LA - Salient points

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The mechanism of local anesthetic(LA) action.

"B" represents an unionized local anesthetic and "BH+" represents an ionized.The blue is a voltage gated sodium channel(NaV) and the ionized anesthetic inhibits it in the intracellular fluid.

When the influx of sodium is interrupted, an action potential cannot arise and signal conduction is inhibited.


Non Ionized - enters at Node of Ranvier

Ionized - blocks Na+ Channel

pKa -- is pH at which Ionized = Non Ionized

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Anaesthetic Potency (related to) Hydrophobicity

Drug with lower pKa is Fast acting (exception - Chloroprocaine which has rapid onset and high pKa)

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To increase Duration 
1. Add Adrenaline

• in Lignocaine - increases both sensory and motor block
• in Bupivacaine - increases Only Sensory block

2. Add phenylephrine

Sodium bicarbonate : Decreases pKa -- increasing availability of unionized form , therefore increase onset of action.

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CNS toxicity  -- Bupivacaine > Etidocaine > Lidocaine  =  4:2:1

All Local Anaesthetics are Negative Ionotropic

All are Vasodialator except Cocaine

Bupivacaine --} CNS:CVS = 1:3

Lignocaine ---} CNS:CVS = 1:7

Large dose of Prilocaine -- causes Methemoglobinemia (Treatment - Methylene Blue )

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LA with Adrenaline is Contraindicated in :

1. Finger
2. Toes
3. Penis
4. Pinna

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