Local Anesthetics Flashcards
What are local anesthetics used for?
To reversibly block nerve conduction in a particular region of the body
Whats are LAs synthetic analogs for?
Cocaine
At high doses what are the negative side effects of LAs?
They can readily enter systemic circulation and cause convulsions or respiratory failure
What are the ways in which a LA can be administered?
Infiltration Nerve Block Epidural Spinal Intravenous
Describe Infiltration administration of a LA
Most common means of injecting, subdermally. Nerve endings are totally paralyzed
Describe nerve block administration of a LA
less predictable and requires precise knowledge of anatomical position.
Describe epidural administration of a LA
Injected near the dorsal root of spinal cord and is capable of blocking all sensation below the segment exposed to the LA. NO NEEDLE PENETRATION OF THE DURA. Intensity and duration are less predictable than spinal
Describe spinal administration of a LA
Injected intrathecally, diffuses through the CSF to motor as well as sensory neurons. VERY PREDICTABLE.
Describe intravenous administration of a LA
Injected into a limb whose venous return has been limited with a tourniquet.
What is the most common usage of a intravenous LA?
Resetting Fractures
What are the three molecular properties of LAs?
- Ionizable head group
- Ester/Amide connecting chain
- Non-polar tail, usually containing a phenyl ring
What can clinically used LAs also be considered? What do they contain?
Weak bases containing a tertiary amino group as hydrophilic cation group
What are the two main sub groups of LAs?
Esters and Amides
What are the Ester type LAs?
Cocaine Procaine Chloroprocaine Tetracaine Benzocaine
What are the amide type LAs?
Lidocaine
Mepivicaine
Bepivicaine
What is the mechanism of LAs?
They preferentially block voltage dependent sodium channels in nerve and muscle membranes.
What is an odd thing about the receptor site for LAs? What importance does this play?
The receptor site occurs within the sodium channel nearest the cytoplasmic side of the membrane. LAs ARE TOO LARGE TO PASS THROUGH THE CHANNEL FROM THE OUTSIDE. The unionized form has to cross the membrane and then dissociate to the cationic form to bind to the receptor.
Why are LAs more effective on active cells?
The intensity of the LA increases with spike frequency, because the opened and inactivated states of the sodium channel (which bind LAs most readily) are increasingly available for occupation
Which form of LAs most rapidly penetrates the cell thus taking effect quciker?
Unionized form
Which for of LAs are more active at blocking the NA channel but take longer to come into effect?
Cationized form
What is a detrimental side effect of Bupivicaine?
Can be cardio toxic because it has a slow rate of dissociation from the myocardial sodium channel. It can modify the normal frequency range of the myocardium
What does Lidocaine selectively block?
tachycardias
Factors controlling the time, course and intensity of local anesthesia:
Method of administrative dose
Lipid solubility (more lipophilic = more potent and longer duration
Membrane permiability barriers
Vascular Absorbtion
What is the major factor determining the duration of LAs?
Vascular absorbtion
What are LAs usually used in combo with to prolong duration and potency?
epinephrine
What LA is unique in that it has a vasoconstrictory response thus not needing epinephrine?
Cocaine
What can happen if LAs are incorrectly injected or absorbed too rapidly?
Can cause CNS convulsions and depression (leading to respiratory depression)
What is a significance of what can happen if LAs are incorrectly injected or absorbed too rapidly?
There are no antagonists to these effects, treat with artificial respiration and anticonvulsant
What is extremely sensitive to LA block?
Synaptic transmission
Allergic reactions are most frequently seen with what type of LAs?
Para amino benzoic acid ester LAs; rare with amide types
How is benzocaine given to a patient?
Applied topically
How and where are ester type LAs metabolized?
They are hydrolyzed by PLASMA (nonspecific) cholinesterase
How and where are amide type LAs metabolized?
They are dealkylinated and hydrolyzed in the liver
What is the metabolism of LAs important in controlling?
the anti arrythmic duration of procainamide and lidocaine
fetal CNS depression due to absorption of LA from maternal circulation
