Local anesthetics (LAs) are drugs that block nerve conduction when applied locally to nerve tissue
in appropriate concentrations. Their action is completely reversible. They act on every type of
nerve fiber and can cause both sensory and motor paralysis in the innervated area. They act on
axons, cell body, dendrites, synapses and other excitable membranes that utilize sodium channels
as the primary mean of action potential generation.

Classification of local anesthetics (LAs) based on route of administration and duration of
action—

• I. Injectable
  1. Short-acting – Procaine,
     chloroprocaine
  2. Intermediate-acting – Lignocaine, prilocaine
  3. Long-acting – Tetracaine(amethocaine),
     bupivacaine — Dibucaine
     (cinchocaine),
     ropivacaine,
     etidocaine.
  4.  
• II. Surface anesthetics — Lignocaine, cocaine,
  anesthetics tetracaine, benzocaine,
  oxethazaine, dibucaine, dyclonine

SYSTEMIC ACTIONS

CNS

: Local anesthetics depress the inhibition from the cerebral cortex. This loss of inhibition results
in unopposed excitatory activity which is manifested as restlessness, tremors and may proceed to convulsions. This central stimulations is followed by generalized CNS depression and death may result from respiratory failure.

CVS

The primary site of action is the myocardium—lignocaine decreases excitability, conduction rate and force of contraction (quinidine like effects). It also causes arteriolar dilatation. Since procaine is short-acting, procainamide is used as antiarrhythmic. Bupivacaine is more cardiotoxic than other LAs.

Smooth muscle:

LAs depress contractions in the intact bowel. They cerebrovascular and bronchial smooth muscles.

ADVERSE EFFECTS

• • Hypersensitivity reactions

Skin rashes, dermatitis, asthma or rarely anaphyledeedemae reactions are more common with ester type of drugs. Intradermal sensitivity test should be done before using these drugs.

• • CNS

Dizziness, auditory and visual disturbances, mental confusion, disorientation, anxiety, muscle tremors, convulsions and respiratory failure can result from large doses. Intravenous diazepam controls
convulsions. Infact, these can be prevented by preanesthetic administration of diazepam, especially if large doses are to be used.

• • CVS

Hypotension, bradycardia, arrhythmias may be encountered. Rarely cardiac arrest can occur.

• • Local irritation

Can be seen with bupivacaine. Wound healing may be delayed.

INDIVIDUAL COMPOUNDS

Lignocaine

—Most widely used LA. It is faster and longer-acting. Action is seen in 3 minutes for nerve block. It is useful for all types of blocks. In contrast to other LAs, lignocaine causes drowsiness and mental
clouding. Xylocaine 4 percent topical solution, 2 percent jelly, 5 percent ointment, 1 percent
and 2 percent injection, 5 percent for spinal anesthesia.

Bupivacaine HCl

Widely used. But it can cause more cardiotoxicity than others. Injection 0.25-0.5 percent with or without adrenaline.

Ropivacaine

Is similar to bupivacaine except that it is less cardiotoxic.

Chloroprocaine HCl

Potency is twice that of procaine and its toxicity is lower because of its more rapid metabolism.

Etidocaine HCl

Its analgesic action lasts 2-3 times longer. It is used for epidural and all types of infiltration and regional anesthesia.

Prilocaine HCl

Onset of action and duration are longer. Because of it’s toxicity, its use is restricted to dental procedures.

Cocaine

Produces euphoria and is drug of dependence and abuse. It is a surface anesthetic. It is a protoplasmic poison and hence cannot be injected. Cocaine is not preferred now due to toxicity.

Procaine

Was widely used once. But is now replaced by other agents. It is hydrolyzed to PABA which interferes with sulfonamides. It is rapidly absorbed following parenteral administration. It is ineffective when applied topically—thus not useful as a surface anesthetic.

Tetracaine

Is a PABA derivative and is 10 times more toxic and more active than procaine. It is used on eye as 0.5 percent drops, ointments 0.5 percent and cream 1 percent for topical use. 0.25 to 0.5 percent injection is used for spinal anesthesia.

LOCAL ANESTHETICS USED ONLY ON THE EYE

Benoxinate HCl

Within 60 seconds of administration it produces corneal anesthesia
enough to perform tonometry.

Proparacaine HCl—

Produces little or no initial irritation—0.5 percent ophthalmic solution is
used.

LOCAL ANESTHETICS USED ON THE SKIN AND MUCOUS MEMBRANES

Local anesthetics used on skin and mucous membranes are lignocaine, dibucaine, dyclonine hydrochloride and pramoxine hydrochloride. These drugs are effective when used topically in the symptomatic relief of anal and genital pruritus, poison ivy rashes, acute and chronic dermatoses.
Dibucaine is the most potent, most toxic and longest-acting LA. It is available as cream and
ointment.

POORLY SOLUBLE ANESTHETIC

These are too slowly absorbed to be toxic. They can be applied to wound directly and ulcerated surfaces as they produce sustained anesthetic effect, e.g. benzocaine.

USES OF LOCAL ANESTHETICS

Local anesthesia is the loss of sensation without the loss of consciousness or impairment of central
control of vital functions. Depending on the site and technique of administration, LA can be:

• • Surface anesthesia:

Anesthesia of mucous membrane of eyes, nose, mouth, tracheobronchial tree, esophagus and genitourinary tract can be produced by direct application of anesthetic solution. Tetracaine 2 percent, lignocaine 2–10 percent n are most often used. Phenylephrine (but not adrenaline) produces vasoconstriction on topical application and prolongs the duration
of action.

• • Infiltration anesthesia

Injection of  local anesthetic solution directly into the tissue can be (i) superficial—only into the skin, or (ii) into deeper structure including intra-abdominal organs.

• • Field block

Subcutaneous injection of a LA solution proximal to the site to be anesthetized, interrupts nerve transmission in the region distal to the injection. Sites such as forearm, scalp, anterior abdominal wall and lower limbs are used for field block. Knowledge of the neuroanatomy of the area is essential.

• • Spinal anesthesia (SA)

Local anesthetic solution is injected into the subarachnoid space between L2-3 and L3-4. The drug acts on nerve roots. Lower abdomen and lower limb are anesthetized and paralyzed. The level of anesthesia can be altered by changing the volume of injection, specific gravity of the solution and posture of the patient. Level of sympathetic block produced is 2 segments higher and motor paralysis is 2 segments lower than sensory or cutaneous anesthesia

• • Epidural anesthesia

LA is injected into the spinal extradural space and it acts on the nerve roots. It is technically more difficult and comparatively larger volumes of the anesthetic
are needed.

• • Intravenous regional anesthesia

This type of anesthesia is useful for rapid anesthetization of an extremity. A rubber bandage is used to force the blood out of the limb (veins) and a tourniquet is applied to prevent the reentry of the blood.