Local Anesthetics Flashcards
Properties of an ideal local anesthetics?
short onset, long action
remain at site of injection
act predictably/reversibly w/o tissue damage
possess lg safety margin
What are the three components of a local anesthetic?
Aromatic ring
Intermediate linkage (ester or amide)
terminal amine
What does the aromatic ring portion of local anesthetic determine?
lipophilicity which determines potency & duration of action
local anesthetics w/ high protein binding have longer duration of action
What are esters and amides metabolized by respectively?
Plasma esterases
Hepatic Amidases
Characteristics of amino esters?
shorter duration of action and increased risk of allergy in comparison to amino amides
Name four amino esters.
Cocaine, procaine, benzocaine, tetracaine
Name six amino amides.
Lidocaine, Mepivicaine, Bupivicaine, Prilocaine, Ropivicaine, Dibucaine
Where are amino amides metabolized?
Liver, increased toxicity risk w/ decreased liver fx
How are amino amides cleared?
kidneys
What does the terminal amine portion of local anesthetics determine?
hydrophilicity which correlates with onset of action
What determines the onset of action?
pKa of the anesthetic and local pH
When can a local anesthetic diffuse across a cell membrane?
When the terminal amine is unionized
When is a local anesthetic active?
When the terminal amine is ionized (undiffusable)
What is the pKa?
Is the pH at which the amount of ionized and unionized drug is equal.
What effect does an acidic local environment have on a local anesthetic?
Favors the ionized form and prolongs onset of action
What effect does mixing a local anesthetic with sodium bicarbonate have?
Favors the unionized diffusible state and speeds up onset.
MOA of local anesthetics?
Blocks intracellular portion of inactivated voltage gated Na+ channels
Name three factors that effect the nerve fiber blockade?
size of fiber- smaller, quicker
degree of myelination - more myelinated, quicker
firing frequency- the higher the firing frequency the greater the # of Na+ channels and easier to block
A higher pKa leads to more ionized or unionized form of local anesthetic?
More ionized form, less unionized (longer time to penetrate membrane)
What three things effect local anesthetic absorption?
Site of injection (highly vasc absorbs more than less vasc)
presence of vasoconstrictors (decreases absorption)
Degree of tissue binding (higher, less absorbed)
How does systemic toxicity progress?
Begins as CNS excitation (decreased inhibition, sensory disturbances, tremor, restlessness)
then CNS depression (lethargy, hypotension, seizures)
finally Cardiovascular problems (reduced conductivity, contractility, and excitability)
What two drugs are at risk of causing methemoglobinemia in large doses?
Benzocaine and prilocaine
Is it normal or unusual for a pt to have allergies to local anesthetics?
Unusual
