8.7:
Local Anesthetics: Clinical Application as Surface, Infiltration, and Conduction Block Anesthesia
Local anesthetics or LAs can anesthetize skin, subcutaneous tissue, and peripheral nerves either by surface, infiltration, or conduction block anesthesia.
They first block sensory impulses which transmit pain signals and then motor impulses, which control movement.
In surface anesthesia, LA is applied topically on injured skin or the mucous membranes of the nose or mouth.
In infiltration anesthesia, LAs are injected directly into the tissue under the skin or into intra-abdominal organs to anesthetize small areas. In surface and infiltration anesthesia, there is no motor function loss. Even at high concentrations, these LAs are effective for only short durations.
On the contrary, conduction block anesthetics can block motor impulses and they last longer, even at low concentrations. They are used to anesthetize distant areas, inducing a field or a nerve block.
In field block anesthesia, LAs are injected subcutaneously, anesthetizing all nerves within a field 2 to 3 cm distant from the injection site.
In nerve block anesthesia, LAs are injected into a network of intersecting nerves to anesthetize larger surrounding areas while paralyzing the associated muscles.
8.7:
Local Anesthetics: Clinical Application as Surface, Infiltration, and Conduction Block Anesthesia
Depending on the target organ, local anesthetics (LAs) can be administered via various routes. In surface anesthesia, LAs are applied directly to the surface of the skin or mucous membranes. It is widely used for topical skin numbing before venipuncture or minor surgical procedures. Commonly used surface local anesthetics are lidocaine or benzocaine sprays or creams. Surface anesthesia occurs within 5 minutes and lasts for about 60 minutes. One of the main disadvantages of topical anesthesia is that the LAs escape into the systemic circulation easily after application on the mucous membranes.
In infiltration anesthesia, the LA is directly injected into the tissues surrounding a surgical site or the area to be anesthetized. This technique is often used for small surgical procedures, wound repairs, and dental procedures. The most frequently used drugs for infiltration anesthesia are lidocaine and bupivacaine. When administered, these drugs have immediate onset of action and can last up to 180 minutes. The main advantage of infiltration anesthesia is that it provides satisfactory anesthesia without disrupting normal functions. One of the disadvantages of this technique is the use of large concentrations of drugs required to anesthetize even small areas.
On the contrary, conduction block anesthetics can block motor impulses and last longer, even at low concentrations. This technique involves the injection of local anesthetics near peripheral nerves or nerve plexuses, effectively blocking the transmission of nerve impulses and providing regional anesthesia to larger areas. It is frequently used for nerve blocks, epidural, and spinal anesthesia. Local anesthetics like lidocaine, bupivacaine, and ropivacaine are commonly employed for conduction block anesthesia. Many factors, like the physical characteristics and amount of the injected LA, the use of vasoconstrictors, and the blood flow rate at the injection site, determine how easily these LAs escape into the systemic circulation.