LOCAL ANESTHETICS
Local anesthetics are partial anesthetics that do not anesthetize the entire body. This is particularly useful for carrying out a surgical operation at a particular area of the body which can easily be desensitized without affecting the rest of the body. Also they are useful to reduce the pain impulses at an injured part of the body such as tooth pain.
TYPES OF LOCAL ANESTHETICS
In general there are two types of local anesthetics divided by their chemical structure. Structurally there are two portions in their molecule such as lipophilic (Affinity for fat) and hydrophilic (Affinity for water). There are two types of local anesthetics available determined by the bond linking the hydrophilic part to the lipophilic part.
They are either esters or amides
Esters
1.Cocaine
2.Benzocaine
3.Procaine
4.Tetracaine
Amides
1.Lidocaine (Xylocaine,or Lignocaine)
2.Mepivacaine
3.Bupivacaine
4.Prilocaine
Mechanism
Local anesthetics block the nerve conduction of pain impulses by inhibiting the voltage-gated sodium channels of the nerve cell membrane.
Small unmyelinated nerves that conduct pain, heat, and autonomic activity are affected first.
With increasing concentration Pain fibers(A and C fibers) sensory(A-fibers) fibers followed by motor fibers(A-fibers) which are myelinated get affected.
Metabolism
Esters are more electrophilic and more rapidly metabolized by blood and tissue esterases and hence they have a shorter half-life and less toxic.
Amides are less electrophilic and slowly metabolized in the liver and hence they have a longer half-life and more toxic.
Clinical Indications
1.For surface anesthesia
2.Nerve blocks
3.Spinal and Epidural anesthesia
4. Lidocaine is also used as an antiarrhythmic systemically
With Epinephrine
To increase the duration of action of local anesthetics epinephrine is added which blocks the area to be anesthetized by vasoconstriction effects and thereby reduces the systemic absorption and enhances the local anesthetic actions.
Side Effects
1. Systemic effects will result in high doses and long time continuous use.
Cardiovascular effects like Depression of myocardial contraction(negative inotropic)
and hypotension.
But on the contrary cocaine causes systemic vasoconstriction and hypertension.
2.Lightheadedness(Headache)
3.Sensory disturbances
4.Convulsions
5. At high dose coma and death can occur.
