Local anesthetics are a group of drugs widely used in many medical departments – surgery, dentistry, dermatology, gastroenterology, pulmonology, obstetrics and gynecology, orthopedics, etc. They are mainly used for local anesthesia of a certain area to perform surgical and invasive manipulations (in dental interventions, gastroscopies, bronchoscopies, cystoscopies, sutures, etc.). Lidocaine is also an antiarrhythmic agent when administered intravenously.
They block sodium channels, stabilize the cell membrane, and block the conduction of sensory and, in higher doses, motor nerve impulses from the site of irritation to the CNS. They act mainly on unmyelinated nerve fibers and in the areas of Ranvier’s pinches in myelinated ones. Their action depends on the acidity of the medium – e.g. in case of inflammation, due to an increase in acidity, their effect weakens; from the size of the nerve fiber, the dose, etc.
All local anesthetics have a similar structure – they have a lipophilic part (aromatic core) which, using an ester or amide bond, is connected to an intermediate hydrocarbon part, which in turn is connected to a hydrophilic (amine group).
Pharmacokinetically, local anesthetics can be applied topically ( surface anesthesia ) – creams, sprays; infiltration – anesthetic is injected layer by layer into the tissue; wire anesthesia – a nerve or nerve plexus is blocked; epidural (for spinal anesthesia ) and subarachnoid (for lumbar ) – often in obstetrics and gynecology during childbirth. Local application of a vasoconstrictor (adrenaline, noradrenaline) reduces the systemic absorption of the local anesthetic and significantly prolongs the time of its effect.
Local anesthetics are divided into two groups: esters and amides. Esters include Cocaine, Procaine, Benzocaine, Tetracaine, Oxybuprocaine, Chloroprocaine, etc., and amides – Lidocaine, Articaine, Mepivacaine, Prilocaine, Bupivacaine, Levobupivacaine, etc.
Mepivacaine is NOT used in gynecology because of its toxic effects on the fetus.
They are also divided according to the strength of the action, the beginning of the effect, and the duration of the effect. For example, Lidocaine has a low potency, rapid onset of action, and medium duration of action.
Local anesthetics are metabolized in the blood and in the liver. Esters are metabolized primarily in the blood by pseudocholinesterase. In patients with defects in this enzyme, a prolongation of the anesthetic effect is observed. In patients with liver failure, the risk of toxic effects increases when using amide local anesthetics, so esters are preferred in such cases. Both types of anesthetics are mainly excreted in the urine.
Often patients report allergies to local anesthetics. In most cases, these reactions are caused by the vasoconstrictor. In another part of the cases, it concerns a psychogenic effect of the stabbing. As for the actual allergic reactions to local anesthetics – to amides, they are extremely rare, while to esters they are much more common. The presence of allergic reactions to one ester precludes the use of other esters because allergic reactions are due to the benzoic acid metabolite derived from all esters.
However, the presence of allergic reactions to one amide does not preclude the use of others. It is not excluded that allergic reactions in part of the cases are due to auxiliary substances in the medicinal product. Therefore, a scarification skin test should still be performed before topical application. Before the administration of a local anesthetic in children, the maximum permissible dose based on body mass should be calculated to prevent possible accidental overdose and systemic side effects (hypotension, arrhythmias, etc.).
Frequent use of local anesthetics leads to tachyphylaxis (weakening of their effect). Other adverse reactions, apart from allergies, are associated with numbness, lack of sensitivity at the site of application, irritation, hypotension, arrhythmias, and changes in mental status (systemic effects are mainly with venous, lumbar, and spinal administration). In older patients, it is always good to keep the administered dose below the maximum allowed due to the risk of hepatic toxicity and the cardiotoxicity of the vasoconstrictor administered.
Local anesthetics are relatively safe drugs with few side effects. They are widely used in medicine for various painful and unpleasant manipulations.
The material is informative and cannot replace consultation with a doctor. Before starting treatment, you must consult a doctor.
