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Local anesthetics

Local anesthetics
Local anesthetic can be defined as a drug that can block in a reversible way the transmission of the nervous stimulus at the place where it is applied, without causing alterations in the level of consciousness

Mechanism of action of local anesthetics.
Local anesthetics block the action of ionic canals in the neuronal cell membrane, preventing the neurotransmission of the action potential. The ionized form of the local anesthetic binds in a specific way to the sodium channels, inactivating them and preventing the propagation of cell depolarization.
However, the specific binding occurs in the intracellular environment, so it is necessary that the local anesthetic in its molecular form exceeds the plasma membrane to then block the sodium channels. It is probable that there is a second mechanism of action of local anesthetics, which involves the inactivation of sodium channels by incorporating their molecules into the plasmatic membrane (plasma membrane expansion theory). This second mechanism of action would be generated by the non-ionized form of local anesthetics, acting from outside to inside. Nerve fibers have different sensitivities to local anesthetics, the small fibers being more sensitive than the large ones, and the myelinated fibers are blocked more quickly than those not mielized in the same diameter.Nerve fibers blockage occurs gradually, initiated with loss of sensitivity to pain, temperature, touch, proprioception and finally loss of skeletal muscle tone. For this reason individuals can still feel the touch at the moment when the pain is already absent after application of the local anesthetic.When administered the local anesthetic is presented in the tissue of ionized and non-ionized forms, depending on the anesthetic used and the pH of the medium that it was inserted, there may be a greater proportion of one shape in relation to another. But what does that interfere with your action?

In physiological ph (7.4) all local anesthetics have their ionized form in greater proportion, lidocaine presents in physiological ph 25% in its non-ionized form. The physiological PH Bupivacaine will have an even smaller fraction of the non-ionized form, approximately 15%.
The drug should cross the lipid membrane of the cell into the intracellular medium acting on the sodium channels. The non-ionized portion crosses the membrane more easily than ionized. Thus, the drug with the highest non-ionized fraction in physiological PH reaches its effector site in a faster way. This explains why Lidocaine has shorter latency time than bupivacaine, but this event leads to lidocaine being metabolized and excreted much faster than bupivacaine, which in turn presents a longer latency period and consequently has its Extended period of action.
How do physical chemical characteristics interfere with the potency of the local anesthetic?

The physicochemical characteristics such as the structure of the aromatic ring and the length of the hydrocarbon chain that determine the liposolubility of the local anesthetic, directly related to its potency. Liposoluble drugs penetrate the cell membrane more easily to exert its effect. The more potent, the lower the amount of drug needed to cause the same effect. Thus, highly liposoluble bupivacaine is approximately four times more potent than lidocaine. This is reflected in the commercially available drug preparations, as bupivacaine is more potent, its presentations are in solutions from 0.1 to 0.5%, while lidocaine is usually presented in solutions of 1 to 2%.

The duration of the action of the local anesthetic is related to its molecular structure, mainly to the length of the intermediate chain that binds the aromatic ring to the amine group, and to the degree of protein binding of the drug. Among the different local anesthetics there is a great variability of intermediate chains and the degree of protein binding. The higher the protein binding, the longer the duration of the anesthetic. Thus, as an example, the degree of protein binding of lidocaine is 65%, while Bupivacaine is 95%, it can be predicted that bupivacaine has a longer duration of action than lidocaine, which in fact occurs. The Procain (ester) has low degree of protein binding, only 6%, so it has ultra fast action.

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Postado Por: Jayme Rocha

Jayme Rocha
Horse lover, trainer and student of Veterinary Medicine, currently works in the marketing and customer relationship department at Univittá Saúde Animal.

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