An Update on Local Anesthetics in Dentistry .pdf

An Update on Local Anesthetics in Dentistry - L.A is induced when propagation of action potentials is prevented. - L.A work by blocking the entry of Na+ into their channels, thereby preventing the transient increase in permeability of t...

An Update on Local Anesthetics in Dentistry - L.A is induced when propagation of action potentials is prevented. - L.A work by blocking the entry of Na+ into their channels, thereby preventing the transient increase in permeability of the nerve membrane to sodium that is required for an action potential Structure of L.A: - Lipophilic group - Amide or ester linkage - Carbon chain which joined to a hydrophilic group L.A are classi ed by their amide or ester linkages: a. Amides: articaine, bupivacaine, lidocaine, mepivacaine and prilocaine b. Esters: procaine (Novocain), topical anesthetic benzocaine The onset and duration of action of local anesthetics are in uenced by several factors, as summarized in Table 1. - The most important factors a ecting onset are pH of the tissue and pKa of the drug. - The pH may drop in sites of infection, which causes onset to be delayed or even prevented. - There are no signi cant di erences in pKa among the amides, exception for bupivacaine, which has a slightly higher pKa and hence a slower onset of action. - Proximity of the deposition is a factor. In ltration is associated with rapid onset whereas the Gow-Gates block is relatively slow. - Nerve morphology is a factor, in that the relatively thin pain bres are usually anesthetized readily. - Higher concentration and greater lipid solubility improve onset to a small degree. - Duration depends on the length of time the drug can stay in nerve to block the Na+ channels. - L.A cause vasodilatation —> rapid di usion from the site —> short duration of action - Di usion can be reduced by the addition of a vasoconstrictor, usually epinephrine. - Bupivacaine is unique in that it provides long duration anesthesia for soft tissue in both the arches and pulp of mandibular teeth. - In general, blocks last longer than in ltrations, and soft-tissue anesthesia lasts longer than pulpal anesthesia. 1 of 5 fl ff fi fi ff ff ff fi fi fi  - Biotransformation of amides occurs primarily in the liver. - Reduced hepatic function predisposes the patient to toxic - Hepatic function does not affect the duration of action of local anesthesia, - A patient with liver disease needs the standard amount of local anesthetic at each site. - It is prudent to treat one quadrant at a time, as the total dose is a concern. - Prilocaine is also metabolized in the plasma and kidney, - Esters are biotransformed by plasma cholinesterase, also known as pseudocholinesterase. - Patients with pseudocholinesterase de ciency can metabolize procaine at a slower rate. - Pseudocholinesterase is synthesized in the liver. - Duration of action of L.A is determined by redistribution and not biotransformation. Adverse Reactions A. Psychogenic Reactions - Anxiety-induced events are the most common adverse reaction associated with L.A in dentistry. - The most common of which is syncope. - They may present with a wide variety of symptoms: - Hyperventilation - Nausea - Vomiting - Alterations in heart rate or blood pressure. - Psychogenic reactions are often misdiagnosed as allergic reactions and may also mimic them, with signs such as urticaria, edema and bronchospasm. B. Allergic Reactions - True allergy to an amide is exceedingly rare - Allergy to epinephrine is impossible. - The ester procaine is somewhat more allergenic. - An allergy to one ester rules out use of another ester - In contrast, an allergy to one amide does not rule out use of another amide. - Patients might be allergic to: - Mthylparaben: a preservatives necessary for multi-dose vials - Allergy to para-aminobenzoic acid would rule out use of esters and methylparabens. - Avoid a vasoconstrictor if there is a true documented allergy to sul tes, - metabisul te is added as an antioxidant whenever vasoconstrictor is present. - Vasoconstrictor can be used in patients with an allergy to the sulfonamide antibacterials, commonly called sulfa, as there is no cross-allergenicity with sul tes. 2 of 5 fi fi fi fi C. Toxicity - The toxicity of local anesthetics is a function of systemic absorption. - This is one reason why aspiration before every injection is so important. - High blood levels of the drug may be due to - Repeated injections - a single inadvertant intravascular administration. Predisposition to toxic effects in any given patient depends on several factors: - Site of administration, - Speed of injection and - Presence of vasoconstrictor. - The high-concentration solutions, prilocaine and articaine, will reach toxic levels with fewer injections than is the case for the other drugs. D. Methemoglobinemia - Associated with prilocaine, may also occur with articaine or the topical anesthetic benzocaine. - These drugs best avoided in patients with congenital methemoglobinemia. - Methemoglobinemia is induced by an excess of the metabolites of these drugs - Manifests as a cyanotic appearance, does not respond to the administration of 100% oxygen. - Cyanosis becomes apparent when methemoglobin levels are low, - Symptoms of nausea, sedation, seizures and even coma may result when levels are very high E. Paresthesia - Prolonged anesthesia or paresthesia of the tongue or lip are known risks of surgical procedures. - Most of these reactions are transient and resolve within 8 weeks, but may become permanent. - Articaine and prilocaine were reported as more likely associated with paresthesia, - Such reactions have most commonly affected the lingual nerve 3 of 5 F. Malignant Hyperthermia - Patients with genetic susceptibility to this condition are exposed to either: - Inhalational general anesthetics - succinylcholine - Today it is well accepted that all L.A are safe for patients with malignant hyperthermia.17 Interactions - L.A have very few clinically signi cant interactions on their own Vasoconstrictors Importance of using Vasoconstrictors - Improving the depth and duration of anesthesia are the most important - Reduce systemic toxic effect - Provide hemostasis A. Epinephrine: - The most common vasoconstrictor agent - Stimulates α1 receptors in mucous membranes. - Stimulates the ß1 receptor in the heart - ↑ heart rate - ↑ strength of contraction and - ↑ myocardial oxygen consumption - Stimulates ß2 receptors, vasodilating blood vessels in the skeletal muscle. - Can be given to patients receiving monoamine oxidase inhibitors - Epinephrine dosage should be minimized in patients with ischemic heart disease. - The recommendation to keep doses below 0.04 mg - Systemic epinephrine has a brief duration of action (approximately 10 minutes), B. Levonordefrin - Available as a 1:20,000 solution and should be considered equivalent to 1:100,000 epinephrine. - Contraindicated for patients receiving tricyclic antidepressants. 4 of 5 fi Topical Anesthetics - Indicated to - Minimize the sensation of needle insertion - For very brief relief from painful mucosal lesions. - Benzocaine in concentrations up to 20% - Lidocaine solution or ointment in concentrations up to 5% or as a spray concentration of 10%. - Excessive doses may lead to toxic effects, particularly in children. Special Patient Populations A. Pregnant and Lactating Women - L.A and vasoconstrictors used are safe, however, aspiration must always be carried out to minimize the likelihood of intravascular injection. - Lidocaine is preferable as L.A and topical - Doses of epinephrine used in L.A so low that they are unlikely to affect uterine blood ow. B. Children - The main concern in pediatrics is the relative ease of inducing an overdose. - Should determine the child’s weight and calculate the maximum dose to prevent overdose. - 2% lidocaine with epinephrine 1:100,000 may be the ideal local anesthetic for a child. - Bupivacaine is best avoided in children because of its long duration of soft-tissue anesthesia. C. Elderly Patients There are no signi cant differences in the response to L.A between younger and older adults. Conclusions The decision regarding which drug to select should be based on - The estimated duration of action required - The patient’s medical history - Potential drug interactions. - Solutions without vasoconstrictor, namely mepivacaine and prilocaine plain, may be selected for short procedures, particularly those involving mandibular block, where vasoconstriction is less important. - These drugs may also be used when epinephrine must be avoided, as in patients with severe ischemic heart disease or recent myocardial infarction. - Bupivacaine can be selected when long duration of action is desired, particularly in the mandible. Lidocaine with epinephrine may be preferred for treatment of children and pregnant patients. - Articaine, lidocaine, mepivacaine or prilocaine may be considered for routine dental Procedures 5 of 5 fi fl