General Anaesthetics Flashcards
(33 cards)
List 2 routes by which general anesthetics can be administered.
When are these routes used?
1 - Inhalation (inhalation anaesthetics - used when administering a single drug).
2 - Intravenous (balanced anaesthetics - used when administering a combination of different drugs for optimal clinical effect with lowest risk).
List 3 early general anaesthetics.
1 - Diethyl ether.
2 - Chloroform.
3 - Nitrous oxide.
Define anaesthesia.
Define analgesia.
- Anaesthesia is the loss of sensation.
- Analgesia is the loss of pain.
What is the triad of general anaesthesia?
- 3 requirements for anaesthesia:
1 - Unconsciousness.
2 - Analgesia.
3 - Muscle relaxation (loss of reflexes).
Which component of the nervous system is targeted by general anaesthetics?
General anaesthetics target the CNS only.
List 3 chemical properties of general anaesthetics.
1 - Simple, short-chain molecules.
2 - Unreactive.
3 - Belong to no single chemical class.
What is the lipid theory for the mechanism of general anaesthesia?
- The lipid theory states that the concentration of agent required to produce general anaesthesia is inversely proportional to the agent’s lipid:water partition coefficient.
- This coefficient is a measure of the lipid solubility of the agent - the higher the coefficient the more lipid soluble.
- I.e. a more lipid soluble general anaesthetic requires a lower concentration to produce general anaesthesia (because it is able to infiltrate the cell membrane more easily).
- This is because the agents act by volume expansion of the lipid membrane, or by increasing the fluidity of the membrane, which interferes with conduction of nerve impulses.
List 3 observations that support the lipid theory for the mechanism of general anaesthesia.
1 - The concentration of a general anaesthetic in a cell membrane must be 0.05mM to produce anaesthesia for any agent.
2 - Anaesthesia occurs when the volume of a lipid (i.e. those found in the cell membrane) expands by 0.4%.
3 - High pressure reverses the anaesthesia.
What is the protein theory for the mechanism of general anaesthesia?
- The protein theory states that the concentration of agent required to produce general anaesthesia is inversely proportional to the agent’s protein affinity.
- Lipid solubility is still required (as seen in the lipid theory), however it is only required for access to membrane proteins.
List 2 observations that support the lipid theory for the mechanism of general anaesthesia.
1 - Protein binding of general anaesthetics exhibits the cut-off phenomenon for long chain compounds. This means that there is a cut-off in chain length where agents abruptly lose their ability to produce general anaesthesia, perhaps due to a loss in protein-binding ability.
2 - The stereoselectivity of general anaesthetics is preserved with protein binding.
What type of membrane proteins are targeted by general anaesthetics?
List 6 examples of these proteins.
- Ion channels. Particularly:
Inhibitory targets:
1 - GABAa receptor.
2 - K+ channels.
Excitatory targets:
3 - Glutamate receptors, specifically NMDA.
4 - Serotonin receptors.
5 - Nicotinic receptors.
6 - Glycine receptors.
*NB the result of general anaesthetics binding to their targets is always depression of the CNS, regardless of whether the target is excitatory or inhibitory.
List 4 functions that are lost with increasing concentrations of general anaesthetics.
List from low concentration to high concentration of general anesthetic.
1 - Memory is impaired first.
2 - Consciousness.
3 - Movement.
4 - Cardiovascular response at high concentrations (bad).
List and describe the stages of general anaesthesia.
1 - Analgesia.
- Drowsiness.
- Intact reflexes.
- Still conscious.
2 - Induction phase.
- Delirium.
- Spasmodic movements.
- Incoherent speech.
- Eventual loss of consciousness
- Unresponsive to non-painful stimuli.
3 - Surgical anaesthesia.
- Unresponsive to painful stimuli.
- Loss of reflexes.
- Muscle relaxation.
- Synchronised electroencephalograph.
4 - Medullary paralysis (overdose).
- Pupillary dilation.
- Respiration ceases.
- Circulation ceases.
- electroencephalograph wanes.
Why is the induction phase of general anaesthesia a particularly dangerous stage?
Due to the risk of:
1 - Cardiac arrhythmias.
2 - Vomiting.
3 - Choking.
List 2 desirable properties of a general anaesthetic drug.
1 - Potent (so not much needs to be administered to achieve general anaesthesia).
2 - Fast acting and fast recovery (to quickly pass the dangerous induction phase).
What is MAC?
What is MAC used for?
- Minimal alveolar concentration.
- It is the concentration of a general anaesthetic in the alveoli (expressed as a % of inspired air) required to produce immobility in 50% of patients when exposed to a noxious stimulus.
- MAC is used as a measure of anaesthetic potency.
How is the MAC of a general anaesthetic used clinically?
2-3x the MAC of a general anaesthetic is given to patients in clinical practice (to be sure that general anaesthesia is produced).
What determines the MAC of a general anaesthetic?
MAC is inversely proportional to lipid solubility.
- I.e. more lipid soluble substances can produce general anaesthesia at a lower concentration - see lipid and protein theory cards.
List 7 factors that increase the speed at which a general anaesthetic induces general anaesthesia, and the speed at which general anaesthesia is reversed.
1 - Low blood:gas partition coefficient.*1
2 - High tissue:blood partition coefficient.
3 - High concentration of anaesthetic inhaled.
4 - High rate and depth of breathing.
5 - High rate of pulmonary blood flow.
6 - Low partial pressure of the drug in venous blood (speed of drug movement into the blood decreases over time as the drug saturates the blood).
7 - Low solubility in blood.*2
8 - Tissue blood flow.
- 1 This is because the drug does not bind strongly to the blood, so it will move more quickly from the blood into the brain tissue.
- 2 This is because the blood will have a lower capacity for a drug that is less soluble in blood, and the blood must be saturated before any will move to the brain, which is easier to achieve if the blood has a low capacity for the drug.
What is the tissue:blood coefficient of all general anaesthetics in lean tissue (such as the brain)?
The tissue:blood coefficient of all general anaesthetics in lean tissue is 1.
Why might the speed of induction of general anaesthesia be lower in a fat (thicc) patient?
- Because general anaesthetics are highly soluble in lipids, so adipose tissue has a high capacity for general anaesthetics.
- This decreases the rate at which the general anaesthetic absorbs into the brain.
- This is also true for reversal of general anaesthesia.
How are general anaesthetics eliminated from the body?
- General anaesthetics are eliminated via the lungs.
- Metabolism is not important for most general anaesthetics. This reduces toxicity).
List 5 general anaesthetics.
For each, list a few advantages and disadvantages.
1 - Halothane (potent, fairly fast but possible liver toxicity).
2 - Enflurane (less liver toxicity but possible seizures).
3 - Isoflurane (rapidly action, effectively relaxes muscles but smells bad).
4 - Sevoflurane (pleasant odour, rapid recovery but possible renal damage).
5 - Nitrous oxide (very rapid, used for strong analgesic properties but low potency and only used in general anaesthetics when combined with other agents).
List 2 advantages of balanced anaesthetics.
1 - Rapid onset.
2 - Short acting (therefore useful for short procedures).
