Lecture 27 : General Anaesthetics Flashcards
(11 cards)
Definition
Induce reversible state of :
- unconsciousness
- muscle relaxation (and no reflexes)
- analgesia (absence of pain in response to a stimulus that would normally be painful)
So patient can undergo surgery without awareness / distress.
Depresses CNS activity.
Lipid Theory
Concentration of GA required is INVERSELY PROPORTIONAL to lipid : water partition coefficient.
Ie. Potency of anaesthetic directly proportional to its lipid solubility. They dissolve in lipid bilayers of neurone membranes.
Alters fluidity of ion channels (Na+ and K+).
Can cause volume expansion of lipid cell membrane.
Causes impaired neuronal excitability, leading to analgesia or unconsciousness.
How do high pressures affect GA
- Reverse it
- Compress bilayer, restoring original structure and function
Protein Theory
Idea that anaesthetics act on membrane proteins, not just lipids.
Eg. Na+ or K+ ion channel proteins.
Reduce neuronal excitability -> analgesia / loss of consciousness.
Some evidence points to protein theory over lipid theory, because there is a ‘cut off’ phenomenon. An increase in chain length means more non-polar / hydrophobic molecules can interact with lipid bilayer, making it easier for it to cross because bilayers like this, and so increasing chain length means more lipid solubility, so more effective anaesthetic. HOWEVER, potency stops beyond a certain chain length, suggesting lipid theory doesn’t make full sense. Protein theory would make more sense here because the larger the molecule, the less it will act on small protein channels like Na+ and K+, which is the phenomenon observed there.
Lots of anaesthetics also exist as stereoisomers, but studies have shown that one isomer is more potent than the other. This would not be the case in lipid theory because stereoselectivity wouldn’t matter.
What tends to be the target site of general anaesthesia?
- GABA receptors (ligand gated channels), when affected increases inhibition
- K+ channel activation, decreases membrane excitability
- Na+ channels, inhibits action potential production
Stages of anaesthesia
- Analgesia, drowsiness, still conscious, reflexes work
- Delirium (induction phase), loss of consciousness, starts working now. Also a dangerous phase, can vomit, spasm
- Surgical anaesthesia, everything is painless. regular breathing, muscles relaxed
- Medullary paralysis if overdosed, can cause death by respiratory / cardiovascular failure
What is MAC
Minimal Alveolar Concentration - a measure of anaeshtetic potency
Conc of anaesthetic in alveoli required to produce immobility in 50% of patients when exposed to noxious stimulant.
Inversely proportional to lipid solubility.
LIPID SOLUBILITY IS THE MAIN DETERMINANT OF ANAESTHETIC POTENCY. The higher lipid solubility, the greater the potency. (less polar too ig)
Speed of induction and gas partition coefficient
Gas partition coefficient tells us how well a gas dissolves in blood. THE MAIN FACTOR in the speed of conduction.
The lower the solubility, the quicker the it reaches the brain.
Hence they are inversely proportional
Anaesthetic examples
IV Anaesthetics
- GABA receptor action
- antagonist for NDMA receptor (KETAMINE)
Premeds taken before anaesthetics
