Induction of Anaesthesia Flashcards
Injectable agents used for general anaesthesia
Propofol Alfaxalone Ketamine Thiopentone/Thiopental Etomidate
Inhaled agents used for general anaesthesia
Isoflurane Sevoflurane Halothane Desflurane Nitrous oxide Xenon
Methods of administration for induction of anaesthesia
Usually injectable (95%) Most commonly intravenous Can be intramuscular Can be inhalational
What may choice of induction drug depend on?
Species and temperament of animal Underlying medical conditions Nature of procedure Available drugs, equipment and conditions Personal preference and experience
Describe the continuum of GA agents
Dose dependent CNS depression
Sedation to anaesthesia
Low dose = sedation
High dose = anaesthesia
What are the stages of anaesthesia?
- Conscious patient (premedicated)
- Administration of anaesthetic agent
- Unconsciousness (phase1)
- Signs of excitement (phase 2)
+ Not if quick induction
+ eg. ketamine - Surgical anaesthesia (phase 3)
- Overdose (phase 4)
What side effects will you see with GA eg. CVRS?
+++ CVS depression
Drop in CO
Vasodilation
Reduced blood pressure when monitoring
+++ Respiratory depression
Decreased respiratory rate
Decreased tidal volume
Reduced minute volume
DOSE DEPENDENT (when animal not stable use benzodiazepine as doesn’t have CVRS effect)
Do anaesthetic agents provide good analgesia?
No with the exception of ketamine.
Need to administer analgesic because nociceptors and pain reflexes still stimulated.
Propofol
- Route of administration
- Use
- Appearance
- Speed of onset
- Metabolism
- IV
- Induction and maintenance
- White emulsion containing lipid
- Rapid onset following injection
- Rapid metabolism
+ Hepatic and extrahepatic eg. lungs, gut, muscles
+ Can be used in animals with liver disease
Side effects associated with propofol
Cardiovascular depression
Vasodilation
Decreased blood pressure
Respiratory depression (dyspnoea)
Stiffness/tremors due to imbalance of neurotransmitters
+ May not want to use again
+ Give benzodiazepine to counteract
Why can’t propofol be given IM?
It is so lipid soluble that it would distribute everywhere and wouldn’t have high enough concentration to have required effect.
Describe the use of propofol in cats
Metabolism is slower than in dogs
Deficient in several conjugation pathways
Recovery slower
Cumulative
Daily anaesthesia may cause Heinz body production and oxidative injury (3-5 days)
Describe dosing of propofol
2-4mg/kg premedicated animals
Give very slowly to effect
Alpha2 agonists cause bradycardia = slower circulation = longer onset time
What is PropoFlo Plus?
Current propofol agent used
No pain on injection
Gives dogs and cats smooth, rapid inductions and recoveries
Milky white appearance
Rarely causes local tissue reaction with inadvertent perivascular administration
Licensed for induction
Use within 28 days
Alfaxalone
- Route of administration
- Use
- Speed of onset
- Speed of duration
- Side effects
- Dose
- IV or IM
- Induction and maintenance
- Rapid onset
- Short duration
+ Can see excitement on recovery if not adequately sedated - Less cardiopulmonary depression than propofol
- Dose 2-5mg/kg IV
Ketamine
- Route of administration
- Use
- Actions
- IM or IV (only licensed for IM in dogs)
- Induction and maintenance
- Analgesic actions at subanaesthetic doses
What happens if you give ketamine alone?
Excitation and rigidity.
Always combined with sedative agents.
Describe the ‘Triple combination’ of ketamine induction
Used in aggressive cats or dogs Ketamine, medetomidine, opioid IM (3-5min onset) Single combined injection Short duration Dose 3-5mg/kg IM
How is ketamine used in horses?
After profound sedation with alpha2 agonist
Dose in combination with benzodiazepine to counteract muscle rigidity
Side effects of ketamine when inducing
CNS = cranial reflexes preserved
+ Makes intubation more difficult
CVS stimulation of sympathetic
+ Mild increase in BP and tachycardia
+ Direct myocardial depression
Mild respiratory depression
Analgesia
Thiopentone/Thiopental
- Licensed
- Onset of action
- Metabolism
- Cumulative?
- Recovery time
- Preparation
- Side effects
- No
- Rapid onset
- Slow metabolism
- Accumulated in body fat with repeated injections
- Slow recovery
- Powder (have to reconstitute with water)
- CV and respiratory depression similar to propofol
What is a problem with the use of thiopentone for induction?
Irritant if administered extravascularly (very alkaline).
Causes necrosis of skin and subcutaneous tissue.
Describe the use of Etomidate
Minimal CV and respiratory depression
Good for sick/compromised patients
Depresses adrenal gland function (cortisol production)
Unlicensed
Describe the use of fentanyl/midazolam combination for induction..
Opioid and benzodiazepine
Good for sick/compromised patients eg. GDV
Muscle relaxation and analgesia
