Premedicant drugs Flashcards
Premedicant Role
Sedation and analgesia
Calm patient
Smooth Induction
Drug Sparing (reducing dose of induction and maintenance agent required)
May help reduce adverse effects of anaesthetic/surgical procedure
Premed considerations
Correct dose tailored to animal
Quiet environment
Enough time for drug to take effect
Route of administration
Phenothiazines
Provide tranquillisation, with sedation at higher doses
They do not provide any analgesia
Often used synergistically with opiods
Dopamine receptor antagonists - located in CNS
e.g acetylpromazine (ACP)
Acetylpromazine
ACP
Phenothiazine
Commonly used pre-medicant usually in combo with opiods
Injectable and tablet formulations
Acetylpromazine Advantages
Tranquillise/sedation - increasing dose may lead to
longer duration of sedation rather then deepening
Long duration of action – up to 6hrs
Smooth recovery
Relatively safe for CVS – ‘protective’ effect against
arrhythmias (at standard doses)
Relatively safe for resp system – at standard doses
Anti- emetic (binds same receptors as metoclopramide)
Smooth muscle relaxation – gut, urethra
Mild anti-histamine effect
Acetylpromazine sensitive/resistant breeds
Sensitive
Boxers, Giant breeds
More resistant
Cats, toy breeds
Acetylpromazine side effects
Causes vasodilation and will lead to a HYPOTENSION –
care with dehydrated/hypovolaemic patients
May cause HYPOTHERMIA - vasodilation and effects
on hypothalamus
Decreased seizure threshold
Acetylpromazine Disadvantages
Slow to peak effect 30mins after i/m inj
Long duration of action up to 6 hrs depending on
dose given
No analgesia
Unpredictable effects
Acetylpromazine - Use with care in
Hypo-volaemic or dehydrated patients
Large breeds = small doses
epileptics
What are alpha-2s?
Drugs that bind alpha 2 receptors in the brain and
spinal cord
Block the transmission of nociceptive impulses
Provide sedation and analgesia
Name two alpha-2 agonists
Medetomidine e.g.. Domitor
Dexmedetomidine e.g.. Dexdomitor
What are alpha 2’s used for?
Sedation - +/- with opioid, reasonably predictable
sedation
Premedication - if used as pre-med then can achieve
profound drug sparing effect i.e. lower doses of other
agents required
Anaesthesia – in combo with ketamine, esp cats
Analgesia with Alpha 2 Agonist
They can be used as part of a multi-modal analgesic
approach
If you reverse them then the analgesic effect is lost
too.
Alpha 2 Agonist Advantages
Duration and level of sedation dose dependent
Profound drug sparing effect
Provides muscle relaxation
Provides analgesia
Complete reversal possible
Alpha 2 Agonist Disadvantages
Profound effects on CVS
Causes vasoconstriction -»_space; pale mm.s
Slows heart rate , may»_space;> reduced CO
Respiratory depression, may cause apnoeic pauses
Reduces liver blood flow
May cause vomiting
Alpha 2 agonist - patients you should avoid/ use with extreme caution in
Patients with cardiovascular disease
Patients with liver disease
Patients where vomiting would be risky
Alpha 2 agonist - suitable patients
Fit & healthy animals
Alpha 2 Reversal
Complete reversal of effects is possible
Drug; atipamezole eg. Antisedan
Reversal of sedation also = removal of analgesia
Opioids
They are ANALGESICS
Act by combing with specific receptors in the brain
and spinal cord
Opioid receptors may also develop at sites of
inflammation
They decrease the transmission of noxious stimuli i.e.
pain across these receptors
Full opioid agonists
Methadone, Morphine, Pethidine, Fentanyl
‘stronger’ produce a maximal effect – bind all
receptors
Can be topped up to increase analgesic effect
For moderate to severe pain
Partial opioid agonist
e.g. buprenorphine
Cant induce a maximal response, not topped up
Used for mild to moderate pain, premedicant in
combo with phenothiazine or alpha2s
Opioid antagonist
Work against the analgesic effects of the agonists
I.e. can be used as an antidote
e.g. naloxone
How do opioids affect the patient?
Provides analgesia and euphoria, sedation
Little effect on respiration at correct dose (caution
with fentanyl iv)
Little effect on CVS – but can cause bradycardia
Drug sparing effect
Pain relief provided may improve ventilation in cases
of thoracic wall injury
Opioid disadvantages
High doses lead to profound respiratory depression
Contraindicated in animals with increased ICP
Vomiting may occur due to morphine
Constipation
Antitussive effect
Urinary retention if given spinally
Antimuscarninics
THEIR USE IS NOT ROUTINE AND GENERALLY
RESTRICTED TO PARTICULAR CASES
They antagonise the parasympathetic nervous system
Prevent bradycardia
Decrease salivation
Antimuscarninic drugs
ATROPINE
GLYCOPYRROLATE
Antimuscarninic Advantages
Reduces salivation and bronchial secretion
Counteracts bradycardia where there is vagal
stimulation likely eg. ocular or laryngeal surgery
Counteracts bradycardia in patients with high vagal
tone eg. brachycephalics
Helps prevent ‘vagal events’ during reversal of muscle relaxants
Antimuscarninic Disdvantages
Cause increased HR and metabolic rate
May cause arrhythmias
Thickens bronchial secretions
Gastrointestinal ileus (counteract PNS)
Pupil dilation»_space; visual disturbance
When are Antimuscarninics used?
Eye surgery
Laryngeal surgery
Brachycephalics
When reversing muscle relaxants
NMBA’s
Muscle relaxants = neuromuscular blocking agents
What do muscle relaxants do?
Interrupt transmission of impulses from motor nerve to muscle synapse
Act at nicotinic receptors to block release of acetylcholine
May be ;
DEPOLARISING
NON-DEPOLARISING
Depolarising muscle relaxant
E.g. Succinylcholine, Suxamethonium
Cause initial surge at NMJ followed by refractory
period where muscle does not respond
Uncommon in veterinary medicine
(rapid onset and short acting, intubation in humans)
Non-depolarising muscle relaxant
E.g. Vecuronium – lasts 20 mins
Atracurium – lasts 45 mins
They block the receptors at the muscle end plates (
there is no initial surge)
Used in vet practice ( but ?frequency)
Can be reversed
Muscle relaxant facts
They are given by slow i/v and work within a few
minutes
They last 10-45 mins and may be topped up if longer
duration of relaxation required
They DO NOT AFFECT CONSCIOUSNESS as they
do not cross the blood brain barrier
They paralyse only skeletal muscle
THIS MEANS THE PATIENT IS UNABLE TO MOVE
OR RESPOND TO INADEQUATE ANAESTHESIA.
If you use them you must provide
adequate general anaesthesia and
be able to provide IPPV
Muscle relaxant order of function loss
Facial/neck muscles >
tail/limb/abdo muscles>
intercostal/diaphragmatic muscles
Circuits for IPPV
Bain
Ayres T-piece
Circle (ADE )
Less suitable
Lack
Magill
Muscle relaxant disadvantages
Hypothermia due to decreased muscle tone
Difficult to assess anaesthetic depth
Some drugs in this group may cause hypotension
Increased effects if used with aminoglycosides eg.
streptomycin, gentamycin
Increased susceptibility if used alongside halothane,
isoflurane, corticosteroids, some diuretics and
epinephrine
Why would we use muscle relaxant
- Facilitate controlled IPPV – eg. for patients
undergoing thoracic surgery - During intraocular surgery ( avoids changes in
eye position) - Where good muscle relaxation is required to
facilitate the surgery eg. ortho ops, laparotomies - As part of balanced anaesthesia technique for
high risk patients
Reversal of NMBA’s
Non-depolarising NMBAs can be reversed
This may make recovery period safer
Drugs used – neostigmine or edrophonium
NMBA reversal side effects
Reversal may lead to bradycardia & excess
salivation
therefore reversal agents are often given with
atropine or glycopyrrolate (anticholinergic drugs) to
counteract this
