Premedicant drugs Flashcards by Vicky Main

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

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Premed considerations

Correct dose tailored to animal

Quiet environment

Enough time for drug to take effect

Route of administration

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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)

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Acetylpromazine

ACP

Phenothiazine

Commonly used pre-medicant usually in combo with opiods

Injectable and tablet formulations

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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

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Acetylpromazine sensitive/resistant breeds

Sensitive
Boxers, Giant breeds

More resistant
Cats, toy breeds

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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

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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

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Acetylpromazine - Use with care in

Hypo-volaemic or dehydrated patients

Large breeds = small doses

epileptics

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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

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Name two alpha-2 agonists

Medetomidine e.g.. Domitor

Dexmedetomidine e.g.. Dexdomitor

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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

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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.

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Alpha 2 Agonist Advantages

Duration and level of sedation dose dependent

Profound drug sparing effect

Provides muscle relaxation

Provides analgesia

Complete reversal possible

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Alpha 2 Agonist Disadvantages

Profound effects on CVS

Causes vasoconstriction -&raquo_space; pale mm.s

Slows heart rate , may&raquo_space;> reduced CO

Respiratory depression, may cause apnoeic pauses

Reduces liver blood flow

May cause vomiting

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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

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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 >> 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

1. Facilitate controlled IPPV – eg. for patients undergoing thoracic surgery 2. During intraocular surgery ( avoids changes in eye position) 3. Where good muscle relaxation is required to facilitate the surgery eg. ortho ops, laparotomies 4. 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