Local Anesthetics and Spasmolytics Flashcards
Definition of local anesthetic
Any agent that can reversibly block electrical activity of excitable tissues
Local analgesic-temporary-w/o loss of cons.
Structure of local anes.
Aromatic (lipophilic) end -Gets molecule to neuronal memb.
Connected via ester or amide linkage
Proparacaine and procaine-ester
Lidocaine-amide
Amino (hydrophilic) end-this end gets ionized
LA mech of action
NI form get through neuronal membrane
Hydrophilic side then ionized and binds (blocks) specific site on the Na channel of excitable membrane
This leads to:
- Decreased nueronal conduction
- Decreased repolarization rate
- Increased refractory period
Effect greater when Na channel open
Neurons that fire more rapidly will have open channels more often so these are affected more than other neurons
LA affects which nerves?
All can be affected
More susceptible:
Smaller neurons (pain) more susceptible than larger (motor)
Myelinated neurons
Faster firing rate
Pain>Cold>Warm>Touch/pressure>motoneurons
What is major difference between the LAs?
Duration of action
Ester link=short duration of action b/c ester metabolized by plasmacholinesterase. T1/2=minutes
Amide link=last longer, metabolized by CYP450. T1/2=hrs
LAs are weak/strong acid/base? Why does this matter?
Weak bases
More acidic means more is ionized form
Matters b/c infected tissues are more acidic so LAs dont enter these cells as easily
Also, repeated use of an epidural anesthetic can lower pH and cause tachyphylaxis (rapid desens).
Routes of administration of LAs
Topical-skin, mucous, eye
Injection:
Infiltration (diffusion)
Direct nerve block
Spinal (subarachnoid space. Wide effect)
Epidural (outside of dura mater. More local than spinal)
May be systemically absorbed regardless of route
Why do we give epi w/ LAs?
Which do we never give epi w/?
LAs may be vasodilators which increases their diffusion away from the site
So we use vasoconstrictor like epi to increase effectiveness
Never give epi w/ cocaine b/c procaine is a vasoconstrictor
*Must also be careful w/ epi and Beta blockers
Cocaine
Ester LA
Ophthalmic and nasal surgery
Mech: Inhibits reuptake of NE from CNS
SE:
Debridement-sloughing of superficial layer if applied directly to eye
Benzocaine
Ester LA
Topically-low solubility
Long duration
Procaine
Ester LA
Short T1/2
Not used topically-does not readily pass through mucous membrane
Benoxinate
Ester LA
Removal of foreign body from the eye
Proparacaine
Ester LA
Removal of foreign body from the eye
Procainamide
Amino Amides
(Contain two i’s)
IV for cardiac arrythmias
SE:
Lupus like in slow acetylators
Lidocaine
Amino Amides
(Contain two i’s)
Intermediate duration
IV for arrythmias
SE-affect CNS:
- Tremors
- Slurred speech
- Drowsiness
