Local Anesthetics

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Ester local anesthetics

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Cocaine
Procaine
Tetracaine
Benzocaine

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Amide Local Anesthetics

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Lidocaine
Mepivacaine
Bupivacaine
Ropivacaine

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Max dose of Bupivicaine

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3 mg/kg
2.5 mg/kg neonates

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Max does of Lidocaine

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5 mg/kg plain
7 mg/kg with epi

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General Mechanism of action of Local Anesthetics

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Local anesthetics (the esters and the amides) block Na channels of excitable membranes. Note--> Na channels can only be blocked if they are OPEN or INACTIVATED (not in resting state). The effect of a LA can be restricted to a certain area (like cornea or arm)

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General Pharmokinetics of locatl anesthetics

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LA's need it BOTH WAYS: nonionized form/basic form to cross membrane; ionized form/acidic form for action in cytoplasm. (So nonionized form=ENTRY, ionized form=ACTION). Most are weak bases, so pK of the drug is about 8ish (around 1 pH unit from physio pH so 50/50 ionized/nonionized). *Lower pK LA's=FASTER onset.

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How does epinephrine or other alpha agonist affect local anesthetics

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the vasoconstriction provided by epinephrine slows the removal of the local anesthetic which not only increases duration of action but requires less LA (reduce toxicity), and dec bleeding.

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Are local anesthetics vasoconstrictors or vasodilators

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vasodilators (except Cocaine)

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How does an infected cell affect the onset of the local

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an infected cell's pH can be 6.4 so that can change the ionization state of the local anesthetic

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Adverse effects of local anesthetics

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plasma cholinesterases make PABA which can cause allergy

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Symptoms of local anesthetic overdose

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affect CNS more than heart producing
seizures
coma
respiratory depression

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Susceptibility to pain blocks

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Nerve susceptibility to block-->smaller fibers before bigger fibers, myelinated fibers before unmyelinated fibers. A-delta fibers "first, fast, pain"; small & myelinated and very susceptible.

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Why use cocaine

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topical use mainly for minor surgeries (nasopharynx, etc)

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Is cocaine an ester or an amide

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Ester, equally as potent as mepivacaine (#2 potency overall)

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MOA of cocaine

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bind & block OPEN or inactivated Na channels; prevent re-uptake of NE at adrenergic terminals
causes vasoconstriction

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Onset of cocaine

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fast

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Duration of cocaine

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medium (1 hr)

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Considerations about Cocaine

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PABA can cause allergy
CNS toxicity
intrinisic vasoconstrictor
topical use

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Why use procaine

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infiltration, nerve block, spinal anesthesia.

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Is procaine an ester or amine

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Ester, MOST POTENT OVERALL (#1) , all potency comparisons are to procaine

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Drug action of Procaine

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bind & block OPEN or inactivated Na channels

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Duration of procaine

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short (30-45 minutes)

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Onset of procaine

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slow

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Considerations for procaine

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PABA can cause allergy
CNS toxicity
Use epi to keep it local

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Why use tetracaine

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spinal anesthetic
less potent than procaine

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Is tetracaine an ester or amide

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ester

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Onset of tetracaine

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slow

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Drug action of tetracaine

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binds open or inactivated Na channels and blocks Na to neuron

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Duration of Tetracaine

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long (2-5 hrs)

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Adverse reactions of Tetracaine

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CNS toxicity
PABA allergy

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Why use Benzocaine

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topical pain usually as an OTC spray

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Is benzocaine an ester or amide

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ester

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Drug action of Benzocaine

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blocks Na channels by binding open or inactivated Na channels

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Onset of Benzocaine

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topical use only

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Adverse events for Benzocaine

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CNS toxicity
PABA allergy

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Why use Lidocaine

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epidural/subarachnoid, infiltration, nerve block

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Is lidocaine an amide or ester

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amide

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Potency of Lidocaine

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third most potent after mepivacaine, cocaine and of course, procaine

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Action of Lidocaine

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block Na channel in nerve by binding open or inactivated form of the channel

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Lidocaine induction time

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FAST

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Lidocaine duration

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medium

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Adverse effects

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Just CNS toxicity

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What happens to nerves in a neuraxial block

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note that in neuraxial blocks, nerves at or below the level of the injection site are blocked!

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Why do you get respiratory arrest in high spinal blocks

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due to hypoperfusion of respiratory centers in the medulla

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Why use Mepivacaine

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epidurals, infiltration, nerve block

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Potentcy of Mepivacaine

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2nd most potent = to cocaine

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Is Mepivacaine an amide or ester

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amide (two i's)

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Action of Mepivacaine

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blocks Na channels in neurons by binding to open or inactivated Na channels

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Onset of mepivacaine

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fast

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Duration of Mepivacaine

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medium

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Adverse events with Mepivacaine

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CNS as usual

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Why use Bupivacaine

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epidural/subarachnoid, infiltration, nerve block

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Is bupivacaine an amide or ester

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amide

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Action of bupivacaine

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blocks Na channels on neurons by binding to the open or inactivated Na channels

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Onset of Bupivacaine

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slow

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Duration of Bupivacaine

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long

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Adverse effects of Bupivacaine

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CNS as usual,MORE CARDIOTOXIC than others in this group!!

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Why use Ropivacaine

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epidural

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Drug action of Ropivacaine

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block Na channels on neurons by binding to Open or inactivated Na channels

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Onset of Ropivacaine

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slow

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Duration of Ropivacaine

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long

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Who are post dural puncture head aches most common in

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young pregnant females

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When would a neuraxial block be contraindicated

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patients with an intestinal blockage because of increased motility

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subarachnoid is also known as

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spinal block

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Side effects of neuraxial blocks

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hypotension and post dural headache

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Order of the block is

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sympathetic (leaves unopposed vagal)>sensory>motor, and is differentiated by TWO dermatomes each

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Why do Hyperkalemic states facilitate cardiac toxicity?

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Sodium channel blockers (local anesthetics) bind more readily to OPEN or inactivated sodium channels. Recall that Hyperkalemia depolarizes the resting membrane potential --> more sodium channels are in the OPEN or inactivated state -->LA effect is potentiated, vasodilation is increased, decreasing BP & venous return.