Analgesia Flashcards
What is the ISAP definition of pain?
An unpleasant secondary and emotional experience associated with or resembling that associated with actual or potential tissue damage.
What is the ISAP definition of nociceptive pain?
Pain that arises from actual or threatened damage to non-neural tissues and is due to the activation of nociceptors.
What is the ISAP definition of neuropathic pain?
Pain caused by a lesion or disease of the somatosensory nervous system.
Why is it important to know the difference between nociceptive and neuropathic pain?
- It can make a difference in how you approach pain management/treatment
- Neuropathic pain can be harder to treat
- Pain may be a consequence of a disease process – will it be neuropathic or nociceptive pain?
- For elective surgical procedures it is important to avoid damaging neural tissue
What is the ISAP definition of hyperalgesia?
Increases pain from a stimulus that normally provokes pain. Reflect increased pain on suprathreshold stimulation.
What is the ISAP definition of allodynia?
Pain due to a stimulus that does not normally provoke pain.
Distinguish allodynia and hyperalgesia?
For a pain evoked by stimuli that are usually not painful, the term allodynia is preferred. Hyperalgesia is preferred for cases with an increased response at a normal threshold or at an increased threshold.
What are the physiological signs of pain?
- Increased heart rate, blood pressure and body temperature
- Altered respiration rate and breathing pattern
- Stress hormones – cortisol, noradrenaline, adrenaline
- EEG activity – but can be pain or nociception
What are the similarities of behavioural signs of pain between cats and dogs?
- Hunched appearance
- Pain face
- Lack of grooming
- Inappetence
- Specific signs like lameness, depending on the condition
What are the differences between the behavioural signs of pain in cats and dogs?
Cats: absence of a normal behavioural repertoire, hide away at the back of cage, unwilling to relax, fear-aggression, resent human contact.
Dogs: positive behavioural signs rather than a reduced repertoire, more likely to seek attention, can be submissive or aggressive, may be more likely to vocalise
What are the behavioural signs of pain in rabbits?
- Immobility
- Profound depression
- Eyes half closed or shut
- Not grooming
- Avoiding attention
- Isolating themselves from other animals
- Bruxism
- Abnormal body position, such as hunched posture and abdominal pressing
- Change in temperament
What are the behavioural signs of pain in horses?
- Low head carriage
- Horse at back of stable
- Vocalisation – groaning and neighing
- Agitation
- Restlessness
- Weight shifting
- Tail swishing when no flies
- Lameness
- Limb lifting
- Abnormal distribution of weight
- Tucked up appearance
- Looking at painful body part
- Pain face
- Bruxism
- Sweating
- Muscle fasciculations
What are some additional indicators of pain in all species?
- Unkempt coat if grooming is difficult
- Loss of condition/weight loss due to appetite loss and/or loss of muscle tone if exercising less
- Slowing down and/or sleeping more
- Conversely could be more restless and fidgety
- May seen signs associated with reluctance to move
- Difficulty in accessing higher places
Describe the Glasgow composite pain scale for dogs.
Score is out of 20 if dog cannot stand and 24 if it can stand. Intervention level is 5/20 or 6/24
What is the drawback of the Glasgow composite pain scale for dogs?
Poor differentiation between pain and sedation, which is an issue in the immediate postoperative period
Describe the Glasgow composite pain scale for cats.
Maximum score is 20. Recommended intervention level is 5/20
What is preventative analgesia?
Administration of effective analgesia before, during and after the surgery/procedure, well into the postoperative recovery period
What are the aims of preventative analgesia?
Prevent upregulation of the nervous system in the face of noxious stimuli by administering effective analgesia. Reduction in both intensity and duration of the acute pain and a reduction in persistent/chronic pain.
What is multimodal analgesia?
Uses different classes of analgesic agents/techniques to attempt to block all the nociceptive/pain pathways.
What analgesic agents are used in veterinary practice?
Opioids
NSAIDs
Local anaesthetics
Alpha -2 agonists
Ketamine
How are opioids classified in terms of legal requirement?
- Most controlled drugs
- Full opioid agonists (methadone and fentanyl) are schedule 2. Special prescription, storage, destruction and record keeping requirements
- Partial opioid agonists (buprenorphine and butorphanol) are schedule 3. Special prescription and some have special storage requirements
Which receptor agonists produce analgesia?
µ agonists associated with analgesia. Full µ agonists provide the most effective analgesia
Name 3 full µ agonists and the species they are used in.
Fentanyl - dog, cat, horse, rabbit
Methadone - dog, cat
Pethidine - horse, less used now in dogs and cats
Name a partial µ agonists and the species it is used in.
Buprenorphine - dog, cat, rabbit
Name a k agonist and the species it is used in.
Butorphanol - dog, cat, rabbit
What are the side effects of opioid use at clinical doses?
- Respiratory depression
- Sedation
- Excitation
- Minimal effect on inotropy
- Bradycardia
- Nausea and vomiting
- Antitussive
- Decreased GI motility
- Various effects on urinary system – generally not of clinical significance unless administered epidurally
When should NSAIDs be given with caution during multimodal analgesia?
Caution needed if administering pre/peri-operatively or if patient is dehydrated/hypotensive. Can’t use more than 1 NSAID at the same time.
What are the side effects of NSAIDs at clinical doses?
- GI ulceration
- Renal ischaemia – during hypotension PGs protect renal blood flow and this action is blocked by NSAIDs
- Hepatopathy – rare idiosyncratic reaction
- Blood clotting
- CNS – dullness and lethargy in cats
What information should be given to owners about safe use of NSAIDs in their dogs and cats?
- GI side effects are most common
- Present as vomiting and/or diarrhoea
- May see digested blood, which looks like coffee grounds, in vomit
- Owners should discontinue medication immediately and ring the practice
- May see general malaise
What information should be given to owners about safe use of NSAIDs in their horses?
- GI side effects are most common. Most commonly occur with overdose, chronic administration and in suspectable populations like foals
- GI ulceration presenting as colic, diarrhoea, dehydration and weight loss
- Renal effects (issue in the dehydrated horse)
- If owners see signs of malaise, colic or other GI signs, ring practice
What information should be given to owners about safe use of NSAIDs in their rabbits?
- No NSAIDs are licensed for use in rabbits therefore the owner should be aware
- GI side effects are most common. Most commonly occurs with chronic administration
- If rabbit appears unwell (anorexia, bruxism, depression, reluctance to move) the owners should discontinue medication and ring the practice
How do local anaesthetic agents produce a nociceptive blockade?
Enter the nerve fibre and block the voltage-operated Na+ channel. This blocks nerve conduction. Said to have a ‘membrane stabilising effect’. C fibres and Aδ fibres are preferentially blocked.
How does pKa affect the onset of action of local anaesthetics?
Weak bases (pKa = 8-9) only the uncharged form can penetrate lipid membranes and enter the nerve cell. Higher Pka will be more ionized in plasma and have a slower onset of action
How does a pH decrease affect local anaesthetic agents?
A greater proportion of the drug is ionized and therefore less drug can penetrate the nerve membrane to bind to the sodium channel. LA less effective in inflamed tissue
When does CNS and CVS toxicity due to LA occur and their treatments?
CNS toxicity occurs at lower doses than CVS toxicity. Don’t exceed safe total dose for LA. Treatment for both CNS and CVS side-effects is symptomatic.
How can paracetamol be used in different species?
- Dogs – when NSAIDs are contraindicated, licensed formulation is co-compounded with codeine
- Horses – adjunctive analgesic in very painful cases and where NSAIDs are contraindicated
- Cats – do not use, toxic
- Rabbits – no published evidence of use
What is tramadol?
Centrally acting analgesic with a multimodal action – µ-opioid system, noradrenergic system, serotonergic system
Describe tramadol use in dogs.
- Oral tramadol unlikely to be effective
- Less well tolerated as sedation, drowsiness, dysphoria
- Shouldn’t be used orally in dogs and even if used parenterally it should be used as a co-analgesic
Describe tramadol use in cats.
Some analgesic effect in cats given parentally for acute pain. May have benefits orally for chronic pain
Describe tramadol use in horses.
- Oral bioavailability is variable
- Short half-life
- Decreased GI motility and a risk of impaction
- Probably only use for laminitis patients that don’t respond to any other analgesics
Describe tramadol use in rabbits.
Need much higher dose than in dogs and cats to get to a plasma level that may produce analgesia
What is gabapentin?
Structural analogue of GABA but has no effect on GABA binding, uptake or degradation
How should gabapentin be given?
Adjunct to other analgesics and/or when NSAIDs are contraindicated. Sedation is the major side effect from gabapentin so reduce the dose or increase the dose interval
What is the effect of gabapentin when given adjunct with tramadol and with xylitol?
- Sedation may be enhanced when combined with tramadol
- Avoid liquid solutions containing xylitol due to the potential for toxicity
What is amantadine?
Oral NMDA receptor antagonist
How should amantadine be given?
Is an antihyperalgesic therefore should be used alongside an analgesic. Use cautiously in patients with reduced renal function, as excreted via the kidneys.
When is clinical benefit seen with amantadine therapy?
3-4 weeks
