Local anesthesia

• Local anesthesia, in a strict sense, is anesthesia of a small part of the body such as a tooth or an area of skin.
• Regional anesthesia is aimed at anesthetizing a larger part of the body such as a leg or arm.
• Conduction anesthesia is a comprehensive term which encompasses a great variety of local and regional anesthetic techniques.

Note: The phrase local anesthesia is colloquially used to describe various local and regional anesthetic techniques; in this article however, conduction anesthesia is used, according to the definition given above.

Many surgical procedures can be done under conduction anesthesia. In some cases (e.g. caesarean section) conduction anesthesia is thought to carry a lower risk and is therefore usually preferred over general anesthesia. In other situations conduction and general anesthesia can be used alternatively or in combination. Conduction anesthesia is also used for purposes of pain therapy and diagnostic procedures.

Techniques

To achieve conduction anesthesia a local anesthetic is injected or applied to a body surface. The local anesthetic then diffuses into nerves where it inhibits the propagation of signals for pain, muscle contraction, regulation of blood circulation and other body functions. With relatively high drug doses and/or concentrations, all qualities of sensation (pain, touch, temperature etc.) as well as muscle control are inhibited. With lower doses and/or concentrations it is possible to inhibit pain sensation, to some degree, selectively, without affecting muscle power. This effect, termed differential block, is made use of in some techniques of pain therapy (e.g. "walking epidural" for labor pain).

Anesthesia persists as long as there is a sufficient concentration of local anesthetic at the nerve or nerves to be blocked. Sometimes a vasoconstrictor drug is added to decrease local blood flow, thereby slowing the transport of the local anesthetic away from the site of injection. Depending on the drug and technique used, the anesthetic effect may persist for less than an hour to several hours. When a catheter is used for continuous infusion or repeated injection, conduction anesthesia can be extended for days or weeks. This is typically done for purposes of pain therapy.

Almost every nerve between the peripheral nerve endings and the central nervous system can be blocked using local anesthetics. The most peripheral technique is topical anesthesia to the skin or other body surface. Small and large peripheral nerves can be anesthetized individually (peripheral nerve blocks[?]) or in anatomic nerve bundles (plexus anesthesia[?]). Spinal anesthesia[?] and epidural anesthesia[?] are applied near the spinal cord where the peripheral nervous system merges into the central nervous system.

Some techniques that are clinically used are:

• surface anesthesia - a local anesthetic spray, solution or cream is applied to the skin or a mucous membrane. The effect is of short duration and is limited to the area of contact.
• infiltration anesthesia - a local anesthetic is injected into the tissue to be anesthetized. Surface and infiltration anesthesia are collectively termed topical anesthesia.
• field block - subcutaneous injection of a local anesthetic in an area bordering on the field to be anesthetized.
• peripheral nerve blocks - a local anesthetic is injected in the vicinity of a peripheral nerve to anesthetize that nerve´s area of innervation.
• plexus anesthesia - a local anesthetic is injected in the vicinity of a nerve plexus[?], often inside a tissue compartment that limits the diffusion of the drug away from the intended site of action. The anesthetic effect extends to the innervation areas of several or all nerves stemming from the plexus.
• epidural anesthesia - a local anesthetic is injected into the epidural space[?] where it acts primarily on the spinal nerve roots. Depending on the site of injection and the volume injected, the anesthetized area can vary from limited areas of the abdomen or chest to extended regions of the body.
• spinal anesthesia - a local anesthetic is injected into the cerebrospinal fluid, usually at the lumbar spine (in the lower back), where it acts on spinal nerve roots and part of the spinal cord. The resulting anesthesia usually extends from the legs to the abdomen or chest.
• intravenous regional anesthesia (Bier block) - blood circulation of a limb is interrupted using a tourniquet (a device similar to a blood pressure cuff), then a large volume of local anesthetic is injected into a peripheral vein. The drug fills the limb´s venous system and diffuses into tissues where peripheral nerves and nerve endings are anesthetized. The anesthetic effect is limited to the area that is excluded from blood circulation and resolves quickly once circulation is restored.
• local anesthesia of body cavities (e.g. intrapleural anesthesia, intraarticular anesthesia)

Virtually every part of the body can be anesthetized using conduction anesthesia. However, only a limited number of techniques are in clinical use. Sometimes conduction anesthesia is combined with general anesthesia for the patient´s comfort and ease of surgery. Typical operations that can be done under conduction anesthesia are:

• eye surgery (surface anesthesia, retrobulbar block)
• ENT operations, head and neck surgery, dental surgery (infiltration anesthesia, field blocks, peripheral nerve blocks, plexus anesthesia)
• shoulder and arm surgery (plexus anesthesia, intravenous regional anesthesia)
• heart and lung surgery (epidural anesthesia combined with general anesthesia)
• abdominal surgery (epidural/spinal anesthesia, often combined with general anesthesia)
• gynecological, obstetrical and urological operations (spinal/epidural anesthesia)
• bone and joint surgery of the pelvis, hip and leg (spinal/epidural anesthesia, peripheral nerve blocks, intravenous regional anesthesia)
• surgery of skin and peripheral blood vessels (topical anesthesia, field blocks, peripheral nerve blocks, spinal/epidural anesthesia)

Acute pain may occur due to trauma, surgery, infection, disruption of blood circulation or many other conditions in which there is tissue injury. In a medical setting it is usually desirable to alleviate pain when its warning function is no longer needed. Besides improving patient comfort, pain therapy can also reduce harmful physiological consequences of untreated pain.

Acute pain can often be managed using analgesics. However, conduction anesthesia may be preferable because of superior pain control and fewer side effects. For purposes of pain therapy, local anesthetic drugs are often given by repeated injection or continuous infusion through a catheter. Low doses of local anesthetic drugs can be sufficient so that muscle weakness does not occur and patients may be mobilized.

Some typical uses of conduction anesthesia for acute pain are:

• labor pain (epidural anesthesia)
• postoperative pain (peripheral nerve blocks, epidural anesthesia)
• trauma (peripheral nerve blocks, intravenous regional anesthesia, epidural anesthesia)

Topical anesthesia is often used for all kinds of punctures (e.g. ascites drainage, amniocentesis, access to blood vessels for angiography and many others). Some diagnostic procedures such as bronchoscopy (visualization of the lower airways) or cystoscopy (visualization of the inner surface of the bladder) are often performed under surface anesthesia.

The leaves of the coca plant were traditionally used as a stimulant in Peru. It is believed that the local anesthetic effect of coca was also known and used for medical purposes. Cocaine was isolated in 1860 and first used as a local anesthetic in 1884. The search for a less toxic and less addictive substitute led to the development of the aminoester local anesthetic procaine[?] in 1904. Since then, several synthetic local anesthetic drugs have been developed and put into clinical use, notably lidocaine[?] in 1943, bupivacaine[?] in 1957 and prilocaine[?] in 1959.

Shortly after the first use of cocaine for topical anesthesia, blocks on peripheral nerves were described. Brachial plexus anesthesia by percutaneous injection through axillary and supraclavicular approaches was developed in the early 20th century. The search for the most effective and least traumatic approach for plexus anesthesia and peripheral nerve blocks continues to this day. In recent decades, continuous regional anesthesia using catheters and automatic pumps has evolved as a method of pain therapy.

Intravenous regional anesthesia was first described by August Bier[?] in 1908. This technique is still in use and is remarkably safe when drugs of low systemic toxicity such as prilocaine are used.

Spinal anesthesia was first used in 1885 but not introduced into clinical practice until 1899, when August Bier subjected himself to a clinical experiment in which he observed the anesthetic effect, but also the typical side effect of postpunctural headache. Within few years, spinal anesthesia became widely used for surgical anesthesia and was accepted as a safe and effective technique. Although atraumatic (non-cutting-tip) cannulas and modern drugs are used today, the technique has otherwise changed very little over many decades.

Epidural anesthesia by a caudal approach had been known in the early 20th century, but a well-defined technique using lumbar injection was not developed until the 1930s. With the advent of thin flexible catheters, continuous infusion and repeated injections have become possible, making epidural anesthesia a highly successful technique to this day. Beside its many uses for surgery, epidural anesthesia is particularly popular in obstetrics for the treatment of labor pain.

New York School of Regional Anesthesia (http://www.nysora.com)

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