• Jeffrey Gadsden, MD
• Cliff Connery, MD

        INTRODUCTION

Many procedures can be performed with the use of local anesthetic alone, instilled at or near the site of surgery. Often this can be done by the surgeon without the use or assistance of an anesthesiologist. Local infiltration is also technically easy to perform and requires minimal postoperative care. Together, these factors contribute to its popularity and nearly ubiquitous application as a means of anesthesia for small minimally invasive procedures and operations. This technique is relatively safe as well, but does require an understanding of basic local anesthetic pharmacology, especially with respect to dosing and toxicity, as well as skill for successful application.

History of Local Anesthesia

There are several references throughout history of efforts to produce local anesthesia by various means.¹ Ancient Egyptians believed that the fat of the crocodile could induce anesthesia if placed on the skin of a patient. The same people also believed that the stone of Memphis could produce local anesthesia if rubbed on the skin with vinegar. Chinese physicians were known to use a mixture of jimson weed, marijuana, deadly nightshade, and mandrake placed into calamus leaves and burned over the operative or painful site to produce anesthesia. In the sixteenth century, Marco Aurelio Severino, an Italian anatomist and surgeon, advocated the use of cold to decrease pain, and this principle was frequently put to use by Napoleon’s military surgeons. Other methods of inducing anesthesia locally included electrical current and superficial application of volatile liquids. The first clinical use of a local anesthetic was in 1884, when Austrian ophthalmologist Carl Koller used raw cocaine topically to anesthetize a patient’s eye. After this, the use of local anesthetics spread quickly, especially with the synthesis of less toxic compounds such as procaine and lidocaine.²

General Principles of Local Infiltration Anesthesia

The aim of local infiltration is to anesthetize nerve endings in a finite area of tissue by the injection of local anesthetics nearby. This stands in contrast to peripheral nerve blocks, in which nerve axons are the target and the injection may take place in an area removed from the surgical site (eg, brachial plexus block for hand surgery). The depth of the area to be operated on typically determines the required extent of infiltration. For superficial skin procedures such as suturing of lacerations and skin biopsies, subcutaneous or intradermal infiltration is sufficient. More extensive operations may demand infiltration into muscle, fascia, and other deep tissues.

        Two general approaches exist for anesthetizing skin and subcutaneous tissue. The first involves injecting local anesthetic directly into the line of incision and nearby tissues, effectively flooding the individual local nerve endings to produce anesthesia. This can be very effective, but may require large volumes of local anesthetic to achieve complete coverage.

        In contrast, field or ring blocks encircle the site of incision with walls of local anesthetic solution through which nerve fibers must pass before branching into terminal nerve endings (Figure 12-1). Field blocks carry several advantages over direct injection into the incision line. First, although discrete nerves are not specifically blocked, less solution is usually required. Furthermore, because the solution is not injected directly in the wound, no anatomic distortion is produced at the surgical site. Wound healing is not affected by field block because it may be from local edema of infiltration. Finally, field blocks are useful when direct injection into the surgical site may be troublesome or harmful (eg, to avoid rupturing a cyst or spreading malignant cells).

        Field blocks can also be performed using a single line of local anesthesia “upstream” from the incision site, thereby anesthetizing any skin distal to the injection. For example, a 5- to 6-cm line of subcutaneous local anesthetic extending across the lateral styloid process at the wrist effectively anesthetizes any superficial terminal branches of the radial nerve beyond the wrist. Clearly, this requires a working knowledge of peripheral nerve anatomy.

Figure 12-1. Skin and subcutaneous tissues can be anesthetized by (A) injecting local anesthetic directly into the line of incision and nearby tissues or (A and B) field or ring blocks that encircle the site of incision with walls of local anesthetic solution through which nerve fibers must pass before branching into terminal nerve endings. C: Area of detail.

Clinical Pearls

Choice of Local Anesthetic Solutions

The choice of local anesthetic agent depends on the size of the area to be anesthetized and on the desired duration of action. If a large surface is involved, large volumes of diluted agents should be used to avoid exceeding the maximum dosage limits of the various agents (Table 12-1).

        Maximum safe dosages of local anesthetics for local and infiltrational anesthesia are controversial. Traditionally, manufacturers have cited these in the form of a single dose (eg, 300 mg for lidocaine) or a weight-related dose (eg, 3 mg/kg for bupivacaine). However, the rate of systemic uptake differs markedly for different injection sites and is related to both the vascularity at the site of administration and the tissue binding of the local anesthetic.³ For instance, local anesthetic administered via an intercostal block results in significantly higher peak plasma concentrations when compared with the epidural or brachial plexus routes. Subcutaneous infiltration appears to produce relatively low plasma levels of local anesthetic compared with equivalent doses used via the epidural, caudal, brachial plexus, and intercostal routes. For example, to produce the same peak plasma level of lidocaine, twice as much local anesthetic is required via the subcutaneous route than for the epidural route. This should not be seen as an invitation to administer subcutaneous local anesthetic without regard for toxicity issues, as systemic toxicity has been described.^4,5 Rather, it highlights the need for further study into site and agent-specific dosing guidelines. More discussion on pharmacology of local anesthetics can be found in Chapter 6.

        Epinephrine is often added to aid in hemostasis, prolong duration of anesthesia, and reduce the absorption rate of local anesthetic, thereby reducing the risk of systemic toxicity when higher doses of local anesthesia are used.⁶ The usefulness of this adjuvant depends on the intrinsic vasoactive properties of a particular local anesthetic. For example, lidocaine in clinical doses is a potent vasodilator and is therefore amenable to the vasoconstrictive properties of epinephrine. In fact, epinephrine 5 mcg/mL (1:200 000) has been shown to reduce peak plasma levels after subcutaneous infiltration by 50%.⁷ On the other hand, ropivacaine is itself a mild vasoconstrictor, and the addition of epinephrine provides a smaller advantage in this context. Bupivacaine, a vasodilator, increases capillary blood flow significantly when injected in the skin.⁸ Typical concentrations of epinephrine added to local anesthetic solution range from 1:200,000 to 1:500,000. Epinephrine is often avoided when local anesthetic solutions are used for extremities such as fingers, toes, nose, or penis to avoid ischemic tissue damage, although such risk remains controversial.

        Although the duration of action varies for various agents, the onset of action is almost immediate for most local anesthetics when administered subcutaneously. In contrast to peripheral nerve blocks, even dilute solutions can provide excellent analgesia and operating conditions. For example, lidocaine 0.5% is a common concentration used to maximize the available volume without exceeding toxic limits.

Clinical Pearls

        Communication with the patient is an important aspect of local infiltration. Frequent reassessment of the quality of anesthesia is reassuring to the patient and helps to prevent unexpected movement as a result of painful stimuli. In addition to the discomfort of a needle puncture, injection of local anesthetic itself can be painful.⁹ Patients often describe an initial burning sensation, which is felt to be due in part to the acidic nature of these solutions or a mechanical pressure to the nerve endings.^10,11 Certain agents such as etidocaine are associated with more severe pain, whereas lidocaine is perceived as less painful.¹² Strategies to lessen the degree of pain on injection include the use of a small needle (eg, 25- to 30-gauge) and slow administration of the local anesthetic.¹³ Buffering of lidocaine solutions with sodium bicarbonate is also effective at reducing the sting of injection and is accomplished by adding 1 mL of an 8.4% solution (1 mEq/mL) to 10 mL of lidocaine.¹⁴

Complications & Side Effects

Complications due to local infiltration fall into two broad categories: allergic reactions and systemic toxicity. True allergy to local anesthetics is rare. Not infrequently a patient may report an “allergy” to local anesthesia when in fact what was experienced was a vagal reaction or a response to intravascular injection of the anesthetic or epinephrine. Genuine allergic reactions typically produce rash, urticaria, and/or upper airway edema.⁹ False-positives also occur and are much more common. For instance, a nonallergic reaction to a local anesthetic injection that produced life-threatening airway swelling was later found on testing to be angioedema secondary to Clesterase inhibitor deficiency.¹⁵ Allergies to local anesthetics are almost always due to the metabolite para-aminobenzoic acid (PABA) in ester anesthetics or the preservative methylparaben in amide anesthetics.¹⁶ These patients may have a history of allergy to various cosmetic preparations (PABA is a common ingredient). If in doubt regarding a patient allergy, it is safest to administer a preservative-free amide anesthetic.

        The toxic profile of local anesthetics primarily affects the central nervous system (CNS) and cardiovascular systems. The risk of toxicity is for the most part related to the potency of the anesthetic (eg, bupivacaine is more potent and therefore more toxic than lidocaine), the total dose administered, and the rate of systemic absorption. Clearance of local anesthetic rarely plays a role in toxicity unless continuous infusions are used. Symptoms of CNS toxicity range from mild complaints such as dizziness, perioral numbness, and a metallic taste to more severe neurologic signs such as restlessness, excitability, and disinhibition, aphasia, and finally seizures and/or coma. Cardiovascular toxicity is due to blockade of sodium channels in cardiac muscle and is manifested by ventricular dysrhythmias, bradycardia, depressed myocardial contractility, and cardiovascular collapse.

        Despite the often-memorized range of symptoms, systemic toxicity can present without warning, and resuscitation equipment must be immediately available wherever local anesthetics are used. This includes oxygen, a bag-valve- mask and other airway management devices, and a cardiac defibrillator—either automated or· manual. Benzodiazepines should also be on hand for treatment of seizures, as well as the equipment and medications for advanced cardiac life support medications. More in-depth discussions of the toxicity of local anesthetics can be found in Chapter 6.

        TECHNIQUES FOR COMMON INDICATIONS

Wound & Laceration Closure

Infiltration of local anesthetic solution into wound edges is usually sufficient to permit cleansing, debridement, and suture repair. Lacerations are typically irrigated with sterile saline or washed with surgical soap before infiltration of local anesthetic solution, but this can be uncomfortable. A more thorough cleansing can be performed after anesthesia. The infiltration of local anesthetic from inside the wound is less painful than injection through intact skin.¹⁷ ‘ The somatosensory innervation of skin is multifaceted and comprises both nociceptive free nerve endings and specialized receptors that transduce mechanical, thermal, and chemical stimuli.¹⁸ It was once thought that the epidermis was not innervated, but this outermost layer is in fact rich in small, unmyelinated nociceptive fibers¹⁹ (Figure 12-2). When anesthetizing skin, it is important to deposit local anesthetic in or just under the dermal layers because administration too deep in the subcutaneous fat may miss some of the more shallow fibers. Adipose tissue is generally poorly innervated, and placing local anesthetic deep in subcutaneous fat simply wastes drug while increasing the total dose administered—hence the risk of systemic toxicity.

Clinical Pearls

        Infiltration of the wound at the end of the operation is an effective method of postoperative pain relief, especially when used as a component of multimodal analgesia.²⁰ This has been demonstrated for a wide variety of procedures, including laparoscopy,²¹ laparotomy,²² thyroid surgery,²³ and craniotomy.²⁴ For this to be successful, all layers of the wound, including muscles in fascial sheaths, must be injected with local anesthesia. Bupivacaine 0.25% can provide good quality pain control for 6 hours or more, depending on the site of the wound and the volume used to infiltrate the site. The anesthetic effect of wound infiltration may be prolonged by continuous infusion of local anesthesia through an implanted subcutaneous catheter system or by irrigation of the wound drain. Occasional reports of the use of this approach have appeared, but this technique has not achieved widespread popularity.^25,26

        In an animal model of wound infection, local anesthetic infiltration reduced wound bacterial counts by more than 70% over controls.²⁷ The addition of epinephrine to the anesthetic solution decreases bleeding from the wound edges, but may be associated with an increased rate of wound infection. When epinephrine is used for wound closure, a clinician may mistakenly believe that adequate hemostasis has been achieved owing to vasoconstriction. For this reason, it is often prudent practice to examine wounds for hematomas up to 24 hours after closure in these instances to ensure that bleeding has not recommenced after the drug effect has worn off.

Figure 12-2. Anatomy of the skin and subcutaneous tissues. When anesthetizing skin, it is important to deposit local anesthetic in or just under the dermal layers because too deep of an administration in the subcutaneous fat may miss some of the more shallow fibers.

Related posts:

Regional Anesthesia in the Patient with Preexisting Neurologic Disease. The History of Local Anesthesia. Stimulating Catheters. Diagnosis & Management of Intraspinal, Epidural, & Peripheral Nerve Hematoma. Histology of Peripheral Nerves. Supraclavicular Brachial Plexus Block.