michael h. ossipov, ph.d. department of pharmacology local/regional anesthetics
Post on 20-Dec-2015
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General concepts
•Cocaine isolated from Erythroxylon coca plant in Andes
•Von Anrep (1880) discovers local anesthetic property, suggests clinical use
•Koller introduces cocaine in opthalmology
•Freud uses cocaine to wean Karl Koller off morphine
•Halstead demonstrates infiltration anesthesia with cocaine
•Rapidly accepted in dentistry
General concepts
• Halstead (1885) shows cocaine blocks nerve conduction in nerve trunks
• Corning (1885) demonstrates spinal block in dogs
• 1905: Procaine (NOVOCAINE) synthesized– analog of cocaine but without euphoric
effects, retains vasoconstrictor effect– Slow onset, fast offset, ester-type (allergic
reactions)
General concepts
• First “modern” LA (1940s): lidocaine (lignocaine in UK; XYLOCAINE) – Amide type (hypoallergenic) – Quick onset, fairly long duration (hrs)– Most widely used local anesthetic in US today,
along with bupivacaine and tetracaine
General concepts
• Cause transient and reversible loss of sensation in a circumscribed area of the body– Very safe, almost no reports of permanent nerve
damage from local anesthetics• Interfere with nerve conduction• Block all types of fibers (axons) in a nerve
(sensory, motor, autonomic)
Local anesthetics: Uses
• Topical anesthesia (cream, ointments, EMLA)• Peripheral nerve blockade• Intravenous regional anesthesia• Spinal and epidural anesthesia• Systemic uses (antiarrhythmics, treatment of
pain syndromes)
Structure•All local anesthetics are weak bases. They all contain:
•An aromatic group (confers lipophilicity)
- diffusion across membranes, duration, toxicity increases with lipophilicity
•An intermediate chain, either an ester or an amide; and
•An amine group (confers hydrophilic properties)
– charged form is the major active form
Structure PKa % RN at PH 7.4
Onset in minutes
Mepivicaine 7.6 40 2 to 4
Etidocaine 7.7 33 2 to 4
Articaine 7.8 29 2 to 4
Lidocaine 7.9 25 2 to 4
Prilocaine 7.9 25 2 to 4
Bupivicaine 8.1 18 5 to 8
Procaine 9.1 2 14 to 18
•Formulated as HCl salt (acidic) for solubility, stability
•But, uncharged (unprotonated N) form required to traverse tissue to site of action
•pH of formulation is irrelevant since drug ends up in interstitial fluid
•Quaternary analogs, low pH bathing medium suggests major form active at site is cationic, but both charged and uncharged species are active
O
COCHH N2
CCH22
H
H
2
N5
C 52
HC 52
HC 52
O
COCHH N2 CH22 N H + H+
Nonionized baseCationic acid
BaseAcid
Log = pH – pKa
(Henderson-Hasselbalch equation)
BaseAcid
0.03=
For procaine (pK = 8.9)at tissue pH (7.4)
a
Base Acid
Lipoid barriers (nerve sheath)
Extracellular fluid
Axoplasm Base Acid
*Nerve membrane
[1.0]
[2.5]
[1.0]
[3.1]
Mode of action• Block sodium channels• Bind to specific sites on channel protein• Prevent formation of open channel• Inhibit influx of sodium ions into the neuron • Reduce depolarization of membrane in response to action potential• Prevent propagation of action potential
Sensitivity of fiber types• Unmyelinated are more sensitive than myelinated nerve
fibers• Smaller fibers are generally more sensitive than large-
diameter peripheral nerve trunks• Smaller fibers have smaller “critical lengths” than larger
fibers (mm range)• Accounts for faster onset, slower offset of local
anesthesia• Overlap between block of C-fibers and A-fibers.
Choice of local anesthetics
• Onset• Duration• Regional anesthetic technique• Sensory vs. motor block• Potential for toxicity
Clinical use Onset Duration Esters Procaine Slow Short Chloroprocaine Fast Short Tetracaine Slow Long Amides Lidocaine Fast Moderate Mepivacaine Fast Moderate Bupivacaine Moderate Long Ropivacaine Moderate Long Etidocaine Fast Long
Technique Appropriate drugsTopical Cocaine, tetracaine, lidocaineInfiltration Procaine, lidocaine, mepivacaine,
bupivacaine, ropivacaine,etidocaine
Peripheral nerve block Chloroprocaine, lidocaine,mepivacaine, bupivacaine,ropivacaine, etidocaine
Spinal Procaine, tetracaine, lidocaine,bupivacaine
Epidural Chloroprocaine, lidocaine,bupivacaine, ropivacaine,etidocaine
I.V. regional anesthesia Lidocaine
Choice of local anesthetics
Factors influencing anesthetic activity
• Needle in appropriate location (most important)
• Dose of local anesthetic• Time since injection• Use of vasoconstrictors• pH adjustment• Nerve block enhanced in pregnancy
Redistribution and metabolism
• Rapidly redistributed• More slowly metabolized and eliminated• Esters hydrolyzed by plasma cholinesterase• Amides primarily metabolized in the liver
Allergy
• Ester local anesthetics may produce true allergic reactions– Typically manifested as skin rashes or
bronchospasm. May be as severe as anaphylaxis– Due to metabolism to ρ-aminobenzoic acid
• True allergic reactions to amides are extremely rare.
Systemic toxicity
• Results from high systemic levels• First symptoms are generally CNS
disturbances (restlessness, tremor, convulsions) - treat with benzodiazepines
• Cardiovascular toxicity generally later
CNS symptoms
• Tinnitus• Lightheadedness, Dizziness• Numbness of the mouth and tongue, metal taste
in the mouth• Muscle twitching• Irrational behavior and speech• Generalized seizures
• Coma
Cardiovascular toxicity
• Depressed myocardial contractility• Systemic vasodilation• Hypotension• Arrhythmias, including ventricular fibrillation
(bupivicaine)
Avoiding systemic toxicity
• Use acceptable total dose• Avoid intravascular administration (aspirate
before injecting)• Administer drug in divided doses
Maximum safe doses of local anesthetics in adults
Anesthetic Dose (mg)
Procaine 500
Chloroprocaine 600
Tetracaine 100 (topical)
Lidocaine 300
Mepivicaine 300
Bupivacaine 175
Uses of Local Anesthetics
•Topical anesthesia- Anesthesia of mucous membranes (ears, nose,
mouth, genitourinary, bronchotrachial)- Lidocaine, tetracaine, cocaine (ENT only)
•EMLA (eutectic mixture of local anesthetics)cream formed from lidocaine (2.5%) & prilocaine
(2.5%) penetrates skin to 5mm within 1 hr, permits superficial procedures, skin graft harvesting
•Infiltration Anesthesia- lidocaine, procaine, bupivacaine
(with or w/o epinephrine)- block nerve at relatively small
area- anesthesia without
immobilization or disruption of bodily functions- use of epinephrine at end arteries
(i.e.; fingers, toes) can cause severe vasoconstriction leading to gangrene
Uses of Local Anesthetics
•Nerve block anesthesia- Inject anesthetic around plexus (e.g.; brachial
plexus for shoulder and upper arm) to anesthetize a larger area- Lidocaine, mepivacaine for blocks of 2 to 4 hrs,
bupivacaine for longer•Bier Block (intravenous)
- useful for arms, possible in legs- Lidocaine is drug of choice, prilocaine can be
used- limb is exsanguinated with elastic bandage,
infiltrated with anesthetic- tourniquet restricts circulation- done for less than 2 hrs due to ischemia, pain
from touniquet
•Spinal anesthesia- Inject anesthetic into lower CSF (below L2)- used mainly for lower abdomen, legs, “saddle
block”- Lidocaine (short procedures), bupivacaine
(intermediate to long), tetracaine (long procedures)- Rostral spread causes sympathetic block, desirable
for bowel surgery- risk of respiratory depression, postural headache
Uses of Local Anesthetics
•Epidural anesthesia- Inject anesthetic into epidural space- Bupivacaine, lidocaine, etidocaine, chloroprocaine- selective action of spinal nerve roots in area of
injection- selectively anesthetize sacral, lumbar, thoracic or
cervical regions- nerve affected can be determined by concentration - High conc: sympathetic, somatic sensory, somatic
motor- Intermediate: somatic sensory, no motor block- low conc: preganglionic sympathetic fibers- used mainly for lower abdomen, legs, “saddle
block”- Lidocaine (short procedures), bupivacaine
(intermediate to long), tetracaine (long procedures)- Rostral spread causes sympathetic block, desirable
for bowel surgery- risk of respiratory depression, postural headache
Uses of Local Anesthetics