Opioids - Anaesthesia Unravelled #9

Hello there! In this Anaesthesia Unravelled video lesson we will be talking about the use of "Opioids", following through the Pain and Analgesia module. Opioids make up one of the most important groups in pain management, both in medicine and in veterinary medicine. Morphine is considered the "gold standard" opioid, which has been used for over 4,000 years.

Currently, there are several choices of opioids available, which act differently on the Mu (µ), Kappa (k) and Delta (δ) receptors. They are located mainly in the CNS, but are also present in several other body organs, especially considering situations of pain. These receptors differ by their resulting effect, acting especially on pain, behaviour (sedation and euphoria) and on the gastrointestinal and cardiovascular systems.

Opioids act differently in each of these receptors. Full opioid agonists have high affinity to the µ, k e δ receptors. The main representatives are morphine, meperidine, methadone, fentanyl, alfentanil and remifentanil. Partial agonists have relative affinity to some of the receptors. In this group we have buprenorphine and tramadol. Agonists-antagonists can activate one receptor, but act as an antagonist to another, with butorphanol and nalbuphine as the main representatives. Antagonists have an affinity for opioid receptors but no intrinsic activity. Here, you have naloxone and naltrexone.

Analgesia is the main desired effect when thinking about using an opioid. Full agonists tend to promote better analgesia than partial opioids or agonist-antagonists. The sedative effect is also of interest, especially in patients who will undergo anaesthesia. However, opioids can promote euphoria in some species, depending on the dose and route.

The impact on the cardiovascular and pulmonary systems is slight, but the use of IV morphine or meperidine should be avoided, as there is a risk of hypotension. The effects on the gastrointestinal system are evident; some opioids promote emesis (dogs and cats) and hypomotility. A widely discussed effect is possible immunosuppression. Some studies support the existence of this effect, others do not, depending on the opioid, use and species. For this reason, the use of "opioid-free" anaesthetic protocols has been widely discussed.

To better understand the action of each opioid in each species, go the the video and check it out the references below!

Further reading
Carregaro et al. Effect of methadone combined with acepromazine or detomidine on sedation and dissociative anesthesia in healthy horses. J Eq. Vet Sci, 86: 102908, 2020.
Carregaro AB, Luna SPL. Opioides. In: Luna SPL, Carregaro AB. Anestesia e Analgesia em Equídeos, Ruminantes e Suínos. Ed MedVet. 1a ed. 2019, 149-172.
Clutton RE. Opioid analgesia in horses. Vet Clin North Am Eq Pract. 26:493-514, 2010.
Kukanich B, Wiese AJ. Opioids. In: Grimm KA et al. Lumb and Jones Veterinary Anesthesia and Analgesia. 5th ed. 2015, 207-226.
Simon BT, Steagall PV. The present and future of opioid analgesics in small animal practice. J Vet Pharmacol Ther, 40:315-326, 2017.
Wagner AE. Opioides. In: Gaynor JS, Muir WW. Manual de Controle da Dor em Medicina Veterinária. Ed MedVet. 2a ed. 2009, 163-182.

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