What is anesthesia? |

Indications and Procedures

Anesthetics are given primarily to prevent the experience of pain during surgical operations. They also are given to reduce fear, relax tissues, and prevent a sympathetic nervous system response to surgery. Some believe, erroneously, that being “put to sleep” with a general anesthetic is the only way an operation can be performed pain-free.

General anesthesia
is a type of anesthesia that produces total unconsciousness and affects the entire body. Regional anesthesia, another type of anesthesia, does not produce unconsciousness but allows surgery to be performed without pain by producing loss of sensation in a region of the body through the interruption of nerve-impulse transmission from the area to be incised.

With general anesthesia, patients receive drugs that are delivered both intravenously and by inhalation. With regional anesthesia, the anesthetic agents are deposited either on the surface of the area to be anesthetized or near a particular nerve or pathway that lies between the area and receptors for painful stimuli that are part of the central nervous system. As a result, transmission of noxious stimuli to the brain is effectively “blocked,” allowing a surgical procedure to be performed without the patient feeling pain. Regional anesthesia is frequently referred to as regional nerve blockade.

Local anesthetics operate in several ways. Those injected near the nerves diffuse into the nerves and bind to receptors on their membranes. Once in the nerve sheath, local anesthetic agents prevent sodium from moving into the nerve interior by physically occluding sodium channels. Impulses traveling from the surgical area to the central nervous system are blocked such that nerves transmitting touch, temperature, and pain sensation are temporarily interrupted
. Nerve impulses traveling from the central nervous system to the surgical area are also blocked, leading to an interruption of motor power to the surgical area.

The number of nerves blocked depends on where the local anesthetic is deposited—that is, on the type of regional anesthetic technique utilized. The duration of the nerve blockade depends on the type and dosage of local anesthetic injected as well as on the technique utilized. Diffusion of the local anesthetic out of the nerve and its absorption into the vascular bed causes the effect of the local anesthetic to be terminated. The blood flowing around the nerve removes the drug from the area. Decreasing the flow of blood to the area by adding vasoconstricting agents, such as epinephrine, to the local anesthetic to be injected is a method commonly utilized to prolong the duration of the nerve block.

Local anesthetics are weak bases whose structure consists of an aromatic moiety connected to a substituted amine through an ester or amide linkage. The two major families of local anesthetics are the amino amides and the amino esters. The clinical differences between the ester and amide local anesthetics involve their potential for producing adverse side effects and the mechanisms by which they are metabolized.

Local anesthetics are also classified on the basis of their potency and duration of action: a short duration of action (thirty to forty-five minutes), an intermediate duration of action (one to two hours), or a long duration of action (four to eight hours). The range in duration of nerve blockade is attributable primarily to two factors: the concentration of the drug used and the addition of vasoconstricting agents, such as epinephrine.

Local and regional anesthesia are excellent ways of supplying surgical and postoperative analgesia (pain control). Local anesthetics are now being given in combination with narcotic analgesics (painkillers). Narcotics injected in combination with local anesthetics work through a different mechanism of action; they bind to narcotic receptors in the area and provide analgesia without interrupting nerve transmission. The combination of local anesthetic agents with narcotics is gaining popularity in postoperative pain control; longer durations of pain relief can be obtained while avoiding the systemic side effects of intravenously administered narcotics.

There are six categories of regional anesthesia: topical anesthesia, local block and field block, nerve block, intravenous (IV) neural blockade, subarachnoid block (spinal anesthesia), and epidural anesthesia, including caudal block. The major differences are the size of the region that is anesthetized and the duration of the neural blockade.

Topical anesthesia. In this technique, also known as surface anesthesia, an anesthetic drug is sprayed or spread onto an area to be desensitized. This short-acting form of anesthesia blocks nerve endings in the skin as well as mucous membranes, such as those of the nasopharynx (nose and throat), mouth, rectum, and vagina. Topical anesthesia is employed in minor procedures such as eye or rectal examinations. The advantages of topical anesthesia include quick onset of action, ease of administration, and general nontoxicity. Disadvantages include lack of deeper-tissue anesthesia and lack of tissue relaxation. A frequently used topical anesthetic is the drug benzocaine, often utilized for traumatic tissue pain secondary to sunburn.

Local blocks and field blocks. In local blocks, the local anesthetic is injected with a needle and syringe into the skin and tissues of an area to be incised. As a result, the nerves in the area of the incision are blocked. Local blocks are used in short, minor operations and prior to the insertion of intravenous or spinal needles. A field block is another type of local block. In a field block, the area surrounding the incision is also injected with local anesthetics, preventing impulses transmitted from a larger area from reaching the central nervous system.

Nerve blocks. Nerve blocks interrupt the transmission of nerve impulses by nerves or by bundles of nerves that are further removed from the surgical site. Nerve blocks may be used to anesthetize a single finger or toe (digital nerve block), a foot (ankle block) or hand, or an entire arm (axillary, supraclavicular, and interscalene blocks) or leg (leg block). In each type of neural blockade, the physician or nurse anesthetist injects local anesthetic agents around the major nerves that supply the area to be incised. The number of injections depends on the location of the nerves to be blocked. For example, in performing a leg block, four nerves—the femoral, sciatic, lateral femoral cutaneous, and obturator nerves—because of their separate locations, may be blocked individually. An arm block can be accomplished by a single injection of a larger volume of local anesthetic into the axillary sheath or between the middle and anterior scalene muscles in the neck. Upper extremity blocks can be performed with a single injection because the brachial plexus (the nerves innervating the arm) is collectively encased in a sheath. Distal to the axillary area, the nerves innervating the arm are no longer encased in a sheath. Consequently, separate injections of the radial, median, and ulnar nerves are required to block the arm at the elbow or wrist.

Intercostal nerve blocks. Intercostal nerves innervate the outer and inner surfaces of the abdominal wall. Intercostal nerve blockade is utilized for postoperative pain control following thoracic or upper abdominal surgeries. A sterile needle is inserted into the skin over the lower margin of the rib along the posterior axillary line. The needle is then directed toward the intercostal groove located inferior to the rib. A local anesthetic is injected into the intercostal space containing the intercostal nerve, vein, and artery. The anesthetic lasts from six to twelve hours and may be prolonged by the addition of epinephrine to the solution.

Intravenous neural blockade. Intravenous (IV) neural blockade was discovered by August Bier in 1908; he was also the first to utilize spinal anesthesia routinely. Today, IV regional neural blockade is also referred to as Bier blockade. With Bier blockade, the local anesthetic agent is injected into a vein, lying distal to a tourniquet, in an upper or lower extremity (instead of around a nerve). Inflation of the tourniquet prevents the local anesthetic from being released into the general
circulation. The local anesthetic, thus contained in the extremity, travels to the major nerves in the limb and blocks neural transmission. The duration of the neural blockade is governed by the length of time that the tourniquet is inflated. Once the tourniquet is deflated, the local anesthetic enters the systemic circulation, the neural blockade recedes, and normal sensation and power to the extremity are rapidly returned. Intravenous regional blockade of the extremities has many advantages: ease of performance, rapid onset, controllable duration of action, and rapid recovery. The disadvantages include possible tourniquet discomfort, possible reaction to the local anesthetic when it is released into the general circulation, and rapid return of sensation, including pain.

Spinal anesthesia. Spinal anesthesia, also called subarachnoid block, is a commonly utilized form of anesthesia. Spinal anesthesia can be used for almost any type of surgical procedure below the umbilicus, such as surgical procedures performed on the legs and hips, hysterectomies, appendectomies, and cesarean sections.

A lumbar puncture
(spinal tap) is performed in the lower back, usually between the second and third lumbar vertebrae, the third and fourth lumbar vertebrae, or the fifth lumbar and first sacral vertebrae. The patient is placed on his or her side in a flexed position (or sometimes in a sitting position). The physician or nurse anesthetist, wearing sterile gloves, prepares the skin in the area to be punctured with a skin antiseptic, such as betadine, and then drapes the area with a sterile towel. The anesthetist then infiltrates the area of the puncture with lidocaine, producing a local block. Once the skin is anesthetized, the lumbar puncture is performed; a needle is inserted through the intraspinous space into the subarachnoid space. The needle passes through the supraspinous ligament, intraspinous ligament, and ligamentum flavum. Proper placement of the needle is identified through an observation of freely flowing spinal fluid. A local anesthetic agent is then injected into the spinal fluid. Cerebrospinal fluid
(CSF) is a clear, colorless ultrafiltrate of the blood that fills the subarachnoid space. The total volume of CSF is 100 to 150 milliliters; the volume contained in the subarachnoid space is 25 to 35 milliliters.

Once injected into the subarachnoid CSF, the local anesthetic agent spreads in both a cephalad (toward the head and anterior) and a caudad (toward the feet and posterior) direction. Factors influencing this spread include the dose and volume of the agent used, patient position, and the specific gravity (weight) of the anesthetic solution relative to the CSF. One of three types of solutions—isobaric, hypobaric, or hyperbaric—can be used. Hyperbaric solutions are heavier than CSF; thus, placing the patient in Trendelenburg’s position (with the head tilted downward) will increase the cephalad spread of the anesthetic. With the patient in Trendelenburg’s position, hypobaric solutions (with a specific gravity less than that of CSF) of local anesthetic agents spread caudally. Spread of the local anesthetic in the subarachnoid space usually stops (fixation) within five to thirty-five minutes after injection. After fixation has occurred, patient position changes will not influence the spread of local anesthetic or the subsequent level of anesthesia.

Within minutes after a subarachnoid injection of a local anesthetic, patients experience a warm sensation in their lower extremities, followed by a loss of sensation and inability to move the legs. The duration of the neural blockade is dependent on the type of local anesthetic utilized, as well as the addition of any vasoconstrictor.

Epidural anesthesia and caudal block. Like spinal anesthesia, epidural blockade can be used for the prevention of pain during surgery. It can also be used to relieve pain after surgery, chronic pain, and the pain in labor; to supplement a light general anesthetic; and to diagnose and treat autonomic nervous system dysfunction. The technique is excellent for the operations performed on the lower abdomen, pelvis, and perineum; for laminectomies; and in obstetrics for the relief of labor pains.

As with spinal anesthesia, the patient is placed on his or her side in a flexed position (or sometimes in a sitting position). The physician or nurse anesthetist, wearing sterile gloves, prepares the skin in the area of the puncture with an antiseptic such as betadine and drapes the area with a sterile towel. The anesthetist infiltrates the area of the puncture with lidocaine, producing a local block. Once the skin is anesthetized, an epidural needle is inserted between the appropriate lumbar vertebrae (occasionally between thoracic vertebrae). With epidural blockade, however, the needle is not advanced into the subarachnoid space; needle advancement is terminated when the needle tip is in the epidural space. Thus, the dura is not penetrated as in spinal anesthesia. (As a result, postdural puncture headache does not occur with a properly placed epidural needle.) Once the epidural space has been identified, local anesthetic agents (in larger volumes than utilized with subarachnoid anesthesia) are injected through the needle or through a small catheter threaded through the needle. Placement of a catheter through the needle allows reinjection to take place without subsequent needle punctures; this is particularly desirable for long surgeries, postoperative pain control, and the control of labor pains. Local anesthetic agents injected through a catheter for postoperative or labor pain control are usually given at lesser concentrations so that nerve motor fibers are not interrupted.

Caudal block is another type of epidural anesthesia. In this case, the needle (with or without a catheter) is placed through the sacroccygeal ligament just superior to the coccyx. The technique is gaining popularity as an adjunct to general anesthesia in children, for the purpose of postoperative pain control.

Uses and Complications

Regional anesthesia has several advantages over general anesthesia. The first is ease of administration: The agents used are injectable, the equipment required is minimal, and the costs are reasonable. Second is relative safety: A localized area of the body can be operated upon while avoiding most of the undesirable and potentially harmful side effects of general anesthesia, such as loss of consciousness and the depression of the cardiovascular and respiratory systems. In addition, advantages include excellent muscle relaxation, which is often required in order to facilitate surgical procedures; improved peripheral blood flow; an antithrombitic effect; a decreased loss of blood in some cases; and postoperative pain relief, a benefit most patients find highly desirable. Regional anesthesia is also utilized in combination with general anesthesia in an effort to increase the benefits of both while decreasing the adverse side effects of each.

Yet regional anesthesia has some disadvantages. First, some operations cannot be performed under regional anesthesia (for example, major surgical procedures involving the brain, heart, and lungs). Second, some patients may be allergic to the local anesthetics. Local anesthetics of the amino ester type may result in allergic reactions because of the metabolite p-aminobenzoic acid. Local anesthetics of the amino amide class are essentially devoid of allergic potential. Many anesthetic solutions, however, contain methylparaben as a preservative, and this compound can produce an allergic reaction in persons sensitive to p-aminobenzoic acid. Third, some patients desire to be unaware of the operation and may be anxious at the thought of being “awake.” They erroneously believe that the total unconsciousness produced by general anesthesia is the only method to produce unawareness. In the majority of cases, patients who receive a regional anesthetic also receive intravenous sedation to decrease their level of awareness. Subsequently, many patients report having no recollection whatsoever of the surgical procedure.

In addition, the advantages of spinal anesthesia
, one of the most popular types of regional anesthesia, far outweigh the disadvantages, which include hypotension, a high level of anesthesia, and postdural puncture headache. Hypotension is treated with intravenous fluids and, if necessary, the administration of vasoconstricting drugs. Postdural puncture headache (spinal headache) is thought to be caused from a loss of spinal fluid (which cushions the brain) through the dural hole produced by the spinal needle. Advances in the technique of spinal anesthesia administration, including the use of very small needles introduced in a manner that separates the dural fibers, have significantly decreased the incidence of postdural puncture headache. Should this complication arise, however, it is treated with analgesics, intravenous fluids, and, if necessary, an injection of saline (or a sample of the patient’s own blood) around the site of the dural puncture, effectively “patching” the dural hole created by the spinal needle.

The first successful demonstration of anesthesia (diethyl ether) by William T. G. Morton occurred in 1846 at the Massachusetts General Hospital. The discovery of anesthesia occurred prior to the discovery of germ theory and aseptic techniques: Although anesthesia made surgery painless in the early nineteenth century, there was still a high rate of surgical morbidity and mortality as a result of infection. In the late 1860s, germ theory had evolved from the work of Robert Koch and Louis Pasteur, and Joseph Lister’s subsequent work on principles of asepsis contributed significantly to a decline in surgical mortality from infection by the late 1880s. There remained, however, a high surgical mortality rate caused by anesthesia. At that time, general anesthetic agents were commonly utilized, and few, if any, practitioners specialized in the administration of anesthesia.

Regional anesthesia was first utilized in 1884 when a German physician named Carl Koller performed an operation to correct glaucoma using a local anesthetic. In this case, cocaine, an alkaloid obtained from the coca plant, was instilled into the eye. This successful operation brought significant acceptance to the principle of local anesthesia. The great advantage of local anesthesia was that it anesthetized only the part of the body on which the operation was to be performed: Patients could be spared the depressive effects of general anesthesia, especially on the cardiovascular and respiratory systems.

By the 1930s, various regional anesthesia techniques had been developed, including subarachnoid block (spinal anesthesia), lumbar epidural, caudal epidural, intravenous, and brachial plexus anesthesia. The occurrence of these regional anesthetic techniques, along with the evolution of local anesthetic agents, allowed anesthetists to tailor the type and duration of regional anesthesia to the requirements of each patient. As a result, regional anesthesia has become a popular choice among surgeons, anesthetists, and patients.

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