Exam 1: Local Anesthetics

how is local anesthesia generally defined

• loss of sensation

• no loss of consciousness

• block of action potential initiation or conduction in nerves

• voltage gated Na+ channels

• synthetic analogs of cocaine

what is the CNS action of -caine

• binds to dopamine reuptake transporters and blocks it

what is te general structure of local anesthetics

• aromatic portion is hydrophobic

• intermediate alkyl chain= ester, amide

  • amine portion = hydophilic, tertiary, secondary amine

what are the ester locals

• cocaine

• procaine

• tetracaine

• chloroprocaine

what are the amide locals

• lidocaine

• mepivacaine

• etidocaine

• prilocaine

• bupivacaine

• ropivacaine

what is the target of local anesthetic drugs

• voltage gated sodium channel

• block of action potential initiation/conduction

what is the effect of pH on locals

• charged cationic form binds to receptor site

• uncharged form penetrates membrane

• effect of drug can be changed by altering extra or intracellular pH

• (cationic form)/ (neutral form) = 10^)pKa-pH)

what are group A nerve fibers

large, myelinated, somatic fibers

what are group B nerve fibers

small, myelinated, autonomic fibers

what are group C nerve fibers

small, non-myelinated, temperature and pain

what is a selective block of differential blockade

• B fibers (vasodilation) → A(delta) fibers (loss of sensation of temperature, sharp pain, and touch) → A(alpha) fibers (loss of motor activity)

what are influencing factors for selective blocks

• myelination

• fiber size

• position of fiber within nerve

• length of nerve exposed to local aka critical length

• choice of local

• concentration of local

what is the use of locals as topical or surface anesthesia

• superficial desensitiation of nerve terminals

• at site of stimulation before it is produced

• sprayed, creams, patches

• lidocaine

• mucosal membranes

how are locals used as infiltration anesthesia

• deposition into or at surgical sites

• targets nerve terminals

• injection or fenestrated catheters

• lidocaine, mepivacaine, bupivacaine, ropivacaine

• line block, inverted block

• easy to perform with minimal equiptment, but incomplete blockade

how are locals used as intraveous regional anesthesia

• aka Bier block

• administration into a peripheral vein

• no longer than 90 min

• lidocaine

how are locals used for neuroaxial anesthesia

• epidural- superficial to dura mater

• spinal or intrathecal- subarachnoid space

• bathing spinal nerve roots for larger area of desensitization, sensory, motor and sympathetic blockade

• operator skill, knowedge of anatomy

• reliant on subjective criteria

• lidocaine, mepivacaine, bupivacaine, ropivacaine

how are locals used for peripheral nerve blocks

• adjacent to peripheral erve

• sensory and or motor blockade

• knowledge of anatomy

• methods to ID location include anatomical landmarks and palpation, nerve stimulators, ultrasound

• lido, mepi, bupi, ropi

• dental blocks, lameness blocks, ect

how are locals used as interfascial plane blocks

• deposition of local between 2 layers of fascia containing and targeting 2+ nerves

• sensory blockage

• ultrasound guided

• Bup, rop

• transversus abdominis plane block

what are causes and major effects f local anesthesia toxicity

• accidental overdose, unitentional intravascular administration, high plasma concentration from decreased biotransformation

• CNS and cardiovascular effects via Na channels

• more lipid solube (bupi) = more toxic

what are the signs of local anesthetic CNS toxicity

• sleepiness, light headedness, dizziness, dorwsiness

• nystagmus, muscular twitching, ataxia

• tonic-clonic convulsions

• CNS depression and unconsciousness

• coma

what are the cardiovascular signs of local anethestic toxicisty

• depression of cardiac automaticity

• reduction of duration of refractory period

• prolonged PR interval, widening of QRS complex

• bradycardia

• hypotension

• asystole

how do you treat CNS toxicity with locals

• treat seizures with benzos (0.5mg/kg midazolam IN)

• O2

  • secure airway if needed

how do you treat cardiac arrest caused by local toxicity

• CPR

• epi 0.01 mg/kg IV

• 20% lipid emulsion