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
2
Q
Examples of opioids
A
- methadone
- fentanyl
- pethidine
- morphine
- buprenorphine
- butorphanol
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
4
Q
Is morphine licenced in veterinary species?
A
- no
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
6
Q
Which 4 receptor types do opioids act on?
A
- mu
- kappa
- delta
- nociceptin
7
Q
Opioid reversal/antagonist
A
- naloxone
8
Q
Which opioids have the best analgesic effects?
A
- full mu agonists
– methadone
– fentanyl
– pethidine
– morphine
9
Q
Which opioids are moderate analgesics?
A
- partial mu agonists
– buprenorphine
10
Q
Which opioids provide good sedation but relatively poor and short-lived analgesia?
A
- butorphanol
– mu antagonist and kappa agonist
11
Q
Which opioid is good for birds and why?
A
- butorphanol
- birds have a high proportion of kappa receptors
12
Q
Examples of alpha-2 agonists
A
- medetomidine
- dexmedetomidine
- xylazine
- detomidine
- romifidine
13
Q
Which alpha-2 agonists are most commonly used in SA practice?
A
- medetomidine
- dexmedetomidine
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
15
Q
Is xylazine licenced in horses and cattle?
A
- yes
16
Q
Is Detomidine licenced in horses and cattle?
A
- yes
17
Q
Which species is romifidine licenced in?
A
- horses/equids only
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
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
Cons of phenothiazines
- 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
Where do phenothiazines act?
- centrally acting by antagonising D1&D2 receptors, also acts on alpha-1 muscarinic and H1 receptors
27
Examples of benzodiazepines
- diazepam
- midazolam
28
Which benzodiazepine is licenced in horses?
- midazolam
29
Which benzodiazepine in licenced in dogs and cats?
- diazepam
30
Why can the available formation of diazepam only be given IV?
- cause pain IM and are poorly absorbed
31
How can midazolam be administered?
- IM
- IV
32
How do benzodiazepines act?
- centrally acting via the GABA-A receptor
33
Benefits of benzodiazepines
- 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
Cons of benzodiazepines
- 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
Benzodiazepine reversal/antagonist
- 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
Example of NMDA receptor antagonist
- ketamine
- methadone and pethidine are opioids which also demonstrate some NMDA receptor antagonism
37
Benefits of ketamine
- 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
Cons of ketamine
- 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
Examples of anti-cholinergics
- atropine
- glycopyrrolate
40
Benefits of anti-cholinergics
- 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
Cons of anti-cholinergics
- not routinely used in premed and do have unwanted effects
-- mydriasis (pupil dilation), reduced gut motility, bronchodilation etc
42
What is aflaxalone?
- a neuroactive steroid
43
When is alfaxalone used for pre-med?
- 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
Cons of alfaxalone
- mild vasodilation when given IV, but this is unlikely to occur IM
- apnoea has been reported when given IV
45
Benefits of alfaxalone
- few adverse effects on the CV system
- cardiac output and tissue perfusion are maintained with its use
- apnoea is not reported when administered IM
