Principles of general anaesthesia Flashcards
Recall 2 effects shared by all general anaesthetcis
Loss of consciousness and suppression of reflexes
What are the 2 possible MOAs of general anaesthetics?
Altered synaptic function
Reduced neuronal excitability
Describe how general anaesthetics can affect neuronal excitability
Only inhaled agents
Increase TREK channel opening
Hyperpolarisation of neurons suppresses the reflex responses
What are TREK channels?
Background leak K+ channels
By what mechanism do IV agents affect synaptic function?
Act on GABA-A receptors
Action on beta 3 subunit suppresses reflexes
Action on alpha 5 subunit causes amnesia
Recall the receptors that are acted on by inhaled agents to alter synaptic function
GABA-A receptors
Glycine receptors
NMDA Glu receptors
Compare the use of inhaled vs IV agents for alteration of synaptic function
IV agents much more selective for GABA-A Rs than inhaled so have a more powerful effect
Inhaled agents affect far more targets due to lower specificity
Describe how inhaled agents tend to affect GABA receptors
Tend to be more selective for GABA receptors with an alpha 1 subunit which are present in the spinal cord and therefore suppress reflex action
What sort of activity to glycine receptors show?
Inhibitory - they are homologous to GABA receptors
Recall an example of an anaesthetic that has an effect on NMDA receptors
Inhaled nitrous oxide
Recall the MOA of nitrous oxide as a general anaesthetic
Competitive inhibitor of NDA Glu receptor
Potently co-agonised by glycine
Recall an example of an IC general anaesthetic
Propofol
What are the main neuroanatomical sites involved in the loss of consciousness due to general anaesthetics?
Hypothalamocortical neurons
RAS
Recall how depression of hypothalamocortical neurons is mediated by local anaesthetics
Inhibits TREK channels
Enhances GABA function
How do general anaesthetics affect the RAS?
Suppress it to reduce its activating influence of hypothalamocortical neurons
What is the main neuroanatomical site responsible for the suppression of reflexes by general anaesthesia?
Dorsal horn of spinal cord
Recall the MOA of general anaesthetics in the dorsal horn of the spinal cord
Many GABA receptors present
Enhancement of GABA decreases activity of dorsal pathways
What are the main neuroanatomical sites responsible for the induction of amnesia by general anaesthesia?
Hippocampus and amygdala
Describe the MOA by which general anaesthetic induce amnesia
Hit Alpha 5 subunit of GABA receptors in the hippocampus and amygdala to decrease synaptic transmission
Compare the methods of excretion of inhaled vs IV agents
Inhalation: via lungs - gives very good control due to ability to remove drug from lungs to alter blood:gas partition coefficient
IV: excretion dependent on hepatorenal function so gives fa less control
How does blood gas partition coefficient affect blood solubility of the drug?
Low BGPC results in large proportion remaining in gaseous form, dissolving in blood and entering brain
High BGPS does opposite
What is the normal pattern of use for IV and inhaled general anaesthetics?
IV for induction, inhaled agent to maintain
What sort of agent is coadministered alongside a general anaesthetic to relieve pain?
Opiate
What sort of agent is coadministered alongside a general anaesthetic to relax muscles?
NMJ blocker like suxamethonium