What does lipid theory say?
When the concentration of general anaesthetic exceeds 0.05mM, anaesthesia is induced as a result of a lipid volume increase of 0.4%. The lipid expansion interferes with nerve impulse conduction and thus anaesthesia is induced.
When the concentration of general anaesthetic exceeds 0.5mM, anaesthesia is induced as a result of a lipid volume increase of 0.4%. The lipid expansion interferes with nerve impulse conduction and thus anaesthesia is induced.
When the concentration of general anaesthetic exceeds 0.5mM, anaesthesia is induced as a result of a lipid volume increase of 4%. The lipid expansion interferes with nerve impulse conduction and thus anaesthesia is induced.
When the concentration of general anaesthetic exceeds 0.1mM, anaesthesia is induced as a result of a lipid volume increase of 0.4%. The lipid expansion interferes with nerve impulse conduction and thus anaesthesia is induced.
What can reverse lipid theory?
High pressure - reduces lipid volume back to normal
Low pressure - reduces lipid volume back to normal
What is protein theory?
A concentration of general anaesthetic that reaches 0.05mM leads to a lipid volume increase of 0.4% which induces anaesthesia. This is because the increase in lipid volume interferes with conduction of nerve impulses.
Lipid solubility of general anaesthetics is required for the general anaesthetic to reach a hydrophobic pocket on a channel protein. Channel proteins targeted are usually ion channels like GABA, K+ or nAchR.
Optical isomers of GA have the same lipid solubility and the same potency.
What are the inhibitory responses caused by general anaesthetics?
Opening of K+ channels (efflux)
Increase GABA activity
Inhibit excitatory channels such as nAchR
Increase glutaminergic activity
Opening of K+ channels (influx)
What is meant by the cut-off phenomenon?
The lipid solubility of a GA (and thus its potency) increases with the number of carbons in a GA but after a certain point (usually >11 carbons) the GA's potency abruptly declines.
After a certain high dose of GA, the patients condition rapidly deteriorates.
The general anaesthetic stops working after a certain period of time.
GA have a large therapeutic window.
The minimum alveolar concentration effectively describes what?
The more lipid soluble a GA is, the lower the concentration required in inspired air.
The more lipid soluble a GA is, the higher the concentration required in inspired air.
The less lipid soluble a GA is, the lower the concentration required in inspired air.
The less lipid soluble a GA is, the higher the concentration required in inspired air.
What increases transfer of GA to the alveoli?
Increased concentration of GA
Increased rate and depth of breathing
Decreased concentration of GA
Decreased rate and depth of breathing
A higher blood:gas partition coefficient means that the GA is highly soluble in blood.
A GA with a high blood:gas partition coefficient will travel to the brain much quicker than one with a lower blood:gas partition coefficient.
Why would pulmonary blood flow increase help with absorption of GA when GA concentration in the body is initially low?
It maintains a high, favourable concentration gradient for the absorption of GA from the lungs into the blood, therefore increasing speed of induction.
It helps to increase blood flow to the brain to increase the concentration of general anaesthetic in the desired place.
What tissue has a high tissue:blood partition coefficient?
Adipose tissue
Muscle tissue
Brain tissue
Overall, what is evidence for lipid theory?
Volume expansion of the lipid bilayer
Cut-off phenomenon
Stereoselectivity
Overall, what is evidence for protein theory?
GA is usually metabolised by the body and excreted in the urine.
Both blood:gas and tissue:blood partition coefficients are inversely proportional to the speed of induction of GA.
What is an advantage of using halothane?
Potent and fast acting
Pleasant odour
Less liver damage
What is a disadvantage of sevoflurane?
Liver toxicity
Bad smell
Possible seizures
Intravenous GA is usually used for induction.
Intravenously administered thiopental and propofol act at which receptors?
GABA-a receptors
GABA-b receptors
NMDA
Intravenously administered ketamine act at what receptors?
GABA-a
GABA-b
Adjuncts are used to enhance the potency of a GA.
Give some examples of adjuncts used for GA administration.
Anxiolytics
Anti-depressants
Anti-psychotics
Anti-emetics
Muscle relaxants
