Analgesia

Question 1

What are the classes of analgesics?

Answer 1

NSAIDs, opioids, local anaesthetics, alpha 2 agonists, ketamine

Question 2

What is the mechanism of action of opioids?

Answer 2

Activate either μ, δ, κ receptors to decrease neuronal excitability.
μ primarily mediates analgesia and side effects

Question 3

What are the effects/side effects of opioids?

Answer 3

Analgesia, sedation, euphoria/dysphoria, nausea +/- v+, decreased gut motility, resp depression

Question 4

Describe the pharmacokinetics of opioids

Answer 4

Poor bioavailability, undergoes extensive first pass metabolism

Question 5

Whata re the indications for using opioids?

Answer 5

Moderate - severe acute pain relief, sedation, to lower the dose of GA, treat d+, control coughing

Question 6

What are the contraindications for opioid use?

Answer 6

Existing hypoventilation (except when this is due to pain), high ICP ( increase in [CO2] = cerebral vasodilation = increased ICP)

Question 7

Describe the mechanism of action of NSAIDs

Answer 7

Inhibit PG production and thromboxanes by COX enzymes

Question 8

What are the effects/side effects of NSAIDs?

Answer 8

Analgesia, decreases inflammation, dyspepsia, GI ulceration, renal and hepatic toxicity

Question 9

Describe the pharmacokinetics of NSAIDs

Answer 9

Marked interspecies variation, good oral obsorption

Question 10

What are the indications for NSAID use?

Answer 10

Management of acute and chronic pain, inflammatory disorders, prothrombotic states, some tumours

Question 11

What are the contraindications for NSAIDs?

Answer 11

Acute/chronic renal disease, impaired hepatic function, GI dz, haemostatic disorders, hypovolaemia, concurrent steroid tx, unstable asthma

Question 12

Describe the mechanism of action of local anaesthetics

Answer 12

Block sodium channels to prevent initiation and conduction of action potentials

Question 13

What are the effects/side effects of local anaesthetics?

Answer 13

Analgesia, dysrhythmia, CNS and CVS toxicity

Question 14

Describe the pharmackinetics of LA

Answer 14

Activity decreases at low pH (inflammation and ionisation delays onset)

Question 15

How can LA be administered?

Answer 15

Topical (mm, skin), infiltration, IV regional, peripheral nerve blocks, epidural, systemic admin (CRI of lidocaine is v painful in dogs)

Question 16

What are the contrainidcations for LA use?

Answer 16

Refers to the route of admin rather than the effect the drug has on the body.
Epidural - haemostatic disorder, pelvic/spinal #, pyoderma

Question 17

Define peripheral sensitisation

Answer 17

CNS is bombarded with noxious stimuli, meaning there is a high sensitisation to pain and an exaggerated response

Question 18

Define central sensitisation

Answer 18

Spinal cord is 1st point of integration of pain
SC changes and shows plasticity in response to repeared /prolonged nociceptive impulses (increased rate of firing of fibres carrying impulses to the brain and increased size of receptive fields)

Question 19

Define allodynia

Answer 19

Induction of pain from a non-painful

Question 20

Define wind up

Answer 20

Exaggeration of a painful stimulus at the cellular level

Question 21

Define hyperalgesia

Answer 21

Lowered pain threshold/increase in pain percieved for a noxious stimulus

Question 22

How to epidural opioids work?

Answer 22

High concentration of drug at the opioid receptors in the SC. Lipid soluble so is quickly taken up by fat surrounding the SC = localised band of analgesia around the injection site.
Hydrophilic opioids are taken up into the CSF and transit up and down the SC to give diffuse and longlasting analgesia.

Question 23

Describe the properties of transdermal opioids

Answer 23

Fentanyl patches - lipid soluble, slow onset, prolonged duration, variable efficacy
Fentanyl spot on - dogs, 2-4hrs before sx, analgesia for 4d, safety issues with handling