LOCAL ANESTHETICS AND REGIONAL ANESTHESIA
UNIFORMED SERVICES UNIVERSITY OF THE HEALTH SCIENCES
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
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 = 1 mg epi = 1 cc
1000cc’s solution
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
1000 mg epi 200,000 cc’s solution
1 cc solution
Local anesthetics - vasoconstrictors
Vasoconstrictors should not be used in the following locations • Fingers • Toes • Nose • Ear lobes • Penis
REGIONAL ANESTHESIA
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”