Injectable Anaesthetics Flashcards
When do we use injectable anaesthetics?
-induction priori to inhalation anaesthesia
-sole agent for short duration procedures
-supplement inhalational agents
-suppresses CNS stimulation in certain conditions
What routes can injectable anaesthetics be given?
I/V
I/M
S/C
I/P
What to consider before choosing a route of administration?
-predictability of affect
-absorption
-need for restraint
-pain on injection
What is total intravenous anaesthesia?
-alternative to gaseous anaesthetic
-good airway and 02 supply must be available
Why might total intravenous anaesthesia be used?
-avoids side effects of inhalational agents
-may be useful where inhalational delivery would compromise access for surgery
What are the 3 things the route of administration affects?
-speed of onset of anaesthesia
-anaesthetic duration
-its peak affect
What does the effect of I/V anaesthetic administration depend on?
-the concentration in plasma
How does unconsciousness occur after the drug has been injected?
-brain receives high concentrations of drug shortly after injection
-unconsciousness occurs once a critical concentration is reached in the brain
What creates a diffusion gradient?
-over a period of time less well perfused organs will start to take up the drug resulting in a reduction of plasma levels creating a diffusion gradient
-this promotes drug levels to move from brain to plasma
-when consciousness returns as the brain drug levels fall below a critical level
What does the duration of action of injectable anaesthetics depend on?
-redistribution of the drug from the brain to less well perfumed tissues
-varies according to cardiac output and mass of tissues available
Where and how are anaesthetics metabolised?
-metabolised in the liver
-converting from lipid to water soluble molecules which are then excreted in bile or urine
What is an example of an injectable agent?
Propofol
What is propofol?
-an oil soluble phenol derivative
-milky white oil emulsion appearance
-results in rapid loss of consciousness
What are the advantages of using propofol?
-rapid metabolism and recovery
-free from hangover if single dose given
-non cumulative so can be used as part of TIVA protocols in dogs
-non irritant peri-vascularly
What are the disadvantages of using propofol?
-may cause apnoea if injected too fast
-may cause hypotension and occasional bradycardia
-may cause twitching, spontaneous muscle activity and excitable recoveries
-slower metabolism in cats, must take care with repeated dosing
What is an example of a steroid anaesthetic?
Alfaxalone
(clear solution)
-suitable for dogs, cats and rabbits
What are the advantages of using alfaxalone?
-rapid smooth induction
-minimal cardiorespiratory depression
-no pain on injection
-no perivascular tissue irritation
-rapidly eliminated from the body
What are the disadvantages of using alfaxalone?
-adequate premed required to ensure a smooth induction
-patients must be kept quiet during recovery period to avoid over excitement as drug is rapidly eliminated from animals system
What is an example of a dissociative anaesthetic?
Ketamine
How can ketamine be administered, difference in dogs and cats?
I/V
I/M
S/C
I/P
Or can be absorbed sublingually
-may be used alone in cats
-must be combined with another drug for dogs
What are the disadvantages of using ketamine?
-muscle relaxation is poor
-slow onset of metabolism
-ocular lubrication required
-can cause dissociative anaesthesia
What are the signs of dissociative anaesthesia?
-eyes remain open and pupils dilate
-protective airway reflexes maintained
-cranial nerve reflex’s less depressed compared to other agents
-heart rate increases, BP maintained
-breathing moderate reduced, and salivation increases
