Local anesthesia

Local anesthesia is any technique to render part of the body insensitive to pain without affecting consciousness.

• 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.

Conduction anesthesia allows patients to undergo many surgical procedures without significant pain or distress. In many situations, such as cesarean section), conduction anesthesia is safer and therefore superior to general anesthesia. In other situations, either conduction or general anesthesia are suitable. Anaesthetists sometimes combine both techniques.

Conduction anesthesia is also used for relief of non-surgical pain, also to enable diagnosis of the cause of some chronic pain conditions.

The most common form of conduction anaesthesia is probably local anaesthesia to enable dental procedures.

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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. Relatively high drug doses or concentrations inhibit all qualities of sensation (pain, touch, temperature etc.) as well as muscle control. Lower doses or concentrations may selectively inhibit pain sensation with minimal effect on muscle power. Some techniques of pain therapy, such as walking epidurals for labor pain use this effect, termed differential block.

Known to block voltage gated sodium channels.

Anesthesia persists as long as there is a sufficient concentration of local anesthetic at the affected nerves. 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, the anesthetic effect may persist from less than an hour to several hours. Placement of a catheter for continuous infusion or repeated injection allows conduction anesthesia to last for days or weeks. This is typically done for purposes of pain therapy.

Local anesthetics can block almost every nerve between the peripheral nerve endings and the central nervous system. 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 block) 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.

Clinical techniques include:

• Surface anesthesia - application of local anesthetic spray, solution or cream to the skin or a mucous membrane. The effect is short lasting and is limited to the area of contact.
• Infiltration anesthesia - injection of local anesthetic into the tissue to be anesthetized. Surface and infiltration anesthesia are collectively topical anesthesia.
• Field block - subcutaneous injection of a local anesthetic in an area bordering on the field to be anesthetized.
• Peripheral nerve blocks - injection of local anesthetic in the vicinity of a peripheral nerve to anesthetize that nerve's area of innervation.
• Plexus anesthesia - injection of local anesthetic 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 varies from limited areas of the abdomen or chest to large 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's 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)

Uses in surgery and dentistry

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

• Dentistry (surface anesthesia, infiltration anesthesia, nerve blocks)
• Eye surgery (surface anesthesia with topical anesthetics, retrobulbar block)
• ENT operations, head and neck 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)

Uses in acute pain

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)

Uses in chronic pain

Chronic pain of more than minor intensity is a complex and often serious condition that requires diagnosis and treatment by an expert in pain medicine. Local anesthetics can be applied repeatedly or continuously for prolonged periods to relieve chronic pain, usually in combination with medication such as opioids, NSAIDs, and anticonvulsants.

Miscellaneous uses

Topical anesthesia, in the form of lidocaine/prilocaine (EMLA) is most commonly used to enable relatively painless venipuncture (blood collection) and placement of intravenous cannulae. It may also be suitable for other kinds of punctures such as ascites drainage and amniocentesis.

Surface anesthesia also facilitates some endoscopic procedures such as bronchoscopy (visualization of the lower airways) or cystoscopy (visualization of the inner surface of the bladder).

History

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.

Adverse Effects

Excessive or inappropriately administered doses may result in Local Anesthetic Toxicity. This includes localized effects of nerve damage and systemic effects of heart and respiration depression even death.