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L4: Injectable anaesthesia - general
Factors effecting, advantages and disadvantages, drugs
26
Veterinary Medicine
Undergraduate 3
05/23/2012

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Term
When may injectable anaes. be given?
Definition
-anaes. induction
-adjuncts to maintaining anaes
-maintain anaes. w/o volatiles
Term
What factors affect
-how rapidly effective plasma conc. are achieved
-what percentage of drug given is available to produce an effect
-duration of action?
Definition
-dose
-route of admin
-volume of distribution
-protein binding
-ionisation
-metabolism and elimination
Term
how does dose affect anaesthesia?
Definition
a higher dose will prodce higher plasma conc. and for longer
Term
Describe and compare IV, IM and SC anaes. admin
Definition
-IV has rapid rise in plasma conc. w. highest plasma peak conc. and longest duration of effective dose in plasma
-IM has slower rise and peak in plasma conc. and shorter duration of effective conc
-SC rise in plasma conc is even slower and may not reach effective plasma conc
Term
Use of IV, IM and SC anaes admin
Definition
IV-most common
IM-esp. difficult aniamls e.g feral cats
SC-rare, used occasionaly in exotics
Term
Why do some inj. drugs produce lower plasma conc. than expected?
Definition
due to uptake and distrubution in other tissues
Term
What does volume of distribution represent?
Definition
the vol of water in which drug would have to be distributed to give its plasma conc
-gives comparision of drugs distribution
Term
what is the relevance of plasma protein binding in injectable anaes. admin?
Definition
if a drug is bound to plasma proteins it cannot exert its effect at a receptor site
-so if plasma protein is dec. e.g. hypoalbuminaemia, there will be a larger free-fraction of drug and so incr. effect of drug - relative OD
-if multiple drugs that bind in the same way are co-admin there may be competition for binding sites - again relative OD
Term
Which drugs of anaesthetic importance are highly plasma protein bound?
Definition
-barbiturates
-propofol
-NSAIDs
Term
What is the relevance of ionisation in injectable anaes. admin?
Definition
Factors affecting ionisation of drugs may affect efficacy
-most drugs are ionised and exist in plasma in ionised and unionised forms
-unionised forms cross memb. more easily
e.g. inflammation decr. pH and so decr. unionised forms of LA - can't reach target organ
Term
What can affect metabolism and elimination of anaesthetics?
Definition
-dz. of liver / affecting BF to the liver will alter metabolism and so duration of many drugs
-drugs can affect other drugs metabolism
e.g. induce enzyme p450 and incr. metabolism of other drugs
Term
Advantages of injectable anaesthesia
Definition
-convenient
-rapid loss of consciousness (IV)
-rapid deepening of anaesthesia possible
-no airway irritation
-no pollution of environment
-some drugs have antagonists
-less equipment required - can perform "in field"
Term
Disadvantages of injectable anaesthesia
Definition
-requires restraint - stressful for animals
-pain of injection in some sites
-not easy to lighten anaesthesia or dec. effects
-rely on organ function for elimination in most cases
-may get drug accumulation
-no airway protection
-risk of self-admin
Term
Which anaesthetic agents can be administered by injection?
Definition
-thiopentane
-propofol
-alphaxalone
-ketamine
Term
Thiopentane summary
Definition
-injectable GA
-rapid onset of action (depending on circulation time) - big advantage
-causes peripheral vasodil. and so dec BP
-myocard depression not marked at normal doses
-arrhythmias common in induction but usually innocuous and not observed
-not analgesic
-slow metabolism - recovery dependent on redistribution
-irritant if perivascular
-decr. intracranial and intraocular P
-effective anticonvulsant
Term
Describe the metabolism and distribution of thiopentane - consequences?
Definition
-slow metabolism
-recovery depends of reedistribution of drug to muscle and fat rather than excretion
-cumulation of drug w. repeated doses (as muscle / fat saturated) - can suddenly OD - do not give repeatedly
-recovry slow in sighthounds due to distrubution
Term
Describe the use of thipentane
Definition
-licensed in D+C and H
-now rarely used for induction
- still popular in horses due to rapid onset of action - if horse needs to be brought under control quickly
Term
Propofol summary
Definition
-injectable GA
-rapid onset but not as rapid as thio
-rapid redistribution and metabolism
-not cumulative
-not irritant perivascular but can occ. cause pain on inj
-dec. intracranial P
-gr. myocardial depression than thio and peripheral vasodil w. decr. BP
-occasional muscl. twitching or rigidity
-can cause post-induction apnoea - less if administered slowly and to effect
Term
Describe propofol preparation
Definition
-emulsion prep (i.e. white)
-some contain preservatives - vial can be kept for up to 28 days once opened
-use non-preservative containing emulsion if giving by long term infusion - to avoid accumulation or toxicity of the preservative (benzyl chloride)
Term
Which spp is propofol licenced for?
Definition
Dog and cat
Term
Alphaxalone summary
Definition
-injectable anaesthetic
-mild CV effects, mild hypotension, mild resp. compression c.f. others
-can also be used to maintain anaesthesia
-expensive
Term
Alphaxalone admin
Definition
-should be given slowly and to effect - may require less than data sheet recommended doses
-generally IV (IM requires large volume)
Term
Ketamine
Definition
-dissociative anaesthetic (can also be used for sedation)
-induction slower after IV than thio
-little cumulation - can be given by infusion or repeat bolus inj
-little resp. depression
-mild INCR. in HR and BP due to sight sympathetic stimulatiomn of CV system
-good analgesia
-increases muscle tone
-incr. salivation
-convulsive activity poss. in dogs, esp. is used alone
-INCR. intracranial and intraocular P
Term
What is unusual about ketamine anaesthesia?
Definition
although unconscious and unaware, may retain reflexes - swallowing, blinking
Term
When is ketamine used?
Definition
-most popular in horses / LA, also dogs and exotics, less in dogs
-rarely alone except some reptiles and birds
Term
Propofol warnings
Definition
-repeated inj. in cats can result in Heinz body anaemia - not <10days between uses
-avoid repeated dosing of form w. benzyl chorlide (preservative) - can cause toxicity
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