Induction of anaesthesia Flashcards
How can anaesthetic drugs be delivered?
IM
IV
SC
Inhaled
Across mucous membranes
Describe Propofol
Rapid onset of action
- rapid uptake by CNS
- 5-8 mins unconsciousness
Mainly Alkyl phenol
Respiratory & cardiovascular depression
Rapid & smooth recovery
Suitable for top ups or TIVA
Only given IV
Licensed for dogs & cats
No analgesia
Works at GABA
What is the key consideration when using propofol in cats?
Cats are sensitive to phenols, & repeated injections of propofol can cause Heinz body anaemia, especially when administered day after day
What are the pharmacokinetics of propofol?
Absorption: Minimal oral bioavailability
Solubility: Minimally water-soluble
Distribution: 98% protein-bound
Metabolism: Liver (via glucuronidation)
Elimination: Renal
What is mechanism of action of propofol at cellular level?
- Binds to GABA-A beta subunit
- Enhances inward Cl⁻ current
- Causes hyperpolarisation of postsynaptic membrane
- Inhibits neuronal depolarization, producing anaesthesia
Describe alfaxalone
Rapid onset of action
- rapid uptake by CNS
Respiratory & cardiovascular depression
Rapid & smooth recovery if premedicated
Neuroactive steroid
Suitable for top ups & TIVA & can be given IM for sedation
No analgesia
Works at GABA
Licensed for dogs, cats & rabbits
What are the key considerations during & after alfaxalone use?
Apnoea may occur, requiring IPPV (intermittent positive pressure ventilation)
Animals shouldn’t be disturbed during recovery, as excitement can occur if aroused
What are the pharmacokinetics of alfaxalone
Absorption: Good bioavailability
Solubility: Water-soluble
Distribution: 30–50% protein-bound
Metabolism: Rapid, occurs in liver & partially in lungs & kidneys
Elimination: Primarily renal, with a small percentage via bile
How do you use propofol & alfaxalone?
- Calculate dose (reduce for premedicated or unhealthy animals (ASA II-V))
- Draw up slightly more than needed
- Administer slowly to effect over 60 seconds
- Observe sedation followed by unconsciousness
- After induction:
- Secure the airway.
- Check for a pulse.
- Provide oxygen if needed
- In event of apnoea, intubate & ventilate animal
*Occasional rigidity & twitching is observed post induction
Describe Ketamine
Dissociative anaesthesia & analgesia
IV or IM (can sting)
Licensed for cats, dogs, ruminants, rodents, rabbits, primates, horses
Poor muscle relaxation & salivation thus combined with other drugs (BZD, a-2)
Schedule 2 drug
Component of feline triple or quad IM protocols
Mechanism of action primarily due to antagonism at NMDA receptor
High therapeutic index
What is the main mechanism of action of ketamine?
Non-competitive NMDA receptor antagonist: Blocks Ca channel pore, providing analgesia & anaesthesia
Reduces presynaptic glutamate release, affecting both CNS & spinal cord
Other mechanisms:
- Interacts with opioid receptors
- Antagonizes monoaminergic, muscarinic & nicotinic receptors, causing effects like tachycardia & bronchodilation
What are the pharmacokinetics of ketamine
Absorption: Good bioavailability
Solubility: Soluble in water
Distribution: 12% protein bound
Metabolism: Liver (to norketamine)
Elimination: Renal (major) & small percentage via bile
Describe volatile agents for induction.
Occasionally used for induction but main use in maintenance
Mask/induction chamber used
Often used in birds, rodents & very compromised animals
e.g Isoflurane
Deliver via precision vaporiser, in oxygen, slowly increase % until animal unconscious
Mechanism of action is GABA & 2PK
What are the subclassifications of mechanism of action of volatile agents?
Macroscopic: Brain & spinal cord
Microscopic: Synapses & axons
Molecular: Pre- & post-synaptic membranes
How are volatile agents absorbed, distributed & eliminated?
Absorption & elimination occur primarily through lungs
Influenced by factors affecting alveolar partial pressure (PA):
- Higher cardiac output (CO): Slows induction (inversely proportional to CO)
- Increased blood solubility: Reduces PA, slowing induction.
What is MS-222
Anaesthetic drug widely used in fish
Induces anaesthesia by blocking Na channels in neuronal membranes, thus reducing action potentials & inducing muscle relaxation
known for crossing blood-brain barrier blocking AP generation in sensory & motor systems as well as in central nervous circuits
Ketamine
- analgesic effects of ketamine mediated primarily via NMDA receptors & partially via opioid receptors
Respiratory depression and apnoea is a common side effect when inducing anaesthesia with propofol. What can you do to avoid this?
Inject slowly to effect
Heart rate is likely to be higher with alfaxalone than with propofol on induction
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?
Ketamine
Alfaxalone
Tiletamine & zolazepam
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 (opioid), ketamine and medetomidine (a2 agonist) by intramuscular injection
Give more romifidine
- important to ensure horses are adequately sedated before inducing anaesthesia with IV ketamine as this may otherwise result in initial excitatory phase
Wait a bit longer
Look for an end tidal carbon dioxide (EtCO2) reading and trace on the capnograph
Watch the chest rise whilst giving a breath
Listen to the lung fields whilst simultaneously giving a breath
Look for condensation forming in the tube
An endoscope can be used in the case of difficult intubation
A mouth gag is used to prevent damage and occlusion of the tube
You are preparing to anaesthetise a 645kg horse with 2.2mg/kg ketamine and 60mcg 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
cephalic vein
Marginal ear vein
Use a squeeze door
Free drop
To replace the nitrogen in the alveoli with oxygen
To increase the fraction of inspired oxygen (FiO2)
To increase the time to desaturation at induction
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 neck
- suitable for longer term fluid administration & cause less turbulence
- make sure they are well secured (disconnection can cause 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
Inhalation anaesthesia with sevoflurane via a mask
Inhalation anaesthesia with sevoflurane in an induction chamber
Subcutaneous ketamine & medetomidine
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
Why is it important to leak test a breathing system before use?
Prevents gas leaks, ensures system function, avoids patient rebreathing CO₂
How do you calculate fresh gas flow (FGF) in a non-rebreathing system?
FGF = Minute Volume (MV) × System Factor (SF).
TV = 10ml/kg
MV = TV × RR (assume 20bpm)
SF:
T-piece = 2.5
Lack = 1
Calculate FGF for a 3.5kg cat on a T-Piece system.
TV = 10ml × 3.5kg = 35ml
MV = 35ml × 20 = 700ml/min
FGF = 700ml × 2.5 = 1750ml/min (1.75 L/min)
What is the recommended initial flow rate for a circle system?
100ml/kg/min for the first 10-15 mins (de-nitrogenation)
What is the maintenance flow rate for a circle system?
50ml/kg/min
How do you calculate the required reservoir bag size?
Bag volume = TV × 4 (or approx. 30ml/kg)
What patients are suitable for an Ayre’s T-Piece?
Patients <10kg, low resistance, requires high FGF (2.5 × MV)
What patients are suitable for a Lack system?
Patients >10kg (or mini version for 2-10kg), FGF = 1 × MV
Why is a Lack system unsuitable for IPPV?
Risk of CO₂ rebreathing
