Chapter 10

1. Chapter 10 Local Anesthetics

2. Local Anesthetics • History of Local Anesthetics • Local anesthetics are derivatives of cocaine which is a derivative of the coca leaf 2

3. Desirable Properties of Local Anesthetics • Potent • Reversible • Absence of local and systemic reactions • Absence of allergic reactions • Rapid onset and satisfactory duration • Adequate tissue penetration • Low cost • Stability in solution 3

4. Site and Mechanism of Action • Local anesthetics are divided into 2 chemical groups; esters and amides. • Esters have a higher rate of allergic reaction. • Local anesthetics slows or blocks depolarization by reducing Na permeability into the nerve cytoplasm, thus inhibiting the flow of K out of the cell. • Nerve impulse travels from node to node. Local anesthetics effectively block nerve impulse travel. 4

5. Ionization Factors • Local anesthetics are weak bases occurring equilibrated between their 2 forms, the fat soluble, free base and water-soluble hydrochloride salt. • The portion of drug in each form is determined by the pKa of the local anesthetic and the pH of the environment. • Once injected into local tissue, the amount of local anesthetic in the free base form increases and allows for greater tissue penetration. • If there is an infection or inflammation, the free base form decreases and less drug penetrates the tissue. • Other factors that can affect tissue penetration include inflammation, vasodilation, and dilution by fluid. 5

6. Local Anesthetics • Absorption • Absorption of local anesthetics is dependent on the route of injection. • Once injected, the rate of absorption is dependent on tissue vascularity. • Tissue vascularity is a function of inflammation, vasodilating properties of the local anesthetic, presence of heat, or the use of massage. • Systemic absorption of the local anesthetic is reduced with the addition of a vasoconstrictor. 6

7. Local Anesthetics • Distribution • The local anesthetic becomes partially bound to plasma and red blood cells. • The unbound drug freely diffuses to other tissue including the CNS and across the placenta. • Distribution allows absorption to occur in 3 phases. • The drug occurs at highly vascular tissues in the lungs and kidneys • It appears in less vascular muscle and fat • The drug is metabolized 7

8. Local Anesthetics • Metabolism and Excretion • Primarily inactivated in the liver • Excretion in urine 2-5% of active drug form • Articaine is removed faster than the other amides 8

9. Local Anesthetics • Pharmacologic Effects • The main pharmacologic effect of the local anesthetic is to reversibly block peripheral nerve conduction. • Local anesthetics also have a direct effect on the cardiac muscle by blocking cardiac Na channels and depressing abnormal cardiac pacemaker activity, excitability, and conduction. 9

10. Local Anesthetics • Adverse Reactions • Adverse reactions and toxicity of local anesthetics are directly related to drug plasma levels. • The factors that influence toxicity include: • Drug itself • Concentration • Route of administration • Rate of injection • Vascularity • Patient’s weight • Rate of metabolism and excretion 10

11. Local Anesthetics • Adverse Effects • Both CNS stimulation and depression can occur. • Local anesthetics can produce myocardial depression and cardiac arrest with peripheral vasodilation. • Local effects include physical injury caused by poor injection technique. • Malignant hyperthermia only occurs in those persons with the inherited autosomal dominant gene. It is not related to amide local anesthetic use. • The incidence of allergic reactions to amide local anesthetics is very low. 11

12. Composition of Local Anesthetics • Local anesthetic solutions are also made up of the following: • Vasoconstrictor that delays local anesthetic absorption, reduces systemic toxicity, and prolongs the duration of action. • Antioxidant that delays the oxidation of epinephrine. Some are used to prolong shelf-life. • Sodium hydroxide adjusts the pH of the solution to between 6 and 7. • Sodium chloride makes the injectable solution isotonic. 12

13. Local Anesthetic Agents • Several different amide local anesthetics are available for use. • Choice of local anesthetic should be based upon onset of action, duration of local anesthesia required and side effects. • Presence or absence of a vasoconstrictor. This is of particular importance if the patient has hypertension. 13

14. Local Anesthetic Agents 14

15. Vasoconstrictors • Benefits of Local Anesthetics • Suppress systemic absorption of anesthetic agent • Increase duration and intensify block • Localize hemostasis • Reduce toxicity • Act on alpha and beta receptors in body tissues, causing the constriction of blood vessels 15

16. Vasoconstrictors • Contraindications • Heart attack in the last 6 months • Uncontrolled high blood pressure • Daily angina • Tricyclic antidepressants • Uncontrolled hyperthyroidism • Uncontrolled arrhythmias 16

17. Vasoconstrictors • Dental Concerns • Vasoconstrictors increase the length of anesthetic effect which increases the chance of accidental patient self-mutilation. • In small quantities, epinephrine acts as a vasodilator thus having the potential to increase post-op bleeding. • Patients with cardiovascular disease, who can receive a vasoconstrictor, should receive the lowest dose possible by means of the best injection technique. • Adverse Reactions • Ischemia • Prolonged pain, numbness or paresthesia • Feelings of nervousness or fast heart rate 17

18. Topical Anesthetics • Several different topical anesthetics are available for use. • Choice should be based upon onset, duration of action, and allergenic potential. • The patient should be instructed to avoid eating for 1 hour after application so that the gag reflex can become fully functional. 18

19. Topical Anesthetics 19

20. Topical Anesthetics 20