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DOH210 S1 > Week Four: Local Anaesthetic Pharmacology > Flashcards

Week Four: Local Anaesthetic Pharmacology Flashcards

1
Q

A sensory neuron is comprised of what?

A 
Axon hillock
Cell body
Myelin Sheath
Axon terminals
Dendrites
Nodes of Ranvier
The axon
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2
Q

What is the axon hillock?

A

This is where the electrical signals from stimuli are summated and reach a threshold of approx -50mv, and action potential commences here.

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3
Q

What are the nodes of ranvier?

A

Na+ and K+ channels are concentrated here

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4
Q

What is the myelin sheath?

A

This is what insulates the axon and influences conduction and velocity

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5
Q

Describe a flow chart of an action potential.

A

IMAGE

Resting Membrane Potential –> Threshold —> Depolarisation —> Repolarisation —> Hyperpolarisation

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6
Q

What is a action potential?

A

An Action potential is a rapid, brief reversal in electrical charge across the axons membrane.

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7
Q

What is signal conduction regarding an action potential?

A
  • Depolarisation on the inside of one node of Ranvier will trigger Na+ channels to open in the adjacent segment, thus triggering another action potential
  • A wave of depolarisation (salutatory conduction) continues, until the signal reaches the axon terminal.
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8
Q

Why is the action potential unidirectional?

A

Unlike graded potentials, the propogation of an action potential is unidirectional, because the absolute refractory period prevents the initiation of an AP in a region of membrane that has just produced an AP

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9
Q

Would blocking the Na+ receptors at one node of Ranvier block an action potential?

A

Ck lecture.

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10
Q

What is a local anaesthetic?

A
  • A local anaesthetic describes a loss of sensation to a circumscribed area of the body
  • Local anaesthetics are used in dentistry to create a reversible blockage of pain/sensation signals in peripheral nerves
  • Natural vasodilators.
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11
Q

What are the actions of local anaesthetics?

A
  • Direct relaxation on smooth muscle, and inhibition of neuromuscular transmission in skeletal muscle.
  • Antidysrhythmic action on the heart
  • Simulation and/or depression of the CNS
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12
Q

What are some properties of LA?

A

In dentistry LA is frequently used topically and subcutaneously.

  • LA is poorly soluble in water: The addition of salt (hydrochlorides) makes the molecule stable and soluble in water
  • LA is a weak base with a pka of 8-9
  • In extracellular fluid it has a pH of 7.4. Here the LA exists simultaneously as an uncharged molecule and a +ve charged molecule called the cation.
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13
Q

Relating to ionisation, what is the effect of speed of onset in LA?

A

This is determined by the pKa of the solution and the pH of the surrounding fluids. Effects how many unionised molecules will be present in order to diffuse across the cell membrane.

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14
Q

Relating to lipid solubility, what is the effect of potency regarding LA?

A

Drugs with a HIGH lipid solubility are more potent as they produce more effective conduction blockade at lower concentrations than do less soluble anaesthetics. (lower % solutions, or smaller volumes of drug required.)

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15
Q

Relating to protein binding, what is the duration of action regarding LA?

A

Drugs with higher protein bonding affinity will bind to plasma proteins for longer, resulting in an additional reservoir once the unbound drug has diffused away from the site, or been metabolised. Also will bind to Na+ channel receptor for longer.

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16
Q

Relating to vasodilator action, what is the duration of action and potency?

A

Increase perfusion of local of local area with blood, therefore anaesthetic agent is carried away from effector site more rapidly for biotransformation.

Potential for increased bleeding at site of injection.

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17
Q

If a tissue has a low pH, in example due to inflammation how does this affect the speed of onset?

A

A low pH outside of the nerve means more L.A solution present in the cation form. Therefore less free base available to penetrate the nerve membrane to act on the Na receptor.

18
Q

If a tissue has increase tissue vascularity, due to inflammation how does this effect the duration of action?

A

More blood vessels present in local area, therefore greater opportunity for L.A to be removed from injection site.

19
Q

How does L.A affect the membrane permeability to sodium Ions?

A

Ck lecture

20
Q

How does a change in extracellular pH affect membrane permeability?

A

Ck Lecture

21
Q

What is the clinical relevance in regards to the intrinsic properties of local anaesthetics?

A

CK LECTURE

22
Q

What is the clinical relevance in regards to the properties of the tissues affecting the mode of action of local anaesthetics?

A

Ck Lecture.

23
Q

What does ADME stand for?

A
  1. Absorption
  2. Distribution
  3. Metabolism/Biotransformation
  4. Elimination/Excretion
24
Q

Absorption?

A

Speed influenced by:
Chemical nature of the drug
Site of injection - local blood flow, vasoconstriction

25
Q

Absorption: The greater the absorption into the blood stream…

A

The greater the potential for systemic effects: Toxic levels will not occur if absorption into the circulation is slow, or if metabolism is high.

The less profound and shorter duration of the drug: Therefore decreased working time.

26
Q

Distribution?

A

Once the drug is absorbed into the bloodstream it is distributed throughout the body to all tissues

27
Q

Distribution is influenced by?

A

Protein binding capacity:
It may act as a reservoir (bound drug, cannot leave the blood)
Presence of other drugs in the body: May compete for binding sites

28
Q

What is the two compartment model used to describe distribution?

A
  • Rapid disappearance phase

- Slow disappearance phase

29
Q

What is the rapid disappearance phase?

A

• Related to uptake by tissues with high perfusion
• i.e. Brain, lung, liver, kidney and heart
• Highly perfused organs will have higher plasma levels of L.A,
which has significant bearing on the potential toxicity of the
drug

30
Q

What is the slow disappearance phase?

A

• Related to uptake by tissues with lower perfusion
• i.e. Muscle and fat- also may act us buffers due to their
lipophilic properties

31
Q

What is metabolism/biotransformation in regard to the ADME of LA?

A
  • Alters the chemical properties of drugs to make them less lipophilic to enable excretion
  • Metabolism of LA can occur in the liver, the blood, plasma and lungs
  • Site of primary biotransformation depends on the drug being broken down
  • Products of metabolism may also have clinical action
32
Q

Give some examples of products of metabolism that may also have clinical action.

A

Methaemoglobinemia: When metabolites of the LA prilocaine induce formation of mathaemoglobin inducing cyanosis like state.

Lidocaine does not produce sedation, but some of its metabolites are currently thought to be responsible for this clinical action.

33
Q

What is excretion/elimination in regard to the ADME process of LA?

A
  • Principal site of excretion of drugs is the
    kidneys
    • LA and its metabolites are excreted via the
    kidneys
34
Q

What is the chemical basic structure of LA?

A

Anaesthetics can be divided into two functional groups based on the structure of the intermediate chain.

The lipophilic part (aromatic group), the intermediate chain and the hydrophilic part (amino terminus)

35
Q

Name functional groups that are esters.

A

• E.g. Procaine/Novocaine, Tetracaine
• Metabolism: Rapidly hydrolyzed in plasma by
pseudocholinesterase
• A metabolite of this para-aminobenzoic acid
(excreted in urine) has been related to allergic
reactions
• ≈ 1/2800 persons has atypical pseudocholinesterase
 unable to metabolise ester prolongation of
higher levels of L.A in blood, may increase potential
for toxicity
• History of difficulty during GA may indicate this
hereditary trait

36
Q

Name functional groups that are amides.

A

• E.g. Lignocaine/Lidocaine, Prilocaine, Articaine,
Bupivacaine, Mepivacaine
• Metabolism: Primarily in the liver
• Primary Biotransformation site is in the liver
• Rate of breakdown in dependant on hepatic blood flow
• May undergo some secondary metabolism in the lungs
• Conditions that affect hepatic blood flow (hypotension,
congestive heart failure) liver function (cirrhosis) are
unable to metabolise amide L.A at a normal rate
• Increases potential for toxicity

37
Q

What factors can influence the plasma concentration of local anaesthetics?

A

Ck lecture

38
Q

What tissues are most likely to be effected when there is an increased plasma concentration of local anaesthesia?

A

Ck lecture

39
Q

What can impact the rate of absorption?

A
  • Accidental intravenous injection
  • Volume of anaesthetic used
  • Concentration of anaesthetic used
  • Addition of vasoconstrictors
  • Local blood flow
40
Q

What can impact the plasma concentration?

A
  • Age and weight
  • Presence of other drugs in the blood stream
  • Lipid solubility and protein binding affinity of drug
  • Rate of metabolism and excretion
41
Q

What can impact metabolism?

A

Liver function

42
Q

What can impact elimination?

A
  • Significant kidney impairment e.g. renal disease 4&5

DOH210 S1 (15 decks)

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