Local Anesthesia Flashcards
What is pKa?
pH at which there is 50% dissociation of the proton
Where are amide LAs metabolised?
The Liver
Where are ester LAs metabolised?
Hydrolysed by pseudo-cholinesterase in the plasma membrane
What is the active form the LA?
RNH+
T/F: RNH+ diffuses through the nerve sheath
False: The uncharged RN base defuses through the nerve sheath
Why is it difficult for LA to work in an infected space?
Bacteria produce H+ which favours equilibrium to form the charged RNH+ OUTSIDE the nerve sheath, meaning less RN can diffuse through the nerve sheath
T/F: the higher the pKA the longer the onset of LA
True
Procaine pKa 9.1, onset = 14-18 minutes
Mepivicaine pKa 7.6, onset = 2-4 minutes
What is the onset duration for Lignocaine (Xylocaine)?
2-4 minutes
What is the onset duration for Mepivicaine (Scandonest)?
2-4 minutes
T/F: Articaine is indicated for IANB
False, although it is indicated for us in the USA
What are additives to LA?
1, Methylparabens (Bacteriostatic agent, preservative)
- Bisulphite (Prevents oxidation of vasoconstrictor and lowers pH (may cause allergies)
- Sodium Chloride (isotonic solution)
- Sodium Hydroxide (increases pH of solution producing more RN)
- Distilled water (increases volume)
- Adrenaline/similar compounds (vasoconstrictor)
What is the role of vasoconstrictors in LA?
- Decrease blood flow
- Slows absorption of LA into blood stream
- Maintain higher local concentrations of LA
- Increases duration of LA action
- Reduces bleeding
What are the 3 structural components of an LA molecule
Lipophilic End
Intermediate Chain
Hydrophilic End
Presence of positive aspiration is indication of what?
- Fast and Bright Red: hit an artery
2. Small and Blue-ish: hit a vein
T/F: 30 Gauge is the thickest needle?
False, it is the thinnest and used in intra-periodontal ligament infilitration
What is a 25 Gauge needle used for?
Gow-Gates Block
Which direction should the bevel be facing when administering an periosteal infiltration?
Away from the operator
What is the topical anesthetic applied in SADS?
Ziagel (50mg/g) Topical Lignocaine
What is the LA toxicity dosage for 2% lignocaine for a 20kg child
2 Carpules
2% of 2.2ml Cartridge = 44mg
Toxicity dosage 4.4mg/kg
4.4 X 20kg = 88mg / 44mg = 2 carpules
What is the LA toxicity dosage for 3% scandonest for a 20kg child
3% of 2.2ml Cartridge = 66mg
Toxicity dosage 4.4mg/kg
4.4 X 20kg = 88mg / 66mg 1.3 carpules
What are the likely causes of LA toxicity?
- Rapid IV Injection
- Administering too large a dose
- Rapid absorption from site of administration
- Inability to biotransform the drug normally
- Inability to excrete the drug normally
What is Methemoglobinemia?
Formation of methemoglobin rather than haemoglobin which contains ferric iron that reduces the RBC capacity to carry oxygen.
Can be triggered and acquired through especially articaine, benzocaine, and prilocaine
What is the likely cause of trismus when administering an IANB?
Injecting too medially / inferiorly into the medial pterygoid muscle
What is the likely cause of sloughing when administering LA?
Result of too much vasoconstrictor. Superficial epithelium has as a lack of oxygen so dies and desquamates.
