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29 Cards in this Set

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  • Back
what do local anesthetic drugs do?
produce a reversible blockade of nerve conduction (sensory and motor)
what are the groups of local anesthetics?
esters and amides
what do all esteres and amides have?
lipophilic portion, intermediate alkyl chain, hydrophilic portion
what determines the pharmacolocial properties of local anesthetic drugs?
the balance between lipophilic and hydrophilic properties
what does increased lipid solubility of a local anesthetic drugs result in?
increased potency, duration, and systemic toxicity
how are local anesthetics used clinically?
topical anesthesia, infiltration anesthesia, field block anesthesia, nerve block anesthesia, intravenous regional anesthesia, spinal anesthesia, epidural anesthesia
name early local anesthetics?
cocaine, procaine, lidocaine
name the local anesthetics most frequently used in vet med?
bupivacaine, lidocaine, proparacaine, tetracaine, mepivacaine
how are esteres metabolized?
by tissue and plasma esterases; usually shorter half-life than amides (tetracaine is exception)
how are amides metabolized?
by liver microsomal enzymes; poor choice for an animal with severe liver disease (lidocaine, bupivicaine, mepivicaine)
what drugs have been used to treat post-op ileus in horses?
Cisapride (difficult to obtain), Domperidone, Metoclopramide (bizarre behavioral effects), Ranitidine (H2-receptor antagonist), Lidocaine (anesthetic action)
how does lidocaine produce a pro-motility effect?
administered IV (same dose as to treat cardiac arrhythmias). Does NOT stimulate intestinal motility; relieves pain from ileus (which stimulates sympathetic output from CNS to reduce motility) decreasing sympathetic output, enabling normal intestinal motility
why are opiod analgesics not suitable for the treatment of post-op ileus?
inhibits intestinal motility
how do local anesthetics produce conduction blockade?
block initiation and ropagation of the action potential by preventing the voltage-dependent increase in Na+ permeability that accompanies a small increase in membrane depolarization
what factors influence the sensitivity of nerves to conduction blockade?
(1) use-dependent block of Na+ channels (greater the frequency of channel openings, the more easily the nerve is blocked) (2) locan anesthetic activity is strongly pH dependent (3) anatomical structure of the nerve
how does use-dependent block of Na+ channels affect the sensitivity of nerves to conduction blockade?
rapidly-firing nerves more likely to be blocked because more channels in open-inactivated state: (a) local anesthetics must access channel from INSIDE cell, not outside (b) resting channels = low affinity for local anesthetics, open channels = high affinity for local anesthetics, inactivated/refractory channels = very high affinity for local anesthetics
how does pH dependency affect the sensitivity of nerves to conduction blockade?
local anesthetics are weak bases with a pKa of 7.9, so if extracellular pH decreases (inflammed tissues has a pH of 6.0) more drug is required to produce conduction blockade because less drug is un-ionized (24x more drug is required to produce a conduction blockade at pH 6.0)
how does diameter of nerve affect the sensitivity of nerves to conduction blockade?
for conduction blockade to occur, a critical length of nerve must be blocked and since smaller diameter nerves have Na+ channels closer together, they are more easily blocked; the nociception-conducting C- and A-delta fibers (smallest) are most easily blocked (can only compare myelinated to myelinated and unmyelinated to unmyelinated, small diameter enteric neurons are more easily blocked than PNS neurons, and pain is easier to block than other modalities like proprioception)
describe the pharmacokinetics of local anesthetics?
designed to work at site of administration; therefore absorption from the site of administration terminates the effects of the local anesthetic and can lead to systemic effects
what affects absorption of local anesthetics from the site of administration ?
dosage (increase = increased absorption), site of injection (more vascular = increased absorption), extent of tissue binding (lipid solubility = decreased absorption), concurrent administration of vasoconstricting substances (= decrease absorption) and all local anesthetics are vasodilators (= enhance own absorption)
what are the systemic effects of local anesthetics?
eventually all local anesthetics are absorbed into the bloodstream; with sufficiently high plasma levels the effects are seen in the CNS, cardiovascular system, and neuromuscular junction and can result in toxicity
what are the systemic and toxic effects of local anesthetics?
produce hypotension (decrease cardiac contractility causing a decrease in stroke volume and cardiac output and causes arteriolar dilation by blocking postganglionic sympathetic neurons) but cocaine's toxic effects are different (vasoconstriction and hypertension)
what class of drug is cocaine?
local anestic ester
what is the action of cocaine?
reversibly block nerve conduction
what are the side effects of cocaine?
blocks reuptake of NE into neuroeffector junction
what class of drug is lidocaine, mepivicaine, bupivicaine, proparacaine?
local anesthetic Na+ channel blocker (prevents voltage-dependent increase in Na+ permeability)
what is the action of lidocaine, mepivicaine, bupivicaine, proparacaine?
reversibly block nerve conduction
what are the clinical uses of lidocaine, mepivicaine, bupivicaine, proparacaine?
post-op ileus in horses, cardiac arrhythmias, topical anesthesia, infiltration anesthesia, nerve block anesthesia, IV regional anesthesia, spinal anesthesia, epidural anesthesia
what are the side effects of lidocaine, mepivicaine, bupivicaine, proparacaine?
CNS effects (tremors, convulsions) & cardiovascular hypotension (arteriodilation by blocking postganglionic SNS nerves), and decreases myocardial contractility/CO