Induction of anaesthetics Flashcards
What are the 2 main methods of administering GAs? Examples of each?
INJECTABLE (usually, IV mainly, or IM): propofol, alfaxalone, ketamine (thiopentone/thiopental, etomidate)
INHALATIONAL: isoflurane, sevoflurance (halothane, desflurane, N2O, xenon)
What affects choice of anaesthetic agent?
- species, temperament
- underlying medical conditions
- nature of procedure
- available drugs, equipment and conditions
- personal preference and experience
Outline basic anaesthesia versus sedation
Dose dependent CNS depression
Continuum: sedation –> anaesthesia
Low dose of sedative = sedation
High dose of sedative = anaesthesia
What are the stages of induction?
Conscious patient –> premeds/sedation –> anaesthetic administration –> (excitement and inadequate anaesthesia) –> deep sedation –> unconsciouness
What are the CVS effects of GAs? 3
OVERALL = CVS DEPRESSION
Drop in CO
Vasodilation
Reduced BP
Respiratory effects of GAs?
OVERALL = RESPIRATORY DEPRESSION
decreased RR
Decreased tidal volume
Reduced MV
Outline pharmacokinetics of GAs - 4
UPTAKE - time to reach adequate blood and brain levels, varies with ROA
DISTRIBUTION INTO TISSUES: depends on BF, disease influence, lowers blood levels
METABOLISM: species and disease influences
ELIMINATION
Define ROA
Route of administration
What is propofol classified as?
a Phenol
Administration - propofol
IV (essential). Give to animal very slowly particularly if v sedated.
Use - propofol
Induction and maintenance of anaesthesia
Appearance - propofol
White emulsion - contains lipid (preparations are now available that contain a lipid so shelf life extended to 28 days)
Pharmacokinetics - propofol
Rapid onset after injection, blood levels decrease by redistribution,
METABOLISM - rapid, hepatic and extra-hepatic (lungs)
EFFECTS: CVS and respiratory depression
Things to bear in mind when using it in cats?
Cats lack the enzymes to conjugate glucoronides (slow propofol metabolis) also problems metabolising triglycerides (metabolism of lipid carrier) –> slower recovery (vs. dogs), also cumulative (increases toxicity risk).
REPEATED USE: oxidative injury and Heinz Body production (–> anaemia and death)
What are Heinz Bodies (HBs)?
Inclusions in RBCs of denatured Hb.
What is Propoclear?
Propofol in a nano-droplet micro emulsion (clear, colourless). 28 shelf life (preservatives). Rapidly withdrawn due to pain on administration.
What is PropofolPlus?
a UK propofol. No pain on injection. Dogs and cats have smooth, rapid inductions and recoveries. 28 day shelf life, Familiar milky white appearance. Rarely causes local tissue reaction with inadvertant perivascular administration.
What benzyl alcohol?
a preservative. Toxicity may lead to prolonged recovery and hyperkinesia in cats and neurological signs in dogs and fatalities in both species.
Alfaxalone:
classification
administration
CLASSIFICATION: steroid, poorly soluble in water so marketed with a solubilising agent (cyclodextrin)
ADMINISTRTION: IV or IM
USE: induction and maintenance of anaesthesia
EFFECTS: rapid onset, short duration of action, less CVRS depression than propfol
KETAMINE type actions administration use
TYPE: dissociative anaesthetic
ACTIONS: analgesic actions at sub-anaesthetic doses (NMDA-Rs), excitation if given alone
ADMINISTRATION: IM or IV
USE: induction and maintenance
What are common ways to use ketamine?
TRIPLE COMBINATION - aggressive cats, ketamine, medetomidine and opioid, IM (onset of action 5 mins), single injection alone suitable for short procedures
HORSES: after profound sedation with alpha2 agonist
Ketamine - effects
CNS - reflexes preserved
CVS - stimulation of SNS (mild hypertension and tachycardia, some direct CV depression)
RESP - some mild depression
ANALGESIA
Thiopentone/thiopental USE ACTION METABOLISM RECOVERY FORM
Used to be a very common induction agent - no longer licensed. Rapid onset of action. Relatively slow metabolism. Cumulative (fat, esp repeated injections), slow recovery, reconstitute with water as it is powdered form .
Effects - Thiopentone/thiopental
Irritant - if administered extravascularly (alkaline)
CV and Resp depression - similar to propofol
Occasionally used in horses to top up during maintenance
Effects etomidate
Minimal CV and Resp. depression
Good for sick/compromised patients
Depresses adrenal gland function (BAD)
unlicensed
What is a fentanyl/midazolam combination? Use?
Opioid + benzodiazepine
For sick/compromised patients
Why use inhalational anaesthesia?
aggressive patients where IV injection impossible
Risks of inhalational anaesthesia?
Increased risk of pollution and personnel exposure
Excitement/stress during induction
Isoflurane - more irritant to respiratory passages (–> breath holding during induction means sevoflurane is preferred)
What causes inhalational agent uptake?
‘Pressure gradient’ fromt he vaporiser to the brain (vaporiser - anaesthetic system - alveoli - blood - brain).
What does brain concentration of an inhalational anaesthetic approximate?
Alveolar concentration (high partial pressure in lungs = high partial pressure in brain. The speed of induction (and recovery) is slower for more SOLUBLE agents since the blood hangs on to the drug more).
What is the blood: gas partition coefficient?
Number of parts of gas in blood versus alveolus. High value means that the gas is very soluble in blood.
Which anaesthetics have the highest partition coefficient?
Halothane > isoflurane > sevoflurance > nitrous oxide > desflurane
What monitoring is needed during induction?
CVS
RespS.
Depth of anaesthesia
Rapid changes in these systems.
What happens after induction of anaesthesia? Why? 4
Endotracheal intubation generally recommended: deliver oxygen deliver inhalational anaesthetic (if required) protect airways (occlusion or aspiration) allow ventilation to be supported if depressed
