Local anaesthetics Flashcards
How do local anesthetics work?
By blocking the influx of Na across voltage gated sodium channels in neuronal membranes.
What is the most important factor for determining the rate of onset of a local anesthetic?
The pKa. Because local anesthetics are all weak bases.
Only free bases can cross membranes and enter cells and the amount of free base present at a physiological pH depends on the pKa.
LAH+ = LA + H+
pKa is the pH at which 50% is LAH and 50%is LA. A pKa higher than the pH = more LAH and less free base.
Local anesthetics don’t work well when injected into an acidic or hypoxic environment.
What determines how potent a local anaesthetic is?
The lipid solubility
What determines a local anaestheics duration of action?
Protein binding, more protein biding means longer duration.
What are the features of lignocaine and what is it used for?
Low lipid solubility (low potency) Low protein binding (short duration) Low pKA (fast acting) Good for short surgical procedures, such as mole removal or short dental procedures.
What are the features of bupivacaine and what is it used for?
Highly lipid soluble (potent)
High protein binding (long duration)
High pKa (slow acting)
Ideal for nerve blocks for analgesia.
What are the adverse events associated with LA?
CNS - seizure and cardiac - heart attack.
The ratio of the concentration require to give a seizure vs that required to cause a heart attack is used.
Usually by injecting into a vein bent.
How can LA be administered what is the use of each route?
Topical to skin - used on children prior to insertion of an IV cannulae. EMLA, a mixture of prilocaine and lignocaine as an oil (lipid base for crossing the skin).
Topical to mucus membranes - cocaine (also causes vasoconstriction)
For soft tissue infiltration. Injections into the skin. For mole removal.
Nerve blocks - peripheral nerve or neuraxial block (spine)
