Opioids

Opioids

What is an opioid?

  • A drug that acts on opioid receptors
  • Opiates are a subset of opioids, which occur naturally, such as morphine
  • They are commonly used to treat acute pain, and also as co-induction agents for general anaesthesia
  • They are also used in regional anaesthesia techniques such as epidural and spinal blocks
  • Side effects include nausea and vomiting, respiratory depression, pruritus, constipation and confusion

Objectives

  • Recall the pertinent chemical, pharmacokinetic and pharmacological properties of Fentanyl, Alfentanil, Remifentanil, Morphine and Diamorphine
  • Compare and contrast the properties of these drugs and their clinical uses in anaesthetic practice
  • Use this knowledge and understanding to select an appropriate opioid for use at induction, for regional anaesthesia, for Awake Fibreoptic Intubation and for intra/post-operative analgesia in various clinical scenarios

Why do we use opioids?

Indications for opioids include

  • Analgesia
  • Anxiolysis
  • Cough suppression
  • Treatment of diarrhoea

What effects and side effects do opioids have in common?

Respiratory

  • Respiratory depression
  • Reduced sensitivity to hypoxia and hypercapnoea
  • Suppression of laryngeal reflexes
  • Chest wall rigidity

Cardiovascular

  • Bradycardia
  • Hypotension

Neurological

  • Analgesia
  • Anxiolysis
  • Drowsiness
  • Miosis

GI/GU

  • Emetogenic
  • Constipation
  • Sphincter of Oddi spasm
  • Urinary retention

Side effects

  • Histamine release
  • Bronchospasm
  • Urticaria and pruritus
  • Hypotension
  • Dependence
  • Tolerance, dependence and addiction can develop with chronic use

Morphine

Fentanyl

Alfentanil

Remifentanil

Diamorphine

What you need to know

  • Diamorphine = Heroin = Diacetylmorphine
  • Diamorphine has no intrinsic activity at opioid receptors
  • Once it is deacetylated, it can exert its effect in the form of morphine
  • 1mg Diamorphine = 2mg Morphine
  • Diamorphine is much more lipid-soluble than morphine, making it ideal for use in neuraxial anaesthesia (spinal / epidural) where barriers such as the dura need to be crossed in order for it to exert its effect

Example OSCE/SOE question:

Tell me about diamorphine

Class and uses

  • Diacetylated morphine derivative prodrug
  • Metabolised to 6-monoacetyl morphine and morphine
  • Used for analgesia, premedication and anxiolysis

Presentation and dose

  • Tablets 10mg
  • White powder for injection
  • 5, 10, 30, 100 or 500mg vials of diamorphine hydrochloride
  • 0.1-0.4mg intrathecal dose
  • up to 3mg epidural dose
  • up to 10mg IV for severe dyspnoea in pulmonary oedema
  • Can also be given intrathecally or via the epidural route

Mechanism of Action

  • μ receptor full agonist

Effects and side effects

Respiratory

  • Respiratory depression
  • Respiratory rate particularly affected
  • Reduced sensitivity to hypercapnoea and to a lesser extent to hypoxia
  • Suppression of laryngeal reflexes
  • Chest wall rigidity
  • Possibly via interaction with dopamine and GABA in the substantia nigra

Cardiovascular

  • Bradycardia
  • Hypotension

Neurological

  • Analgesia
  • Better for visceral pain than sharp pain
  • Anxiolysis
  • Drowsiness and sedation
  • Triggers the most euphoria of all the opioids hence its addictive nature
  • Miosis
  • Stimulates the Edinger–Westphal nucleus

GI/GU

  • Emetogenic
  • Constipation
  • Sphincter of Oddi spasm
  • Urinary retention

Endocrine

  • Inhibits ACTH release
  • Increases ADH release

Side effects

  • Histamine release
  • Bronchospasm
  • Urticaria and pruritus
  • Particularly after epidural and intrathecal injection
  • Hypotension
  • Dependence
  • Tolerance, dependence and addiction can develop with chronic use

Pharmacokinetics

Absorption

  • Highly lipid soluble so well absorbed from the GI tract but significant first pass metabolism
  • pKa 7.6 so 37% unionised in the blood
  • Bioavailability of 30%
  • Significant first pass metabolism
  • Peak effect after 10mins when given IV
  • 30mins when given IM

Distribution

  • Higher lipid solubility than morphine makes it faster acting and safer for intrathecal injection, with reduced risk of cranial spread and delayed respiratory depression
  • 40% protein bound

Metabolism

  • Ester hydrolysis in plasma, liver and brain to 6-monoacetylmorphine and morphine
  • Half life of 5 minutes

Excretion

  • Renal excretion of all metabolites, which accumulate in renal failure

Side effects

Tell me about naloxone

Opioid antagonist at three opioid receptors, used for reversal of opioid overdose or toxicity

  • μ is main target
  • κ
  • δ

Effects

  • Hypertension
  • Pulmonary oedema
  • Arrhythmias

Dose

  • 1-4 μg/kg IV
  • Short duration of action of 30 mins

A really useful reference


Download our free opioid summary pdf


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