HD22&25 Local Anaesthetic Flashcards
(65 cards)
describe the ideal LA?
- Specific and reversible action
- Non-irritant
- Produces no permanent damage
- No systemic toxicity
- Active topically and by injection
- Rapid onset
- Suitable duration of action
- Chemically stable and sterilisable
- Combinable with other agents
- Non-allergic
- Non-addictive
What would be the therapeutic ratio of the ideal LA?
High therapeutic ratio (difference in dose between the effect that would give you the dose you want and the dose that would cause toxic effects) = want it to be high
LA is not specific, what will affect transmission in any excitable tissue. What are the 3 excitable tissues
- CNS
- Cardiac
- Motor
What are the uses of LA in dentistry (4)?
- Operative pain management
- Post-operative pain management - give LA while in GA so when wake aren’t in pain
- Diagnosis – pulp sensibility tests (in the past), now used for atypical pain or to identify which tooth is causing the pain if a number are carious
- Haemostasis – reduce bleeding during procedures
A chemical roadblock between the tooth and the brain
TRUE OR FALSE
true
Explain the theory of membrane expansion as the way LA works:
• The LA diffuses into cell membrane, cuasing it to expant. Therefore membrane is less permeable to Na and changes in polarisation do not occur.
what happens normally during membrane potential?
Sodium channel has 2 gates
• At rest – m gate closed/ h gate open
• Depolarisation – m gate opens which allows sodium to move inside cell + ve inside
• Repolarises – H gate closes which stops sodium moving into nerve cell membrane
• Potassium diffuses back out – electrochemical gradient shifts back (+ outside)
• Now in refractory period – cannot fire another impulse
What is the specific receptor theory of LA ?
- Receptor on inside of H gate – LA attaches to receptor holding the gate closed
- This holds it in the refractory period so that no more impulses can be fired
what are the 3 components of LA molecule?
- Aromatic group (lipophilic)
- Intermediate chain (ester or amide link)
- Substituted amino terminal (hydrophilic)
What part of the LA molecule allows:
spatial separation of lipid and water-soluble components
Intermediate chain
What part of the LA molecule allows:
classification of LA into 2 major groups:
- Esters
- Amides
Intermediate chain
What 2 major groups can LA belong to (determined by Intermediate chain ):
Esters
- Amides
What 2 major groups can LA belong to (determined by Intermediate chain ):
2) Which are at greater risk of an allergic reaction?
Esters
- Amides
2) esters
Name the amide LAs:
- Lidocaine
- Prilocaine
- Mepivacaine
- Articaine
- Bupivacaine
- Levobupivacaine
- Ropivacaine
Name the esters LAs:
which of these is a topical anaesthetic?
- Benzocaine – topical anaesthetic
- Amethocaine
- Procaine
The binding site of LA is intracellular , what is the significance of this?
2) What is required for specific binding to any receptor by LA?
1) • Therefore LA needs to be lipophilic and uncharged to cross the cell membrane
2) • Specific binding to any receptor to achieve LA requires a charged molecule
• Therefore the LA needs to be in a charged form
• How can LA be uncharged (lipophilic) and charged at the same time?
2) What does this achieve?
• LAs are weak bases
• In solution the LA molecule will exist as both -
- Uncharged base
- Charged cation
2) - Lipid soluble to enter cells to work
- Charged form for specific binding once in cell
which is the more effective LA once injected, the one with a higher proportion of uncharged molecule or lower?
- The quicker the LA enters the cell the more effective it is and the quicker it acts
- Therefore, LA with a high proportion of uncharged molecules after injection are most effective
what is ionisation?
ratio of charged to uncharged
what governs ionisation?
- pH – can be controlled usually
* pKa (dissociation constant) – set within each LA (cannot change)
what is the affect on LA uptake when the pH around the neurone is low?
2) what tissues have a lower pH?
• Lower pH - less uncharged molecules present (difficult to cross membrane as not uncharged)
2)infected tissues
what affect does pKa have on the onset of action of LA?
2) list the pKa of these from high to low:
lidocaine
bupivacaine
procaine
articaine
1) the lower the pKa the better
2) procaine -9.1
bupivacaine- 8.1
articaine- 7.8
lidocaine- 7.9
For more ionisation does pH need to be high or low?
2) for pKa ?
1) high
2) low
What chemico-physical properties influence the onset of LA?
- Ionisation (pH and pKa) - onset
* Partition coefficient - onset
