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Principles of anaesthesia > Pain pathophysiology, recognition, and management > Flashcards

Pain pathophysiology, recognition, and management Flashcards

1
Q

Pain definition

A

An unpleasant sensory and emotional experience associated with, or resembling that associated with actual, or potential, tissue damage

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2
Q

Definition of nociception

A

Neural processes of encoding and processing noxious stimuli

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3
Q

Pain physiology

A

Nociceptors (specialised nerve endings)

High activation threshold

Skin, muscles, joints, viscera, meninges

Noxious mechanical, thermal, electrical, chemical stimuli -> nociceptor activation -> action potential -> dorsal horn spinal cord

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4
Q

Types of fibres involved in pain transmission

A

As(sigma) fibres

C fibres

AB(beta) fibres

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5
Q

As(sigma) fibres

A

Medium diameter, lightly myelinated, rapid conduction

Well localised, sharp pain

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6
Q

C fibres

A

Small diameter, unmyelinated, slow conduction

Poorly localised, slow, dull pain, burning sensation

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7
Q

AB(beta) fibres

A

Large diameter, thick myelination, very rapid conduction

Non-noxious stimuli, touch, pressure, proprioception, noxious stimuli (neuropathic pain)

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8
Q

Modulation

A

Spinal cord recieves and processes somatosensory info

Synapsis with 2nd order neurons

Release of excitatory or inhibitory neurotransmitters to activate 2nd order neuron

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9
Q

Excitatory neurotransmitters

A

Glutamate
Substance P
Nerve growth factor

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10
Q

Inhibitory neurotransmitters

A

Gamma-amino-butyric acid (GABA)
Enkephalin
Glycine
Serotonin
Dopamine
Opioids

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11
Q

Projection

A

2nd order neurons from dorsal horn of spinal cord project info to:

Thalamus -> somatosensory cortex, frontal motor cortex

Lymbic system -> emotions

Reticular formation + hypothalamus, pons -> autonomic responses, sympathetic nervous system activation, catecholamines

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12
Q

Pain modulation

A

Either inhibitory or facilitatory

Peripheral nociceptors
Spinal cord
Supraspinal structures

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13
Q

Descending inhibitory pathways

A

Inhibition of interneurons (stimulated by 1st order neurons)

Secretion of inhibitory neurotransmitters (opioids, noradrenaline, GABA, serotonin, dopamine

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14
Q

Acute pain

A

Results of traumatic, surgical, or infectious events

Begins abruptly

Resolves in days/weeks

Self limiting

Serves a biological purpose

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15
Q

Chronic pain

A

Persists beyond normal time of healing/pain caused by conditions where healing has not occurred

> 1-3mo in duration

A disease, no biological purpose, usually involves changes in CNS

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16
Q

Somatic pain

A

Superficial - pain associated with skin

Deep - associatedd with muscles, joints, tendons, bones

Well localised, aching, sharp, intense

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17
Q

Visceral pain

A

Pain associated with visceral organs

Stretching capsule, distension, contration, ischaemia, inflammation

Dull, diffused, poorly defined

Often associated with feelings of nauseau, vomiting, change in autonomic system

Referred pain

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18
Q

Inflammatory pain

A

Associated with tissue injury, immune cells activation

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19
Q

Cancer pain

A

It has characteristics of both inflammatory and neuropathic pain

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20
Q

Neuropathic pain

A

Caused by injury of the nervous sytem (peripheral nerves, spinal cord, or CNS)

Increased activation of peripheral nociceptors

Increased CNS neurons excitability

Peripheral or central sensitisation

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21
Q

Peripheral sensitisation

A

Activation, sensitisation, change of nociceptors caused by tissue injury/inflammation

Reduction in activation threshold, increase in responsiveness of nociceptors, amplified response with recruitment of other nociceptive fibres

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22
Q

Central sensitisation

A

Increase efficiency in pain signal transmission, even after nociceptors have stopped signalling

Change in membrane excitability, decreased inhibition, increase secretion of excitatory neurotransmitters, increase responsiveness

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23
Q

Allodynia

A

Pain sensation in response to an innocuous stimulus

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24
Q

Hyperalgesia

A

Exaggerated pain sensation in response to a normally painful stimulus

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25
Wind up
Spontaneous activity and temporal summation of sub-threshold stimuli -> increased response
26
Consequences of pain
Sympathetic nervous system activation (tachycardia, arrhythmias, increased respiratory rate, increased blood pressure) Stress response Immunodepression Delayed wound healing Inflammation Deterioration of QOL Decrease appetitie/anorexia
27
Challenges of pain assessment
Absence of verbal communication Aggressive/stoic patients Individual normal behaviour Behaviour during hospitalisation vs with owner Species variations Subjectivity Fear/anxiety Sedation Emergency delirium Dysphoria/euphoria Nausea/vomiting Distended bladder
28
Physiological parameters used to measure pain
Heart rate Arterial BP Resp rate Plassma cortisol, catecholamine levels, endorphines
29
How often should you do pain assessments?
During recovery phase: ideally q15min then hourly for first few hours, then q4 Before next analgesic medications is due In between dosages If additional analgesia has been administered - reassess after 15mins
30
Facial pain signs
Orbiatal tightening Ear position Nose bulge Cheeks (Whiskers)
31
Pain assessment in rodents
Difficult Teeth grinding, changes in posture, locomotion, or gait Decreased activity levels and behaviour display Food and water consumption and weight changes Faecal output Knowing animals normal behaviours/owners involvement
32
Pain assessment in reptiles
Difficult Facial signs? Vocalisation? Owner, history, subtle changes Consider temperature Anorexia, weight loss Immobility, abnormal posture/locomotion Dull colouration Aggression
33
Pain assessment in birds
Difficult Change in behaviour/appearance - drooping - fluffed up - eyes closed - poor appearance/feather quality/hunched - lameness, decreased weight-bearing, one legged standing - difficulty perching/climbing/falling - inappetance
34
Pre-emptive analgesia
To prevent central sensitisation To limit subsequent pain experience Reason to administer analgesic drugs in premed
35
Options for analgesia
Opioids Alpha2 agonists NSAIDs Local anaesthetics Ketamine Gabapentin Amantadine
36
Opioids
Reduce pre-synaptic neurotransmitter release Hyperpolarise post-synaptic membrane Activates descending inhibitory pathways Inhibits ascending nociceptive input Located in midbrain, spinal cord, periphery etc. Most effective analgesic: used to treat mild to severe pain Distribution of opioid receptors and response to treatment is species dependent
37
Side effects of opioids
Cardiovascular - decrease HR Respiratory depression GI effects
38
Alpha 2 agonists
Sedation and analgesia - decreased sympathetic discharge (decreased noradrenaline) - modulation nociceptive transmission - inhibit neurotransmitter release from nociceptive neurons - alteration in transmission ascending nociceptive signals - descending inhibitory pathways Synergistic with opioids, ketamine and local anaesthetics Reversible (atipam)
39
Side effects of alpha 2 agonists
Vasocontriction Bradycardia Hypotension Vomiting Hyperglycaemia In ruminants: bronchoconstriction, increase in pulmonary vascular resistences, oedema
40
NSAIDs
Ant-inflammatory, analgesic, antipyretic action Effective on acute and chronic pain Standard perioperative use for inflammatory pain and acute pain unless contraindicated Wide availability, long duration of action, low cost, easy to administer
41
Examples of NSAIDs
Meloxicam Carprofen Robenacoxib Phenylbutazone Flunixin Meglumine Grapiprant (galliprat) - not fully understood but used for osteoarthritis in dogs
42
Side effects of NSAIDs
Vomiting Diarrhoea Renal injury Hepatic injury
43
Paracetamol
Acetaminophen Poor anti-inflammatory properties Exact MoA unknown - prostaglandin inhibition - COX-3 inhibition - Serotinergic pathway activation - Endocannabinoids enhancement DO NOT USE IN CATS
44
Ketamine
NMDA receptor antagonist Anaesthetic Analgesic Anti-inflammatory Local anaesthetic properties Interaction with opioid receptors Good for acute and chronic pain Bolus or CRI Peri- and post-operatively
45
Side effects of ketamine
Dysphoria Muscle rigidity SNS stimulation + negative inotropic effect - increase in HR and BP, care if they have heart conditions Apneustic breathing
46
Local anaesthetics
Na+ channel blockers Block transmission of nociceptive inputs to the spinal cord Gold standard Decrease other analgesic drug needs Low cost, but needs practice Narrow therapeutic index, work below toxic doses to avoid side effects on CNS and CV system Lidocaine and bupivacaine are most used
47
Lidocaine vs bupivacaine
Lidocaine: fast onset, shorter duration Bupivacaine: slower onset, longer duration
48
Lidocaine
Can be administered IV in dogs Bolus/CRI Cats high toxicity level so not commonly used Antiarrythmic (class 1B) Anti-inflammatory (sepsis) Visceral, somatic analgesia Inhibitory descending pathways
49
Side effects of lidocaine
Nausea Vomiting CNS depression Seizures CV depression Arrhythmias
50
Monoclonal antibodies anti-nerve growth factor
Frunevetmab - solensia (cats), bedinvetmab - librela (dogs) Feline/canine monoclonal Ab (mAb) targets nerve growth factor (NGF) which inhibits NGF mediated cell signalling to reduce pain Monthly SC injection - easy but high cost Only licenced for pain associated with osteoarthritis
51
Cannabidiol oil (CBD oil)
Found in cannabis and hemp Does not contain THC Interacts with endocannabinoid receptors Not allowed to promote or advertise in UK Different formulations Reduces acute/chronic pain, anxiety, inflammation More research needed
52
Gabapentin
Anticonvulsant drug Used to treat chronic neuropathic pain Blockage of calcium channels - decreased calcium influx - decreased release of excitatory neurotransmitters Use in association with NSAIDs, opioids Sedation (helpful with stressed cats before vet visits)
53
Amantadine
Antagonist of N-methyl-D-aspartate (NMDA) receptors: blockage of pain transmission Decreases central sensitisation: long onset of action (at least 20min) Use in association with other drugs (opioids, gaba, NSAIDs)
54
Side effects of amatadine
Lethargy Restlessness GI upset Seizures (rare)
55
Non-pharmacological pain treatment
Acupuncture Laser therapy Transcutaneous electrical nerve stimulation (TENS) Massage Exercise Hot/cold therapy May help to decrease amount of analgesic drugs administered, esp in older/sicker patients
56
Acupuncture
Insertion of small, flexible needles in specific areas of the body MoA not fully understood - endorphine release - wound healing - immunomodulation - modulation of descending inhibitory pathways - gate control theory
57
Laser therapy
Light Amplification by Stimulated Emmision of Radiation Low intensity light therapy: application of near infra-red laser light on tissues Photochemical effect not thermak Light triggers biochemical changes within cells: increases ATP production - endorphin release - vasodilation - decreased inflammation - faster wound healing and tissue repair Needs to be high frequency to penetrate skin so not really good for that?
58
Transcutaneous electrical nerve stimulation (TENS)
Low voltage electric currents to treat acute pain and inflammation Electrode pads connected with a machine Passes electrical currents across intact surface of skin to activate underlying nerves Gate control theory (inhibitory signal to pain gate) Endorphines released Adjunct treatment
59
Ice therapy
Decrease in transmission of painful stimuli from periphery and spinal cord Decreases inflammation Decreases tissue metabolism and O2 demand Acute/inflammatory pain
60
Hot therapy
Muscle spasm relief Stiffness of joints (arthritis) Improves blood circulation - helps with tissue healing Chronic pain
61
Analgesic/anaesthetic protocol for spay of healthy 1yo bouncy vizla
Pre-med - opioid (methadone) - alpha 2 (medetomidine) - ACP Induction - propofol Maintenance - iso in O2 Locoregional - Transversus abdominus plane (TAP) block / splash blodk with bupivacaine Other - NSAIDs - ketamine bolus - fentanyl bolus
62
Analgesic/anaesthetic protocol for castration of a 6mo cat
Pre-med - opioids (methadone/buprenorphine) - Alpha 2 (medetomidine) Induction - alfaxalone Maintenance - iso in O2 Locoregional - intratesticular nerve block lidocaine Other - NSAIDs

Principles of anaesthesia (9 decks)

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