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Anaesthesia & analgesia > Types of premedication > Flashcards

Types of premedication Flashcards

1
Q

What different drug classes are used for premed?

A
  • opioids
  • alpha-2 agonists
  • phenothiazines
  • benzodiazepines
  • NMDA receptor antagonists
  • anticholinergics
  • alfaxalone
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2
Q

Examples of opioids

A
  • methadone
  • fentanyl
  • pethidine
  • morphine
  • buprenorphine
  • butorphanol
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3
Q

Benefits of using opioids

A
  • provides sedation and analgesia
  • generally cause minimal cardiovascular depression
  • often used in very sick pts
  • respiratory depression is usually minimal at clinical doses
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4
Q

Is morphine licenced in veterinary species?

A
  • no
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5
Q

Cons of using opioids

A
  • some vagally mediated bradycardia can be seen at high doses
  • respiratory depression can be seen with fentanyl or high doses of methadone given IV
    – if give these peri-operatively can cause apnoea
  • they reduce GI motility and decrease gastric emptying
  • morphine can induce emesis
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6
Q

Which 4 receptor types do opioids act on?

A
  • mu
  • kappa
  • delta
  • nociceptin
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7
Q

Opioid reversal/antagonist

A
  • naloxone
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8
Q

Which opioids have the best analgesic effects?

A
  • full mu agonists
    – methadone
    – fentanyl
    – pethidine
    – morphine
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9
Q

Which opioids are moderate analgesics?

A
  • partial mu agonists
    – buprenorphine
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10
Q

Which opioids provide good sedation but relatively poor and short-lived analgesia?

A
  • butorphanol
    – mu antagonist and kappa agonist
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11
Q

Which opioid is good for birds and why?

A
  • butorphanol
  • birds have a high proportion of kappa receptors
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12
Q

Examples of alpha-2 agonists

A
  • medetomidine
  • dexmedetomidine
  • xylazine
  • detomidine
  • romifidine
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13
Q

Which alpha-2 agonists are most commonly used in SA practice?

A
  • medetomidine
  • dexmedetomidine
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14
Q

Why is xylazine less commonly used in SA practice?

A
  • poor affinity for the alpha-2 receptor (cf. medetomidine & dexmedetomidine)
  • therefore significantly more side effects
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15
Q

Is xylazine licenced in horses and cattle?

A
  • yes
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16
Q

Is Detomidine licenced in horses and cattle?

A
  • yes
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17
Q

Which species is romifidine licenced in?

A
  • horses/equids only
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18
Q

Benefits of alpha-2 agonists

A
  • profound dose dependent sedation
    – therefore can be useful in very lively or aggressive animals at higher doses
  • marked drug sparing effects
    – reduce MAC, therefore reduce amount of inhalation required and amount of induction agent
  • good analgesics
  • cause minimal respiratory depression
  • good muscle relaxant
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19
Q

Cons of alpha-2 agonists

A
  • increase the amount of time it takes for injectable anaesthetics to travel to the CNS so care re injecting too quickly if already given alpha-2 agonists
  • reduce blood flow to the liver and reduce hepatic metabolism of other agents
  • analgesia is relatively short lived
  • be cautious of the degree of sedation caused in brachy breeds as there’s potential for URT obstruction
  • significant cardiovascular effects
    – associated with marked bradycardia / hypotension
    – not used in sick pts or those with CV dz
  • can cause emesis
    – particularly in cats if used alone
  • reduce endogenous insulin production
    – therefore result in transient hyperglycaemia
  • urine production is increased
    – as the result of reduced renin and vasopressor secretion
  • analgesic effect shorter than sedative
20
Q

What are the cardiovascular effects of alpha-2 agonists

A
  • biphasic
  • initially peripheral vasoconstriction results in an increase in bp
  • in response there’s a reflex bradycardia and reduction in cardiac output
  • after ~15-20 mins bp and hr return to normal
21
Q

Alpha-2 agonist reversal/antagonist

A
  • atipamezole
22
Q

What is the only phenothiazine licenced for veterinary use?

A
  • acepromazine (ACP)
23
Q

What can ACP be combined with as part of a premed?

A
  • opioid +/- alpha-2 agonist
24
Q

Benefits of phenothiazines

A
  • contributes to sedation/tranquilisation
  • provides anxiolysis
  • lasts ~6-8h in healthy animals
  • respiratory depression is minimal in healthy animals
  • considered anti-arrhythmic due to effects of alpha-1 receptor antagonism in the heart, cell membrane stabilisation and sympathetic tone reduction, but the clinical effects of this are unknown
25
Q

Cons of phenothiazines

A
  • no analgesic properties
  • slow to taking effect
    – ~30-40mins when administered IM
  • vasodilation caused by alpha-1 antagonism can result in hypotension and hypothermia
  • cannot be reversed/antagonised
  • effects on vasodilation can be hard to manage
  • not suitable for pts with sepsis, hypovolaemia, CV compromise
  • reports of syncope in boxers
  • giant breeds are considered more susceptible to the CV effects of ACP
  • duration of action may be significantly prolonged in some sight hounds (24h)
  • dogs with MDR1 mutations may be more sensitive
  • priapism reported in horses
    – but risk of permanent penile dysfunction is low
  • oral absorption can be erratic and unpredictable, hence IM or IV preferable
  • rapid distribution and will cross BBB and placenta
26
Q

Where do phenothiazines act?

A
  • centrally acting by antagonising D1&D2 receptors, also acts on alpha-1 muscarinic and H1 receptors
27
Q

Examples of benzodiazepines

A
  • diazepam
  • midazolam
28
Q

Which benzodiazepine is licenced in horses?

A
  • midazolam
29
Q

Which benzodiazepine in licenced in dogs and cats?

A
  • diazepam
30
Q

Why can the available formation of diazepam only be given IV?

A
  • cause pain IM and are poorly absorbed
31
Q

How can midazolam be administered?

A
  • IM
  • IV
32
Q

How do benzodiazepines act?

A
  • centrally acting via the GABA-A receptor
33
Q

Benefits of benzodiazepines

A
  • anxiolytic
  • sedative
  • causes hyponosis
  • minimal CV and respiratory depression
    – therefore potentially good for sick/compromised pts e.g. in combination with an opioid or ketamine
  • reduce the amount of induction and inhalation agent
  • in sick pts midzolam can be used as a co-induction agent rather than as a part of premed
  • contribute to muscle relaxation
  • cause amnesia in people so may do this for our spp
  • anticonvulsant
  • high lipid solubility aiding oral and IM absorption
  • widely distributed in body with rapid central activity
34
Q

Cons of benzodiazepines

A
  • not analgesic
  • degree of sedation in healthy animals is often poor and can result in disinhibition, esp if given IV
    – these pts may become aggressive
    – therefore, not given normally to a healthy pt as part of premed
35
Q

Benzodiazepine reversal/antagonist

A
  • flumazenil
    – competes for the receptor without producing effects
    – but is short-lived and doses may need to be repeated
    – expensive and probably isn’t available in most 1st opinion practices
36
Q

Example of NMDA receptor antagonist

A
  • ketamine
  • methadone and pethidine are opioids which also demonstrate some NMDA receptor antagonism
37
Q

Benefits of ketamine

A
  • at high doses ket is a dissociative anaesthetic and appears to interrupt association pathways to the brain and results in sensory blockade
    – Depresses the thalamoocortical system and activates the limbic system
  • At lower doses results in sedation and at even lower doses can be used e,g, in an infusion for analgesia
  • profoundly reduces the amount of induction agent and inhalation anaesthetics
  • good cardiovascular stability and is stimulatory or a sympatheticomimmetic i.e. stimulation is by noradrenaline release
  • it causes a transient increase in cardiac output and blood pressure
  • extreme lipid solubility ensures its rapid transfer across BBB
38
Q

Cons of ketamine

A
  • in some sick patients administration has caused a reduction in blood pressure probably due to myocardial depression in the presence of altered sympathetic tone (?catecholamine depletion).
  • therefore in theory some animal might not be able to cope with the stimulation produced by higher (i.e. anaesthetic doses)
  • results in variable dose dependent respiratory depression, although this is probably not of huge concern at subanaesthetic doses
  • relatively short acting (30-45 minutes) due to high hepatic clearance
  • when used alone it’s a poor muscle relaxant
  • drying to the corneas
  • ome patients who have received higher doses can be excitable/ very reactive to light/ noise/movement on recovery, but this is usually manageable +/- a sedative
39
Q

Examples of anti-cholinergics

A
  • atropine
  • glycopyrrolate
40
Q

Benefits of anti-cholinergics

A
  • reduce bradycardia and are sometimes used intraoperatively in response to alpha-2s
  • reduce the amount of respiratory tract secretions and are therefore sometimes added into a premed in circumstances where this is a potential concern e.g. brachy animal undergoing respiratory surgery
41
Q

Cons of anti-cholinergics

A
  • not routinely used in premed and do have unwanted effects
    – mydriasis (pupil dilation), reduced gut motility, bronchodilation etc
42
Q

What is aflaxalone?

A
  • a neuroactive steroid
43
Q

When is alfaxalone used for pre-med?

A
  • can be used to sedate sick pts, e.g. in combination with an opioid when ket is not a good option
  • occasionally given IM to pts where a robust premed hasn’t provided sufficient sedation to place an IV cannula
44
Q

Cons of alfaxalone

A
  • mild vasodilation when given IV, but this is unlikely to occur IM
  • apnoea has been reported when given IV
45
Q

Benefits of alfaxalone

A
  • few adverse effects on the CV system
  • cardiac output and tissue perfusion are maintained with its use
  • apnoea is not reported when administered IM

Anaesthesia & analgesia (14 decks)

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