Specific local anesthetic techniques are described in detail in the following topics:
- Anesthesia, Regional, Digital Block
- Anesthesia, Topical
- Local Anesthetic Agents, Infiltrative Administration
- Nerve Block, Deep Peroneal
- Nerve Block, Dorsal Penile
- Nerve Block, Dorsal Penile, Neonatal
- Nerve Block, Inferior Alveolar
- Nerve Block, Infraorbital
- Nerve Block, Oral
- Nerve Block, Median
- Nerve Block, Mental
- Nerve Block, Posterior Tibial
- Nerve Block, Radial
- Nerve Block, Saphenous
- Nerve Block, Superficial Peroneal
- Nerve Block, Supraorbital
- Nerve Block, Sural
- Local effects are usually a result of the injection technique. These effects include pain, ecchymosis, hematoma formation, infection, and nerve laceration. Pain is always felt when a local anesthetic is injected; however, associated discomfort can be minimized by using good technique. Several factors, including needle puncture of the skin, tissue irritation resulting from the anesthetic, and distention of tissues caused by infiltration, are responsible for the discomfort associated with the use of local anesthetics.
- Pediatric patients and patients who are extremely anxious may benefit from pretreatment of the injection area with a topical anesthetic. Pretreatment eliminates the initial pain that occurs when the needle perforates the skin. Small-diameter needles also decrease the pain associated with injection. Fortunately, for most dermatologic procedures, a 30-gauge needle can be used to infiltrate tissue.
- Tissue irritation caused by local anesthetics is related to the acidity of the infiltrated solution; therefore, increasing the pH of the mixture can decrease associated discomfort. The addition of epinephrine to an anesthetic solution decreases the pH of the solution, making it more acidic (pH 3.5-4.5) and leading to a more painful injection. The solution can be neutralized by the addition of sodium bicarbonate 8.4% to minimize discomfort. For example, sodium bicarbonate 8.4% can be added to lidocaine with epinephrine in a 1:10 ratio to achieve a solution pH similar to that of tissue fluid (pH 7.3-7.4).
- Discomfort associated with distension of the tissues during the injection of local anesthetics is caused by the rate of injection and the volume of fluid injected. To limit the pain, the anesthetic should be slowly administered to allow the stretch receptors time to accommodate the new volume of fluid.In addition, the volume of solution injected should be the smallest volume needed to achieve a loss of sensation at the surgical site.
- The formation of ecchymosis or a local hematoma is a result of the perforation of cutaneous blood vessels. These complications are encountered more commonly in areas of high vascularity, including the mucous membranes, head, and genitalia. Ecchymosis and hematoma are even more pronounced when the patient has a bleeding diathesis or when the patient has been taking aspirin or other anticoagulants. If ecchymosis occurs, the patient should simply be reassured. If hematoma formation occurs, the patient should be evaluated. The hematoma may require drainage with an 18-gauge needle, followed by the application of a pressure dressing.
- Infection is an additional local complication of anesthetic use that usually occurs when proper sterile technique is not used. Cleansing the skin surface with alcohol is adequate in otherwise clean or noninfected areas. If signs of infection are noted, treatment includes appropriate culture studies and antimicrobial therapy. If abscess formation occurs, drainage may also be required.
- Nerve laceration, although rare, may occur during the infiltration of a local anesthetic. This complication more commonly occurs during the placement of regional blocks than the placement of other blocks. Clinical indications of nerve laceration include paresthesias, shooting or sharp stinging sensations, and excessive pain during needle insertion. Paresthesias of the infraorbital nerve are characterized by sharp or shooting sensations involving the upper lip, nasal ala, and upper teeth.
- If the needle is suspected to have entered or lacerated a nerve, it should be withdrawn slowly and deliberately by 1-2 mm, until the paresthesias are no longer present. The needle should never be advanced further, moved laterally, or inserted into the foramen, because these maneuvers further increase the risk of nerve laceration. Although dysesthesias may remain for an extended duration, in most patients, the nerve regenerates and sensation normalizes over time.
- Tendon injury is an inherent aspect of transthecal digital anesthesia since the needle is pushed through the tendon. This may cause persistent discomfort lasting 1-2 days post surgery. Tendon sheath infection and late occurrence of trigger finger have also been reported.
- Cutaneous adverse effects that have been reported with the most commonly used topical anesthetic EMLA include itching, burning, pain, pallor, erythema, edema, and purpura. Irritant dermatitis, allergic contact dermatitis, and contact urticaria have also been reported, but these are very unusual.
- Amethocaine may induce an urticarial reaction at the site of application, and the risk of such a reaction seems to be significantly higher when amethocaine is used over the antecubital fossa and in younger children.
- Systemic effects usually occur when blood concentrations of local anesthetic increase to toxic levels. Effects are most often encountered after the unintentional intravenous injection or administration of an excessive dose of an anesthetic. Adding a vasoconstrictor (eg, epinephrine) can reduce the systemic absorption of an anesthetic. When using topical anesthetics, strict adherence to the maximal dose or area recommended is advised; additionally, great caution must be exercised when using topical anesthetics on mucosal surfaces because of the much greater absorption.
- Importantly, remember that (1) the metabolism of ester anesthetics is decreased in patients with deficient pseudocholinesterase activity and (2) the metabolism of amide anesthetics in patients who are taking medications that inhibit the cytochrome P-450 system is decreased. In addition, the potency of an anesthetic is directly correlated with the potential for toxicity. Allergic reactions, although systemic, are not related to serum levels of the anesthetic, but rather, they are considered idiosyncratic and can occur at any dose.
- Maximal safe doses of lidocaine for local anesthesia have been determined. For adults, a maximum of 4.5 mg of lidocaine per kilogram of body weight can be administered, whereas as much as 7 mg/kg can be used if the lidocaine solution has 1:100,000 epinephrine added as a vasoconstrictor. For children, lower maximal doses are recommended; only 1.5-2.5 mg/kg of plain lidocaine and 3-4 mg/kg of lidocaine with epinephrine should be used.
- Systemic toxicity resulting from excessive blood levels of anesthetics is clinically manifested as adverse reactions in the CNS and cardiovascular system. The CNS is affected in a predictable and dose-dependent fashion. As serum levels of lidocaine increase, effects on the CNS become more severe.
- Any physician who uses local anesthetics must be aware of the signs and symptoms of systemic toxicity. At serum lidocaine levels in the range of 1-5 mcg/mL, patients may complain of tinnitus, lightheadedness, circumoral numbness, diplopia, or a metallic taste in the mouth. In addition, they may complain of nausea and/or vomiting, or they may become more talkative. As serum levels increase to 5-8 mcg/mL, nystagmus, slurred speech, localized muscle twitching, or fine tremors may be noticed. Patients also have been noted to have hallucinations at these levels. If blood lidocaine levels reach 8-12 mcg/mL, focal seizure activity occurs; this can progress to generalized tonic-clonic seizures. Respiratory depression occurs at extremely high blood levels (20-25 mcg/mL) and can progress to coma.
- If signs of CNS toxicity are noted, steps must be taken to reduce hypoxia and acidosis, because these states increase the toxicity of local anesthetics. The patient’s airway should be maintained, and supplemental oxygen provided. If blood levels of carbon dioxide increase, protein binding of lidocaine decreases and results in higher levels of free lidocaine in the blood. Increased respiration and respiratory alkalosis increase the seizure threshold and decrease the uptake of the local anesthetic into the CNS. If convulsions occur, the patient’s airway should be maintained, and supplemental oxygen administered. If seizure activity is sustained, 5-10 mg of diazepam should be administered slowly (1-2 mg/min) until the seizures cease.
- Compared with the CNS, the cardiovascular system is less susceptible to the effects of local anesthetics. Most adverse effects of the cardiovascular system that occur with the administration of local anesthetics are a result of the addition of epinephrine rather than direct effects of the anesthetic. However, high blood levels of local anesthetics directly reduce cardiac contractility. In addition to the direct vasodilatory effects of most local anesthetics, the decrease in cardiac function can cause hypotension. Atrioventricular blocks, bradycardia, and ventricular arrhythmias also are reported; these are more common in patients with known conduction disturbances and requiring antiarrhythmic medications.
- The treatment of conduction disturbances should be appropriately tailored to the type of reaction. The treatment of hypotension requires the physician to initiate advanced cardiac life support protocols, that is, he or she should ensure that the patient has a patent airway, provide supplemental oxygen, and elevate the patient’s legs. If necessary, intravenous fluid should be administered, and the use of vasopressor agents such as ephedrine should be considered. Ephedrine can be intravenously administered in 5-mL incremental doses to a total of 15-30 mg, until a blood pressure response is noted.
- Lidocaine and the FDA-approved topical anesthetics EMLA and LMX are pregnancy category B medications.
- Allergic reactions to local anesthetics are extremely rare, especially with amide local anesthetics, and account for less than 1% of the reactions caused by local anesthetics. Reactions can be type 1 (ie, anaphylactic) or type 4 (ie, delayed-type hypersensitivity) reactions. These reactions are not dose related, but, rather, they are idiosyncratic. Skin prick and intradermal test results are negative in the vast majority of patients, but some authors recommend testing with the most commonly used amide local anesthetic (lidocaine).
- Type 1 reactions are usually caused by ester-type anesthetics. The ester group of local anesthetics have a much greater allergenic potential than that of the amide group. Pseudocholinesterases, which produce the highly allergenic metabolic product PABA, break down ester-type anesthetics. Cross-reactivity exists among ester anesthetics; therefore, the use of all anesthetics in this structural group should be avoided in a patient with an established sensitivity to one ester-type anesthetic.
- No cross-reactivity appears to exist between ester and amide anesthetics; however, cross-reactivity in anaphylactic reactions has not been investigated thoroughly. In addition, reactions to preservatives, specifically methylparaben and sodium metabisulfate (found in multiple-dose vials of amide anesthetics), may cause adverse reactions in a patient who is allergic to an ester-type anesthetic. Preservative-free single-dose vials of lidocaine are available for use if an amide anesthetic is to be used in a patient with a true hypersensitivity reaction to ester-type anesthetics.
- Clinical signs of type I reactions include pruritus, urticaria, facial swelling, wheezing, dyspnea, cyanosis, laryngeal edema, nausea, vomiting, and abdominal cramping. Epinephrine with a concentration of 1:1000 should be subcutaneously administered at a dose of 0.3-0.5 mL. This dose can be repeated every 20-30 minutes to a maximum of 3 doses. If anaphylaxis ensues, a 5-mL dose of epinephrine 1:10,000 should be administered intravenously.
- Type IV (ie, delayed-type hypersensitivity) reactions account for 80% of allergic reactions to local anesthetics. They are more common with the use of topical anesthetics and may occur with anesthetics of the amide and ester subtypes. Clinical manifestations are similar to those of allergic contact dermatitis and include erythema, plaques, and pruritus. Patients with a history of type IV reactions are not at an increased risk of type I reactions due to amide-type anesthetics. Contact dermatitis caused by topical anesthetics should be treated with topical steroid preparations.
- Alternative agents for use as anesthetics in patients with a known allergy to both ester- and amide-type local anesthetics include isotonic sodium chloride solution and injectable antihistamines. An intradermal injection of 0.9% sodium chloride solution can provide temporary anesthesia suitable for shave or punch biopsy. Physical pressure on the nerve endings resulting from the volume injected is postulated to be responsible for the anesthetic effect. Nonbacteriostatic sodium chloride solution should be used if the patient has an allergy to the methylparaben preservative in the local anesthetic. A bacteriostatic solution, which contains benzyl alcohol, has known anesthetic properties and can be used for limited procedures such as punch biopsy.
- Injectable antihistamines, such as diphenhydramine, have been administered to patients who are allergic to local anesthetics. The mechanism of anesthetic action is unknown. Injectable diphenhydramine is effective, but it has a short duration of activity, it is sedating, and its injection is painful. In addition, tissue necrosis is reported after the local injection of 5% diphenhydramine. If used for injection, diphenhydramine should be diluted to 1% by mixing 1 vial of 50-mg diphenhydramine with 4 mL of a bacteriostatic sodium chloride solution.
Reactions to local anesthetic additives
- With the exception of cocaine, local anesthetics directly cause relaxation of the vascular smooth muscle, which leads to vasodilation. This effect increases bleeding at the surgical site. Vasoconstrictors, such as epinephrine, are often added to anesthetic solutions to counteract this effect. The vasoconstrictor effect of epinephrine is maximal at 7-15 minutes, and this effect is clinically evident as blanching of the skin. This blanching also is useful in determining the area that is anesthetized.
- Vasoconstriction not only decreases bleeding but also slows the rate of systemic absorption of the anesthetic, which allows the body more time to metabolize the anesthetic and prolongs anesthesia. Therefore, larger volumes of anesthetic can be injected when epinephrine is added to a solution. A premixed solution of lidocaine with epinephrine in a concentration of 1:100,000 (1 mg/100 mL) is available. Concentrations greater than this are associated with a higher rate of adverse effects, including an increased risk of tissue necrosis as a result of prolonged ischemia.
- Systemic effects of epinephrine can occur with a dose as little as 2 mL of an anesthetic solution containing epinephrine in a concentration of 1:100,000. The most common clinical manifestation is transient tachycardia. At higher doses and with an inadvertent intravascular injection, palpitations, diaphoresis, angina, tremors, nervousness, and hypertension can occur. The maximum dose of epinephrine is 1 mg or 100 mL of a 1:100,000 solution. In patients with a history of heart disease, especially unstable angina and arrhythmias, the maximum dose should be decreased to 0.2 mg or 20 mL of a 1:100,000 solution (recommendation of the NY Heart Association).
- Epinephrine is contraindicated in patients with pheochromocytoma, hyperthyroidism, severe hypertension, or severe peripheral vascular occlusive disease. Relative contraindications include pregnancy and psychological instability; epinephrine can induce an acute psychotic episode in predisposed patients.
- The FDA designates epinephrine as a pregnancy category B medication (ie, usually safe but benefits must outweigh the risks). No known adverse effects on the fetus are reported; however, during the first trimester, vasoconstriction may cause fetoplacental ischemia and affect organogenesis. In the last trimester, epinephrine can induce premature labor if placental ischemia occurs. If possible, surgery should be performed without epinephrine, or it should be postponed until after delivery.
- Epinephrine must be used with caution in patients taking propranolol because life-threatening reactions have been reported; these include hypertension, myocardial infarction, and stroke. Epinephrine stimulates alpha-receptors to cause vasoconstriction and increase vascular resistance. Beta-receptors balance this effect by causing vasodilation (beta2-receptors) and an increased heart rate (beta1-receptors). Like other nonselective beta-blockers, propranolol antagonizes both beta1-receptors and beta2-receptors. Therefore, in the presence of propranolol, the effects of epinephrine on alpha-receptors are unbalanced, and the result is pure alpha stimulation, which leads to severe hypertension and reflex bradycardia.
- Although propranolol is the only nonselective beta-blocker reported to have this effect, probably all nonselective beta-blockers have the potential to cause severe hypertension and reflex bradycardia in the presence of epinephrine. A significant risk does not appear to be associated with the use of epinephrine and cardioselective beta-blockers. Although the use of epinephrine in patients who are taking nonselective beta-blockers is not contraindicated, it should be avoided if possible. Apparently, the effect may be dose related, and caution should be exercised because individual variability is reported.
- In addition to nonselective beta-blockers, monoamine oxidase inhibitors, tricyclic antidepressants, butyrophenones, and phenothiazines can cause hypotension or hypertension in patients who are taking epinephrine.
- Pain resulting from the infiltration of a local anesthetic can be reduced by using a solution with a pH close to physiologic range (ie, pH 7.3-7.4). The pH of plain lidocaine is 6.3-6.4. When epinephrine is added to lidocaine, the pH decreases to 3.5-4.5. The pH of the solution must be acidic to prevent the degradation of epinephrine.
- Sodium bicarbonate
- To reduce the pain of an injection of lidocaine and epinephrine, 1 mL of sodium bicarbonate 8.4% is added to 10 mL of the anesthetic solution to neutralize the solution. Buffered solutions should be discarded after 1 week because the effectiveness of epinephrine decreases by almost 25% during this time.
- Hyaluronidase is a bovine-derived enzyme that hydrolyzes hyaluronic acid in the connective tissue and facilitates the diffusion of the anesthetic. Although it can increase the spread of anesthesia, hyaluronidase also decreases the duration of action of the anesthetics because it increases absorption. As expected, this increased absorption leads to the potential for a greater incidence of toxic reactions that correspond to elevated blood levels. To decrease distortion of the surgical site, the addition of hyaluronidase is useful for nerve blocks and procedures around the orbit.
- Hyaluronidase is marketed in ampules. One ampule is equivalent to 150 United States Pharmacopeia (USP) units per milliliter. The usual dilution is 150 U in 30 mL of anesthetic. A patient can have an allergy to hyaluronidase. Hyaluronidase is a foreign protein, and its use is contraindicated in patients with a known allergy to bee stings. In addition, hyaluronidase contains the preservative thimerosal, which is a known allergen. To evaluate the potential for an allergic reaction before infiltration, a test dose should be injected intradermally. If urticaria is observed at the site of the test injection, the use of hyaluronidase is contraindicated.