Local anesthetics Flashcards
What is the mechanism of action of local anesthetics?
Blocks voltage-gated Na channels in neuronal cell membrane; slows rate of depolarization (threshold potential not reached; action potential not propagated)
True/False: in general, smaller diameter fibers and those with less myelination are blocked first.
True
What is the exception to the order of nerve blockade with local anesthetics?
Brachial plexus block. Motor fibers are blocked first
What is the order of nerve blockade when using local anesthetics?
1) B fibers (preganglionic sympathetic)
2) A-delta and C fibers (pain)
3) A-gamma (proprioception)
4) A-beta (touch, pressure)
5) A-alpha (motor)
True/False: all local anesthetics are weak acids.
False. All are weak bases; pK ~7.7-9
How does pK affect local anesthetics?
Lower pK (closer to tissue pH) –> faster onset
How does protein binding affect local anesthetics?
Higher protein binding –> longer duration of action
How does lipid solubility affect local anesthetics?
Higher lipid solubility –> higher potency
What are some commonly used amides?
Lidocaine, bupivacaine (Marcaine), mepivacaine (Carbocaine), ropivacaine, prilocaine (with lidocaine in EMLA cream)
True/False: all local anesthetics can be given IV or used for intravenous regional anesthesia (IVRA).
False. Lidocaine is the only LA that can be given IV or used for IVRA
What are the pharmacodynamics of lidocaine?
Fast onset, short duration of action (1-2 hours when used in loco-regional techniques
What are the effects of lidocaine when used systemically (IV)?
Anti-arrhythmic; decreases MAC (MAC-sparing); analgesic; free radical scavenger; improves GI motility
What are the pharmacodynamics and adverse effects of bupivacaine?
Intermediate onset; duration of 3-8 hours; has the highest CV toxicity
What are the pharmacodynamics and uses of mepivacaine?
Fast onset; duration of 1.5-3 hours; used for intra-articular nerve block (common in large animal)
What are the pharmacodynamics and adverse effects of ropivacaine?
Intermediate onset; duration of 3-8 hours; less cardiotoxic than bupivacaine
What are some commonly used esters?
Procaine (procaine penicillin G (PPG)); tetracaine and proparacaine (ophthalmic preparations); benzocaine (historically used in laryngeal spray for intubation)
What blocks have the fastest absorption?
Intercostal blocks (highest peak plasma concentration)
How does epinephrine affect LAs?
Added to prolong duration of block; causes vasoconstriction, SLOWING systemic absorption and increasing duration of action at local site (most LAs cause vasodilation on their own)
What is another additive used with LAs?
Bicarbonate. Faster onset of action and prolonged duration; less sting on injection
True/False: when combining two LAs, it is ok to use the max dose of both drugs.
False. Toxicity is additive; can’t use max dose of both agents.
What are some types of toxicity caused by LAs?
Methemoglobinemia; neurotoxicity; chondrotoxicity; systemic toxicity
What LAs cause methemoglobinemia?
Benzocaine and prilocaine; seen in many species
Neurotoxicity caused by LAs
Concentration dependent; permanent nerve injury extremely rare with normal clinical use; spinal lidocaine seems to be worse than bupivacaine; preservative-free versions should be used for epidurals and spinal anesthesia
Chondrotoxicity caused by LAs
Concentration and time dependent; bupivacaine most damaging (not administered IA anymore for this reason); mepivacaine (Carbocaine) leas damaging (used for equine lameness diagnosis IA)
What causes systemic toxicity from LAs?
Due to excessive plasma concentrations (inadvertant intravascular injection; excessive dose, intolerant patient)
What are the systemic toxicity signs of lidocaine?
Depression/sedation, twitching, seizures, then CV signs; not clinically apparent if animal is anesthetized
What are the systemic toxicity signs of bupivacaine?
Supra ventricular tachycardia, ventricular tachycardia, AV block, wide QRS complexes; hypotension; first sign is CV collapse –> death
How do LAs cause CV toxicity?
Block cardiac Na channels
What will you see if epinephrine is included with LAs?
Increasing heart rate as first sign of intravascular injection; can alert to IV injection and prevent additional drug administration
Risk factors of LAs
IV injection; increased absorption (intercostal > epidural > brachial plexus); patient factors (hypoproteinemia; pregnancy; Beta- or Ca channel-blockade; hypoxemia, acidosis
How do you treat arrest secondary to LA?
Start CPR; low dose epinephrine; give 20% lipid emulsion (Intralipid); avoid lidocaine, Ca-channel blockers, Beta-blockers, vasopressin
True/False: allergic reactions to LAs occur most commonly with procaine in PPG.
True. PPG metabolized to PABA which is a common allergen. Much less commonly occur with amides
What are the goals of local anesthesia?
Provides analgesia before, during, and after procedure; may allow decreased dose of systemic drugs (MAC-sparing; decreased hypotension and other adverse effects of general anesthesia); prevents central sensitization; important part of multimodal anesthesia
What the different types of local anesthesia?
Topical (epidermis, eye, laryngeal); subcutaneous (mass removal, wounds); line block (ventral midline, flank)
What are the different types of regional anesthesia?
Intravenous regional (IVRA); peripheral (individual nerve; plexus; intercostal; paravertebral); central (epidural; spinal)
How is an IVRA (“Bier block”) done?
Limb is wrapped tightly from distal to proximal and tourniquet placed; local anesthetic injected into peripheral vein, diffuses into target tissues; released into systemic circulation when tourniquet is removed (lidocaine ONLY); procedure time limit of 60-90 min
What is epidural anesthesia used for?
Tail/perineum, hindlimb, abdominal/thoracic
What are the contraindications for an epidural?
Coagulopathy; hypovolemia; infection at injection site; neoplasia at injection site; anatomy (inability to palpate landmarks); sepsis. CHINAS
What are adverse effects of epidurals?
Depends on drug used (LAs, opioids, alpha-2 agonists); hypotension (block of sympathetic trunk –> decreased vascular tone); motor block/paralysis (not a big deal in small animal, but is for a horse); hematoma, infection, neurotoxicity
