What is local anesthesia
Local anesthesia is used when pain control (pain relief) is required in small areas (e.g., for repair of minor lacerations, skin biopsies). An anesthetic drug (which can be given as a shot, spray, or ointment) numbs only a small, specific area of the body (for example, a foot, a single tooth, a hand, a toe or a patch of skin). With local anesthesia, a person is awake and comfortable or sedated, depending on what is needed. Local anesthesia lasts for a short period of time and is often used for minor outpatient procedures (when patients come in for surgery and can go home that same day). For someone having outpatient surgery in a clinic or doctor’s office (such as the dentist or dermatologist), this is probably the type of anesthetic used. The medicine used can numb the area during the procedure and for a short time afterwards to help control post-surgery discomfort.
Local cutaneous infiltration is the most commonly used anesthetic technique and involves direct injection into the area requiring anesthesia. The anesthetic solution is infiltrated to the deep dermis, where the sensory plexus supplying the skin begins to branch. The amount of solution used depends on the area that needs to be infiltrated.
Sometimes the local anesthetic agents are applied topically to relieve pain and itching caused by conditions such as sunburn or other minor burns, insect bites or stings, poison ivy, poison oak, poison sumac, and minor cuts and scratches.
Local and topical anesthetics deaden the nerve endings in the skin. They do not cause unconsciousness as do general anesthetics used for surgery.
Furthermore, most topical anesthetics are available without a prescription; however, your doctor may have special instructions on the proper use and dose for your medical problem.
Topical anesthetic product is available in the following dosage forms:
- Patch, Extended Release
- Patch, Device Assisted
How does local anesthesia work
There are two classes of infiltrative anesthetics, amides and esters, which create a reversible blockade of sodium channels within the nerve fibers by inhibiting sodium influx through sodium-specific ion channels in the neuronal cell membrane, in particular the so-called voltage-gated sodium channels 1). When the influx of sodium is interrupted, an action potential cannot arise and signal conduction is inhibited. The receptor site is thought to be located at the cytoplasmic (inner) portion of the sodium channel. Local anesthetic drugs bind more readily to sodium channels in an activated state, thus onset of neuronal blockade is faster in rapidly firing neurons. This is referred to as state-dependent blockade.
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 merge into the central nervous system.
When choosing an anesthetic agent, it is important to consider the type of procedure, the length of time required for anesthesia, and the pharmacodynamics of each medication. Table 1 is an overview of commonly used infiltrative anesthetic agents 2). True allergies to local anesthetics are rare, especially with amide preparations 3). However, evidence suggests there is cross reactivity between agents within the same class 4). In patients with a possible allergy, skin testing should be considered when immunoglobulin E–mediated reactions cannot be ruled out using the history 5).
Local anesthesia drugs
Lidocaine (Xylocaine), an amide, is the most commonly used infiltrative anesthetic and is available in several concentrations 6). For most procedures, a 0.5% or 1% solution is appropriate. Higher concentrations of lidocaine do not improve onset or duration of action and may increase the risk of toxicity 7). Adding epinephrine (concentration of 1:100,000 or 1:200,000) prolongs the duration of anesthesia, increases the maximum dose, and may aid hemostasis 8). Contrary to longstanding belief, the use of lidocaine with epinephrine on the nose, ears, digits, and penis appears to be safe 9), 10). However, many physicians still choose to avoid epinephrine use in these areas. Epinephrine should not be used in patients with peripheral artery disease.
Bupivacaine (Marcaine) is a widely used amide. It has a prolonged duration of action, but this also increases the risk of toxicity (4:1 risk of toxicity compared with lidocaine) and can cause a dose-dependent widening of the QRS interval, leading to ventricular fibrillation 11). Bupivacaine is contraindicated in pregnant women because of the increased bio-availability from decreased venous return 12).
Procaine (Novocain) and tetracaine (Pontocaine) are most often used for dental, topical, spinal, and epidural anesthesia.
Table 1. Commonly Used Infiltrative Anesthetic Agents
|mg per kg||mL|
0.5%, 1%, or 2%†
Rapid: < 2 minutes
30 to 60 minutes
4 (up to 300 mg per dose)
0.5%: 60 1%: 30 2%: 15
Lidocaine with epinephrine‡
1% or 2%
Rapid: < 2 minutes§
1 to 4 hours
7 (up to 500 mg per dose)
1%: 50 2%: 25
0.25% or 0.5%
Slow: 5 minutes
2 to 4 hours
2 (up to 175 mg per dose)
0.25%: 70 0.5%: 35
1% or 2%
Moderate: 2 to 5 minutes
15 to 60 minutes
7 (up to 600 mg per dose)
1%: 60 2%: 30
Slow: 5 to 10 minutes
2 to 3 hours
1.4 (up to 120 mg per dose)
*—Similar for all concentrations of each agent.
†—Higher concentrations provide no additional anesthetic effects.
‡—Epinephrine concentration may be 1:100,000 or 1:200,000.
§—May take up to 5 minutes for epinephrine to be effective.
Uses of Local anesthetics
Local anesthetics uses include:
- Surface anesthesia is the application of an 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 is infiltration of local anesthetic into the tissue to be anesthetized; surface and infiltration anesthesia are collectively topical anesthesia
- Field block is subcutaneous injection of an local anesthetic in an area bordering on the field to be anesthetized.
- Peripheral nerve block is injection of local anesthetic in the vicinity of a peripheral nerve to anesthetize that nerve’s area of innervation.
- Plexus anesthesia is 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 is an local anesthetic 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 is an local anesthetic 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) is when 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 includes intrapleural anesthesia and intra-articular anesthesia.
- Transincision (or transwound) catheter anesthesia uses a multilumen catheter inserted through an insicion or wound and aligned across it on the inside as the incision or wound is closed, providing continuous administration of local anesthetic along the incision or wound.
Local anesthesia in dentistry
A nerve block is achieved by targeting a specific nerve that supplies sensation to the desired location.
The inferior alveolar nerve, a branch of the mandibular nerve, supplies all of the teeth in one half of the mandible; it is often anesthetized in dental procedures. The same procedure will anesthetize the lower lip because the mental nerve is a branch of the inferior alveolar nerve. Because the lingual nerve runs very close to the inferior alveolar nerve near the mental foramen, it too is often anesthetized at the same time. For anesthesia to the upper teeth, the superior alveolar nerve endings, which are branches of the maxillary nerve, are blocked by inserting the needle beneath the mucous membrane. The anesthetic solution is then infiltrated slowly throughout the area of the roots of the teeth to be treated.
Figure 1. Mandibular nerve (branch of Trigeminal nerve or cranial nerve #5)
Figure 2. Inferior alveolar nerve block
Figure 3. Superior alveolar nerve block
References [ + ]
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