Induction of anaethesia Flashcards
Describe the pharmacokinetics of propofol: absorption, solubility, distribution, metabolism, elimination
Absorption: Given IV as not effective if given orally (extensively metabolised in gut + liver)
Solubility: lipophilic
Distribution: 98% protein bound (distributes rapidly to brain => fast induction
Metabolism: metabolised by glucuronidation in liver
Elimination: renal
Describe the mechanism of action of propofol and alfaxalone
GABA beta subunit:
- enhances GABA-A receptor activity
- opens Cl- channel => Cl- influx => hyperpolarisation
=> sedation, hypnosis and anaesthesia
Describe the induction of anaesthesia via propofol
Rapid onset of action
rapid uptake by CNS
5-8 mins unconsciousness (induction, not maintenance) - good for ‘top ups’ or TIVA
What is the route of administration of propofol?
slow IV - cats and dogs
What chemical is propofol?
Milky alkyl phenol
Describe the pharmacodynamics of propofol: CNS, Respiratory and Cardiovascular effects
CNS Effects:
Anaesthesia only (no analgesia)
↓ CMRO₂ (brain O₂ consumption)
Respiratory Effects:
Respiratory depression
Cardiovascular Effects:
Depressed cardiovascular reflexes
Sympathetic depression causes:
- Stable cardiac output
- ↓ Heart rate (blunted baroreceptor reflex)
- ↓ MAP, SVR, CVP
Describe the pharmacokinetics of alfaxalone: absorption, solubility, distribution, metabolism, elimination
Absorption - good systemic absorption when given IV or IM
Solubility - soluble in water => improves IM absorption and stability
Distribution - 30-50% protein bound
Metabolism - hepatic (Rapid), lungs and kidneys
Elimination - renal, and small % bile
Describe the pharmacodynamics of alfaxalone
Anaesthesia (no analgesia)
respiratory and cardiovascular depression (less than propofol)
decreased CMRO2 (brain O2 consumption)
Haemodynamic effects minimal
- Stable cardiac output
- Stable heart rate
- Stable MAP, SVR, CVP
Describe the recovery from alfaxalone
Rapid and smooth if premedicated
Should not be disturbed during recovery as excitement can occur
What type of drug is alfaxalone?
Clear, colourless neuroactive steroid
What is the route of administration of alfaxalone?
slow IV (or IM, not licensed) - good for ‘top ups’ or TIVA
For dogs, cats, rabbits
Describe the use of alfaxolone and propofol in unhealthy animals
Dose is reduced in unhealthy animals (ASA II-V)
Describe how alfaxolone/propofol is used
Inject either drug slowly to effect over 60 seconds
Animal will become sedated and then unconscious
In most cases the next step will be securing the airway
Always feel for a pulse following induction
Oxygen can be delivered before and during induction for compromised animals
In the event of apnoea, intubate and ventilate the animal
Occasional rigidity and twitching is observed post and during induction
What is ketamine licensed for?
cats
dogs
ruminants
rodents
rabbits
primates
horses
Describe the use of ketamine on its own
Dissociative anaesthesia - increased muscle tone, salivation and animals that did not really appear to be unconscious
Good analgesia
Combined with other drugs
What is the route of administration of ketamine?
IV or IM (can sting)
Describe the pharmacodynamics of ketamine
Dissociative anaesthesia
increased CMRO2
Haemodynamic effects minimal:
-Stable cardiac output
- Increased heart rate
- Increased MAP, SVR, CVP
Describe the pharmacokinetics of ketamine
Absorption, solubility, distribution, metabolism, excretion
Absorption - good bioavailability => well absorbed via multiple routes
Solubility - soluble in water
Distribution - 12% protein bound => large proportion remains free and active => rapid CNS penetration
Metabolism - liver converts to norketamine (still active, less potent)
Elimination - renal, and small % bile
Describe the mechanism of action of ketamine
Primary Mechanism:
- Non-competitive NMDA receptor antagonist.
- Blocks NMDA receptor Ca²⁺ channel pores.
- Reduces presynaptic glutamate release.
Other Mechanisms:
Opioid receptor interaction:
- Weak affinity for mu and kappa receptors.
- Naloxone (opioid antagonist) does not antagonize ketamine’s analgesic effects
Antagonistic interactions:
- Monoaminergic, muscarinic, and nicotinic receptors.
- Produces anticholinergic effects (e.g., tachycardia, bronchodilation).
Local anaesthetic properties:
- Inhibits neuronal sodium channels at high doses.
What is drawback of ketamine?
Rocky recovery
What is a volatile agent?
Inhalation agents e.g., isoflurane, sevoflurane
What is the use of volatile agents?
Maintenance (can be used for induction)
What is the drawback of using volatile agents as an induction agent?
stress
What is the mechanism of action of volatile agents
potentiation of GABA receptors
Inhibition at NMDA receptors
What are some less commonly used induction agents?
‘Zoletil’ (zolazepam & tiletamine)
MS-222
Thiopentone
‘Ketofol’ (ketamine + propofol)
Etomidate
Describe the pharmacokinetics of volatile agents?
Absorbed and eliminated through the lungs
Influenced by alveolar partial pressure (PA)
Speed of induction is inversely proportional to the CO due to negative effect of high CO on PA
Which of the following has analgesic properties?
Ketamine
Respiratory depression and apnoea is a common side effect when inducing anaesthesia with propofol. What can you do to avoid this?
Inject slowly
You are restraining a 25kg cross breed dog for induction of anaesthesia. How can you assess the patient whilst your colleague is injecting the induction agent?
Palpate a peripheral pulse, observe the respiratory rate and effort, keep an eye on mucous membrane colour.
You are inducing anaesthesia in a 3-year-old female French bulldog for an ovariohysterectomy. The owner reports occasional regurgitation. What can you do to reduce the risk of aspiration during induction?
Keep the head up and inflate the cuff on the endotracheal tube
You have been brought a tom cat from a farm in a trap. He is feral, very fearful and is hissing at you. You need to anaesthetise him for neutering. How are you going to induce anaesthesia?
Give methadone, ketamine and medetomidine by intramuscular injection
You have been asked to induce anaesthesia in a 450kg gelding for joint arthroscopy with ketamine and midazolam. He has been given acepromazine, romifidine and morphine. He is not quite as sedated as you expected and is lifting his head and responding to noises outside the knockdown box. What do you want to do
Wait a bit longer
Give more romifidine
You have just intubated a cat with an appropriately sized endotracheal tube following induction of anaesthesia with intravenous propofol. What can you do to help you confirm correct placement of the tube
Watch the chest rise whilst giving a breath
Listen to the lung fields whilst simultaneously giving a breath
Look for an end tidal carbon dioxide (EtCO2) reading and trace on the capnograph
Look for condensation forming in the tube
You have just induced anaesthesia in a horse and are about to place an endotracheal tube. How can this be done?
A mouth gag is used to prevent damage and occlusion of the tube
An endoscope can be used in the case of difficult intubation
You are preparing to anaesthetise a 645kg horse with 2.2mg/kg ketamine and 60mcg/kg of midazolam. Ketamine comes as 100mg/ml solution and midazolam as a 5mg/ml solution. What volume of each drug will you draw up?
14.2mls ketamine and 7.7mls midazolam
A 2-year-old 1.2kg dwarf rabbit has been admitted for ovariohysterectomy. You plan to place an intravenous cannula before induction of anaesthesia. Where might you place this
Marginal ear vein
Cephalic vein
What is the purpose of pre-oxygenation
To increase the time to desaturation at induction
To replace the nitrogen in the alveoli with oxygen
To increase the fraction of inspired oxygen (FiO2)
You are about to place a jugular catheter in a colicking horse prior to induction of anaesthesia. In which direction do you place the catheter and can you explain this?
Pointing down the neck
Catheters pointing down the neck are suitable for longer term fluid administration and cause less turbulence. You should make sure they are well secured though as disconnection can result in an air embolus with catastrophic consequences.
You have pre-medicated an adult pot-bellied pig using azaperone, butorphanol and ketamine. You now want to induce anaesthesia to perform castration and trim his feet. Where do you place the cannula?
Auricular vein
You have been asked to induce anaesthesia in a guinea pig. What are some suitable options?
Subcutaneous ketamine and medetomidine
Inhalation anaesthesia with sevoflurane via a mask
Inhalation anaesthesia with sevoflurane in an induction chamber
You are inducing anaesthesia with sevoflurane in oxygen via a mask in a rat. The rat is sick, and you suspect it has low cardiac output. How will this affect induction?
Induction will be quicker
You’ve decided to use a mixture of propofol and ketamine (Ketofol) at a ratio of 1:1 to induce anaesthesia. Propofol comes as a 10mg/ml solution and ketamine as a 100mg/ml solution. The dog weighs 22.5kgs. You want to draw up 4mg/kg propofol. How much propofol do you draw up and how much ketamine do you add to this.
9mls propofol, 0.9ml ketamine
