Reviewed and revised 26 August 2014
- An opioid is any psychoactive chemical that resembles opiates in their pharmacological effects by binding opioid receptors.
- They may be endogenous or exogenous compounds, and may be naturally occurring or synthetic
CLASSIFICATION OF OPIOIDS
Opioid analgesics may be:
- Pure agonists of specific opioid receptors (notably the mu receptor)
- Mixed agonist – antagonist drugs with opposing effects at distinct receptor subtypes
- Partial mu agonists
The affinity of individual opioid analgesics for receptors varies
- No ceiling effect to analgesia is found with the pure agonists, in contrast to the partial agonist and mixed agonist – antagonist opioids, which demonstrate a ceiling response above which an increase in dose does not produce any additional increase in effect.
OPIOID RECEPTORS AND EFFECTS
Opioid receptors are distributed widely in the:
- Brain (supraspinal sites)
- Spinal cord
- Digestive tract (peripheral sites)
The three principle opioid receptors are:
|Receptor||Opioid class||Location||Possible Functions|
|Mu subtypes include: μ1, μ2, μ3||Endorphins||Analgesia/ physical dependence/ respiratory depression/ miosis/ Euphoria/ reduced GIT motility/ physical dependence / Possible vasodilation|
Kappa subtypes include: κ1, κ2, κ3
|Dynorphins||Analgesia/ convulsant effects/ dysphoria/ Respiratory depression/ reduced GIT motility|
Delta subtypes include: δ1, δ2
|Enkephalins||Analgesia, (less than mu)|
Mechanisms of opioid analgesia (primarily via mu receptors, though delta and kappa receptors also mediate analgesic effects)
- Presynaptic inhibition of neurotransmitter release from C-fiber terminals
- Postsynaptic inhibition of evoked activity in nociceptive pathways
- Disinhibition of other circuits regulating nociceptive transmission
- Supraspinal opioids increase descending inhibition of spinal nociceptive transmission
The endogenous opioids include:
- Natural opiates (alkaloids contained in the resin of the opium poppy) e.g. morphine, codeine
- Esters of morphine opiates e.g. diacetylmorphine (heroin)
- Semi-synthetic opioids (created from natural opiates or morphine esters) e.g. hydromorphone, oxycodone, and buprenorphine
- Synthetic opioids e.g. fentanyl, alfentanil, remifentanil, pethidine, levorphanol, methadone, tramadol and dextropropoxyphene
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.