General Anaesthesia Flashcards
In clinical practice the ‘triad of general anaesthesia’ is a term that is often used, describe this.
• The need for unconsciousness.
• The need for analgesia.
• The need for muscle relaxation – the body still responds to painful stimuli even if the person is unconscious.
o Can get away with lighter anaesthesia.
Describe the characteristics of ideal general anaesthetics.
Ideal general anaesthetics should be readily controllable so that induction and recovery are rapid, allowing the level of general anaesthesia to be adjusted as required.
What are the stages of anaesthesia?
Analgesia, excitement, surgical anaesthesia, medullary paralysis.
Describe the first stage of anaesthesia - analgesia.
- Patient conscious but drowsy.
- Response to painful stimuli reduced.
- Degree of analgesia depends upon agent (ether ~ N2O > Halothane).
Describe the second stage of anaesthesia - excitement.
• Very hazardous state:
• Modern anaesthetic procedures are designed to counter effects which include.
o Loss of consciousness.
o No response to non-painful stimuli but reflex response to painful stimuli.
• Other reflexes (cough-/gagging in response to pharyngeal stimulation) exaggerated.
• May move around, talk incoherently, “voluntary” apnoea, choke, vomit.
• Irregular breathing may affect the intake of gaseous anaesthetic agents.
Describe the third stage of anaesthesia - surgical anaesthesia.
- Spontaneous movement stops.
- Respiration becomes regular.
- While anaesthesia is light, responses to pharyngeal and peritoneal stimulation remain intact.
- Deepening anaesthesia leads to progressive loss of reflexes, increased muscles relaxation.
- Shallowed respiration with intercostal respiratory muscles failing before diaphragm.
Describe the fourth stage of anaesthesia - medullary paralysis.
- Respiration and vasomotor control cease. Death occurs within minutes.
- BS paralysed.
What are the aims of surgical anaesthesia?
• Produce rapid unconsciousness.
o Intravenous injection of a rapid onset, short acting barbiturate (thiopentone).
• Maintain unconsciousness.
o With one of more inhalation agents (e.g. N2O and Halothane).
• Supplement analgesia.
o With an intravenous analgesic agent (e.g. fentanyl).
• Produce muscle paralysis.
o With a neuromuscular blocking agent such as tubocurarine (nicotinic antagonist).
What are the two main theories as to how general anaesthetics work?
There are two main theories as to how general anaesthetics work, these being the ‘Lipid Theory of Overton and Meyer’ and the ‘Protein (Receptor) Theory’.
What observations were made when the lipid theory of anaesthetic action was proposed?
Between 1899 and 1901, Overton and Meyer found that there was a close correlation between anaesthetic potency and lipid solubility.
They took a diverse series of simple and un-reactive organic compounds and determined the concentration needed to produce reversible immobilization of swimming tadpoles.
This was then compared with the olive oil to water partition coefficient (a determiner of lipid solubility).
What is the hypothesis behind the lipid theory of anaesthetic action?
“Narcosis commences when any chemically indifferent substance has attained a certain molar concentration in the lipids of the cell. This concentration depends on the nature of the animal or cell but is independent of the narcotic.”
What evidence was gathered to back up the lipid theory of anaesthetic action?
The data gathered by Meyer and Overton showed that there was an inversely proportional relationship between the minimum alveolar concentration (MAC) of a given anaesthetic and the olive oil to gas partition coefficient at 37°C.
What two explanations were given for the lipid theory of anaesthetic action?
Lipid bilayer volume expansion, increased membrane fluidity.
What arguments are there against the lipid theory of anaesthetic action?
The cut-off phenomenon and in-vitro immobilization of halothane during anaesthesia.
The cut-off phenomenon is an argument against the lipid theory of anaesthetic action, explain it.
Anaesthetic potency increases steadily through a homologous series of chemical compounds as the length of the hydrocarbon chain is increased. Beyond a certain point potency suddenly drops even though lipid solubility continues to increase.
