Local anesthesia means to render only one area of the body insensible to pain. Consciousness is not altered.
Principle. In order for pain to be felt in an area, an impulse must be passed up the nerve supplying that area, to the pain center in the brain. The impulse travels rapidly, and as soon as it registers in the pain center, the sensation of pain in the area is experienced. The principle of local anesthesia is to block the impulse by placing a drug which temporarily paralyzes the nerve, somewhere throughout the course of the nerve. So in local anesthesia the site of action of the anesthetic agent is on the peripheral nerves or a segment of a nerve, rendering it incapable of transmitting its impulses to the central nervous system. The site of the block of the nerve’s impulses is exactly where the drug is placed along the course of the nerve. The impulses of sensations of all sorts pass up to the site of the paralyzed nerve section but cannot pass farther, so they do not reach the brain to register their impressions.
There are three main types of local anesthesia: topical, infiltration, and regional. There are many sites at which anesthetizing drugs are applied along the course of a nerve to block the pain impulses in each of the types. Several drugs are used in each type, but their chemical names and properties are for the most part not important from the patient’s point of view.
These drugs act directly on the nerve with which they come in contact to paralyze its ability to relay impulses. The length of action varies with different drugs or combinations of drugs. The paralyzing effect is only temporary with complete return of nerve function when the drug is chemically neutralized or absorbed from the area so that the effect “wears off.” When the drug is first instilled there is a feeling of numbness in the affected area, shortly followed by the loss of all sensations. The area just “goes to sleep.” The return to normal as the drug is rendered by the body no longer effective is the opposite course; the area feels numb, and then gradually all sensations are felt. (One form of topical anesthesia employs a drug which literally freezes the surface to anesthetize the nerve endings; from this has evolved the phrase commonly applied to all forms of local anesthesia, to freeze” the area; this term may still be aptly used, but with infiltration and regional anesthetics and most topical anesthetics there is no temperature change or freezing action.)
With local anesthetics there is no alteration in the state of consciousness. There may be, however, some changes in the ability for movement in the area affected by the local anesthetic, especially with the regional nerve block type. This is because there may be nerve fibers within the blocked nerve which carry motor or movement impulses down from the brain to the muscles in the area, and these impulses, of course, are blocked also. This loss of movement is only temporary, lasting only as long as the anesthesia.
Local anesthesia is most often rendered by the surgeon who is to perform the operation, as a preliminary step in the procedure. When this type anesthesia is indicated, it is very effective in all respects. Local anesthesia has a high margin of safety with very rare complications, is adaptable to a wide variety of operations and renders the patient free from all pain during the operation. In most cases more of the agent may be added at any time during the course of an operation, so the patient need have no fear that the anesthetic will wear off before the completion of the procedure. When the first signs of return of sensations in the area appear, the surgeon will administer more of the anesthetic drug.
If you are to have an operation under local anesthesia, try to realize the advantages of this type and do not anticipate that you are going to be put through unbearable pain. Never try to influence your surgeon in his choice of anesthesia; he knows what is best for your case. Throughout the procedure you will be awake and alert, but try to remain calm. Everyone in the operating room is directing his attention to you, and each person can do his part better if everyone remains calm. If you become anxious and apprehensive, others may also. In most instances the patient’s role is entirely passive. There will be a specific position for you to assume and maintain. Answer all questions asked of you, but do not try to strike up a conversation of your own. Everyone is quite busy with a definite job to do, even though it may not appear so to you. Be co-operative and be calm.
Topical anesthesia is that obtained over a surface by blocking the nerve endings by application of an anesthetic agent on the surface, thereby rendering the nerve endings (the very tips of the nerves) incapable of perceiving pain and initiating impulses up the nerve to the brain. The drug is applied by means of a medicine dropper, cotton swabs, atomizer or other spraying device. The depth of the anesthetized area is not very great, so it is limited to superficial procedures. Topical anesthesia is used most commonly in surgery of the eye, but it is also useful for operations on the mucous linings of the upper air passages and the genitalia. It is also very effective for anesthetizing surfaces prior to the insertion of instruments for diagnostic or treatment purposes, such as in the throat or the urinary bladder. The instrumentation is then less uncomfortable.
On the skin a drug is often used which literally freezes the area on which it is placed. As it is sprayed onto the surface it readily evaporates so that the heat in the area is utilized, and the tissue actually freezes and turns white. This lasts but a few minutes, and then the tissue thaws to normal. This procedure is very useful in opening skin abscesses, removing splinters, and other short procedures.
Topical anesthesia, where indicated, is very effective and may be prolonged by simply adding more of the drug. Its greatest usefulness is for operations on the eye, where it makes possible procedures which otherwise would be impossible. Topical anesthetic drugs are often incorporated in ointments which are to be used on painful surface lesions to alleviate pain while the area is healing.
This type of anesthesia is that accomplished by injecting an anesthetizing drug directly into the tissues where anesthesia is desired, so that the function of all the small nerve branches in the area is blocked. The onset of anesthesia is almost immediate, and more may be injected at any time during an operation as it is absorbed or as the surgery involves deeper areas. Infiltration requires several injections, but usually the first needle sting is all that is felt, because the surgeon progresses with the injections so that the anesthetized region overlaps onto each succeeding injection point. Infiltration anesthesia is very practical for many operations. It finds its greatest usefulness in minor procedures, but may be used in major operations as well. It is commonly used for repairing lacerations and other superficial wounds, removing surface tumors and cysts, skin grafting, removing scars, and many other superficial operations. It is also commonly used in setting displaced broken bones by injection of the anesthetic directly into the fracture site. Infiltration anesthesia has almost unlimited use, especially in minor surgery.
Nerve Block Anesthesia
This is effected by injecting an anesthetizing drug about a nerve trunk which branches into the area where anesthesia is desired. Here the injection site may be some distance from the operative area. None of the sensory impulses initiated in the area can pass the paralyzed section of nerve to reach the brain to register pain. The region supplied by each nerve in the body is very specific, so the region anesthetized by blocking a nerve is just as specific. Nerve block anesthesia is also called regional anesthesia. This form of anesthesia is adaptable to almost every region of the body, but in some regions it has greater practicability than others. It is very commonly used for operations on the fingers and toes by blocking the nerves at the base of the digit. It may be employed for much larger areas as well, such as the entire upper extremity from shoulder to fingertips, by injecting about the nerves in the shoulder.
The anatomy of the nervous system is very definite, and the surgeon employs an exact injection site to produce anesthesia over a specific area. The area is always more than adequately large to cover the operative field. Frequently infiltration anesthesia is used in conjunction with nerve block.
Spinal anesthesia is essentially a type of nerve block anesthesia. The anesthetizing agent is placed about the nerves to prevent their sensory impulses from passing up to the central nervous system. In this case, however, the block level is higher along the course of the nerves than the ordinary nerve block, that is, nearer the nerves’ origin off the spinal cord.
To realize this anatomical difference, one must understand the structure of the nervous system within the spinal canal. The brain is contained within the bony skull which rests on the top of the spinal column, or spine. The spinal column is made up of segments of bones called vertebrae, each of which has an opening through it. These are so aligned that there is a passageway within the spine called the spinal canal. Off the base of the brain arises the spinal cord, which lies within the spinal canal. The cord is composed of thousands of very small nerve fibers. Some of these fine fibers carry impulses to the brain (sensory fibers) and others carry impulses away from the brain to promote an action somewhere in the body (motor fibers). Along the spinal cord these nerve fibers become grouped together and lead off the cord, giving rise to nerves. The motor nerve fibers come off the front of the cord, and the sensory fibers come off the posterior of the cord (anterior and posterior nerve roots). These then reunite to form one nerve within which the fibers become intermingled. The nerves make exit from the spinal canal between the vertebrae. But their exit point is at a lower level than their origin from the cord. Thus, the nerves are within the spinal canal for a short distance. It is at this part of the nerves that spinal anesthesia is effected. The anesthetizing drug is placed within the spinal canal so that all impulses are blocked at this part of the nerve.
Spinal anesthesia may be produced to any level, simply by controlling the location of the anesthetic drug after it is introduced into the spinal canal. The lumbar puncture is done in the low back region, well below the lowest level of the spinal cord. The solution injected is of greater specific gravity than the spinal fluid within the spinal canal, and it therefore sinks in the spinal fluid. As soon as the material is injected, the patient is placed in a lying position on the operating table, and then by tilting the table so that the feet are higher than the head, or vice versa, the desired level of anesthesia may be reached. When the correct level is obtained, the table is then placed flat again, so that there will be no further ascent or descent of the drug in the spinal cord. In this way spinal anesthesia may be limited to the very lowest nerves in the canal or may be made to extend up to high levels. It is rarely used for operations above the nipple level, however. Most frequently a small amount of spinal fluid is withdrawn at the time of the lumbar puncture, to be used as the solvent for the anesthetic drug. The crystals of the drug are dissolved in the fluid, and then it is replaced in the canal through the same needle. This portion of the spinal fluid is heavier than the normal spinal fluid, since the drug crystals are dissolved in it, and therefore tends to sink. Then by patient positioning the exact desired level of anesthesia is obtained. Often the anesthetic is already in a heavy solution form, however. (Occasionally anesthetic solutions with specific gravities less than that of the spinal fluid are employed, and then of course the anesthetist will use just the reverse positioning schemes to get the desired level.)
When the nerves in the spinal canal are surrounded by the anesthetic solution, the transmission of all impulses in the nerves is blocked. Not only is there a block of sensations, but there is interruption of the motor impulses as well. The muscles receive no impulses to contract and are therefore temporarily paralyzed and flaccid. This provides the muscular relaxation needed at the site of operation. Brain impulses to the heart and breathing organs exit very high off the spinal cord, so there is no danger of blocking these vital processes.
It can be seen that spinal anesthesia requires very exacting technique. But, with co-operation from the patient, it is very effective and very pleasant. The duration of the anesthesia varies with many factors, but mainly with the drug and the dosage. The surgeon always calculates the duration of the anesthesia well in excess of the anticipated time of the operation. The return to normal is quite gradual, and the patient is required to remain flat in bed for at least four hours after the operation. There is rarely nausea. Complications from this form of anesthesia are extremely rare. Frequently spinal anesthesia is used in conjunction with general anesthesia and other forms of local anesthesia.
Refrigeration anesthesia is employed when it is inevitable that an extremity must be amputated, such as in severely crushed or gangrenous limbs. In this method the diseased arm or leg is simply packed in ice, either with or without a tourniquet about the upper end. This is done by several means right in the patient’s bed. Special ice packing tanks or boots, large rubber sheets, or multiple ice bags may be used to keep the ice in constant contact with the extremity. This local reduction of tissue temperature produces certain definite effects: the tissue chemical reactions (metabolism) and oxygen utilization are slowed; the blood flow is decreased; absorption of tissue byproducts is lessened; conduction of nerve impulses is abolished, and inflammation and healing are slowed. Refrigeration for anesthesia purposes depends on the loss of the nerves’ conduction of impulses, and if amputation is to be done the temperatures will be allowed to fall without limitation and the tourniquet may be left in place. Complete anesthesia will be produced in a couple of hours, but may be continued for a day or longer if there is reason to delay the operation. The other physiological effects are used to advantage also. In toxic conditions, such as gangrene, the absorption of the toxins into the rest of the body is decreased, so the patient’s general condition may be improved before operation. Time can be gained to improve the operative risk in every way possible, and the patient’s pain is relieved in the meantime. Diabetic patients need not forego their diets as is always necessary with general anesthesia. Infection and gangrene are delayed until the optimal time for operation. Whether or not a tourniquet is used depends on the individual case and how soon amputation is anticipated. Refrigeration anesthesia is not uncomfortable and affords prompt relief in painful limbs.
(Use of ice packs and other cooling devices on an extremity does not always mean, however, that the surgeon plans amputation or other surgery. Controlled lowering of tissue temperature is used for other purposes. In arterial diseases, for example, the metabolism and oxygen needs may be decreased by lowered temperatures, so blood supply demands are not so great. But in such cases the temperature is not allowed to fall to very low levels.)