Anaesthetic agents Flashcards
Anaesthesia is a triad of what 3 things?
Unconsciousness (sleep)
Muscle relaxation (immobilisation)
Analgesia
What injectable drugs are used to cause unconsciousness? (All GABA agonists except one - which one?)
Propofol
Alfaxalone (not alfoxolaner)
Ketamine - NMDA agonist
Thiopental
What drugs are used for maintenance of unconsciousness? (inhalation)
Sevoflurane
Isoflurane
What are the advantages of injectable induction agents of anaesthesia, cf inhalation?
Injectable don’t require specialised equipment, don’t pose risk to personnel
Inhalational require specialised equipment and potential environmental contamination and risk to personnel
What are the advantages of using inhalation agents compared to injectable?
Easily and quickly eliminated
Easy to adjust depth
(Injectables need metabolising and excretion, difficult to adjust depth)
How does propofol work? How does dose affect propofol effects?
GABA agonist
Low dose = sedation
High dose = anaesthesia
How is propofol administered? Why?
IV only - not lipid soluble so must be made into emulsion, can cross BBB
IM - metabolism faster than rate of uptake
Why should propofol not be used to top up anaesthesia?
Some contains preserves - accumulate and cause prolonged seizures, haemolysis and Heinz body anaemia (anaemia due to formation of Heinz bodies after haemolysis)
Which tissues does propofol enter?
Crosses BBB to cause unconsciousness
Also enters other tissues
How does propofol distribution cause an animal to wake up?
As drug distributed, levels in blood drop
Causes propofol to leave BBB down new concentration gradient
Requires more or another agent
Where is propofol metabolised?
Liver
GI
Which species should not be given propofol (or can be given with care)?
Cats
Cats struggle to metabolise propofol = accumulation and prolonged anaesthesia
How does propofol affect BP?
Causes vasodilation
Baroreceptors would usually detect this and cause increased HR
Baroreceptors blocked by propofol = drop in blood pressure
Propofol can cause post-induction apnea. What is this? Why dos this happen?
Respiratory depression due to anaesthetic
CNS thinks lower blood CO2 is normal - stimulus to breathe removed until CO2 increases
How does alfaxalone work?
GABA agonist
Where is alfaxalone metabolised? How can it be administered?
Liver Lipid soluble (IM, SC) but can be made water soluble (IV)
Both alfaxalone and propofol are GABA inhibitors used for anaesthesia. They both cause vasodilation, how do they differ in terms of baroreceptors?
Propofol - blocks baroreceptors, vasodilation causes decreased BP
Alfaxalone - baroreceptors intact, compensatory increase in HR causes increased blood pressure
What are the adverse effects of alfaxalone?
Tachycardia (due to preserved baroreflex)
Excitable recoveries
Respiratory depression
Why is alfaxalone often the agent of choice in cats?
Doesn’t accumulate
Doesn’t cause Heinz body anaemia
How does NMDA work? It causes dissociative anaesthesia. What does this mean?
NMDA antagonist
Disconnects higher brain functions from body
Where is ketamine metabolised ? What is metabolised to? Which species is the exception?
Metabolised - liver (into nor-ketamine, less potent but can prolong anaesthetic)
Cats - not metabolised, excreted unchanged in urine
What are the side effects of ketamine?
Analgesia (good)
Direct myocardial depression
Respiratory depression
Muscle hypertonicity
Why may animals still be able to swallow and blink under ketamine anaesthesia?
Ketamine maintains cranial nerve reflexes
How does thiopental work? When is it used?
GABA agonist
Equine only
How is thiopental adminsitered? How is it metabolised?
IV only as irritant
Slow hepatic metabolism - accumulates if repeated dose
What are the adverse effects of thiopental?
Respiratory depressio
Myocardial depression
Increased incidence of arrhythmias
Inhalational agents are used for maintaining anaesthesia. Can they be used for induction?
Yes but dangeous
What is the minimum alveolar concentration? (MAC)
Concentration at which 50% of patients will not respond to noxious stimuli
What is MAC expressed as? Describe its relationship with potency
% of atmospheric pressure
As MAC increases, potency decreases
If not using other drugs (rare), what multiple of MAC should be used for surgery?
1.2-1.5X mac
How do premedication agents, opioids and induction agents affect the MAC and amount of inhalation agents needed?
Other drugs decrease MAC and amount of other drugs needed
What is the alveolar concentration?
% of inhalation agent in alveoli
Inhalational agents are taken from the alveoli into the blood and to the brain. Not all of the agent is active - some is dissolved in the blood. What type of the drug (dissolved or undissolved) has anaesthetic effect?
Undissolved has anaesthetic effect - NOT dissolved
What affects the amount of drug that dissolves? How does this affect amount of drug needed?
Solubility
More soluble- more dissolves - more needed to have an effect
How do less soluble agents allow a faster wake up?
Less soluble - more likely to go from the blood stream and to the lungs to be exhaled
What are adverse effects of isoflurane and sevoflurane?
Cause vasodilation
Cause respiratory depression
Is respiratory depression more of an issue with sevoflurane or isoflurane? Why?
Isoflurane - more pungent, breathed easily
Sevoflurane not breathed as easily - less respiratory depression
Isoflurane causes more respiratory depression than sevoflurane. Why is isoflurane still commonly used?
Cheaper
Low MAC - requires less - even cheaper
