local anasthetics Flashcards
How do local anasthetics work?
- Block excitation of nerve endings
- Lipid soluble so can pass easily into cells
- Ionised within cells + bind to Na+ receptors
- Depolarisation of cell membrane and action potential production
Describe the basic structure of local anaesthetics
Aromatic lipophillic portion
Intermediate chain: Amide or ester linkage
Amino hydrophillic portion
Compare amino esters vs amino amides
Amino esters: Metabolised in plasma
- more likely to produce allergic reactions
- unstable
Amino amides: Metabolised in the liver
- They are more stable and less likely to cause reactions
What is the most common local anasthetic used?
Why is it used?
Ligocaine = amino amide
- 1/2% solution = 10-20 micro grams per ml
- Fast onset and long duration ( 1 hour)
What is the pKa?
pH at which the drug is 50% ionised and 50% unionised
local anaesthetics need to be ionised to work
What are some side effects of local anaesthetics?
Pain+ discomfort Sedation Allergic reactions Methaglobinaemia : Fe2+ -> Fe3+ Systemic toxicity
How do adjuvants work?
Increase efficiency of drug by increasing the number of ionising particles in a cell
What are some properties of adjuvants?
- Minimal systemic absorption, high protein binding, high renal clearance, lipid soluble, increase pH/Pka, long shelf life, no side effects, enhance duration and onset of drug
How do adjuvants decrease uptake at site of injection?
Vasoconstrictors
Activate alpha 2 receptors
Vasoconstriction
Less release of NT in substantial gelatinous from A delta + C fibres
How do adjuvants change pH?
Sodium bicarbonate increases no of lipid soluble molecules
Increases tissue penetrability
How do adjuvants increase permeability across a tissue?
Hyluronidases break down hyluronic acid in the tissues
How do adjuvants prolong the duration of analgesia?
Alter transduction/transmission/modulation/perception
