Induction Flashcards
Which inhalational agents can be used to induce anaesthesia?
- isoflurane
- sevoflurane
- halothane
- desflurane (not licensed)
- N20 (nitrous oxide)
- xenon (new, expensive, specialised equipment needed)
Which injectable agents can be used to induce anaesthesia?
- propofol
- alfaxalone
- ketamine
> thiopentone/thiopental (old, can’t get vet form anymore)
> etomidate (not licensed, human drug)
What is the most common route of induction?
IV
What do GA agents cause?
Dose dependant CNS depression
- continuum from sedation -> anaesthesia
eg. use of propofol as a sedative (michael jackson!)
How is CV and respiratory system affected by anaesthetic agents?
> CV - v CO - vasodilation - reduced BP - bad for kidneys etc. > resp - v RR - v TV (tidal volume) = reduced minute volume *dose-dependant*
Do GA drugs provide analgesia?
No - so must administer analgesics eg. opioids
What does uptake of GA depend on?
- route of administration
What does distribution into tissues depend on?
- blood flow (eg. injecting into sublumbar fat)
- may be influenced by disease
- lowers blood levels if injected into muscle etc.
Which species are particularly sensitive to a2 agonists?
Goats
What is metabolism of drugs influenced by?
- Species
- Disease state (esp. liver and kidneys, lungs for inhalation agents)
> thiopental in a thin dog will maintain for a long time due to lack of body fat
How does propofol’s action differ in cats?
take a long time to recover
- enzyme lacking to conjugate glucuronides
- problems metabolising triglyceride lipid carrier of drug
> cumulative effects of the drug, can be TOXIC if given repeatedly
> Oxidative injury, Heinz body production, anaemia and death
Where does the induction agent ultimately act?
In the brain
How does age affect amount of induction agent required?
- extremes of age require less
Should doses of GA drugs be learnt?
NO NOT FOR EXAMS!!! Know where to find them
WHat type of drug is propofol? What is it’s appearance and ROA?
- a phenol
- white opaque liquid (rich medium for bacterial growth)
- must be given IV (not IM)
What can propofol be used for?
Induction AND maintainence eg. if cannot use inhalation agents (TIVA)
Pharmacodynamics of propofol
- rapid onset after injection
- blood levels v by redisribution
- rapid metabolism (hepatic and extra-hepatic eg. lungs)
Effects of propofol
- CV depression (vasodilation and v BP)
- respiratory depression
> so give SLOWLY over one minute
What is propclear?
Nanodroplet clear solution - no longer available
- caused extreme pain on injection
What is propofol plus?
Preservative (benzyl alcohol) means can be used once opened (doesnt have to be discarded on the day)
> benzyl alcohol toxicity ->
- prolonged recovery and hyperkinesia in cats
- tremors in dogs
* DO NOT USE FOR TIVA*
What type of drug is alfaxalone? What s it mixed with in preparations?
steroid - poorly soluable
- mixed with solubilising agent (cyclodextrin)
ROA alfaxolone?
IV or IM
How does alfaxolone differ to propofol?
- rapid onset of action, short duration
- less CV and respiratory depression than propofol
Potential side effects and cons of alfaxolone?
More expensive
Can see excitement on recovery if not adequately sedated
What type of drug is ketamine?
- Dissociative anaesthetic
- Analgesic actions at sub-anaesthetic doses (NMDA receptors)
ROA ketamine
IM or IV
What can ketamine be used for?
Inuction and maintainence of anaesthesia
Why is ketamine NEVER GIVEN ALONE?
Excitation and muscle tension if given alone, always requires a sedative (benzodiazepine or a2 ag)
- also need to lubricate eyes as will remain open
What situations is ketamine useful for?
- angry cats
- brachycephalic breeds
What is the triple combination? Which species is this commonly used in?
- Aggressive cats > Ketamine, medetomidine, opioid - IM - onset of action ~5 mins - single injection alone suitable for short procedures
Which species if ketamine commonly used in following sedation with a n a2 agonist?
Horses after PROFOUND sedation with an a2 ag
- given IV combination with a benzodiazepine (diazepam or midazolam)
How does ketamine affect the CNS, CB and resp systems?
> CNS reflexes are preserved (palpebral, jaw tone, swallowing)
CV: SNS stimulated -> hypertension and tachycardia mildly, but some direct CV depression also
resp: mild resp depression
Where is thiopental usually used nowadays?
Horses
- not licensed for vet use anymore
- used for induction (to effect) and as top up during maintainence
Effects and potential side effects of thiopental?
- irritant and necrosis if administered extravascualrly (alkaline)
- CV and resp depression similar to propofol
Can thiopental be used once vial is opened?
Yes for ~5d (pH so high nothing can grow)
What drug or drug combination is best for high risk, sick patients? POtential problem with this drug?
*Etomidate
- minimal CV and resp depression
- problem: depresses adrenal gland function and cortisol production
> NOT LICENSED
* Fentanyl/midazolam combination
- very potent opioid
Which inhalational agents are used for induction? Why would these be used over injectables?
- sevoflurane or ioflurance delivered in oxygen
> aggressive patients if IV impossible
What are the downsides to inhalational indcution?
- ^ risk pollution and personnel exposure
- may see excitement/stress during induction
Which inhalational agent is more irritating to respiratory passages and what effect may this have?
Isoflurane
-> breath holding
(but is cheaper)
How can the smell of isoflurane be disguised to prevent mask avoidance?
Smear food inside mask
What concentration roughly approximates brain concentration of anaesthetic drug?
ALveolar concentration
What does partial pressure mean?
Amount in blood
If an agent is very soluble in blood how is the partial pressure or tension of gas in the brain affected?
Lower in brain (doesnt ant to leave blood!)
What is the partition coefficient?
Blood:Gas partition coefficient = no. particles gas in blood vs. alveolus
- high number means gas is very soluble in blood (and therefore induction and recovery will take longer, as moving agent from blood to brain and back takes long time)
- eg. halothane 2.4 [long induction and recovery], N2O and desflurane ~0.4 [short induction and recovery]
Why is endotracheal intubation advocated?
- o2 delivery
- inhalational anaesthetic if required
- protect airway from occlusion or aspiration
- allow ventilation to be supported if depressed
