Analgesia and pain assessment

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Analgesia and pain assessment
Aleksandar Popovic DVM, CertLAS, MRCVS
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Pain and Analgesia

• Pain may be defined as an unpleasant sensory and
  emotional experience associated with actual or
  potential damage or described in terms of such a
  damage (IASP, 1979).
• Pain can be classified as acute and chronic, can
  arise from somatic tissues, viscera and nervous
  tissues.
• Analgesia means the absence of pain in response
  to the stimulation that would normally be painful
  (IASP, 1979).

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Nociception

• Transmission of the signal provoked by the
  noxious stimulus to the brain is nociception.
• It involves
• Detection (peripheral) of the noxious stimuli
• Transmission by afferent sensory nerve fibers
  (myelinated Ad or unmyelinated C)
• Spinal Cord (processing, modulation,
  transmission)
• Central connection (brain)

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Pain vs nociception
0330-3.jpg
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Misconceptions

• Animals do not feel pain the same way as humans
• Side-effects prevent usage of opioids in animals
• Analgesic results in skin suture removal
• Alleviation of pain post-op leads to self
  mutilation
• All animals should receive the same dose and rate
  of analgesics
• Analgesia will adversely affect the outcome of
  experiment

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Negative effects of pain

• Animal suffers
• Catabolism is increased
• Increase in sympathetic tonus
• Impaired respiratory functions
• Immune suppression
• Self-mutilation
• Potential for chronic pain
• Delayed recovery
• Increased morbidity

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How to assess pain?

• Objective means
• Pain threshold
• Biochemical
• Physiological
• Subjective means
• Scales (e.g.,Visual Analogue Scale, Interactive
  V.A.S)
• Behavioural observation
• Pain studies (ethical dilemma to use placebo
  controls)

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Pain Relief

• Methods of alleviation
• Opioids (Controlled drugs)
• NSAIDs
• Miscellaneous drugs
• Local

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• Excitation
• glutamate
• substance P
• neurokinin A
• other
• neuropeptides
• prostaglandins
• nociceptin?
• dynorphins?

• Inhibition
• b endorphin
• noradrenaline
• dynorphins
• endomorphin
• adensoine
• 5-HT?
• GABA?

descending inhibition

• Excitation/sensitisation
• prostaglandins
• bradykinin
• H K
• histamine
• purines
• leukotrienes
• growth factors
• substance P
• other neuropeptides

pain signals
Pain signals
Thalamus

• Inhibition
• anandamide
• b endorphin?

Periaqueductal grey matter
Nucleus raphe magnus
Peripheral sensory nerves
From Pain Management in animals P. Flecknell A.
Waterman-Pearson
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Intervention and block
Opioids (epidural subarachnoid) Local
anaesthesia, a2 agonists, NMDA antagonists
Local anaesthesia
Systemic opioids
Pain signals
Opioids
NSAID
Peripheral sensory nerves
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Goals of pain management

• Pre-emptive analgesia will prevent the priming
  effect of nociceptive stimulation on the CNS and
  reduce the amount of pain in the post-op period
• To return the animal to its old self
• Is the animal in pain or not? versus
• Will it benefit from analgesics?
• If in doubt reassess or administer analgesic!

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Pain Relief

• Opioids - problems
• Short duration of action
• Controlled drugs
• Side-effects

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Opioids Side Effects

• Rarely of significance in laboratory animals
• Respiratory depression
• Raised intracranial pressure
• Hypotension
• Constipation

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Opioid Actions

• Receptor type
• agonist/antagonist/partial agonist
• tissue/body system
• Species differences
• Durations of action
• Severity of pain?

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Opioid Receptors, Agonism Antagonism (i)
Opioid receptor family
m
k
d
Mu
Kapa
Delta
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Opioid selectivity
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Local anaesthesiacomplications

• Toxicity
• Accidental injection IV, IA
• Excessive dosage leading to CV, CNS toxicity
• Calculate total dose to lt 6mg/kg lignocaine,
  lt3mg/kg bupivacaine
• Localised tissue reaction, delayed healing
• Short acting in rodents
• Use of adrenaline may prolong action, increase
  toxicity.

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Polymodal (Multidrug) Approach

• Action at different levels
• diminish peripheral nociceptor activation (NSAID)
• block peripheral nerve transmission (e.g. local
  anaest.)
• interfere with nociceptive processing in spinal
  cord (e.g., NMDA receptor antagonists, local
  anaesthetics, opioids, a2 agonists)
• reduce emotional/cognitive component of pain in
  CNS (e.g., opioids, benzoidazepins)

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Different classes of analgesics

• Opioids (e.g. Morphine, Fentanyl, Buprenorphine)
• NSAID (e.g. Carprofen, Meloxicam)
• Local anaesthetics (e.g. Bupivacaine, Lidocaine)
• NMDA-antagonists (e.g. Ketamine)
• Alpha-2 agonists (e.g. Medetomidine, Xylazine)
• Miscellaneous
• Antidepressants (amitryptyline, clomipramine)
• Anticonvulsivants and antyarhytmics

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Pain (re)assessment

• Reassess frequently
• Requirements for analgesics are not static
• There are important inter-individual differences
• Compare before and after analgesics

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Administer NSAID if not given preoperatively
Allow for anaesthetic-sparing effect of
preoperative opioids
Consider use of additional doses of opioids or
other analgesics such as ketamine
Appropriate nursing care
Induction of anaesthesia are induction agents
analgesics?
Administer NSAID?
Anaesthesia and surgery
Postoperative period
Premedication
Or give analgesic dose of opioid separately
Monitor the efficacy of pain relief give
additional analgesic if needed
Preoperative local anaesthetic, nerve block
Additional local anaesthetic block e.g.
intercostal, intra-articular
Adjust timing so opioid is most effective during
surgery
From Pain Management in animals P. Flecknell A.
Waterman-Pearson