premedication Flashcards
What is premedication?
the provision of sedation & anxiolysis before induction of anaesthesia
What are the benefits of premedication?
Reduction of dose of induction agent
Reduction of dose of maintenance agent
Provide pre-emptive analgesia & preventive analgesia
Ensure a smooth recovery
Prevent side effects associated with other anaesthetic drugs
Premedication/sedation can start at home (anxious/fearful animals)
What is pre-emptive analgesia?
the administration of analgesic drugs prior to the onset of noxious stimulation which has the potential to be more efficacious
What is the function of pre-emptive analgesia?
Reduced immediate post-op pain
Lowers the intensity and duration of subsequent pain
makes pain easier to control because it helps prevent the nervous system from becoming overly sensitive to pain (reduces peripheral and central sensitisation to pain)
What is multimodal analgesia?
combining different analgesics acting by different mechanisms or at different sites in the nervous system
What is the purpose of multimodal analgesia?
Additive and improved analgesia
minimises overall side effects by reducing dose of each drug
What is preventive analgesia?
combines multimodal and pre-emptive concepts but continues into the early post-operative period
What species is acepromazine (a phenothiazine) used as a premed for?
dogs
cats
horses
What is neuroleptanalgesia?
a state of sedation and analgesia induced by combining neuroleptic and opioid analgesics
What are the common drugs used to produce neuroleptanalgesia?
Phenothiazine agents
a2-adrenoceptor agonists
opioid analgesics
How does acepromazine provide sedation?
interferes with dopamine transmission
What are the contraindications of acepromazine?
Acepromazine causes a reduction in oxygen delivery to tissues via a reduction in haemoglobin concentration, caused by splenic sequestration of red blood cells therefore, it is not advised to administer acepromazine to patients with anaemia.
Why must acepromazine be used in conjunction with other drugs?
does not have antinociceptive or analgesic properties
What are the routes of administration of acepromazine?
SC
IM
IV
oral
What are the adverse effects of acepromazine?
reduced BP
reduced oxygen delivery to tissues
Give examples of alpha 2 agonists used as premedication in small animals
Medetomidine
dexmedetomidine (preferred)
Give examples of alpha 2 agonists used as premedication in horses
Xylazine
detomidine
romifidine
Give examples of alpha 2 agonists used as premedication in cattle
Xylazine
detomidine
How do alpha 2 agonists cause sedation?
Inhibit adenylyl cyclase => reduces levels of cyclic adenosine monophosphate => hyperpolarisation of noradrenergic neurons
As cyclic adenosine monophosphate is inhibited, potassium efflux through calcium-activated channels prevents Ca ions from entering the nerve terminal => suppressed neural firing => inhibit norepinephrine release => reduces activity of ascending noradrenergic pathways => hypnosis and sedation
How do alpha 2 agonists cause analgesia?
Stimulation of a-2 receptors in the dorsal horn of the spinal column => inhibits nociceptive neurons and reduces the release of substance P => analgesia
What are the side effects of alpha 2 agonists?
profound cardiovascular effects and reduced cardiac output
Increased urine output
miosis (pupil constriction), decreased intraocular pressure and decreased ciliary body perfusion
vomiting
What drug is the antagonist of alpha 2 agonists?
atipamezole
Give examples of benzodiazepines used as premeds
Midazolam
diazepam
zolazepam
alprazolam
What are the effects of benzodiazepines?
anxiolytic (reduces anxiety)
Muscle relaxation
Cardiovascular system stability
No analgesia
What animals should benzodiazepines be used in?
ASA 3-5
Not healthy animals (causes excitement)
What is the antagonist of benzodiazepines?
flumazenil
What drugs should benzodiazepines be combined with for premedication?
opioids
or ketamine
or alfaxalone
What is the drawback of using alfaxalone in large animals?
Large volume given IM can be painful
What is the function of opioids in premedication?
provide pre-emptive (and preventive) analgesia
synergism with sedatives
Describe the sedative effects of opioids
ASA 1+2 - mild sedation when given alone
ASA 3-5 - more profound
Give examples of different opioids
Butorphanol, buprenorphine (partial agonists - lower analgesic effect)
Methadone, fentanyl, morphine, pethidine (pure agonists)
Describe the regulation of opioids
Schedule 2 controlled drugs
What species can morphine be used in?
horse
cat
dog
What is the route of administration of morphine?
IM
SC
Slow IV
What is the duration of effect of morphine?
dog - 4hrs
cat - 6-8hrs
What receptors does morphine act on?
Full MOR agonist
What are the side effects/considerations of morphine?
Vomiting
Histamine release
What are the target species of methadone?
horse
cat
dog
What are the routes of administration of methadone?
IM
SC
Slow IV
What is the duration of effect of methadone?
~4hrs
What type of receptors does methadone act on?
Full MOR agonist
NMDA antagonist
NRI (noradrenaline uptake inhibitor)
What are the considerations of methadone?
high licensed doses
What are the target species for fentanyl?
dog
cat
What are the routes of administration of fentanyl?
IV
IM
What is the duration of effect of fentanyl?
0.3hrs
What type of receptor does fentanyl act on?
Full MOR agonist
What are the side effects of fentanyl?
Decreased HR
Decreased RR
What are the target species of butorphanol?
horse
dog
cat
What are the routes of administration of butorphanol?
IM
IV
SC
What is the duration of effect of butorphanol?
0.75-1hrs
What receptors does butorphanol act on?
KOR agonist
MOR antagonist
What are the relative effects of butorphanol?
limited analgesia
Good sedation
What are the target species of buprenorphine?
horse
cat
dog
What are the routes of administration of buprenorphine?
IM
IV
SC
What is the duration of effect of buprenorphine?
6-8hrs
Slow onset
What types of receptors does buprenorphine act on?
Partial MOR agonist
KOR antagonist
Where are opioid receptors found?
Throughout the CNS in somatic and visceral sensory neurons (throughout the brain and dorsal horn of the spinal cord) and in the periphery (especially after inflammation)
What types of opioid receptors are there?
MOR - mu opioid receptors
KOR - kappa opioid receptors
beta-receptors
What is the effect of activation of MORs?
produces supraspinal and spinal analgesia
euphoria
sedation
miosis (e.g. dogs)
mydriasis (e.g. cats)
respiratory depression
decreased gastrointestinal motility due to inhibition of acetylcholine (ACh).
What is the effect of KOR activation?
spinal and supraspinal analgesia
mild sedation
dysphoria
diuresis
miosis
What is the effect of beta opioid receptor activation?
spinal and supraspinal analgesia
cardiovascular depression.
Give examples of other drugs possibly given before anaesthesia
anticholinergics
antihistamines
H2 receptor antagonists + gastroprotectants (brachy?)
Anti-nausea/vomiting medication and NK1 antagonism
Lidocaine
Steroids or NSAIDs
Antibiotics
What premedication should be given to a ASA1 animal?
Acepromazine or alpha 2 agonist + opioid
Describe premedication of ASA 2 animals
Low dose ACP/alpha 2 and opioid usually tolerated
Care with ACP in liver disease
Care with alpha 2 agonists in CVS disease - ACP may be a better option
Alpha 2 agonists may be of use in HCM - hypertrophic cardiomyopathy (low dose)
Describe pre-medication of ASA 3-5 animals
Avoid the alpha 2 agonists in compromised animals
V low doses of alpha 2 agonist may be used if animal very distressed
Opioid alone may be sufficient to sedate patient (but patients are often quiet/compromised) e.g. methadone or butorphanol
Fentanyl and midazolam (IV) useful
Ketamine and midazolam (IM or IV) cats (not in HCM)
Alfaxalone and opioid is an option (volume of injectate is disadvantage), can add BZD too if necessary
Describe the premedication of aggressive patients
Alpha 2 agonists alone (even high doses) often insufficient to sedate these animals
Alpha 2 + ketamine + opioid (+/- acepromazine) combination IM
What premedication can be given at home for aggressive patients?
‘Chill’ protocol (Gabapentin + trazodone or melatonin oral day before and morning of consult, and oral (injectable ACP) or oral transmucosal alpha 2 on arrival)
Alprazolam or diazepam
