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List some of the targets of the general anaesthetics
- GABA receptor
- NMDA receptor
- Two-pore domain K+ channels
- Glycine recptors
- Na+ channels
Are these receptors normally inhibitory/excitatory? a) GABA b) NMDA
- a) inhibitory
- b) excitatory
What is the overall mechanism of the general anaesthetics?
They decrease neurotransmission in the CNS which leads to loss of consciousness
Anaesthesia = loss of feeling
What is the difference between general anaesthetic and local anaesthetic?
General anaesthetic = loss of feeling in the whole bodyLocal anaesthetic = loss of feeling to the part of the body where the anaesthetic is applied
Define general anaesthesia
General anaesthesia is a state of reversible unconsciousness with reduced sensitivity and response to stimuli
What is the mechanism of action of local anaesthetics? (revision of module 7)
They block sodium channels which reduces AP firing in nerves
List some reasons why we anaesthetise animals
- To perform painful surgical or diagnostic procedures
- To minimise patient suffering
- To reduce risk to the vet and other individuals
- To facilitate the procedure by immobilising the patient
Anaesthesia is usually given as a combination of different drugs, what are these?
Premedicant drugs, induction drugs and maintenance drugs
What type of drug will make a better general anaesthetic: water soluble or lipid soluble?
Lipid soluble drugs - the more lipid soluble the drug the faster it crosses the cell membrane and the faster it reaches the CNS to have its action
What type of anaesthesia is typically used for induction/maintenance? Intravenous or inhalational?
- Induction - intravenous
- Maintenance - inhalational
What are the advantages and disadvantages of using IV anaesthetics for induction?
- Advantages - rapid smooth induction, rapid protection of the airway, no environmental pollution
- Disadvantages - IV access required
What does TIVA stand for?
Total intravenous anaesthesia - anaesthesia maintained by intermittent boluses or continuous infusion of an IV agent
List some reasons for using TIVA
- Easy to administer
- Pharmacokinetics are known/predictable
- Inhalational anaesthetics may be unsuitable in some individuals
- Avoids risk to people administering drugs i.e. no environmental pollution
What are the advantages/disadvantages of using inhalational anaesthetics for maintenance?
- Advantages - delivery/elimination depends on ventilation, rapid adjustment of anaesthetic depth
- Disadvantages - equipment required (endotracheal tube, carrier gas, vaporiser, breathing system, etc), environmental pollution
What are the advantages/disadvantages of using inhalational anaesthetics for induction?
- Advantages - IV access can be secured after induction
- Disadvantages - environmental pollution, takes longer and delay in securing airway may be a problem in some cases
What is the blood:gas partition coefficient?
The b:g partition coefficient compares whether the drug would rather be dissolved in the blood or alveolar air
What would a low b:g/high b:g partition coefficient give with regards to induction?
- Low - gives a rapid induction and recovery
- High - gives a slow induction and recovery
Would a high or low oil:gas coefficient give a drug a high potency?
A high o:g coefficient
What brain regions do the general anaesthetics inhibit?
The reticular formation and hippocampus
What is the minimum alveolar concentration?
The MAC describes the minimum alveolar concentration at which 50% of patients will not respond to a particular stimulus
The MAC compares the ... of different inhalational anaesthetics?
The higher/lower the MAC the more potent the agent?
Give examples of halogenated inhalational agents
Halothane, isoflurane, desflurane, and sevoflurane
What is the most widely used inhalation agent?
Why may isoflurane not be ideal for induction?
It has a pungent odour, meaning animals are likely to hold their breath
What are some of the advantages of isoflurane?
Minimal metabolism, cardiac output better maintained, less arrythmogenic
What animal is sevoflurane licensed in?
True or false: sevoflurane has a pungent odour?
False: it has a pleasant odour and minimal airway irritation so suitable for induction
Why can nitrous oxide not be used as an anaesthetic agent on its own?
As the MAC is >100% which means you cannot get full anaesthesia with N2O on its own
List some IV anaesthetic agents
Propofol, Alfaxalone, Ketamine, Barbituates eg thiopentone, pentobarbitone, imidazole derivatives eg etomidate
Is propofol a liquid or oil at room temperature?
What receptor does propofol act on?
Propofol enhances GABA transmission
Is propofol high/low plasma protein bound?
High plasma protein bound
Does propofol have a high/low volume of distribution?
What type of metabolism does propofol undergo?
Phase II metabolism in the liver prior to excretion in the urine
Is propofol suitable for TIVA?
Under what condition may the pharmacological effects of propofol be prolonged?
What species may show prolonged propofol effects?
How is alfaxolone presented?
It is insoluble in water so is presented in a cyclodextrin vehicle
What receptor type does alfaxalone act on?
It enhances the inhibitory effect of GABA
Does alfaxalone have a high/low therapeutic index?
Is alfaxalone suitable for TIVA?
What receptor does ketamine act on?
It inhibits NMDA receptors (interrupts the association between the limbic and cortical regions)
True or false: ketamine causes sensory loss as well as analgesia?
What effect does ketamine have on muscles?
It causes enhanced muscle tone
Describe what an animal anaesthetised with ketamine will present like
- Eyes open +/- slow nystagmus
- Active reflexes eg laryngeal and pharyngeal reflexes
- Less profound CVS and respiratory depression
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