Novel data on opioid effect on breathing and analgesia

Modeling Insights into Potential Mechanisms of Opioid-Induced Respiratory Depression within Medullary and Pontine Networks

The opioid epidemic is a pervasive health issue and continues to have a drastic impact on the United States. This is primarily because opioids cause respiratory suppression and the leading cause of death in opioid overdose is respiratory failure (i.e., opioid-induced respiratory depression, OIRD). Opioid administration can affect the frequency and magnitude of inspiratory motor drive by activating μ-opioid receptors that are located throughout the respiratory control network in the brainstem. This can significantly affect ventilation and blunt CO2 responsiveness, but the precise neural mechanisms that suppress breathing are not fully understood. Previous research has suggested that opioids affect medullary and pontine inspiratory neuron activity by disrupting upstream elements within this circuit. Inspiratory neurons within this network exhibit synchrony consistent with shared excitation from other neuron populations and recurrent mechanisms. One possible target for opioid suppression of inspiratory drive are excitatory synapses. Reduced excitability of these synaptic elements may result in disfacilitation and reduced synchrony among inspiratory neurons. Downstream effects of disfacilitation may result in abnormal output from phrenic motoneurons resulting in distressed breathing. We tested the plausibility of this hypothesis with a computational model of the respiratory network by targeting the synaptic excitability in fictive medullary and pontine populations. The synaptic conductances were systematically decreased while monitoring the overall respiratory motor pattern and aggregate firing rates of subsets of cell populations. Simulations suggest that highly selective, rather than generalized, actions of opioids on synapses within the inspiratory network may account for different observed breathing mechanics.

Pathophysiology, assessment, and management of pain in critically ill adults

PURPOSE

The pathophysiology of pain in critically ill patients, the role of pain assessment in optimal pain management, and pharmacologic and nonpharmacologic strategies for pain prevention and treatment are reviewed.

SUMMARY

There are many short- and long-term consequences of inadequately treated pain, including hyperglycemia, insulin resistance, an increased risk of infection, decreased patient comfort and satisfaction, and the development of chronic pain. Clinicians should have an understanding of the basic physiology of pain and the patient populations that are affected. Pain should be assessed using validated pain scales that are appropriate for the patient's communication status. Opioids are the cornerstone of pain treatment. The use of opioids, administered via bolus dosing or continuous infusion, should be guided by patient-specific goals of care in order to avoid adverse events. A multimodal approach to pain management, including the use of regional analgesia, may improve patient outcomes and decrease opioid-related adverse events, though there are limited relevant data in adult critically ill patient populations. Nonpharmacologic strategies have been shown to be effective adjuncts to pharmacologic regimens that can improve patient-reported pain intensity and reduce analgesic requirements. Analgesic regimens need to take into account patient-specific factors and be closely monitored for safety and efficacy.

CONCLUSION

Acute pain management in the critically ill is a largely underassessed and undertreated area of critical care. Opioids are the cornerstone of treatment, though a multimodal approach may improve patient outcomes and decrease opioid-related adverse events.

Inhaled carbon dioxide causes dose-dependent paradoxical bradypnea in animals anesthetized with pentobarbital, but not with isoflurane or ketamine

INTRODUCTION

In spontaneously breathing mice anesthetized with pentobarbital, we observed unexpected paradoxical bradypnea following 5% inhaled CO2.

METHODS

Observational study 7-8 week CB6F1/OlaHsd mice (n = 99), anesthetized with 30 mg/kg intraperitoneal pentobarbital. Interventional study: Adult male Wistar rats (n = 18), anesthetized either with 30 mg/kg intraperitoneal pentobarbital, 100 mg/kg intraperitoneal ketamine or 1.5% isoflurane. Rats had femoral artery cannulas inserted for hemodynamic monitoring and serial arterial blood gas measurements.

RESULTS

Observational study: There was a marked reduction in respiratory rate following 4 min of normoxic hypercapnia; average reduction of 9 breaths/min (p < 0.001) (17% reduction from baseline). Interventional study: increasing CO2 caused dose-dependent increase in respiratory rate for ketamine-xylazine (p = 0.007) and isoflurane (p = 0.016) but dose-dependent decrease in respiratory rate for pentobarbital (p = 0.046). Increasing inspired CO2 caused dose-dependent acidosis following pentobarbital and isoflurane (p = 0.013 and p = 0.017, respectively); but not following ketamine-xylazine (p = 0.58).

CONCLUSIONS

Inhaled CO2 caused paradoxical dose-dependent bradypnea in animals anesthetized with pentobarbital, an observation not hitherto reported as a part of anesthesia-related respiratory depression.

Neural control of the upper airway: integrative physiological mechanisms and relevance for sleep disordered breathing

The various neural mechanisms affecting the control of the upper airway muscles are discussed in this review, with particular emphasis on structure-function relationships and integrative physiological motor-control processes. Particular foci of attention include the respiratory function of the upper airway muscles, and the various reflex mechanisms underlying their control, specifically the reflex responses to changes in airway pressure, reflexes from pulmonary receptors, chemoreceptor and baroreceptor reflexes, and postural effects on upper airway motor control. This article also addresses the determinants of upper airway collapsibility and the influence of neural drive to the upper airway muscles, and the influence of common drugs such as ethanol, sedative hypnotics, and opioids on upper airway motor control. In addition to an examination of these basic physiological mechanisms, consideration is given throughout this review as to how these mechanisms relate to integrative function in the intact normal upper airway in wakefulness and sleep, and how they may be involved in the pathogenesis of clinical problems such obstructive sleep apnea hypopnea.

Morphine has latent deleterious effects on the ventilatory responses to a hypoxic challenge

The aim of this study was to determine whether morphine depresses the ventilatory responses elicited by a hypoxic challenge (10% O₂, 90% N₂) in conscious rats at a time when the effects of morphine on arterial blood gas (ABG) chemistry, Alveolar-arterial (A-a) gradient and minute ventilation (V_M) had completely subsided. In vehicle-treated rats, each episode of hypoxia stimulated ventilatory function and the responses generally subsided during each normoxic period. Morphine (5 mg/kg, i.v.) induced an array of depressant effects on ABG chemistry, A-a gradient and V_M (via decreases in tidal volume). Despite resolution of these morphine-induced effects, the first episode of hypoxia elicited substantially smaller increases in V_M than in vehicle-treated rats, due mainly to smaller increases in frequency of breathing. The pattern of ventilatory responses during subsequent episodes of hypoxia and normoxia changed substantially in morphine-treated rats. It is evident that morphine has latent deleterious effects on ventilatory responses elicited by hypoxic challenge.

Opioid receptor mechanisms at the hypoglossal motor pool and effects on tongue muscle activity in vivo

Opioids can modulate breathing and predispose to respiratory depression by actions at various central nervous system sites, but the mechanisms operating at respiratory motor nuclei have not been studied. This study tests the hypotheses that (i) local delivery of the mu-opioid receptor agonist fentanyl into the hypoglossal motor nucleus (HMN) will suppress genioglossus activity in vivo, (ii) a component of this suppression is mediated by opioid-induced acetylcholine release acting at muscarinic receptors, and (iii) delta- and kappa-opioid receptors also modulate genioglossus activity. Seventy-two isoflurane-anaesthetised, tracheotomised, spontaneously breathing rats were studied during microdialysis perfusion into the HMN of (i) fentanyl and naloxone (mu-opioid receptor antagonist), (ii) fentanyl with and without co-application of muscarinic receptor antagonists, and (iii) delta- and kappa-opioid receptor agonists and antagonists. The results showed (i) that fentanyl at the HMN caused a suppression of genioglossus activity (P < 0.001) that reversed with naloxone (P < 0.001), (ii) that neither atropine nor scopolamine affected the fentanyl-induced suppression of genioglossus activity, and (iii) that delta-, but not kappa-, opioid receptor stimulation also suppressed genioglossus activity (P = 0.036 and P = 0.402 respectively). We conclude that mu-opioid receptor stimulation suppresses motor output from a central respiratory motoneuronal pool that activates genioglossus muscle, and this suppression does not involve muscarinic receptor-mediated inhibition. This mu-opioid receptor-induced suppression of tongue muscle activity by effects at the hypoglossal motor pool may underlie the clinical concern regarding adverse upper airway function with mu-opioid analgesics. The inhibitory effects of mu- and delta-opioid receptors at the HMN also indicate an influence of endogenous enkephalins and endorphins in respiratory motor control.