STRUCTURE-ACTIVITY RELATIONSHIPS lipophillic group hydrophilic group ester linkage amide linkage potency Cm MLAC MEAC onset of action duration of action lipophilic group

-- STRUCTURE-ACTIVITY RELATIONSHIPS lipophillic group hydrophilic group ester linkage amide linkage potency Cm MLAC MEAC onset of action duration of action lipophilic group: benzene ring hydrophilic group: tertiary amine: usually carrys a positive charge at physiologic pH (making it a weak base) ester linkage intermediate chain amide linkage intermediate chain properties of local anesthetics are deteremined by: -substitutions on the aromatic ring -intermediate chain linkage (ex. ester vs amide) -alkyl groups attached to the nitrogen amine potency is increased by: -association with lipid solubility -ability of the local anesthetic to penetrate through a hydrophobic (lipophillic) environment -greater number of carbon atoms within the molecule -addition of a halide group to the aromatic ring 2-chloroprocaine more potent than procaine -ester linkage procaine more potent than procainamide -large alkyl groups on the tertiary amide nitrogen etidocaine more potent than lidocaine Cm -minumun concentration of a local anesthetic that will block nerve impulse conduction -measure of potency affected by: -nerve fiber size -nerve fiber type -myelinated vs nonmyelinated fibers -pH environment of the fiber/tissue -frequency of stimulation to the nerve -electrolyte plasma concentration MLAC -minimum local analgesic conentration -median effective local analgesic concentration within a 20 ml volume for epidural analgesia during the 1st stage of labor -measure of relative potency MEAC -minimum effective anesthetic concentration -concentration which a spinal anesthetic agent produces surgical anesthesia within 20 minutes of administration in 50% of patients Onset of Action depends on: pKa: -relative concentration of the nonionized (lipid soluable) and ionized (water soluable) form of the local anesthetic -pH when the amount of the nonionized and ionized form of local anesthetic are in equilibrium -rapid onset when the pKa close to physiologic pH will have more concentration of nonionized (lipid soluable) local anesthetic Addition of epinephrine -creates vasocontriction in nearby vessels preventing uptake intravascularly -less intravascular uptake of local anesthetic creates higher concentration at the site of injection therefore more rapid onset of action Alkalinization of local anesthetics by theaddition of bicarbonate may: -speed the onset of action -improve the quality of anesthetic block -prolong the anesthetic block -decrease pain upon injection of local anesthetic Duration of Action depends on: -correlation with plasma protein binding (ex. alpha1-acidic glycoprotein bind to local anesthetics)