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29 Cards in this Set
- Front
- Back
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With what molecule on the surface of neurons do local anesthetics interact?
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They interact with the INNER SURFACE of the OPEN sodium channels
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What is the main action of local anesthetics on the nerve?
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They cause a decrease in the impulse conduction of the nerve which is dose dependent.
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Describe how local anesthetics act differently depending on the size of the nerve fiber.
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Smaller nerve fibers are more susceptible to anesthetic affect than larger, heavily myelinated fibers.
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What are the two main classes (or types) of local anesthetics?
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Amides and Esters
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What property of local anesthetics is most significant in determining it's anesthetic ability, and which drugs are most potent?
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The lipid solubility is the most significant property, and bupivacain and etidocain have the highest potency due to their large solubility.
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The duration of the conduction block of a local anesthetic is generally determined by what parameter?
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The capacity of the drug to bind to proteins. Agents with higher binding capacity are associated with longer action
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Describe the duration of conduction block for both procaine and bupivacaine
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Procaine has a short duration of block due to it's low protein binding ability, but bupivacaine has a long duration of block due to it's high degree of protein binding
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The onset of conduction block is determined by what parameter?
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Onset is determined by the pKa because the drug needs to be in a non-ionized form in order to penetrate the nerve cell membrane. Most have high pKa's (7.7-9.0)
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What effect do all local anesthetics have on blood vessels (there is one exception, can you name it?)?
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They cause vasodilation, except for cocaine
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Why is vasodilation important to consider when giving a local anesthetic?
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It will cause a decrease in the duration of action and have a longer onset time because the drug will enter the bloodstream more readily and distribute throughout the body
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The toxicity of a local anesthetic is directly related to what factor? Which local anesthetic would most be toxic?
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Toxicity is directly related to lipid solubility. The most lipid soluble drug and therefore most toxic of the local anesthetics is bupivacaine.
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What is important about the administration of a local anesthetic in regards to it's toxicity?
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The site of injection is important because a more vascular site of injection will lead to increased uptake into the plasma, leading to more intense CNS toxicity
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What are examples of CNS toxicity with local anesthetics?
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Initially - lightheadedness, dizziness, tinnitus
Later - drowsiness, disorientation, convulsions, loss of consciousness Finally - respiratory depression |
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What drugs are commonly added to local anesthetics, and why are they added?
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Epinephrine, ephedrine, and phenylephrine are commonly added to act as adrenergic agonists, decreasing the vasodilation and inducing vasoconstriction which causes less of the anesthetic to access the bloodstream
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What toxicity is associated with the drugs which are added in local anesthetic preparations?
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Hypertension, tachycardia, cardiac arrhythmia, and ischemia; all of which are caused by the effects of the added adrenergic agonists (epinephrine, ephedrine, phenylephrine)
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What local anesthetic is the most cardiotoxic?
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Bupivacaine
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All local anesthetics have what sort of effect on the heart?
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They are all direct myocardial depressants, especially bupivacaine and etidocaine
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Which local anesthetic is associated with methemoglobinemia and what is given to reverse this?
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Prilocaine causes the oxidation of ferrous iron to ferric iron on Hg, leading to MetHg. It can be reversed by methylene blue
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Describe the therapeutic index of local anesthetics
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It is a very narrow TE, leading to toxic effects at lower concentrations
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Which type of local anesthetic is most associated with allergy and why does this occur?
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Ester agents are more associated with allergy, which occurs due to the preservative in the vial called methylparaben.
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Describe the half life of ester local anesthetics. Patients with what atypical enzymes cause descreased metabolism of these esters?
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Ester local anesthetics have very short half lives. Patients with a genetically atypical cholinesterase have decreased metabolism of esters.
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How are amide local anesthetics metabolized?
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THey are metabolized in the liver by N-dealkylation and hydrolysis
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Describe the effect a local anesthetic has on a nerve that is very active verses one that is not as active.
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Resting nerves are less sensitive and active nerves are more sensitive because active nerves have Na+ channels which are more likely to be open. This better facilitates the binding of the drug to the inside of the channel
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What are the differences in metabolism of ester and amide local anesthetics?
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Esters are metabolized by cholinsterase and amides are metabolized in the liver by the CYP450 system
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What are the ester local anesthetics?
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Procaine, cocaine, tetracaine,chlorocaine, and benzocaine
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What are the amide local anesthetics?
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lidocaine, mepivacaine, bupivacaine, etidocaine
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What is the most commonly used local anesthetic? Describe it's onset and duration of action
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Lidocaine has a rapid onset with an intermediate duration of action
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Describe the onset, duration and toxicity of bupivacaine
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It has a slow onset, long duration, and is a highly toxic agent (due to it's high solubility)
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In what situations would you want to use a local anesthetic with a pre-mixed adrenergic agonist?
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You would want to use a local with an adrenergic agonist for peripheral nerve blocks to induce vasoconstriction, but NOT for epidural, caudal, and infiltration nerve blocks because the agonist could gain access to the blood and cause systemic affects.
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