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How do local anesthetics work?
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They block axonal sodium channels blocking sodium entry
They are nonselective and usually nonmyelindated axons are blocked first, pain stops first, then cold, warmth , touch, and deep pressure |
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What are the three determinents to the movement of an anesthetic to it's site of action?
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Molecular size, lipid solubility, and degree of ionization at tissue pH
Anesthetics with low molecular size, high lipid solubility, and a low degree of ionization move quicker across the nerve membrane therefore drugs with these qualities have the quickest onset This being said, the drugs that easily cross the membrane in can easily cross the membrane out, therefore they have a short duration of action and those with a high molecular size, low lipid solubility, and high degree of ionization are longer acting. regional blood flow also plays an important role. |
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Why are anesthetics often given with a vasoconstrictor?
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Because a vasoconstrictor slows systemic absorption of the drug which prolongs the anesthesia and reduces the risk of toxicity because less of the drug is used and there is a better balance b/w systemic drug and inactive metabolites.
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For a patient in liver which local anesthetic would you rather administer: ester or amide?
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Ester, because these are metabolized in the blood by esterases. In contrast amide-type anesthetics are metabolized in the liver (maybe b/c they need to be deaminated).
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Adverse Effects (local anesthetics)
CNS: sometimes excitation precedes depression and convulsions may occur. IV depressants may be used i.e. benzodiazepines (diazepam) or barbituates are given. CV: when absorbed in sufficient amounts they can effect the heart and smooth muscle of blood vessels resulting in vasodilation leading to hypotension. heart: it can effect the myocardium causing bradycardia, heart block, reduced contractile force, and cardiac arrest. Allergic rxns: Esters are more likely to cause an allergic reaction in this case amides are used. there has been no sign of cross-hypersensitivity |
Procaine (novocain): ester-type drug, injection only (therefore more likely to cause an allergic reaction but not a high risk for toxicity because it is an ester and esterases rapidly metabolize the drug in blood).
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Lidocaine: amide-type, topical and injection, allergic reactions are rare but systemic reactions can occur) can also be used to control cardiac dysrhythmias
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