General Anastheasia

Question 1

What are the five clinically desirable effects of general anaesthetics? State which two effects are caused by ALL general anaesthetics.

Answer 1

Loss of consciousness (ALL) 
Suppression of reflex responses (ALL) 
Relief of pain 
Muscle relaxation 
Amnesia

Question 2

Name 4 inhalational general anaesthetics.

Answer 2

Nitric oxide
Diethyl ether
Halothane
Enflurane

Question 3

Name 2 IV general anaesthetics.

Answer 3

Propofol

Etomidate

Question 4

What is the Meyer/Overton correlation?

Answer 4

For a long time it was believed that anaesthetic potency increases in direct proportion with the oil/gas partition coefficient
In other words: anaesthetic potency is directly correlated with lipid solubility

Question 5

What were the proposed mechanisms of the meye/overton correlation? What problems with this are there?

Answer 5

The drugs disturbed the lipid bilayer

At therapeutic doses, the changes to the lipid bilayer were minute
How would the change in membrane impact on membrane proteins anyway?

Question 6

What are the two real mechanisms of action of general anaesthetics?

Answer 6

Reduced neuronal excitability

Altered synaptic function

Question 7

Describe the difference in the selectivity of IV and inhalational agents.

Answer 7

IV agents are more selective for GABA-A

Inhalational agents are far less selective - large antagonistic action on NMDA and Nicotinic Ach

Question 8

Which specific subunits of the GABA-A receptor do IV agents that alter synaptic function target and what are their effects?

Answer 8

Beta 3 – important in suppression of reflex responses

Alpha 5 – important in amnesia

Question 9

What are the two main targets of inhalational agents that alter synaptic function?

Answer 9

GABA-A receptors

Glycine receptors

Question 10

Which subunit of the GABA-A receptor do inhalational agents seem to be more selective for?

Answer 10

Alpha 1 – important in suppression of reflex responses

Question 11

Explain how nitrous oxide induces its anaesthetic effect.

Answer 11

Nitrous oxide competes for the glycine-binding site on NMDA receptors (glutamate receptors)
Glycine is an important co-agonist of NMDA receptors – it allows the full receptor response to be transduced

Question 12

What is the effect of blocking neuronal nicotinic acetylcholinereceptors with regards to general anaesthesia?

Answer 12

Blocking nAChR leads to reduced nerve conduction

This is important for amnesia and relief of pain

Question 13

Which channels are important in reducing neuronal excitability via the action of inhalational agents?

Answer 13

TREK – background leak K+ channels

These lead to hyperpolarisation of neurones and inhalational agents facilitate the opening of these channels

Question 14

Inhalational agents are less selective than IV agents. What effect does this have on the dose needed to induce general anaesthesia?

Answer 14

Generally speaking, a higher dose of inhalational agents is required

Question 15

Explain how general anaesthetics cause a loss of consciousness.

Answer 15

Loss of consciousness is caused by depressing the excitability of thalamocortical and reticular formation neurones

This is mediated by TREK channels and GABA-A

Question 16

Explain how general anaesthetics suppress reflex responses.

Answer 16

Depression of reflex pathways in the dorsal horn of the spinal cord
This is done by anaesthetics that enhance GABA-A and glycine function

Question 17

Explain how general anaesthetics cause amnesia.

Answer 17

There are a lot of GABA-A receptors in the hippocampus and amygdala that have the alpha 5 subunit

General anaesthetics stimulate these receptors to decrease synaptic transmission in the hippocampus

Question 18

Explain how blood: gas partition coefficient affects the speed at which the general anaesthetic penetrates the brain.

Answer 18

If you have a GA that dissolves really well in the blood, then there is less GA in the gas phase in blood and hence less GA penetrates the blood-brain barrier and reaches the brain

It is only anaesthetic that remains in the gas phase that diffuses easily into the brain

Question 19

Would a general anaesthetics with a high or low blood: gas partition coefficient be useful for anaesthetics and why?

Answer 19

LOW
This means that less of the GA will dissolve in the blood so more will be left in the gas phase
Hence more of the drug will penetrate the blood-brain barrier and reach the brain

Question 20

What are the benefits of inhalation anaesthetics?

Answer 20

Rapidly eliminated

Good control of the depth of anaesthesia

Question 21

What are the benefits of IV anaesthetics?

Answer 21

Fast induction

Less coughing/excitatory phenomena

Question 22

What types of drugs can be used to achieve:

a. Relief of pain
b. Muscle relaxation
c. Amnesia

Answer 22

Relief of pain = Opioids

Muscle relaxation = Neuromuscular blockers

Amnesia = Benzodiazepines