded in the masseter muscle and 1st upper incisor (tooth), as well as in the fingertip, before, during, and 5 and 15 min after a cold pressor task to the contralateral hand immersed in ice-cold water for a maximum of 5 min.

RESULTS

With the exception of the EPT in the orofacial region, all pain thresholds increased during the HNCS and then returned to baseline during the 15 min follow-up. The significant changes in EPT were greater in the finger than in the tooth, while the changes in PPT were greater in the masseter muscle than in the finger. Electrical stimuli in the finger induced greater significant changes of pain thresholds than pressure. In the orofacial region, pressure induced greater significant changes in pain thresholds during HNCS than electrical stimuli did. The HNCS induced pain of high intensity and unpleasantness, i.e. varying between 5 and 10 on the numeric rating scale (NRS). There were no gender differences in the response to the HNCS.

CONCLUSION

We conclude that, in general, HNCS induced by cold pressor stimulation increases pain thresholds, but the magnitude of the effect differs between the orofacial region and the finger and is influenced by the tissue and type of test stimuli.

Comparison of the sedative, cognitive, and analgesic effects of nitrous oxide, sevoflurane, and ethanol

BACKGROUND

Anaesthetics which work by different mechanisms may have different patterns of effect. Measurement of these patterns thus may elucidate their mechanisms of action and allow therapeutic choices between the agents.

METHODS

We compared the effects of ethanol (approximately 80 mg per 100 ml), and different end-tidal concentrations of nitrous oxide (15% and 25%) and sevoflurane (0.3% and 0.5%) in volunteers. We measured speed and accuracy in psychomotor tests, reaction time and memory, touch and pain sensitivity to von Frey filaments, and subjective mood for a range of descriptors.

RESULTS

All treatments caused the same degree of overall abnormal feelings, but sevoflurane caused more obtunding (subjective drowsiness, slow reaction times, and loss of memory function) and nitrous oxide was more analgesic. Ethanol caused a marked feeling of drunkenness, but little drowsiness or analgesia.

CONCLUSIONS

In the same volunteer subjects, direct comparison of sub-anaesthetic doses of these agents showed a clear and characteristic pattern of effects. These support the possible mechanisms for these disparate agents and may help choose appropriate agents for specific desired anaesthetic outcomes such as sedation or analgesia.

[Premixed 50% nitrous oxide and oxygen: theoretical recalls and practical modalities]

The concept of premixed 50% nitrous oxide and oxygen dated back to 1961 in England, where it is commercialised under the name Entonox. In France, after a so marginal use, premixed 50% nitrous oxide and oxygen now knows such a revival since we first consider the pain provoked by the cure. To use correctly premixed 50% nitrous oxide and oxygen, we need to know the main properties of the nitrous oxide. The four commercial versions, now on the market, are presented (Kalinox, Medimix, Antasol, Oxynox. Except a few contraindications, these indications are large, as well at hospital as outside. In less than 10% of the cases, side effects are possible. Conditions of administration are given in details. The limits of the technic and the particular precautions of use are precised.

Nitrous Oxide vs Periprostatic Nerve Block With 1% Lidocaine During Transrectal Ultrasound Guided Biopsy of the Prostate: A Prospective, Randomized, Controlled Trial

PURPOSE

We compared the efficacy of Entonox (BOC Gases, Manchester, United Kingdom), a mixture of 50% nitrous oxide and oxygen, with periprostatic infiltration of 1% lidocaine to provide analgesia during transrectal ultrasound (TRUS) guided biopsy of the prostate.

MATERIALS AND METHODS

The study included 235 consecutive men undergoing TRUS guided biopsy of the prostate for elevated prostate specific antigen or abnormal digital rectal examination. Patients were randomized to 3 groups, including group 1-84 controls who did not receive any form of analgesia prior to the procedure, group 2-75 who received periprostatic infiltration with 10 ml 1% lidocaine with biopsies performed 5 minutes after infiltration and group 3-76 who received Entonox for 2 minutes through a breath activated device prior to the procedure and thereafter according to patient preference. All patients were asked to indicate the level of pain experienced before and after the procedure on a 10 cm, nonnumerical, horizontal visual analog score. Results were analyzed using 1-way ANOVA.

RESULTS

Mean patient age was 68.8, 64.9 and 65.2 years, and mean visual analog score was 2.9, 1.6 and 2.2 in groups 1 to 3, respectively. Patients in groups 2 (1% lidocaine infiltration) and 3 (Entonox) experienced significantly less pain during the procedure compared with group 1 controls (p <0.001 and 0.028, respectively). There was no statistical difference in pain scores between groups 2 and 3 (p = 0.08).

CONCLUSIONS

Inhalation of Entonox or periprostatic infiltration with 1% lidocaine can be used for analgesia during TRUS guided biopsy of the prostate since each provides significant and similar pain relief.

Neurobiology of nitrous oxide-induced antinociceptive effects

Nitrous oxide (N2O), or laughing gas, has been used for clinical anesthesia for more than a century and is still commonly used. While the anesthetic/hypnotic mechanisms of N2O remain largely unknown, the underlying mechanisms of its analgesic/antinociceptive effects have been elucidated during the last several decades. Evidence to date indicate that N2O induces opioid peptide release in the periaqueductal gray area of the midbrain leading to the activation of the descending inhibitory pathways, which results in modulation of the pain/nociceptive processing in the spinal cord. The types of opioid peptide induced by N2O and the subtypes of opioid receptors that mediate the antinociceptive effects of N2O appear to depend on various factors including the species and/or strain, the regions of the brain, and the paradigms of behavior testing used for the experiments. Among three types of descending inhibitory pathways, the descending noradrenergic inhibitory pathway seems to play the most prominent role. The specific elements involved are now being resolved.

Enkephalin release and opioid receptor activation does not mediate the antinociceptive or sedative/hypnotic effects of nitrous oxide

In previous studies using Fos expression as a marker of neuronal activation, we showed that nitrous oxide (N(2)O) activates bulbospinal noradrenergic neurons in rats and that destruction of these neuronal pathways leads to loss of N(2)O antinociceptive action. Based on previous rat studies it has been proposed that these noradrenergic neurons are activated through opioid receptors through the release of endogenous opioid ligands in the periaqueductal gray. Using mice with a disrupted preproenkephalin gene (Penk2 -/-) and the opioid receptor antagonist naltrexone, we investigated the role of enkephalinergic mechanisms and opioid receptor activation in the behavioral and bulbospinal neuron responses to N(2)O in mice. The antinociceptive response to N(2)O was investigated using the tail-flick, hot-plate, and von Frey assays, the sedative/hypnotic response was measured using rotarod and loss of righting reflex, and bulbospinal neuronal activation was assessed with pontine Fos immunostaining. No differences were observed between wild-type and Penk2 -/- mice for the antinociceptive, sedative/hypnotic, and pontine neuronal activation effects of N(2)O. Similarly, naltrexone did not block N(2)O-induced antinociception, sedation, or hypnosis. We conclude that neither enkephalin nor opioid receptors participate in either the antinociceptive or the sedative/hypnotic actions of N(2)O in mice.

Effect of naloxone on primary and secondary hyperalgesia induced by the human burn injury model

BACKGROUND

Opioid antagonists may change the responses in models of experimental hyperalgesia. This indicates a possible involvement of the endogenous opioid system in these models. The aim of the present study was to evaluate whether activation of the endogenous opioid system could be demonstrated in the human burn injury model of cutaneous hyperalgesia, using an intravenous challenge with the non-selective opioid antagonist naloxone.

METHODS

We studied 25 healthy male volunteers aged 20-31 yrs in a randomised, double-blind, triple crossover design. A 25x50 mm rectangular burn injury was produced on the calf on 3 separate days, at least 1 week apart. Subjects received an intravenous bolus dose of naloxone 0.4 mg, 10 mg or placebo 3 h after induction of the burn injury.

RESULTS

Primary and secondary hyperalgesia was induced by the burn injury. Naloxone did not affect any of the measured variables: heat pain detection threshold in non-injured or injured tissue, pain produced by short or prolonged noxious heat in non-injured or injured tissue, secondary hyperalgesia elicited by pin prick or stroke, or pain produced by short or prolonged noxious mechanical stimulation in non-injured tissue. No significant adverse effects of naloxone were encountered.

CONCLUSIONS

Activation of an endogenous opioid response following induction of hyperalgesia in human volunteers by a burn injury could not be demonstrated with an intravenous naloxone challenge. These findings suggest that the endogenous opioid response is not a confounding factor in this model.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.