Analgesia Notes

Analgesia

1. Introduction

Analgesics may act at different sites. They may act at the site of injury and decrease the pain associated with an inflammatory reaction, e.g. NSAIDS.

They may alter nerve conduction, e.g. local anaesthetics.

They may modify transmission in the dorsal horn, e.g. opioids.

They may affect the central component and the emotional component of pain, e.g. opioids and anti-depressants.

The more sites an analgesic acts at, the better the analgesia.

Post-injury analgesia will reduce pain and hypersensitivity for a while, but pain increases again when the analgesic wears off. Pre-emptive analgesia blocks impulses before the injury occurs, therefore preventing hypersensitivity and reducing pain much more effectively.

2. Opioids

Opioids can be natural or synthetic drugs, and can be endogenous.

Opioid receptors that have been identified include mu, delta, kappa and nociception. Delta and kappa receptors mostly modulate mu receptors.

Side effects of opioids include cardiovascular effects, pruritus, urinary retention, vomiting and nausea.

1. Morphine

Morphine is the most effective opioid at relieving pain. It is a full agonist at mu, delta and kappa receptors.

Duration of action is between 4-8 hours. Oral bioavailability is around 25%.

Morphine significantly reduces MAC and MIR.

It is not licensed for veterinary use and is a schedule II controlled drug.

2. Methadone

Methadone is a synthetic full mu agonist and has affinity for NMDA receptor. It also acts as a norepinephrine and serotonin re-uptake inhibitor.

Following IV administration, duration of action if around 4 hours. Oral bioavailability is poor.

It is licensed for use in dogs and cats and is a schedule II controlled drug.

3. Pethidine (Meperidine)

Pethidine is a short acting synthetic agonist at the mu receptor. It also blocks sodium channels and is an agonist at alpha-2 subtype B receptors.

Pethidine has significant inotropic effects and tends to increase heart rate. It can also induce histamine release.

It can be given intramuscularly only. It should not be given to dogs receiving selegiline.

It is license for use in dogs, cats and horses and is a schedule II controlled drug.

4. Fentanyl

Fentanyl is a highly lipid soluble short acting mu opioid agonist.

It may be given intra-operatively as a bolus, at induction in combination with a benzodiazepine or as transdermal fentanyl patches.

It can cause significant bradycardia.

It is a schedule II controlled drug.

5. Other opioids

Codeine has been used in dogs to treat mild to moderate pain post-operatively.

Oxycodone has also been used in dogs post-operatively.

Naloxone is used for reversal of pure mu opioids.

Tramadol is a popular drug for use in cats and dogs but is not actually licensed in animals. It is a synthetic analogue of codeine, and is a low potency mu selective partial agonist. It also has an alpha-2 adrenergic effect and inhibits 5HT re-uptake.

6. Partial and mixed agonist/antagonist opioids

Buprenorphine is an agonist-atagonist with a strong affinity for mu receptors, and kappa antagonist...

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