University:
Michigan State University
Course:
ANES 621 | Concepts in Clinical Anesthesiology I (Interim New)
Academic year:

2020
Views:

126

Pages:

19
Author:

teespagmaxrtf

Local Anesthetics- History ● ● ● ● 1860 - cocaine isolated from erythroxylum coca Koller - 1884 uses cocaine for topical anesthesia Halsted - 1885 performs peripheral nerve block with local Bier - 1899 first spinal anesthetic Local Anesthetics - Definition A substance which reversibly inhibits nerve conduction when applied directly to tissues at nontoxic concentrations ● ● Local anesthetics block generation, propagation, and oscillations of electrical impulses in electrically excitable tissue. Mainly by acting on Na channels. PHARMACOLOGY AND PHARMACODYNAMICS Clinically used local anesthetics consist of lipidsoluble, substituted benzene ring linked to amine group via alkyl chain containing either an amide or ester linkage. • Type of linkage separates local anesthetics into either aminoamides (metabolized in liver) or aminoesters (metabolized in liver or by plasma cholinesterase). PHARMACOKINETICS Systemic toxicity depends on blood levels of local anesthetic. • End organs of main concern for toxicity are CV and CNS. • Determinants of systemic absorption » Site of injection (intercostal > caudal > brachial plexus > sciatic/femoral) » Dose » Physiochemical properties (lipid solubility, protein binding) » Addition of epinephrine Local anesthetics - Mechanism Limit influx of sodium, thereby limiting propagation of the action potential. Local Anesthetics - Classes Esters Local anesthetics - Classes Esters Cocaine Chloroprocaine Procaine Tetracaine Amides Bupivacaine Lidocaine Ropivacaine Etidocaine Mepivacaine Bupivacaine Amide • Infiltration: use 0.25%, fast onset, 2- to 8-hr duration, max dose 175 mg (225 mg with epinephrine) • Peripheral nerve block: use 0.25-0.5%, slow onset, 4- to 12-hr duration, max dose 175 mg (225 mg with epinephrine Bupivacaine ● Epidural anesthesia: Use 0.5-0.75%, moderate onset, 2- to 5-hr duration, max dose 175 mg (225 mg with epinephrine) • Spinal anesthesia: Use 0.5-0.75%, fast onset, 1- to 4-hr duration, max dose 20 mg levo (-) bupivacaine less cardiotoxic than racemic bupivacaine, same Lidocaine Amide • Infiltration: use 0.5-1%, fast onset, 2- to 8-hr duration, max dose 300 mg (500 mg with epinephrine) • Peripheral nerve block: use 1.0-1.5%, fast onset, 1- to 3-hr duration, max dose 300 mg (500 mg with epinephrine) Lidocaine Epidural anesthesia: use 1.5-2%, fast onset, 1- to 2-hr duration, max dose 300 mg (500 mg with epinephrine) • Spinal anesthesia: use 1.5-2%, fast onset, 0.5- to 1hr duration, max dose 100 mg • Topical anesthesia: use 4%, fast onset, 0.5- to 1-hr duration, max dose 300 mg • IV regional: Use 0.25-0.5%, fast onset, 0.5-1 hr duration, max dose 300 mg Ropivacaine Amide • Infiltration: use 0.2-0.5%, fast onset, 2- to 6-hr duration, max dose 200 mg • Peripheral nerve block: use 0.5-1.0%, slow onset, 5- to 8-hr duration, max dose 250 mg • Epidural anesthesia: Use 0.5-1.0%, moderate onset, 2- to 6-hr duration, max dose 200 mg · less cariac toxicity than bupivacaine Local anesthetics - Formulation Biologically active substances are frequently administered as very dilute solutions which can be expressed as parts of active drug per 100 parts of solution (grams percent) Ex.: 2% solution = _2 grams__ = _2000 mg_ = __20 mg__ 100 cc’s 100 cc’s 1 cc Local Anesthetics - Allergy ● ● ● True allergy is very rare Most reactions are from ester class - ester hydrolysis (normal metabolism) leads to formation of PABA - like compounds Patient reports of “allergy” are frequently due to previous intravascular injections Local Anesthetics - Toxicity ● ● Tissue toxicity - Rare Can occur if administered in high enough concentrations (greater than those used clinically) Usually related to preservatives added to solution ● ● Systemic toxicity - Rare Related to blood level of drug secondary to absorption from site of injection. Range from lightheadedness, tinnitus to seizures and CNS/cardiovascular collapse Local anesthetics - Duration ● ● ● Determined by rate of elimination of agent from site injected Factors include lipid solubility, dose given, blood flow at site, addition of vasoconstrictors (does not reliably prolong all agents) Some techniques allow multiple injections over time to increase duration, e.g. epidural catheter Local anesthetics - vasoconstrictors Ratios Epinephrine is added to local anesthetics in extremely dilute concentrations, best expressed as a ratio of grams of drug:total cc’s of solution. Expressed numerically, a 1:1000 preparation of epinephrine would be 1 gram epi 1000 cc’s solution 1000 mg epi = 1000cc’s solution = 1 mg epi 1 cc Local anesthetics - vasoconstrictors Therefore, a 1 : 200,000 solution of epinephrine would be 1 gram epi 200,000 cc’s solution = or 5 mcg epi 1 cc solution 1000 mg epi 200,000 cc’s solution Local anesthetics - vasoconstrictors ● ● ● ● ● Vasoconstrictors should not be used in the following locations Fingers Toes Nose Ear lobes Penis

Local Anesthetics

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