Analgesia Flashcards
What are the classes of analgesics?
NSAIDs, opioids, local anaesthetics, alpha 2 agonists, ketamine
What is the mechanism of action of opioids?
Activate either μ, δ, κ receptors to decrease neuronal excitability.
μ primarily mediates analgesia and side effects
What are the effects/side effects of opioids?
Analgesia, sedation, euphoria/dysphoria, nausea +/- v+, decreased gut motility, resp depression
Describe the pharmacokinetics of opioids
Poor bioavailability, undergoes extensive first pass metabolism
Whata re the indications for using opioids?
Moderate - severe acute pain relief, sedation, to lower the dose of GA, treat d+, control coughing
What are the contraindications for opioid use?
Existing hypoventilation (except when this is due to pain), high ICP ( increase in [CO2] = cerebral vasodilation = increased ICP)
Describe the mechanism of action of NSAIDs
Inhibit PG production and thromboxanes by COX enzymes
What are the effects/side effects of NSAIDs?
Analgesia, decreases inflammation, dyspepsia, GI ulceration, renal and hepatic toxicity
Describe the pharmacokinetics of NSAIDs
Marked interspecies variation, good oral obsorption
What are the indications for NSAID use?
Management of acute and chronic pain, inflammatory disorders, prothrombotic states, some tumours
What are the contraindications for NSAIDs?
Acute/chronic renal disease, impaired hepatic function, GI dz, haemostatic disorders, hypovolaemia, concurrent steroid tx, unstable asthma
Describe the mechanism of action of local anaesthetics
Block sodium channels to prevent initiation and conduction of action potentials
What are the effects/side effects of local anaesthetics?
Analgesia, dysrhythmia, CNS and CVS toxicity
Describe the pharmackinetics of LA
Activity decreases at low pH (inflammation and ionisation delays onset)
How can LA be administered?
Topical (mm, skin), infiltration, IV regional, peripheral nerve blocks, epidural, systemic admin (CRI of lidocaine is v painful in dogs)
What are the contrainidcations for LA use?
Refers to the route of admin rather than the effect the drug has on the body.
Epidural - haemostatic disorder, pelvic/spinal #, pyoderma
Define peripheral sensitisation
CNS is bombarded with noxious stimuli, meaning there is a high sensitisation to pain and an exaggerated response
Define central sensitisation
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)
Define allodynia
Induction of pain from a non-painful
Define wind up
Exaggeration of a painful stimulus at the cellular level
Define hyperalgesia
Lowered pain threshold/increase in pain percieved for a noxious stimulus
How to epidural opioids work?
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.
Describe the properties of transdermal opioids
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
