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41 Cards in this Set
- Front
- Back
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What are local anesthetics?
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Drugs that produce reversible conduction blockade of impulses along central and peripheral nerve pathways after regional anesthesia.
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Describe the structure of local anesthetics?
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They consist of lipophilic and hydrophilic portion separated by a connecting hydrocarbon chain. The hydrophilic portion is a tertiary amine, such as diethylamine, whereas the lipophilic portion is usually an unsaturated aromatic ring, such as paraaminobenzoic acid. In almost all instance, an ester (-CO-) or and amide (-NHC-) bond links the hyrdocarbon chain to the lipophilic aromatic ring.
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Which of the local anesthetics are chiral?
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Mepivicaine, Bupivacaine, Ropivacaine, and Levobupivacaine (these are all amides too!)
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Name the ester local anesthetics.
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Procaine
Chloroprocaine Tetracaine (all others are amides) |
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What is the mechanism of action of local anesthetics?
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They prevent transmission of nerve impulses (conduction blockade) by inhibiting the passage of sodium ions through ion-selective sodium channels in nerve membranes. They dont alter resting transmembrane potential or threshold potential.
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Describe the structure of sodium channels.
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It consists of a large sodium conducting pore (aplha subunit) with varying numbers of adjacent smaller beta subunits. The alpha is divided further into 4 subunits. H is the alpha subunit that allows ion conduction and binds to local anesthetics.
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Name other sites of action for local anesthetics.
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Potassium-ion channels, calcium ion channels (L-type), and there is evidence regarding action on G-protein coupled receptors
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What is the minimum concentration of local anesthetics?
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Cm is the minimum concentration of local anesthetic necessary to produce conduction blockade or nerve impulses.
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What affects minimum concentration?
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1. Larger nerve fiber diameter require higher concentrations of local anesthetic
2. Increased tissue pH and higher frequency of nerve stimulation decreases Cm 3. Unmyelinated fibers require less Cm than myelinated 4. increased sensitivity during pregnancy d/t protein binding (except bupivacaine) |
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Do local anesthetics act like bases or acids in the body?
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Bases with pK values above physiologic pH so <50% exists in lipid-soluble nonionized form (acidosis increases lipid solubility->
poor quality of locals at infected areas) |
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Which local anesthetics are the lungs capable of extracting?
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Lidocaine, Prilocaine, and Bupivacaine--> limits the concentration of drug that reaches systemic circulation
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How is lidocaine metabolized?
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Through oxidative dealkylation in the liver to monoethylglycinexylidide (has 80% of activity of lidocaine for protecting against cardiac dysrhythmias) followed by hydrolysis of this metabolite to xylidide
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What can decrease the metabolism of lidocaine?
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Hepatic disease, decreases in hepatic blood flow d/t volatile anesthetics, and in the presence of pregnancy-induced hypertension.
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What is prilocaine metabolized to and what is the concern with this metabolite?
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Orthotoluidine. It is an oxidizing compound that is capable of converting hemoglobin to its oxidized form, methemoglobin. Prilocaine >600mg can cause sufficient methemoglobin to make the patient appear cyanotic and decrease oxygen-carrying capacity.
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What is the treatment for methemoglobinemia?
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Methylene blue 1-2mg/kg IV over 5 minutes (not to exceed 7-8mg/kg)
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Name the amide local anesthetics.
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Lidocaine
Etidocaine Prilocaine Mepivacaine Bupivacaine Levobupivacaine Ropivacaine |
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What protein does bupivacaine bind to?
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Alpha1-acid glycoprotein (concentration of this protein is increased in many situations including post-op traumas; ropivacaine binds to this protein too)
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Which of the amide locals do not cause much vasodilation and thus dont need epinephrine in mixture?
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Prilocaine
Mepivacaine |
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Which local anesthetic is the most slowly eliminated?
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Dibucaine
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How is ropivacaine metabolized?
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It is metabolized to 2,6-pipecoloxylidide and 3-hydroxyropivacaine by hepatic CYTOCHROME P-450 enzymes; less than 1% is excreted unchanged in urine (dosage adjustments to renal impairment may not be necessary but in pts with uremia the 2.6 metabolite may accumulate and be toxic)
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How are ester local anesthetics metabolized?
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They undergo hydrolysis by cholinsterase enzyme principally in the plasma and to a lesser extent in the liver. The rate of hydrolysis varies with chrlorprocaine being the most rapid, procaine being intermediate, and tetracaine being the slowest. The metabolites are inactive although paraaminobenzoic acid may be an antigen responsible for allergic reactions.
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What is the exception to the ester local anesthetic metabolism in plasma?
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Cocaine which undergoes metabolism mainly in the liver.
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What is benzocaine good for and why?
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It is ideal for topical anesthesia because it is a weak acid and exists nonionized at physiologic pH--rapid onset and duration of action of 30-60 minutes
*Methemoglobinemia is rare but a potential complication |
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What is the benefit of mixing epinephrine in local anesthetic solutions?
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It produces vasoconstriction which limits systemic absorption (by 1/3) and lengthens the duration of action (not used with ropivacaine)
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What are the principal side effects related to the use of local anesthetics?
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Allergic reactions (rare-less than 1%) and Systemic toxicity (due to excess plasma concentrations)
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What are the side effects in CNS toxicity from local anesthetics?
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Numbness of tongue and circumoral tissue, restlessness, vertigo, tinnitus, difficulty in focusing; more dosage results in slurred speech and skeletal muscle twitching that first evident in face and extremities--> tonic clonic seizures
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What is the treatment for local anesthetic induced seizures?
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Ventilation of lungs with oxygen because arterial hypoxemia and metabolic acidosis can occur in seconds. Also IV administration of a benzodiazepine such as midazolam and diazepam is effective in suppressing local anesthetic-induced seizures.
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What are transient neurologic symptoms?
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Moderate to severe pain in lower back , buttocks, and posterior thigh that appears 6-36 hours after complete recovery from single-shot spinal anesthsia and full recovery may take between 1 and 7 days.
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What are the symptoms of cauda equina syndrome?
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Diffuse injury across lumbosacral plexus produces varying degrees of sensory anesthesia, bowel and bladder sphincter dysfunction, and paraplegia
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What is anterior spinal artery syndrome?
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It consists of lower extremity paresis with a variable sensory deficit that usually diagnosed as neural blockade resolves.
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Which local anesthetic must be used with caution in patients who are on antidyrhythmic drugs or other cardiac medications known to depress impulse propagation?
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Bupivacaine
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There is a risk fo methemoglobinemia with these drugs:
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Prilocaine, Benzocaine, Cetacaine, Lidocaine, nitrogylcerin, phenytoin, and sulfonamides (neonates at greater risk because of more readily oxidized fetal hemoglobin)
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What are some uses of local anesthetics?
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Regional anesthesia, Topical anesthesia, Peripheral nerve block anesthesia, Intravenous Regional Anesthesia (Bier block), Epidural anesthesia, Spinal anesthesia, and Tumescent liposuction
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What drugs are used most often for topical anesthesia?
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Cocaine, tetracaine, and lidocaine
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What is the eutectic mixture of local anesthetics (EMLA)?
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It is a 5% combination of lidocaine-prilocaine cream that is used for skin grafting, venipuncture, arterial cannulation, lumbar puncture, myringotomy in children, circumcision
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What is the benefit of dyclonine?
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It is used to provide topical anesthesia of the airway in a patient that is presumed to be allergic to bupivacaine and procaine.
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How is peripheral nerve block anesthesia achieved?
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By injection of local anesthetic solutions into tissues surrounding individual peripheral nerves or nerve plexuses such as brachial plexus.
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Which locals are recommended for IV regional anesthesia?
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Lidocaine, prilocaine, mepivacaine
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How is epidural anesthesia done and how does it work?
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Local anesthetic solutions are placed in the epidural or sacral caudal space. They first diffuse across dura to act on nerve roots and spinal cord then it also diffuses into the paravertebral formania producing multiple paravertebral nerve blocks.
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How is spinal anesthesia produced?
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It is produced by injection of local anesthetics into the lumbar subarachnoid space; principal site of action are the preganglionic fibers as they leave the spinal cord in the anterior rami
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What is tumuscent liposuction?
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Subcutaneous infiltration of large volumes (5 or more liters) of solution containing highly diluted lidocaine with epi (1:100,000)--> sufficient local anesthesia for liposuction and prolonged post-op analgesia
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