Pain and analgesia Flashcards
Define pain
an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
What is adaptive pain (protective)?
- protective pain
- nociceptive pain
- inflammatory pain
What is maladaptive (pathological) pain?
- serve no protective function
- neuropathic pain (abnormal)
Define acute pain
often has an obvious cause and is relatively short duration (hours to days)
Define chronic pain
pain lasting longer than 1 month, associated with a wide range of often subtle behavioural disturbances
Outline the 3 processes of pain
- NOCICEPTION: sensory process by which a noxious stimulus is transmitted to the brain (transduction, transmission, modulation)
- PERCEPTION of an unpleasant sensation in CNS
- BEHAVIOURAL response to pain -varies according to species
Define allodynia
painful response to a normally innocuous stimulus
Define hyperalgesia
increased response to painful stimulus
What does hypersensitisation mean?
- level of pain perceived is more severe
- analgesic drugs may be less effective if given once pain is present
- one type of analgesic may not be effective on its own
What causes variations in behaviour to pain?
- species (prey vs. predator)
- breed (labrador vs greyhound)
- demeanour
How can pain be assessed?
- scales: visual analogue or numerical rating
- simple descriptive: no pain to worst pain imaginable
- composite scoring system (based on different behaviours)
- multidimensional scoring system
What pain scoring systems are validated in horses? 4
- composite orthopaedic pain
- equine acute abdominal pain scale (EAAPS)
- complex numerical rating scale - postop colic
- post abdominal surgery pain assessment scale (PASPAS)
Ideal features of pain assessment in practice
- simple
- repeatable and reproducible
- minimal time
- allows effective pain management
Multimodal analgesic options
- opioids
- NSAIDs
- ketamine
- alpha-2-agonists
- local anaesthetics
What receptor does ketamine antagonise?
NMDA-R
Describe multi-modal analgesia
- attack pain pathway at multiple sites
- each method helps in overall control of individual pain
- more effective than using single method of analgesia
- allows lower doses of each drug to be used, reducing the risk of side effects
Use - gabapentin
usually for chronic pain
List systemic analgesia options
- opioids
- NSAIDs
- ketamine
- lidocaine
- alpha 2 agonists
- (tramdol)
- (gabapentin/ pregabalin)
- (chronic pain)
List local analgesia options
- local anaesthetics
- opioids
- alpha 2 agonists
Define opium
mixture of alkaloids from poppy plant
Define opioid
any naturally occurring, semi-synthetic or synthetic compound that binds to opioid receptors and shares the proerties of the natural occuring endogenous opioids
Define opiate
any naturally occuring opioid, derived from opium (e.g. morphine, codeine)
Define narcotic
from the Greek meaning ‘to numb’. It was used to denote an opioid but also to describe non-opioid drugs of addiction
Name 2 naturally occurring opioids
- morphine
- codeine
Name 4 semi-synthetic opioids
- diamorphine (heroid)
- dihydromorphone
- buprenorphine
- hydromorphone
Name 7 synthetic opioid compounds
- pethidine
- methadone
- fentanyl, alfentanil, remifentanil
- butorphanol, tramadol
What are the 4 receptors opioids mediate their effects through?
- delta
- kappa
- miu
- (nociceptin)
- kappa and miu are the most important, miu provides most analgesic effects but also some side effects
List some full miu agonist (opioids)
- morphine
- methadone
- pethidine (licensed)
- fentanyl / remifentanil
- etorphine (Immobilon)
- Papaveretum (Omnopon)
Example of a partil miu agonist
Buprenorphone (licensed)
Example - mixed opioid agonist-antagonist
Butorphanol (licensed)
Name 2 opioid antangonists
- naloxone
- diprenorphine (Revivon)
What is a partial agonist?
- same overall actions as agonists
- doesn’t produce maximal effects
- Buprenorphine
- partial miu agonist
- analgesia but not as profound as full agonists
- use for mild to moderate pain
What is a mixed agonist-antagonist?
- agonist at some Rs, antagonist at others
- Butorphanol
- antagonist at miu receptors: relatively poor analgesic
- agonist at kappa receptors: some analgesia, sedation, dysphoria
Define dysphoria
state of unease or dissatisfaction with life (can be a side effect of opioid analagesics in cats, horses, ruminants). You see pupil dilation. Tx with a partial agonist (buprenorphine)
ROA - opioid analgesics
- IV (NOT pethidine)
- IM
- SC (variable absorption)
- Oral (high 1st pass metabolism)
- Oral transmucosal (OTM) - i.e. cats that don’t like injections, best way to give buprenorphine but you don’t want the cat to swallow
- Spinal/ epidural
- intra-articular
- transdermal
Side effects - opioid analgesics in dogs, monkeys, people
CNS depression translated into sedative effect
Side effects - opioid analgesics in cats, horses and ruminants
CNS stimulation (excitement, locomotor activity) –> euphoria and dysphoria
Why are the side effects of opioids on arousal different in dogs, monkeys, people vs. cats, horses and ruminants?
- interspecies differences in type and distribution of receptors in various regions of brain
- presence or absence of pain
- dose and ROA
- specific opioid administered.
Side effects - opioid analgesics
- nausea and vomiting
- variation in pupillary diameter (cats develop mydriasis d/t catecholamines and are very light sensitive, dogs develop miasis)
Why do opioid analgesics cause nausea and vomiting?
- direct CTZ stimulation
- antiemtic effects on vomiting centre
- dogs +++, cats +
- morphine +++, methadone + (difference in lipid solubility)
- rarely occur if pain present
How do opioids affect thermoregulation?
- dogs: decrease in thermoregulatory set point and panting
- CATS, horses, swine, ruminants: hyperthermia d/t increase mm activity
How do opioids affect respiratory system?
- -> respiratory depression
- miu mediated effect in respiratory centre
- d/t decreased responsiveness to CO2
- especially if co-administration of a sedative
How do opioid analgesics affect the heart?
- -> bradycardia
- vagal stimulation
- response to anticholinergics
- pethidine an exception
What is the Controlled Drugs Legislation?
- 1971 Misues of Drugs Act
- SCHEDULE 2: Pure agonists (controlled drugs):
- must be kept in locked cupboard
- records kept of purchase and use
- special prescription requirements
- must be disposed of according to legislation
- SCHEDULE 3: partial agonist (buprenorphone)
- should be kept in locked cupboard with schedule 2 drugs, but no record of use necessary
- BUTORPHANOL - currently free from restrictions
Outline ketamine as an analgesic
- an adjuvant to pain management
- sub-anaesthetic doses –> blocks NMDA-R
- INDICATIONS: skin sx, grafts, burns, neurogenic pain (amputation).
- USE: infusion during sx but car d/t dysphoria
What is tramadol?
- mix of 2 stereoisomers (each has 5 different metabolites)
- cheap, currently not controlled
- no studies into efficacy
- can’t be reversed
- most of action mediated by inhibition of uptake (e.g. serotonin)
Summarise NSAID effects
- anti-inflammatory (not paracetamol)
- analgesic
- anti-pyretic
- acute and chronic use
