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Physiology and Pharmacology of Cells > L9 - General Anaesthetics > Flashcards

L9 - General Anaesthetics Flashcards

1
Q

What is the history of general anaesthetics?

A 
1815 – Horace Wells – Nitrous oxide 
1819 – William Morton – Ether 
- Volatile anaesthetic 
- Highly flammable 
1820s – Warren – first use of Ether as a genera anaesthetic in surgery
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2
Q

What is chloroform used for?

A

Used to ease pain in labour – widely shown in media
Volatile anaesthetic
Highly flammable

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3
Q

Who discovered chloroform?

A

1842 - Robert Glover - began testing chloroform effects in dogs
Caused a loss of consciousness and response to painful stimuli

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4
Q

How are general anaesthetics divided into groups?

A

Depending on their route of administration

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5
Q

What are the two groups of general anaesthetics?

A 
Chemical 
- Inhalation 
  - Nitrous oxide – laughing gas 
- Intravenous 
  - Halogenates hydrocarbons – isoflurane 
  - Barbiturates – thiopental 
  - Steroids – alphaxalone 
Physical 
- Low pressure 
   - Hypothermia – shown in frogs
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6
Q

What is the lipid solubility of general anaesthetics?

A

They bind to hydrophobic domains of proteins

They are very lipophilic – enter lipid compartments easily

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7
Q

What is the graph plotted to show the effects of lipid solubility of general anaesthetic potency?

A

Minimum alveolar concentration plotted against oil:gas partition coefficient
- Amount of drug required to achieve anaesthesia in 50% of patients
Drugs with the highest oil:gas partition coefficient have the lowest minimum alveolar concentration
- Drugs with highest lipid solubility don’t induce anaesthesia as well

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8
Q

What are the two theory’s of the way anaesthetics work?

A

Membrane expansion
Increasing lipid/membrane fluidity
Both change activity of proteins within lipids impacting neuronal excitability

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9
Q

What factors agree with the lipid theory?

A

Obey the Meyer-overton rule – relationship between potency and oil:gas coefficient
Pressure effect – reducing atmospheric pressure increases anaesthetic potency
Diverse drug structures – only important factor is that they enter lipid compartment

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10
Q

What factors disagree with the lipid theory?

A

Temperature effect – lower temperatures reduce fluidity
Binding sites
Loss of activity with homologous series of lipophylic compounds
Increase GABAA receptor affinity for agonists

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11
Q

What is the protein theory?

A

Must be some specific receptors that the drugs bind to

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12
Q

General anaesthetics regulate activity of?

A

Neuronal ion channels

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13
Q

What are some example of general anaesthetics that interact with different ion channels?

A

Many increase action of GABA at GABAA receptors
Volatile ones
- Bind at interface of α and β subunits of GABAA
- Low concentrations activate two pore domain K channels
Intravenous ones bind only β subunit of GABAA
Ketamine and nitrous oxide block NMDA receptors

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14
Q

Where are the amino acids the anaesthetics interact with found?

A

Buried within the plasma membrane

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15
Q

All general anaethetic interaction with receptors effects?

A

Neuronal excitability

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16
Q

What is the effect of isoflurane?

A

Isoflurane depresses nerve terminal action potentials – measured using voltage clamp

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17
Q

The effects of isoflurane on presynaptic ion channels have been analyzed in?

A

Isolated rat neurohypophysial nerve terminals

Rat calyx of Held

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18
Q

What does isoflurane do in Rat calyx of Held?

A

Reversibly

  • Reduces synaptic vesicle exocytosis
  • Reduces action potential amplitude evoked by small current injections
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19
Q

What does isoflurane do in isolated rat neurohypophysial nerve?

A

Reversibly

  • Blocks Na+ currents
  • Blocks action potentials evoked by small current injections
20
Q

What are the effects of low concentrations of general anaesthetics on neurotransmission?

A

Synaptic transmission in CNS decreased – decrease vesicle fusion
Reticular formation - loss of consciousness
Hippocampus – short term amnesia
Thalamic sensory relay nuclei parts of cortex – analgesia
Some volatile anaesthetics inhibit spinal reflexes

21
Q

What are the effects of high concentrations of general anaesthetics on neurotransmission?

A

All brain functions affected - loss of motor control, reflexes, respiration, autonomic regulation
In absence of artificial respiration - death

22
Q

What are the 4 stages of anaesthesia?

A
  1. Analgesia – reduced responsiveness to painful stimuli
  2. Excitement – exaggerated reflex responses – dangerous phase
  3. Surgical Anaesthesia
    - Unconsciousness
    - Loss of response to painful stimulation
    - Loss of reflexes
    - Short-term amnesia
  4. Medullary Paralysis - loss of cardiovascular reflexes and respiratory paralysis – death
23
Q

What are the ideal conditions of anaesthesia?

A

Rapid induction and loss of consciousness
Analgesia
Muscle relaxation (common to give neuromuscular blockers as well)
Rapid recovery
Avoid stages 2 and 4

24
Q

Why is anaesthesia hard to control in overweight people?

A

The general anaesthetic is absorbed into fat rather than the CNS

25
Q

What are some examples of intravenous anaesthetics?

A

Propofol
Thiopental
Etomidate

26
Q

What are intravenous anaesthetics useful for?

A

Induction of anaesthesia

27
Q

What are the advantages of intravenous anaesthetics?

A

Easy to administer
Rapid induction - 20-30 sec
Propofol
- Rapid metabolism, good for induction of anaesthesia
- Rapid recovery - useful for day-case surgery

28
Q

What are the disadvantages of intravenous anaesthetics?

A

Pain at site of injection
Complex pharmacokinetics
Thiopental
- High lipid solubility
- Short duration of action due to redistribution
- Hangover due to accumulation in body fat
Side effects
- Respiratory depression
- Cardiovascular depression (not etomidate)

29
Q

What is Ketamine similar to?

A

PCP (phencyclidine)

A dissociative anaesthetic

30
Q

What are the advantages of Ketamine?

A 
Sensory loss 
Powerful Analgesia
NMDA receptor blocker 
Amnesia
No complete loss of consciousness
No respiratory depression
31
Q

What are the disadvantages of Ketamine?

A

CV excitement, involuntary movements
Increased intracranial pressure
Hallucinations
Irrational behaviour on recovery

32
Q

What are some examples of anaesthetics that are inhaled?

A

Nitrous Oxide
Isoflurane
Desflurane
Sevoflurane

33
Q

What are inhaled anaesthetics useful for?

A

Maintaining anaesthesia

34
Q

What are the differences between inhaled anaesthetics?

A

Differences arise from solubility of different agents in blood and toxicity associated with metabolism

35
Q

What are some problems with inhaled anaesthetics?

A

Only route of entry and exit is via lungs

Problem if lungs are diseased or damaged

36
Q

How can you alter the concentration of inhaled anaesthetic in the blood and brain?

A

By altering

  • Ventilation rate of the patient
  • Concentration of drug in inspired air
37
Q

What does the speed of induction and recovery depend on?

A

Solubility in blood and fat

38
Q

The depth of anaesthesia is determined by?

A

Tension of inhalational anaesthetic in brain, blood, alveolar air
- Not inspired air

39
Q

The rate at which tension of anaesthetic in blood and brain reaches tension in inspired air is dependent on?

A

Solubility in blood

40
Q

What does low blood solubility of a general anaesthetic allow?

A

More rapid induction and recovery because concentration in alveolar space = concentration in inspired air

  • Equilibrates more rapidly with concentration in blood
  • Rate of equilibration regulated through rate of alveolar ventilation
41
Q

What does high lipid solubility of a general anaesthetic allow?

A

Increased potency but slowed recovery

42
Q

What is malignant hyperthermia triggered by?

A

Exposure to certain drugs (halogenated general anaesethics)

- Volatile anesthetic agents and succinylcholine (neuromuscular blocker)

43
Q

In susceptible individuals to malignant hyperthermia what does exposure to the halogenated general anaesthetic induce?

A

A drastic and uncontrolled increase in oxidative metabolism in skeletal muscle
Overpowers ability to supply oxygen, remove carbon dioxide, and regulate body temperature
Leads to circulatory collapse and death if not immediately treated

44
Q

What is Dantrolene?

A

RYR antagonist and muscle relaxant

Given ahead of anaesthesia to people that may have condition

45
Q

How is susceptibility to malignant hyperthermia inherited?

A

Autosomal dominant disorder

Ryanodine receptor gene one of the most important

Physiology and Pharmacology of Cells (13 decks)

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