Least useful in Treatment of local anesthetic induced convulsions/seizures?
Correct Answer: Phenytoin sodium
Description: Ans. a (Phenytoin sodium) (Ref. Katzung pharma, 9th/Chapter 26; Anaesthesia by Ajay Yadav 3rd/pg. 111)When local anesthetic-induced seizures occur, hypoxia, hypercarbia and acidosis develop rapidly. ABC (Airway, Breathing and Circulation) is the mainstay of treatment. Administration of O2 by mask, or ventilation support by bag and mask, is often all that is necessary to treat seizures. If seizures interfere with ventilation, benzodiazepines, small dose propofol or thiopental can be used. The use of succinylcholine effectively facilitates ventilation and, by abolishing muscular activity, decreases the severity of acidosis. However neuronal seizure activity is not inhibited and therefore, cerebral metabolism and oxygen requirements remain increased.LOCALANAESTHETICSMechanism of action of local anesthetics# Diffusion of the unionized (base) form across the nerve sheath and membrane# Re-equilibration between the base and cationic forms in the axoplasm# Binding of the cation to a receptor site inside of the Na+ channel, resulting in its blockade and consequent inhibition of Na+ conductance.Order of blockade: small fibre > large fibres; myelinated fibres > unmyelinated fibres. Modality blocked: autonomic and pain > touch/pressure > motor. There are two main classes: amides and esters.Local anesthetic potencyAgentLipid SolubilityRelative PotencyProteinDuration Binding (%)pKaOnset TimeProcaine<115Short8.9Slow2-Chloroprocaine>13-Short9.1Very QuickquickMepivacaine11.575Medium7.7QuickLidocaine3265Medium7.8QuickBupivacaine28895Long8.1ModerateTetracaine80895Long8.4SlowEtidocaine140895Long7.9QuickRopivacaine14894Long8.1ModerateLocal anaesthetic Toxicity:The following are accepted plasma levels of selected local anesthetics, above which systemic effects are expected in humans: Lidocaine 5 mcg/mL; mepivacaine 5 mcg/mL; bupivacaine 1.5 mcg/mL; ropivacaine 4 mcg/mL.A. CNS Toxicity# Lightheadedness, tinnitus, perioral numbness, confusion (especially known with Lignocaine)# Muscle twitching, auditory and visual hallucinations# Tonic-clonic seizure, unconsciousness, respiratory arrest# Benzodiazepines or rapidly acting barbiturates administered intravenously are the drugs of choice for both the prevention and arrest of convulsions.# Transient neurological symptoms is side-effect of LIGNOCAINE# Seizures induced by local anesthetics can also be treated with small doses (given intravenously) of thiopental 1-2 mg/ kg, propofol 0.5-1 mg/kg, midazolam 2-4 mg total dose, or diazepam 0.1 mg/kg.B. Cardiotoxicity# Hypertension, tachycardia,can occur or# Decreased contractility and cardiac output, Sinus bradycardia, ventricular dysrhythmias, circulatory arrest# Bretylium tosylate is drug of choice for lignocaine-induced arrhythmias.^C. Methemoglobinemia# Prilocaine doses of more than 600 mg are needed to produce clinically significant methemoglobinemia.# Depending on the degree, methemoglobinemia can lead to tissue hypoxia. 02Hb curve shifts to left (P50 < 27 mmHg).# Rx:Methylene blue (l-2mg/kg of 1% solution over 5 min) or less successfully with ascorbic acid (2 mg/kg).Educational points:Ropivacaine# Ropivacaine is a new amide local anesthetic that is structurally and behaviorally similar to bupivacaine.# Like bupivacaine, it is highly protein bound and has a lengthy duration of action.# It is, however, less cardiotoxic.# With ropivacaine, sensory anesthesia may be provided without a significant degree of motor blockade.# These characteristics may make ropivacaine an ideal anesthetic for use in obstetric procedures.
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Anaesthesia
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