Anaesthesia and induction Flashcards
Mode of action: propofol
- Acts on (beta subunit of) GABA receptor
- This leads to inward direct Cl current hyper polarising the post-synaptic membrane and inhibiting neuronal depolarisation
Characteristics of propofol
Milky alkyl phenol
Pharmacokinetics of propofol
- Minimal oral bioavailability means IV route is needed
- Minimally soluble in water (so is given as an emulsion)
- Distribution: 98% is protein-bound
- Metabolism is in the liver via glycoronidation
- Metabolite elimination is renal
Pharmacodynamics of propofol
i.e. effect on CO, MAP, etc.
- Produces anaesthesia, respiratory depression, decreased CMRO2, depressed cardiovascular reflexes
- Haemodynamic effects are largely a result of sympathetic depression
- There is stable CO
- Decreased heart rate (due to blunted baroreceptor reflex)
- Decreased MAP, SVR, CVP
Onset of action of propofol
- Rapid uptake by the CNS
- Induces unconsciousness in 5-8 mins
Effects of propofol
(i.e. broadly: CV effects? Recovery? Analgesia?)
- Respiratory and cardiovascular depression
- Rapid and smooth recovery
- No analgesic effects
Suitability of propofol
- Suitable TIVA (top ups or CRI)
- Can be injected by IV only
- Consider carefully if the animal already has CV depression
Practical delivery of propofol and common side effects to anticipate
- Titrate to effect
- Use premed to reduce dose
- Give slowly over 60s
- Apnoea is common! Be prepared to rapidly intubate, connect to breathing system and perform IPPV.
- Repeat doses for several days a week can cause problems in cats (e.g. anorexia, anaemia, unkemptness) because propofol is not being adequately metabolised
- Cats struggle with phenols
- Twitching and myoclonus can occur. This can be dramatic and frightening, but is often mild and self-resolving.
Which species is propofol licensed for?
Dogs
Cats
Mode of action: alfaxalone
i.e. which receptor
- GABA allosteric modulator (acts on the GABA receptor)
Characteristics of alfaxalone
Clear, colourless, neuroactive steroid
Pharmacokinetics of alfaxalone
- Good bioavailability (absorption)
- Soluble in water
- Distribution: 30-50% protein bound
- Metabolised rapidly in liver, also in lungs and kidney
- Metabolite excretion: mainly renal elimination, small % through bile
Pharmacodynamics of alfaxalone
i.e. effect on HR and others
- Produces anaesthesia
- Decreased CMRO2
- Haemodynamic effects are minimal:
- stable CO
- stable HR
- stable MAP, SVR, CVP
Onset of action: alfaxalone
Rapid uptake by CNS
Effects of alfaxalone
(Broadly: CV effects? Recovery? Analgesia?)
- Respiratory and cardiovascular depression (probably less than propofol)
- Recovery rapid and smooth if animal has had premed; if not, can be stormy
- No analgesic effects
Suitability of alfaxalone
- Suitable for TIVA (CRI or top-ups)
- Can be given IM for sedation
- Good for cats, especially if compromised/difficult to secure IV access
Practical delivery of alfaxalone
- Titrate to effect: give IV slowly over 60s
- Reduce required dose by premed; lack of premed can result in agitated recovery
- Apnoea is much less common than with propofol :)
Mode of action: ketamine
NMDA receptor antagonist
NMDA receptor is located across the cortex, thalamus, striatum and brainstem
Characteristics of ketamine
Clear colourless solution with pH 3.5
Pharmacokinetics of ketamine
- Good bioavailability
- Soluble in water
- 12% protein bound
- Metabolised in the liver to norketamine (an active metabolite)
- Eliminated through renal system and a small % in bile
Pharmacodynamics of ketamine
i.e. effects on HR etc.
- Dissociative anaesthesia
- Increased CMRO2
- Increases cerebral blood flow and ICP
- Minimal haemodynamic effects:
- Stable CO
- Increased HR
- Increased MAP, SVR, CVP
- Provides somatic analgesia
What is the only analgesic induction agent?
Ketamine
When would ketamine be contraindicated and when?
- In cases where ICP is a problem
- Because ketamine increases cerebral blood flow and ICP
Onset of action: ketamine
- Rapid uptake by the CNS
- Induces unconsciousness in 5-8 mins
Effects of ketamine
(Broadly: anaesthesia? Analgesia? etc.)
- Produces dissociative anaesthesia
- Provides somatic analgesia
- Animals typically exhibit eyes that are wide open and central rather than rotated
- Poor muscle relaxation, but can be combined with other agents to improve this
- e.g. horses are given alpha-2 agonist followed by ketamine
- Can also be combined with benzodiazepines
Suitability and usage of ketamine
- Schedule 2 drug: must be kept in a locked cabinet and a record kept of its use
- Component of the feline triple or quad IM protocols
- Often used in highly compromised dogs and cats as it increases HR and BP rather than causing CV depression
Practical delivery of ketamine
- Can be given IM, IV, or squirted into the mouth of a hissing cat
Species licensing: ketamine
- Licensed for cats, cattle, dogs, guinea pigs, hamsters, horses, mice, pigs, rabbits, rats, sheep, sub-human primates
- Induction agent of choice in horses
- Used in exotic and lab animals commonly
Examples of volatile agents
Isoflurane, sevoflurane
Pharmacokinetics of volatile agents
- Absorption via the lungs
- Solubility in tissues/blood varies according to partition coefficient
- Distribution is via blood to brain
- Minimal metabolism
- Elimination is via the lungs
Pharmacodynamics of volatile agents
(i.e. effect on HR etc.)
- Produce anaesthesia
- Cause profound cardiopulmonary depression‼️
- Whether used for induction or maintenance, only use the absolute minimum.
- Many problems in anaesthesia arise from overdose:
- Poor BP → poor perfusion → can be fatal
- These drugs are basically poisons
- Reduced CO
- Reduced HR
- Decreased MAP, SVR and CVP
Onset of action: volatile agents
- Role of lungs causes the drugs to behave differently to injectable agents:
- IV agents: speed of induction is proportional to cardiac output
- Inhalants: speed of induction is inversely proportional to cardiac output due to negative effects of high cardiac output on the alveolar partial pressure
- i.e. if very compromised animal receives inhalant, induction can be v. rapid. If very compromised animal receives IV agent, induction can be v. slow.
- Reverse is true for recovery.
Suitability and usage of volatile agents
- Can be used for induction and maintenance
- Mask or chamber induction is used whenever IV/IM not possible
- Beware of stress of restraint/breathing in agent
- Requires a precision vaporiser to slowly increase % and avoid overdose
Species suitability of volatile agents
- Birds
- Small furries
- Very compromised animals
What should you consider when anaesthetising a brachycephalic animal?
- Have a large selection of ET tubes available
- Use an induction agent that is not likely to cause apnoea (i.e. alfaxalone rather than propofol)
- Pre-oxygenation of 100% for 3-5 mins prior to induction
CMRO2
cerebral metabolic rate
SVR
systemic vascular resistance
CVP
central venous pressure
