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 non-toxic concentrations
Local anesthetics - Mechanism Limit influx of sodium, thereby limiting propagation of the action potential.
Local Anesthetics - Classes Esters
Local anesthetics - Classes (Rule of “i’s”) Esters Cocaine Chloroprocaine Procaine Tetracaine Am”i”des Bupivacaine Lidocaine Ropivacaine Etidocaine Mepivacaine
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
1000 mg epi 1000cc’s solution 1 mg epi 1 cc 1 gram epi 1000 cc’s solution = = 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 200,000 cc’s solution 1000 mg epi 200,000 cc’s solution Local anesthetics - vasoconstrictors Therefore, a 1 : 200,000 solution of epinephrine would be = or 5 mcg epi 1 cc solution
Local anesthetics - vasoconstrictors Vasoconstrictors should not be used in the following locations • Fingers • Toes • Nose • Ear lobes • Penis
Regional anesthesia - Definition Rendering a specific area of the body, e.g. foot, arm, lower extremities, insensate to stimulus of surgery or other instrumentation
Regional anesthesia - Uses • Provide anesthesia for a surgical procedure • Provide analgesia post-operatively or during labor and delivery • Diagnosis or therapy for patients with chronic pain syndromes
Regional anesthesia - types • Topical • Local/Field • Intravenous block (“Bier” block) • Peripheral (named) nerve, e.g. radial n. • Plexus - brachial, lumbar • Central neuraxial - epidural, spinal
Topical Anesthesia • Application of local anesthetic to mucous membrane - cornea, nasal/oral mucosa • Uses : • awake oral, nasal intubation, superficial surgical procedure • Advantages : • technically easy • minimal equipment • Disadvantages : • potential for large doses leading to toxicity
Local/Field Anesthesia • Application of local subcutaneously to anesthetize distal nerve endings • Uses: • Suturing, minor superficial surgery, line placement, more extensive surgery with sedation • Advantages: • minimal equipment, technically easy, rapid onset • Disadvantages: • potential for toxicity if large field
IV Block - “Bier” block • Injection of local anesthetic intravenously for anesthesia of an extremity • Uses • any surgical procedure on an extremity • Advantages: • technically simple, minimal equipment, rapid onset • Disadvantages: • duration limited by tolerance of tourniquet pain, toxicity
Peripheral nerve block • Injecting local anesthetic near the course of a named nerve • Uses: • Surgical procedures in the distribution of the blocked nerve • Advantages: • relatively small dose of local anesthetic to cover large area; rapid onset • Disadvantages: • technical complexity, neuropathy
Plexus Blockade • Injection of local anesthetic adjacent to a plexus, e.g cervical, brachial or lumbar plexus • Uses : • surgical anesthesia or post-operative analgesia in the distribution of the plexus • Advantages: • large area of anesthesia with relatively large dose of agent • Disadvantages: • technically complex, potential for toxicity and neuropathy.
Central neuraxial blockade - “Spinal” • Injection of local anesthetic into CSF • Uses: • profound anesthesia of lower abdomen and extremities • Advantages: • technically easy (LP technique), high success rate, rapid onset • Disadvantages: • “high spinal”, hypotension due to sympathetic block, post dural puncture headache.
Central Neuraxial Blockade - “epidural” • Injection of local anesthetic in to the epidural space at any level of the spinal column • Uses: • Anesthesia/analgesia of the thorax, abdomen, lower extremities • Advantages: • Controlled onset of blockade, long duration when catheter is placed, post-operative analgesia. • Disadvantages: • Technically complex, toxicity, “spinal headache”