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Pharmacology of opioids and application to practice

02 February 2021
Clinical
12 mins read
02 February 2021
Volume 12 · Issue 1
Figure 1. Primary afferent neurons.

Abstract

This article analyses the pharmacological considerations when administering opioids in practice. Beginning with a basic review of pain pathways, followed by a more in-depth analysis of opioid pharmacology. The focus will be on the three most commonly used full mu agonist opioids (methadone, morphine, fentanyl). The aim of this article is to show the importance of applying pharmacology to a clinical situation, promoting individual pain management assessment. This involves the application of the most appropriate opioid, as part of a more extensive pain management plan to improve analgesic efficacy and patient outcomes.

The inhibition of nociceptive pathways reduces the sensory and emotional aspects of pain. Understanding these pathways, allows the selection of appropriate agents and the ethical treatment of a variety of species in veterinary practice. This article will concentrate on the pain pathways, the pharmacology of opioids and their role in administration of analgesia. As veterinary literature can be limited this analysis will use a combination of human and veterinary studies to provide a more comprehensive understanding.

Pain is an unpleasant experience providing sensory and emotional components to actual or potential tissue damage (McKune et al, 2015).

Physiologic pain is detected and interpreted by a nociceptive sensory system, responding to pain through transduction, transmission, modulation, and perception (McKune et al, 2015).

Peripheral nociceptors detect noxious (thermal, mechanical, and chemical) stimuli and an action potential is transmitted by two types of primary afferent neuron: C-fibres and A-δ fibres (Dubin and Patapoutian, 2010). A-δ fibres are lightly myelinated fibres with a diameter of 2–5 µm (Figure 1), and have a low threshold for noxious stimuli (thermal and mechanical), providing rapid delivery of action potentials in response to acute pain. C-fibres are unmyelinated and have a diameter of <2 µm (Figure 1), and provide slow delivery of action potentials. C-fibres have a high threshold and are polymodal; therefore, they respond to chemical, mechanical, and thermal noxious stimulation (Reddi et al, 2013).

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analgesia
anaesthesia
pharmacology
opioids