Local Anesthetics Overview

Local Anesthetics

• Local anesthetics reduce pain during medical procedures.
• They were used as early as 1884.
• Cocaine was the first local anesthetic used in 1886.
• Procaine was introduced in 1905, and other local anesthetics followed in 1946.
• Local anesthetics are classified based on their chemical structure, duration of action, biological site, and mode of application.

Chemical Structure

• Local anesthetics are categorized as either esters or amides.
• Esters include benzocaine, cocaine, procaine, and propoxycaine.
• Amides include articaine, bupivacaine, lidocaine, mepivacaine, and prilocaine.

Duration of Action

• Local anesthetics can be short, intermediate, or long acting.
• Short-acting: Mepivacaine HCl 3%.
• Intermediate-acting: Lidocaine HCl 2% + epinephrine 1:100,000.
• Long-acting: Bupivacaine HCl 0.5% + epinephrine 1:200,000 and etidocaine.

Biological Site and Mode of Action

• Local anesthetics block sodium channels on the nerve cell membrane to prevent nerve impulse conduction.

Mode of Application

• Local anesthetics can be injected (e.g., procaine, lidocaine) or used topically.
• Topical anesthetics can be soluble (e.g., cocaine, lidocaine) or insoluble (e.g., benzocaine).

Pharmacokinetics

• Uptake: Local anesthetics are absorbed into the bloodstream and distributed throughout the body.
• Distribution: Highly perfused organs (e.g., brain, liver, kidneys, lungs, spleen) receive the anesthetic first.
• Metabolism:
  • Esters are hydrolyzed in the blood plasma by pseudocholinesterase.
  • Amides are metabolized in the liver.
• Excretion: Local anesthetics and their metabolites are primarily excreted through the kidneys.

Systemic Actions

• Central Nervous System: Local anesthetics can cross the blood-brain barrier.
  • Low blood levels do not affect the CNS.
  • High blood levels can lead to generalized tonic-clonic convulsions.
• Cardiovascular System: Local anesthetics produce myocardial depression.
  • They decrease electrical excitability of the myocardium.
  • They reduce the force of contraction.
  • They can lead to hypotension due to vasodilation.
• Respiratory System: Local anesthetics can have a direct relaxant effect on bronchial smooth muscles.
  • Overdose levels can lead to respiratory arrest.

Composition of Local Anesthetics Solutions

• Local anesthetics solutions contain:
  • Local anesthetic drug: Blocks nerve conduction.
  • Sodium chloride: Makes the solution isotonic to prevent cell damage.
  • Sterile water: Provides volume.
  • Vasoconstrictor (e.g., epinephrine): Increases the depth and duration of anesthesia. It also reduces the absorption of local anesthetics, reducing toxicity.
  • Sodium meta bisulfite: An antioxidant for the vasoconstrictor.
  • Methyl paraben: A bacteriostatic agent.

Procaine

• Classification: Ester.
• Potency: 1 (procaine = 1).
• Toxicity: 1 (procaine = 1).
• Onset of action: 6 to 10 minutes.
• Anesthetic half-life: 0.1 hour (6 minutes).
• Effective dental concentration: 2% to 4%.
• Uses:
  • Management of accidental intraarterial injection.
  • Breaking arteriospasm.
• It produces the greatest vasodilation of all local anesthetics.

Propoxycaine

• This agent is usually combined with procaine for a more rapid onset and prolonged anesthesia.
• It is not used alone due to high toxicity.
• Combined with procaine when amide agents are contraindicated.

Lidocaine

• Classification: Amide.
• Potency: 2 (compared to procaine).
• Toxicity: 2 (compared to procaine).
• Effective dental concentration: 2% with Epinephrine.
• Onset of action: Rapid, 3 to 5 minutes.
• Pulpal anesthesia (with 1:100,000 epinephrine): 60 minutes (intermediate duration).
• Soft tissue anesthesia (with 1:100,000 epinephrine): 180-300 minutes (3-5 hours).
• Maximum recommended dose (with epinephrine): Adult: 7.0 mg/kg body weight.
• Pregnancy classification: B (considered safe during pregnancy).

Mepivacaine

• Classification: Amide.
• Potency: 2 (procaine = 1; lidocaine = 2).
• Toxicity: 1.5 to 2 (procaine = 1; lidocaine = 2).
• Effective dental concentration: 3% plain (short duration) or 2% with a vasoconstrictor (intermediate duration).
• Onset of action: Rapid, 3 to 5 minutes.
• Pulpal anesthesia: 20-40 minutes for 3% and 60 minutes for 2%.
• Soft tissue anesthesia: 2-3 hours for 3% and 3-5 hours for 2%.
• Maximum recommended dose: Adult: 6.6 mg/kg body weight.
• Pregnancy classification: C (not considered safe).
• 3% mepivacaine plain is recommended for pediatric & geriatric patients or shorter dental procedures.
• 2% mepivacaine with vasoconstrictors (e.g. levonordefrin, epinephrine) provides intermediate duration.

Prilocaine

• Classification: Amide.
• Prilocaine 4% with or without epinephrine (1:200,000) is used for patients with epinephrine sensitivity.
• It is considered safe due to its fast biotransformation.
• Maximum recommended dose: 8 mg/kg.
• Contraindications:
  • Idiopathic or congenital methemoglobinemia.
  • Hemoglobinopathies (e.g., sickle cell anemia).
  • Cardiac or respiratory failure.

This quiz explores the fundamentals of local anesthetics, including their historical development and classification. You'll learn about different chemical structures, duration of action, and their biological effects. Test your knowledge on these essential medical agents.