General Anaesthesia Flashcards
What are the three types of anaesthesia?
general anaesthesia:
- total loss of sensation
regional anaesthesia:
- loss of sensation to a region or part of the body
local anaesthesia:
- topical infiltration
What is an anaesthetic drug?
a drug that induces partial or total loss of sensation
What are the 3As as components to balanced general anaesthesia?
amnesia:
- lack of response and recall to noxious stimuli (unconsciousness)
analgesia:
- pain relief
akinesis:
- immobilisation / paralysis caused by muscle relaxation
What are the three components of the triad of balanced general anaesthesia?
- unconsciousness
- analgesia
- muscle relaxation
What is meant by balanced anaesthesia?
Why is it used?
it involves a combination of agents being used
this is safer than a large dose of a single agent
it minimises adverse effects
What are the stages involved in one episode of general anaesthesia?
monitoring
intravenous access:
- to give anaesthetic agents
start the process:
- induction agents used for induction of anaesthesia
- start the analgesia and muscle relaxation
maintain the process:
- maintenance agents for amnesia / analgesia / muscle relaxation
- replace fluid and blood loss
reverse the process:
- reverse muscle relaxation
- maintain post-operative analgesia
What are the roles of induction agents?
How quickly do they work?
induce loss of consciousness in one to two arm-brain circulation times
this is around 10-20 seconds
What compounds are used in amnesia as induction agents?
a diverse range of compounds:
- nitrous oxide
- halothane
- sodium thiopentone
- enflurane
- isoflurane
- sevoflurane
- desflurane
- propofol
- xenon
What is the onset and duration of intravenous agents like?
When are they commonly used?
they have a quick onset and short duration
they are most commonly used as a bolus for induction
(but also for maintenance as infusion)
What is the potency of intravenous agents like?
How is it measured?
measured by Cp50
this is the minimal steady state plasma concentration of an intravenous agent required to prevent a somatic response in 50% of patients following skin incision
What is the main intravenous anaesthetic that has been used in the past?
How does it work?
barbituates
they work by prolonging the action of the neurotransmitter GABA on its receptors
In general, how do general anaesthetics work?
they modulate the activity of transmitter-gated ion channels
What are the sites of action of general anaesthetics?
- peripheral nervous system and cerebral cortex
- reticular activating system
- basal ganglia, cerebellum, medullary centres, motor pathways
- afferent neurones + monosynaptic pathways
What are the ideal properties of an intravenous induction agent?
- simple preparation
- compatible with other agents & IV fluids
- painless on administration
- high potency and efficacy
- predictable action within one circulation time
- minimal cardiovascular effects or other toxicity
- depression of airway reflexes for intubation
- rapid and predictable offset of effect
- rapid metabolism for minimal hangover
What are the 4 main intravenous induction agents that are used?
- propofol
- thiopentone
- ketamine
- etomidate
What does propofol look like?
What are the benefits of its use?
it is a lipid-based white emulsion
it has excellent suppression of airway reflexes
it decreases incidence of postoperative nausea and vomiting (PONV)
What are the unwanted effects of propofol?
- marked drop in heart rate and blood pressure
- pain on injection
- involuntary movements
What is the dose for propofol?
1.5 - 2.5 mg / kg
What are the benefits of using thiopentone?
What is it mainly used for?
- faster than propofol
- antiepileptic properties and protects the brain
it is mainly used for rapid sequence induction
What are the unwanted effects of thiopentone?
- drop in blood pressure but rise in heart rate
- rash / bronchospasm
- intra-arterial injection - thrombosis and gangrene
- contraindicated in porphyria
What dose of thiopentone is given usually?
it is a barbiturate and a dose of 4 - 5 mg/kg is used
When is ketamine used?
dissociative anaesthesia
anterograde amnesia and profound analgesia
it is the sole anaesthetic for short procedures
What is the onset of ketamine like?
it has a slow onset of 90 seconds
it causes a rise in heart rate and blood pressure and bronchodilation
What are the unwanted effects of ketamine?
- nausea
- vomiting
- emergence phenomenon
What dose of ketamine is given?
1 - 1.5 mg/kg
What is dose of etomidate is given?
What is its onset like?
it has a rapid onset
the dose is 0.3 mg / kg
What are the benefits of using etomidate?
haemodynamic stability
lowest incidence of hypersensitvity reaction
What are the unwanted effects of etomidate?
- pain on injection
- spontaneous movements
- adreno-cortical suppression
- high incidence of PONV
What are examples of situations when ketamine should be used?
a patient requiring a burn dressing change
What are examples of situations when propofol should be used?
a patient undergoing arm operation under GA with an LMA
a patient with porphyria comes for an inguinal hernia repair
What are examples of situations when etomidate and thiopentone would be used?
etomidate:
- a patient with a history of heart failure requires a general anaesthetic
thiopentone:
- a patient with intestinal obstruction requires emergency laparotomy
How long do induction agents last for?
How can amnesia be maintained?
they last from 4 to 10 minutes
amnesia can be maintained through:
propofol infusion:
- this is total intravenous anaesthesia
inhalation agents:
- this is inhalational anaesthesia
When are vapours used for amnesia?
How are they administered?
inhalation agents are used to maintain amnesia
they are started after induction
they are administered via vaporisers or breathing circuits
What are the physical properties of an ideal inhalation agent?
- non-flammable
- stable with materials (e.g. plastic, metal) with long shelf life
- environmentally friendly
- cheap and easy to manufacture
What are the biological properties of an ideal inhalation agent?
- pleasant to inhale, non-irritant
- fast onset
- high potency
- minimal effects on other systems
- no biotransformation
- non-toxic to theatre personnel
What is meant by the Meyer-Overton theory?
it describes the correlation between lipid solubility of inhaled anaesthetics and MAC
it suggests that anaesthesia occurs when a sufficient number of inhalational anaesthetic molecules dissolve in the lipid cell membrane
What is meant by “MAC” of a general anaesthetic?
minimum alveolar concentration
this is the concentration of a vapour in the alveoli of the lungs that is needed to prevent movement (motor response) in 50% of subjects in response to surgical (pain) stimulus
it is used to compare potency (or strength) or anaesthetic vapours
How was Meyer Overton theory proven to be false?
- some predicted anaesthetics ineffective
- lipid membrane effect reproduced by small changes in temperature in vitro
- differing effects of stereoisomers
- large molecular weight compounds less potent than predicted
What are other theories about how anaesthetics produce unconsciousness?
- critical volume theory
- mean excess volume theory
- multisite expansion theory
- protein theory of anaesthesia
- effect on channels
- combination of above
What is meant by critical volume theory?
solubilisation of a lipophilic general anaesthetic in the lipid bilayer of a neurone causes its malfunction and anaesthetic effect when a critical concentration of anaesthetic is reached
What is meant by mean excess volume theory?
bulky and hydrophobic anaesthetic molecules accumulate inside the neuronal cell membrane
this causes distortion and expansion (thickening) due to volume displacement
membrane thickening reversibly alters function of membrane ion channels, producing an anaesthetic effect
What is meant by multisite expansion hypothesis?
general anaesthesia may be the result of expansion of different molecular sites that are of a finite size and ultimately reach saturation
What is meant by the protein theory of general anaesthetics?
general anaesthetics bind directly to a small number of targets in the CNS
these are mostly ligand-gated ion channels in the synapse and G-protein coupled receptors
this alters their ion flux
What are the neurobiological effects of anaesthetics?
How can they be classified?
- amnesia
- hypnosis
- immobility
can be classified by their effects on different receptors
there is an effect of genetic inter-individual variability
What are group 1 general anaesthetics?
What are their clinical features?
etomidate, propofol, pentobarbital
clinical features:
- strong hypnotics
- strong amnestics
- weak immobilisers
- slow cortical EEG
molecular targets:
- GABAA receptors
What are the group 2 general anaesthetics?
What are their clinical features and molecular targets?
nitrous oxide, ketamine, xenon, cyclopropane
clinical features:
- weak hypnotics
- weak immobilisers
- potent analgesics
- no EEG slowing
molecular targets:
- NMDA receptors
- AMPA receptors
- neuronal nAChRs
- 2-pore K+ channels
What are the group 3 anaesthetics?
What are their clinical features and molecular targets?
halogenated ethers and alkanes
clinical features:
- strong hypnotics
- strong amnestics
- strong immobilisers
- slow cortical EEG
molecular targets:
- GABAA receptors
- glycine receptors
- glutamate receptors
- neuronal nAChRs
- 2-pore K+ channels
What are the partition coefficients and recovery time for nitrous oxide?
blood: gas = 0.5
oil: gas = 1.4
MAC = 104%
recovery is fast
What are the partition coefficients and recovery time for halothane?
blood: gas = 2.4
oil: gas = 220
MAC = 0.8%
recovery time is medium
What are the partition coefficients and recovery time for desflurane?
blood: gas = 0.4
oil: gas = 23
MAC = 6.1%
recovery is fast
What are the partition coefficients and recovery time for sevoflurane?
blood: gas = 0.6
oil: gas = 53
MAC = 2.1%
recovery is fast
What are the 4 unconsciousness (amnesia) inhalation agents?
What are they used for?
- isoflurane
- sevoflurane
- desflurane
- enflurane
they are used for maintenancce
What is the definition of MAC (minimum alveolar concentration)?
concentration of the vapour
that prevents the reaction to a standard surgical stimulus (traditionally a set depth and width of skin incision)
in 50% of subjects
it measures potency
What is one MAC of the unconsciousness inhalation agents?
one MAC of:
nitrous oxide is 104%
sevoflurane is 2%
isoflurane is 1.15%
desflurane is 6%
enflurane is 1.6%
When are inhalation agents used until?
What inhalation agents are used?
they can be continued until the end of operation
any inhalation agent can be used
commonly used agents are sevoflurane, desflurane and isoflurane
What determines the characteristics and onset/offset of inhalation agents?
physical properties determine the characteristics of general anaesthetics
solubility in the blood determines the onset / offset
What does it mean if an inhalation agent has low solubility?
low solubility means fast equilibration
agents with low solubility have a fast onset and quick recovery
they are most commonly used for maintenance (but can be used for induction)
What is the problem with highly fat-soluble agents?
GA given for a long time accumulates in fat
this can result in a “hangover effect” for hours
What is the difference in perfusion and equillibration of GAs in fat and lean tissues?
fat:
- slow perfusion
- large partition coefficient
- slow equilibration
lean tissues:
- fast perfusion
- small partition coefficient
- rapid equilibration
When is sevoflurane used?
it is a sweet smelling inhalation agent
used for inhalational induction
When is desflurane used?
What is its onset and offset like?
it is an inhalational agent used for long operations
it has low lipid solubility and rapid onset and offset
What is isoflurane?
What is a benefit to its use?
an inhalational agent
it has the least effect on organ blood flow
What are the general and specific adverse effects of inhalational agents?
general:
- vasodilation
- decrease cardiac contractility
- can potentially affect organ perfusion
specific:
- malignant hyperthermia
- hepatotoxicity (halothane)
What would be the preferred inhalational agent in an 8 hour long finger re-implantation?
desflurane
What is the preferred inhalational agent in a chubby child with no intravenous access?
sevoflurane
What would be the preferred inhalational agent in organ retrieval from a donor?
isoflurane
After balanced anaesthesia is acheived, why is analgesia required?
- insertion of airway
- laryngeal mask airway
- intubation
- intraoperative pain relief
- postoperative pain relief
What category of drug tends to be used for analgesia?
opioids
What short-acting drugs are used for analgesia?
- remifentanil
- alfentanil
- fentanyl
- remifentanyl given via IV infusion
What analgesics are used for long-acting effects?
these are used for intra-op and post-op analgesia
examples are morphine and oxycodone
What other opioids and analgesics are used in balanced anaesthesia?
- paracetamol
NSAIDs:
- diclofenac
- parecoxib
- ketorolac
weaker opioids:
- tramadol
- dihydrocodeine
What is performed after amnesia and analgesia have been achieved as part of balanced anaesthesia?
muscle relaxation
this is required for intubation and surgery
what is meant by akinesia?
the loss of the ability to create muscular movement
What are examples of depolarising and non-depolarising muscle relaxant drugs (akinesis)?
depolarising:
- suxamethonium
non-depolarising:
- short-acting - atracurium, mivacurium
- intermediate-acting - vecuronoium, rocuronium
- long-acting - pancuronium
What drugs are used to reverse the effects of muscle relaxants?
neostigmine & glycopyrrolate
What are the stages involved in assessing unconsciousness?
- clinical signs
- measure level MAC
- BIS monitor
- isolated forearm
- evoked potentials
