Local Anesthetics Flashcards
3 structural components of local anesthetics
Lipophilic side (benzene ring) Hydrophilic side (quarternary amine) Intermediate chain linkage -Ester or -Amide
Ester metabolism
Via cholinesterase, rapid
Tend to be shorter acting than amides
Ester allergies
Higher potential than amides
If allergic to one ester, all esters should be avoided
Amide metabolism
In liver by CYP1A2 and CYP3A4
Longer acting generally because they’re more lipophilic and protein bound and require transport to the liver for metabolism
Amide allergies
Extremely rare No cross allergy among class or with esters
Nerve Fibers A alpha, A delta, B, and C function
A alpha: Motor
A delta: Fast pain
B: Preganglionic SNS
C: Slow pain
Side of chemical structure of local anesthetic that can be ionized or not (Lipophillic or hydrophilic)
Hydrophillic side
-Can be ionized (charged) or nonionized (not charged)
Mechanism of action of local anesthetics
- Nonionized portion (lipid soluble) enters the nerve
- Drug reequilibrates
- Ionized fraction attaches to receptor on the inside of the sodium channel, blocking it
Protein binding of local anesthetics determines:
Duration of action
- Higher percent protein binding = longer duration of action
- Binding stronger to proteins ~=binding stronger to tissues/nerves
Lipid solubility of local anesthetics determines:
Potency
Lipid solubility means they are nonionized and able to penetrate the myelin/nerve bilayer to get into the nerve to ionize and work
pKa of local anesthetics determines:
Onset of action
Lower pKa = faster onset
-Closer to 7.4 means theres a larger fraction that nonionized to enter the nerve
*Exception: Chloroprocaine has a high pKa but its fast because it’s given in such high concentrations
Effects on local anesthetics when epinephrine is added
- Shorter onset time (faster) *Except during spinal anesthesia
- More depth motor and sensory blockage
- Longer duration of blockade
- Larger area of blockade
- Lower peak plasma concentration (less toxicity)
Ester name
1 I
Amide name
2 I’s
Ropivacaine characteristics
Similar to bupivacaine but doesn’t have the cardiac effects/risks
Bupivacaine problem, max dose
Preferentially binds to cardiac cells over others, can cause sudden cardiac arrest
- .75% no longer allowed in US
- Max dose = 3mg/kg