Local Anaesthetics Flashcards
How do local anaesthetics work?
By blocking the voltage gated sodium channel, they enter the channels by entering a neuron unionised, then ionising the cell and blocking the channel
What is the base structure of a local anaesthetic?
Hydrophobic group,
Amine group
Amide or ester linkage
What is important about the ester/amide linkage?
Determines site of metabolism, esters are more rapidly metabolised and thus shorter acting, and allergenic
What are all local anaesthetics?
Weak bases, meaning they exist as free base or ionised form.
What will produce the amount of free base at a physiological pH?
Note free base is what enters cells.
The pKa of the drug (i.e the pH at which 50% of the drug is free baseand vice versa.)
The closer the physiological pH is to the pKa of the drug, the more free base and thus faster onset. A ‘lower’ pKa means more free base
What happens if you lengthen the alkyl chain?
Makes them more lipid soluble, meaning more potent
How does protein binding affect duration of action?
More protein binding means a longer duration of action
How are LA’s metabolised?
Esters: plasma cholinesterases
Amides: liver
Lignocaine: type; lipid sol; ; protein binding; pka; use
Type: amide Lipid sol: low, low potency Protein binding: low, short duration pKa: low, more free base, fast onset Use: short surgical procedures
Bupivicaine: type; lipid sol; protein binding; pKa; use
Type: amide Lipid solubility: High, potent Protein binding: high, long duration pKa: High, less free base, slower onset Use: nerve block analgesia
Other LA’s
Cocaine: topical to nose, vasoc.
Prilocaine: like lignocaine, used in IV regional anaesthesia. Bier’s block. Not toxic
Ropivacaine: like bupivicaine, less cardiac toxicity
LA toxicity: Allergy; CNS; CVS
note these are dose dependent
Allergy somtimes with esters
CNS: can have seizures
CVS: Vfib, bupivicaine can be culprit
Dose dependent toxicities usually inadvertent IV administration
Topical administration
EMLA, oil of lignocaine and prilocaine. for children getting IV’s
Cocaine for mucous membranes
Soft tissue administration
Lignocaine for small procedures
Bupivicaine for postop pain relief, as slow acting long duration
Peripheral Nerve block
LA’s is injected around specific nerve or nerves to produce numbing. E.g brachial plexus block for arm
