Injectable anaesthetic agents

Question 1

Name 4 injectable agents and what they are used for

Answer 1

• Propofol
• Alfaxalone
• Ketamine
• Etomidate

Question 2

What is the use of Total Intra Venous Anaesthesia?

Answer 2

• Use of injectable agents to induce and maintain anaesthesia
• Intermittent boluses
• Constant rate infusion

Question 3

What is partial Intravenous anaesthesia?

Answer 3

• co-administration of injectable and inhalational anaesthetics to maintain anaesthesia
• creates a balanced anaesthesia

Question 4

What is the function of propofol?

Answer 4

• Hypnotic alkyl phenol
• lipid water macroemulsion
• licensed in dogs and cats
• concentration 1%

Question 5

What are the indications of propofol?

Answer 5

• induction and maintenance of anaesthesia
• status epilepticus
• fish and reptile anaesthesia

Question 6

What is preservatiive free propofol?

Answer 6

• Supports bacterial growth
• Discard at the end of the day
• Constant rate infusion

Question 7

What is preservative based propofol?

Answer 7

• PropoFlo Plus
• benzyl alcohol preservative
• induction and short-term maintenance
• last 28 days
• no CRI

Question 8

What are the key factors surrounding propofol?

Answer 8

• IV administration
• Highly protein bound to albumin and RBC
• highly lipid soluble -> rapid onset of action
• equilibration between the CNS and plasma
• Induction and recovery -> usually smooth and excitement free
• Extrapyramidal signs

Question 9

What is the propofol method of action?

Answer 9

• Hypnotic agent and GABA enhancer
  1. Binds to the B subunits of the GABA receptor
  2. Prolonged opening of Cl- channels
  3. Hyperpolarisation of the post-synaptic neurons
  4. inhibits NMDA receptors

Question 10

How does propofol work in cats?

Answer 10

• Slower hepatic metabolism
• Feline Hb is prone to oxidative injury by repeated daily administration

Question 11

What is alfaxalone?

Answer 11

• synthetic neuroactive steroid
• Insoluble in water
• Preservatives- last 28 days
• licensed in dogs, cats and rabbits
• concentration = 10mg/ml

Question 12

What are the indications for alfaxalone?

Answer 12

Induction & maintenance of anaesthesia
* Sedation (not licensed) * Suitable for immersion anaesthesia in amphibians, reptiles & fish

Question 13

What is the mechanism of action for alfaxalone?

Answer 13

• Enhanced inhibitory effect on GABA
• high dose is a GABA agonist

Question 14

How might you dose alfaxalone?

Answer 14

IV, IM, SC administration (occasional pain on injection)
* Rapid induction: 30-60’ (IV)
* Dose premedicated animal: 1-2 mg/Kg (to effect)
* IM sedation/anaesthesia (7-10’)
* Alpha 2 agonist, opioids, ACP, midazolam: ↓ induction dose

Question 15

How does the alfaxalone induction work?

Answer 15

Induction usually smooth without excitement
* Recovery quality ~ to propofol
* Prolonged recoveries if long infusions
* Excitable recovery (Paddling, rigidity, vocalisation possible, especially if used alone)
* Rapid metabolism (hepatic)

Question 16

What is ketamine?

Answer 16

Phencyclidine derivative
* Licensed: Dog, cat, horse, cattle, sheep, goat, pig, rabbits…
* Used also for exotic & wildlife anaesthesia
* IV, IM, SC, transmucosal absorption
* Aqueous solution: Acidic PH: pain on IM injection
* 10% solution (most commonly used)

Question 17

What are the indications for ketamine?

Answer 17

Induction ( + other agent(s) to provide myorelaxation)
* Analgesia
* Sedation ( aggressive/ painful patients

Question 18

How might ketamine be administered?

Answer 18

Can be administered as a bolus, CRI
* Sedation: 0.5-3 mg/kg IM
* Induction dose: * ~ 2.5 up to 5 mg/Kg (higher dose for exotics)
* Rapid onset IV (60-90’’)
* Pain (CRI 10-20 mcg/kg/min intra-op, 2-5 mcg/kg/min post-op)
* HepaƟc metabolism → acƟve metabolite: Norketamine
* Ketamine & Norketamine excreted unchanged (urine) in : careful if renal dx * Dog, horse: Norketamine further metabolised (inactive compounds), urinary + biliary
excretion

Question 19

What is the ketamine mechanism of action?

Answer 19

• Interacts with many differet receptors
• non-competitive NMDA receptor antagonist
• pain, memory, learning, processing and modulation of neural activity

Question 20

What is ketamine used for?

Answer 20

Chronic pain/ central sensitisation

Question 21

What is etomidate?

Answer 21

• Imidazole derivative
• not licensed for animals in the UK

Question 22

What are the indications for etomidate?

Answer 22

Induction (with premed +/- co-induction) * 1-3 mg/Kg IV

Question 23

What is etomidate used for?

Answer 23

Rapid induction & recovery (poor quality)
* Metabolism (hepatic & plasma estereses)
* Inactive metabolites excreted in urine, bile, faeces

Question 24

What is co-induction/ what is the purpose of co-induction?

Answer 24

Administration of two or > drugs together for induction of anaesthesia
* Drug synergism
* Dose sparing effect (< side effects)
* Cost sparing
* Improve cardiovascular stability
* Helpful properties of the non-induction agents (i.e. muscle relaxation, analgesia,
cough response to intubation

Question 25

How might you obtain central myorelaxation?

Answer 25

Depress internuncial transmission at spinal cord & brainstem

Question 26

How might you obtain peripheral myorelaxation?

Answer 26

Action at neuromuscular junction (acetylcholine receptors)