Introduction:
Anesthetic drugs have been used for centuries in the medical and dental fields to anesthetize an area by preventing nerve endings from being excited or by blocking conduction in peripheral nerves to reduce the sensation of pain at the site of application. To avoid pain from regular dental procedures, minor surgeries, incisions, biopsies, and obstetrical operations, as well as pain from wounds, local anesthetics are frequently utilized.
The characteristic feature of local anesthesia is that it produces the temporary loss of sensation without inducing loss of consciousness. They can be used as a single injection at the site, and epidural or wound-based catheters can be used as infusions over several days or hours of postoperative pain management. They are safe and have excellent tolerance. Short-acting local anesthetics are usually administered with Epinephrine, which slows the absorption rate, reducing systemic exposure and lengthening the duration of effect.
What Are the Ideal Properties of an Anesthetic Drug?
An anesthetic solution's most desirable characteristics include
-
It should not irritate the tissue.
-
It should not cause any permanent damage to the nerves.
-
It should have a low systemic.
-
Whether it is injected into the tissue or administered topically to mucous membranes, it must be effective.
-
Its action should begin quickly.
-
It should have a long duration of action to permit the completion of the procedure.
-
It should have sufficient potency to achieve a complete anesthesia effect without the additional concentrated solutions being hazardous.
-
It should not cause any sort of allergic reaction.
-
It should be stable in solution and quickly undergo a metabolic transformation in the body.
-
It should be capable of withholding heat sterilization methods.
How Are Anesthetics Classified?
Depending on the presence of the chemical bond, local anesthetics are classified as
-
Amino Esters: (for example, Cocaine, Procaine, Chloroprocaine, and Tetracaine).
-
Amino Amides: (for example, Lidocaine, Prilocaine, Bupivacaine, Mepivacaine).
What Is the Structure of Amide Anesthetics?
Most of the local anesthetics are water-soluble salts of lipid-soluble alkaloids. The basic structure of amide anesthetics consists of three components:
-
A lipophilic-aromatic group.
-
Intermediary link - amide link.
-
A hydrophilic-amine group.
What Are the Various Examples of Amide Anesthetics?
The most frequently used amide anesthetics include - Lidocaine, Mepivacaine, Prilocaine, Bupivacaine, Etidocaine, Ropivacaine, and Levobupivacaine.
What Is the Onset of Local Anesthetic Drugs?
Local anesthesia's onset is based on lipid solubility and ionization constant (pKa). When exposed to physiological pH, the ionization constant (pKa) converts to the lipid-soluble structure. In an anesthetic solution with high pKa, very few molecules are available in their lipid-soluble form - the onset of action is delayed. To anesthetize a patient with an infection is difficult because the environment pH is much lower (around pH of 5.2) - which favors the water-soluble state. For the faster onset of action, the pKa of the local anesthetic should be low, the more ionized form of the drug is present in normal tissue, and the faster the onset.
What Is the Duration of Action of Local Anesthetics?
As the local anesthetic is bound to the nerve membrane, recovery from the nerve block is considerably longer than the rapid onset of the action. Bupivacaine, Etidocaine, and Ropivacaine are firmly bound to the nerve cell membrane (increased protein binding). These amide anesthetics are released more slowly from receptor sites in the sodium channels. The local anesthetic drug's duration of action depends on the rate at which an anesthetic is removed from the nerve cell membrane.
How Is Amide Local Anesthetic Drug Metabolized?
The existence of an intermediate chain not only establishes a proper classification but also determines the elimination pattern of local anesthetics. Amide-type local anesthetics are biotransformed by hepatic microsomal enzymes into water-soluble metabolites in the liver and then eliminated by the kidney.
How Are Amide Local Anesthetics Excreted?
As their biotransformation is more complicated than esters, amides are often seen in urine as the parent compound. Patients with renal impairment cannot eliminate the drug leading to an increased level of anesthetic solution and their metabolites in the blood, causing systemic toxicity.
What Is the Maximum Dose of Administration of Local Anesthetics?
Administration of local anesthetics depends on individual patient characteristics, the dosage of the drug, with or without Epinephrine, the technique used and speed of administration, and the tissue's vascularity should be considered.
Table Shows the Dosage of Amide Local Anesthetic Drugs:
The dose to be administered, the patient's weight and any prior cardiac problems should all be taken into account by the surgeon when calculating the maximum tolerated dose.
What Are the Contraindications of Amide Anesthetics?
Amide local anesthetics have methylparaben as a preservative, which causes severe allergic reactions. It is advised to avoid an amide group of local anesthetics drugs in patients with altered hepatic function because they are metabolized in the liver.
What Are the Side Effects of Local Anesthetics?
The common side effects of Amide local anesthetics include - headache, pain at the site of injection, nausea, vomiting, drowsiness, infection at the site of injection, facial swelling, loss of sensation, tingling, and prickling sensation, inflammation of the gums, bleeding, and convulsions.
Apart from the symptoms listed above, some multisystem adverse effects are as follows:
-
Neurological Symptoms: Tinnitus (ringing in the ear), dizziness, and twitching.
-
Gastrointestinal Symptoms: Constipation.
-
Cardiovascular Symptoms: In higher doses, ventricular arrhythmias (a heart rhythm problem caused by irregular electrical signals in the heart's lower chambers).
Conclusion:
As the local anesthetics initially target the voltage-gated sodium channels in the nerve cell membrane, a complete understanding of the usage, pharmacological effects, and action of the local anesthetics are necessary for the surgeon to attain desired anesthetic effect to carry out the procedures.
