Definition and mechanism of action
Local anesthetics are agents that reversibly block sodium channels and abolish nerve impulse conduction. Unlike analgesics, which suppress only pain sensation, local anesthetics sequentially turn off all types of sensation: first pain, then tactile, gustatory, olfactory, and temperature.
To block the channel, the anesthetic molecule must penetrate inside the cell and bind to the sodium channel from the cytoplasmic side. As a result, membrane depolarization is disrupted, the action potential is not formed, and impulse conduction ceases.
Local anesthetics are weak bases. In the acidic environment of an inflammatory focus (especially purulent), they become ionized, fail to penetrate the neuronal membrane, and lose efficacy. Therefore, their use in purulent inflammation is impractical.
Classification
Based on their chemical structure, local anesthetics are divided into two groups:
- Esters are hydrolyzed by plasma cholinesterase and are preferred in liver disease. They include: procaine (novocaine), tetracaine, cocaine, benzocaine.
- Amides are metabolized by cytochrome P450 enzymes in the liver. This group includes: lidocaine, trimecaine, mepivacaine, ropivacaine, bupivacaine, and articaine (ultracaine).
Application features and side effects
Local anesthetics are often combined with adrenaline: vasoconstriction delays drug absorption into the bloodstream, prolongs its duration of action, and reduces systemic (resorptive) side effects. Noradrenaline must not be used for this purpose — when administered subcutaneously or intramuscularly, it causes tissue necrosis (the antidote is phentolamine).
Resorptive side effects:
- nausea, vomiting;
- slowed atrioventricular conduction (second- to third-degree AV block is a contraindication for intravenous administration);
- paresthesia, visual floaters, tinnitus;
- CNS excitation followed by depression of consciousness;
- in severe intoxication — sharp drop in blood pressure, tonic-clonic seizures, cardiac and respiratory arrest.
Management of toxicity: adrenaline to support hemodynamics and diazepam to control seizures.
Types of local anesthesia
- Surface (topical) anesthesia — application of an anesthetic to the skin or mucous membranes. Used in ophthalmology, ENT practice, EGD, intubation, and bronchoscopy. Lidocaine, benzocaine, bupivacaine, articaine, and trimecaine are used.
- Infiltration anesthesia — layer-by-layer infiltration of tissues with an anesthetic solution during surgical procedures.
- Nerve block (regional) anesthesia — injection of an anesthetic into the area of a nerve (avoiding direct injection into the nerve itself to prevent neuralgia). Widely used in dentistry and Lukashevich surgery. Bupivacaine, articaine, procaine, and lidocaine are used.
- Epidural anesthesia — injection into the epidural space of the spinal cord. Used for pelvic surgeries, Cesarean delivery, and labor pain relief.
- Subarachnoid (spinal) anesthesia — injection into the cerebrospinal fluid under the arachnoid mater. Used less frequently due to the risk of central side effects and the danger of brainstem herniation with significant CSF leakage.
Key drugs
- Lidocaine — a universal anesthetic for all types of anesthesia, more toxic than novocaine. The only local anesthetic administered intravenously: used as a class IB antiarrhythmic in myocardial infarction and arrhythmias caused by cardiac glycoside, antidepressant, or atropine poisoning. For emergency use only.
- Benzocaine (anesthesin) — insoluble in water, used topically only: in ointments for itching, scabies, and allergies; orally for reflux esophagitis; in suppositories for hemorrhoids.
- Procaine (novocaine) — duration of action is 30–60 minutes, highly water-soluble, less toxic than lidocaine; virtually non-toxic in combination with adrenaline. Suitable for all types of anesthesia.
- Articaine (ultracaine) — duration of action is 1–2 hours, used for all types of anesthesia except topical. Highly effective and low-toxicity with adrenaline; widely used in dentistry.
- Bupivacaine (marcaine) — duration of action is 3–4 hours, has a latency period of about 20 minutes, 16 times more potent than novocaine. Liposomal bupivacaine releases over up to 72 hours and is considered an alternative to morphine for postoperative analgesia.
- Ropivacaine — duration of action is 4–6 hours, primarily used for epidural and subarachnoid anesthesia.
- Cocaine — the first historical local anesthetic, derived from the coca plant. Unique in that it blocks the reuptake of dopamine, noradrenaline, and serotonin, causing profound euphoria, tachycardia, and increased blood pressure. Causes psychological dependence only; withdrawal manifests as depression and fatigue due to depleted neurotransmitter stores.
- Ethyl chloride — used as a spray: cools the skin upon evaporation, reducing pain from sports injuries.
Key points
- Local anesthetics block sodium channels from inside the cell, sequentially turning off all modalities of sensation starting with pain.
- In the acidic environment of an inflammatory focus, these drugs lose activity, rendering them ineffective in purulent processes.
- Esters (novocaine, tetracaine) are metabolized by plasma cholinesterase; amides (lidocaine, articaine, bupivacaine) are metabolized by hepatic cytochrome P450 enzymes.
- Adrenaline prolongs and enhances the effect of the anesthetic while reducing its systemic toxicity; noradrenaline is not used for this purpose.
- In case of resorptive toxicity, adrenaline and diazepam are administered; second- to third-degree AV block is a contraindication for intravenous administration.
- Lidocaine is the only local anesthetic for intravenous use, utilized as a class IB antiarrhythmic.
- Bupivacaine has the longest duration of action; the liposomal formulation works for up to 72 hours.
- Cocaine inhibits monoamine reuptake, producing pronounced psychological dependence without physical dependence.