General Anaesthetics Flashcards
what are the targets of general anaesthetics
1. GABAa receptor
2. NMDA receptor
- two-pore domain K+ channels
- glycine receptors
- Na+ channels
what is the overall mechanism of general anaesthesia
decrease in neurotransmission in the CNS –> loss of consciousness
what is general anaesthesia
reversible unconsciousness with reduced sensitivity and response to stimuli
what are the 3 required components of general anaesthesia
- unconsciousness
- analgesia
- muscle relaxation
why do we anaesthetize animals?
- perform painful surgical or diagnostic prodecures
- minimize patient suffering
- reduce risk to vet and other individuals
- facilitate the procedure by immobilizing the patient
what are the stages of anaesthesia
- voluntary movement
- involuntary movement or excitement
- surgical anaesthesia
- medullary paralysis
modern anaesthetic drugs/protocols aim to avoid stages 1 & 2
GAs have narrow therapeutic index –> want to avoid stage 4
what are intravenous anaesthetic agents used for
to induce anaesthesia
occasionally used to maintain anaesthesia
what are the advantages IV anaesthetic agents used for induction
- rapid smooth induction (minimal excitement)
- rapid protection of airway (important in dyspnoeic patients & those risk of regurgitation/aspiration)
- no environmental pollution
what are the disadvantages of IV induction agents
- IV access required
what are the advantages of inhalational anaesthetic agents used for maintenance
- delivery/elimination depends on ventilation
- rapid adjustment of anaesthetic depth
what are the disadvantages of inhalational anaesthetic agents used for maintenance
- equiptment required
- endotracheal tube, carrier gas (oxygen), vapourizer, breathing system, etc
- environmental pollution
what are inhalational anaesthetic agents used for
maintenance of anaesthesia
what are the advantages of inhalational anaesthetic agents used for induction
- IV access can be secured after induction
what are the disadvantages of inhalational anaesthetic agents used for induction
- environmental pollution
- takes longer & delay in securing airway may be a problem in some cases
what are the effects of GAs on CVS and respiration
decreased contractility of isolated heart preperations
effects on CO and BP vary
potentially arrhythmogenic
decrease respiration
increase arterial PCO2
what are IV anaesthetic agents used in vet med
- propofol
- steroid anaesthetics (alfaxolne)
- dissociative agents (ketamine)
also
- barbiturates (thiopenone, pentobarbitone), imidazole derivatives (etomidate)
what is TIVA
total intravenous anaesthesia
what is redistribution
concentration of drug determined by lipid solubility, composition of tissue and blood flow
what is redistribution
concentration of drug determined by:
- lipid solubility
- composition of tissue
- blood flow
what is redistribution for IV anaesthetic agents
upon IV injection rapidly goes to brain because it is highly lipid soluble and has high blood flow –> rapid loss of consciousness –> but begins to redistribute and leave brain –> consciousness returns to normal
what are the effects of GAs on the nervous system (3)
- inhibit conduction of action potentials
- inhibit transmission at synapses (decrease transmitter release, decrease action of transmitter, decrease excitability of post-synaptic cell)
- brain regions (reticular formation, hippocampus)
what is the chemical structure of propofol
hindered phenol
what is propofol at room temp
oil (emulsion)
what is the mechanisms of action of propofol
enhanced GABA transmission
why is propofol used
short acting
smooth and rapid recovery
what are the pharmacokinetics of propofol (7)
- highly plasma protein bound
- large volume of distribution (>3 l/kg)
- redistribution and metabolism
- phase II metabolism: conjugated in the liver and another site (sulfate and glucuronide) 5. prior to excretion in urine
- rapidly cleared (>40ml/kg/min)
- suitable for TIVA
what are the clinical relevance of pharmacokinetics for propofol
- effect may be enhanced in hypoproteinemia
- effect not prolonged if repeated IV doses are administered (suitable for maintenance)
- effect not prolonged in dogs with hepatic dysfunction
- a prolonged effect may be seen in cats
what are the effects of ketamine (6)
- sensory loss with analgesia
- increased muscle tone
- eyes open +/- slow nystagmus
- active reflexes include laryngeal/pharyngeal reflexes
- less profound CVS & resp depression
- hallucinations/emergence delirium
what is TIVA
total intravenous anaesthesia
anaesthesia maintained by intermittent boluses or continuous infusion of an IV agent
why use TIVA (4)
- easy to administer (minimal equipment)
- pharmacokinetics are known/predictable
- inhalation anaesthetics may be unsuitable in some individuals
- avoids risk to people administering drugs (no environmental pollution)
what are halogenated compounds
halothane, isoflurane, sevoflurane, desflurane
nitrous oxide, xenon
list 5 pharmacokinetic features of isoflurane
licensed in non-food producing animals
- good speed of induction/recovery/change of depth
- pungent odour (not ideal for induction)
- minimal metabolism
- CO better maintained
- less arrhythmogenic
list 3 pharmacokinetic features of sevoflurane
- rapid induction/recovery/change of anaesthetic depth
- pleasant odour & minimal airway irritation so suitable for induction
- low rate of metabolism
licensed in dogs
gaining popularity in vet anaesthesia despite higher cost
what is the strucure of nitrous oxide
N2O
N=N
O
colourless gas, without taste or odour
stored under pressure in cylinders
what are the pharmacokinetic properties of nitrous oxide
MAC (%) >100
oil: gas partition coefficient 1.4
blood: gas partition coefficient 0.47
% metabolized <0.01
