Induction of anaesthesia Flashcards
What is induction of anaesthesia?
induction is the transition from an awake state to an anaesthetised state
How can we induce anaesthesia?
usually injectable: most common IV, can also be IM
inhalational
What does the choice of induction method depend on?
species and temperament of animal
underlying medical conditions
nature of procedure
available drugs, equipment and conditions
personal preference and experience
What are the injectable induction agents?
propofol
alfaxolone
ketamine
thiopentone/thiopental
etomidate
What are the inhalational induction agents?
isofluorane
sevoflurane
desflurane
What are the 4 pharmacokinetic characteristics important to know with general anaesthesia agents?
uptake: time to reach adequate blood and brain levels, effect of route administration
distribution into tissues: depends on blood flow, may be influence by dz, lowers blood levels
metabolism: species variable, influenced by dz
elimination
What do we need to consider with general anaesthetic agents?
not only dz increase anaesthetic risk
anaesthesia can impact morbidity
evaluate if ptx has problem that can affect the anaesthetic or if there is a problem the anaesthesia could worsen
What are the actions in the CNS by GA agents?
dose dependent nervous system depression
continuum: sedation becomes anaesthesia
What is the difference between low and high doses of anaesthetic agent?
Low: sedation (reduction of agitation to facilitate a medical procedure)
High: anaesthesia (unaware and unresponsive to painful stimuli - analgesia, unconsciousness and muscle relaxation)
What are the effects of GA agents on the CVS?
CVS depression
drop in CO
vasodilation
reduced BP when monitoring
What are the effects of GA agents on resp syst?
Resp depression
decrease resp rate
decrease tidal volume
reduced minute volume
What are the analgesic effects of GA agents?
some provide little analgesia
administer analgesic for sx procedures
What is nociception?
CNS and pain processing pathways are still stimulated during sx under anaesthesia
What do we need to monitor during induction and why?
critical period w/ rapid changes as large amounts of anaesthetic agent are administered
CVS: HR, pulses, BP
Resp: rate/depth of resp
Depth of anaesthesia: changes in reflexes during induction
What is the difference between dose and concentration?
Dose = mg/kg your ptx will need
Concentration = mg/ml in a solution
What is inhalational induction?
induction of anaesthesia using gas
When should we use inhalational induction?
aggressive ptx when IV/IM injection impossible
What is important to not about inhalational anaesthesia?
increased risk of pollution and personnel exposure
may see excitement/stress during induction
What are the different inhalant induction agents?
isoflurane
sevoflurane
nitrous oxide
halothane
desflurane
What are the factors that affect inhalational agent uptake?
alveolar concentration of inhalant agents (inspired concentration, fgf, breathing syst vol, alveolar ventilation)
drug uptake from lungs (solubility)
What is the blood-gas partition coefficient?
it describes the relative affinity of an anaesthetic for 2 phases
number of parts of gas in blood vs alveolus
What does the partition coefficient tell us about the inhalational agent?
high nb means gas is very soluble in blood
speed of induction and recovery is SLOWER for more soluble agents
Which inhalational agent is quicker?
halothane ++++
isoflurane++
What are the different IV induction agents?
propofol
alfaxolone
ketamine
thiopentene/thiopental
etomidate
fentanyl/midazolam
What is propofol?
IV induction agent
a phenol
can be used for induction and maintenance of anaesthesia (dogs and cats)
must be given IV
How is propofol formulated?
white emulsion: containing lipid (= more soluble)
soya oil, egg, lecithin
either no preservatives = discarded same day
with preservatives= 28 day shelf life
What is benzyl alcohol and what are its effects?
a preservative often used in propofol
can cause prolonged recovery and hyperkinesia in cats
can cause neuro signs like tremors in dogs
What is the danger on using propofol with preservatives?
can cause fatalities
no antidote: supportive tx
dogs: lethal dose = max every h for 9h
cats: lethal dose within 6.5 h of administration
only licensed for induction
What are the pharmacokinetics and properties of propofol?
rapid onset after injection
blood levels decrease by redistribution
rapid metabolism: hepatic and extra-hepatic
CVS depression (vasodilation, lower BP)
resp depression
What should we know about the use of propofol in cats?
cats are deficient in several conjugation pathways
some may have problems metabolising triglycerides due to lipoprotein lipase deficiency
recovery might be slower than dogs
can cause heinz body production and oxidative injury if used daily
Why should we give propofol slowly?
within a minute
particularly if animal deeply sedated
alpha 2 agonist cause bradycardia -> slower circulation -> longer onset time
What is alfaxalone?
IV induction agent
neuro steroid
for induction and maintenance of anaesthesia
What are the pharmacokinetics and properties of alfaxalone?
poorly soluble in water so marketed with a solubilising agent (cyclodextrin)
rapid onset of action, short duration
cardiopulmonary depression is shorter in time to recover than propofol
What is important to not when using alfaxalone as induction agent?
can see excitement on recovery if not adequately sedated
What is ketamine?
IV induction agent
dissociative anaesthetic
induction and maintenance of anaesthesia
also has analgesic actions at sub-anaesthetic doses
How can ketamine have analgesic effects?
at sub-anaesthetic doses
binding NMDA receptors
What is important to note when using ketamine as an induction agent?
excitation and rigidity if given alone
always combined with other sedative agents
What is a common way to use ketamine in cats and dogs?
“triple combo” (aggressive cats and dogs)
- ketamine, medetomidine, opioid
IM
onset of action 3-5 min, short duration procedures
What is a common way to use ketamine in horses?
after profound sedation with alpha 2 agonist
IV in combo with a benzodiazepine (diazepam or midazolam) to counteract muscle rigidity
What are the effects of ketamine as an induction agent on the body systems?
CNS effects: cranial reflexes preserved
CVS: stimulation of sympathetic NS (mild increase in BP and tachycardia, direct myocardial depression)
mild resp depression
analgesia
What are the properties of thiopentene/thiopental?
rapid onset of action
relatively slow hepatic metab
cumulative: accumulates in body fat, repeated injection
slow recovery (hungover effect)
What effects are important to note about thiopentone/thipental as an induction agent?
irritant if administered extravascularly
CV and resp depression
may be seen used in horses as top up
dos for induction is to effect
What are the properties of etomidate?
minimal cv and. resp depression
depresses adrenal gland function (cortisol production)
When/how do we administer etomidate as an induction agent?
good for sick/compromised patients
usually administered with midazolam
unlicensed
When do we use fentanyl/midazolam as an induction agent?
good for sick compromised ptx
opioid and benzodiazepene
How can you reduce the dose of your induction agent?
use premedication
use co-induction technique (no evidence that this will reduce CV side effect)
administer induction agent slowly to effect
What IV induction agent is suggested for normal animals?
propofol
alfaxalone
What IV induction agent is suggested for CV dz (mild)?
propofol or alfaxalone with midazolam
What IV induction agent is suggested for CV dz (severe, like sick sinus syndrome)?
etomidate
