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While many drugs have some local anesthetic action as a side effect, the substances that are clinically used belong to two related classes: aminoamide and aminoester local anesthetics. These so-called synthetic local anesthetics are structurally related to cocaine. They differ from cocaine mainly in that they have no abuse potential and do not act on the sympathoadrenergic system , i.e. they do not produce hypertension or local vasoconstriction.
Local anesthetics vary in their properties and they are used in various techniques of local anesthesia such as:
The local anesthetic Lidocaine is also used as an antiarrhythmic drug.
Local anesthetic drugs act mainly by inhibiting sodium influx through sodium-specific ion channels in the neuronal cell membrane, in particular the so-called voltage-gated sodium channels. When the influx of sodium is interrupted, an action potential cannot arise and signal conduction is thus inhibited. The receptor site is thought to be located at the cytoplasmic (inner) portion of the sodium channel.
Local anesthetics are weak bases and are usually available as hydrochloride solutions. At physiologic pH the protonated and unprotonated forms of the molecule exist in an equilibrium. Only the unprotonated molecule diffuses readily across cell membranes, but only the protonated form is able to block sodium channels. It may be possible to enhance local anesthetic activity by altering the pH of a drug preparation.
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The conduction of electric impulses follows a similar mechanism in peripheral nerves, the central nervous systemThe human central nervous system consists of the brain and spinal cord. These lie in the midline of the body and are associated with the skull and vertebrae respectively. see Nervous System) The central nervous system along with the peripheral nervous sys, and the heartThis page is about the muscular organ, the Heart . For other meanings of the word, see Heart (disambiguation). The heart ( Latin cor is a hollow, muscular organ that pumps blood through the blood vessels by repeated, rhythmic contractions. The term cardia. The effects of local anesthetics are therefore not specific for the signal conduction in peripheral nerves. Side effects on the central nervous system and the heart may be severe and potentially fatal. However, toxicity usually occurs only at blood plasma levels which are rarely reached if proper anesthetic techniques are adhered to.
Depending on local tissue concentrations of local anesthetics, there may be excitatory or depressant effects on the central nervous system. At lower concentrations, a relatively selective depression of inhibitory neurons results in cerebral excitation, which may lead to generalized convulsions. A profound depression of brain functions occurs at higher concentrations which may lead to asphyxia, coma and death. Such tissue concentrations may be due to very high plasma levels after intravenous injection of a large dose. Another possibility is direct exposure of the central nervous system through the cerebrospinal fluidCerebrospinal fluid CSF in short, is the clear fluid that occupies the subarachnoid space (the space between the skull and cortex of the brain). It acts as a "cushion" or buffer for the cortex. Also, CSF occupies the ventricular system of the brain and th, i.e. overdose in spinal anesthesia or accidental injection into the subarachnoid space in epidural anesthesia.