Chapter 2. Anesthesia

Anesthesiology as a special field of endeavor has made clear the many physiologic changes occurring in the patient during anesthesia. The pharmacologic effects of anesthetic agents and techniques on the central nervous system and the cardiovascular and respiratory systems are now better understood. New drugs have been introduced for inhalation, intravenous, spinal, and regional anesthesia. In addition, drugs, such as muscle relaxants and hypotensive or hypertensive agents, are used for their specific pharmacologic effect. Older anesthetic techniques, such as spinal and caudal anesthesia, have been improved by the refinement of the continuous technique and more accurate methods of controlling the distribution of the administered drug. Marked advances in anesthesia have taken place in pulmonary, cardiac, pediatric, and geriatric surgery. Improved management of airway and pulmonary ventilation is reflected in the techniques and equipment available to prevent the deleterious effects of hypoxia and hypercarbia. An increased understanding of the altered hemodynamics produced by anesthesia in the ill patient has resulted in better fluid, electrolyte, and blood replacement preoperatively in patients with a decreased blood volume and electrolyte imbalance, thus allowing many patients once thought to be too ill for surgery the opportunity for safe operative care.

Although the number of anesthesiologists has increased within recent years, it still is not enough to meet the increased surgical load. Surgeons, therefore, may find that they have to rely upon less trained assistants to administer anesthesia. They must bear in mind that in the absence of a trained anesthesiologist, the surgeon is legally accountable should catastrophe from any cause compromise the outcome of the surgical procedure. Under these circumstances, the surgeon should be knowledgeable about the choice of anesthetic agents and techniques and their indications and complications. Further, he or she should be familiar with the condition of the patient under anesthesia by observing the color of blood or viscera, the rapidity and strength of the arterial pulsation, and the effort and rhythm of the chest wall or diaphragmatic respirations. Knowing the character of these conditions under a well-conducted anesthesia, the surgeon will be able readily to detect a patient who is doing poorly.

It is this point of view that has caused us to present in this practical volume the following short outline of modern anesthetic principles. This outline makes no pretense of covering fully the physiologic, pharmacologic, and technical details of anesthesiology, but it offers to the surgeon some basic important information.

The intraoperative role of the anesthesiologist as a member of the surgical team is severalfold, including the assurance of adequate pulmonary ventilation, maintenance of a near-normal cardiovascular system, and conduction of the anesthetic procedure itself. One cannot be isolated from the other.

Preventing the subtle effects of hypoxia is of prime importance to the anesthesiologist. It is well known that severe hypoxia may cause sudden disaster, and that hypoxia of a moderate degree may result in slower but equally disastrous consequences. Hypoxia during anesthesia is related directly to some interference with the patient's ability to exchange oxygen. This commonly is caused by allowing the patient's tongue partially or completely to obstruct the upper airway. Foreign bodies, emesis, profuse secretions, or laryngeal spasm may also cause obstruction of the upper airway. Of these, aspiration of emesis, although rare, represents the greatest hazard to the patient. General anesthesia should not be administered in those patients likely to have a full stomach unless adequate protection of the airway is assured. A common guideline in adults with normal gastrointestinal motility is a 6- to 8-hour interval between the ingestion of solid food and induction of anesthesia. In addition, members of the surgical team should be capable of performing endotracheal intubation. This will reduce the possibility of the patient's asphyxiating, as the endotracheal tube is not always a guarantee of a perfect airway. Other conditions known to produce a severe state of hypoxia are congestive heart failure, pulmonary edema, asthma, or masses in the neck and mediastinum compressing the trachea. As these conditions may not be directly under the anesthesiologist's control, preoperative evaluation should be made by the surgeon, the anesthesiologist, and appropriate consultants. In complex airway cases, the patient may be intubated using topical anesthetics and a flexible fiberoptic bronchoscope that serves as an internal guide for the overlying endotracheal tube.

Before any general anesthetic technique is commenced, facilities must be available to perform positive-pressure oxygen breathing, and suction must be available to remove secretions and vomitus from the airway before, during, and after the surgical procedure. Every effort should be expended to perform an adequate tracheobronchial and oropharyngeal cleansing after the surgical procedure, and the airway should be kept free of secretions and vomitus until the protective reflexes return. With the patient properly positioned and observed, all these procedures will help to reduce the incidence of postoperative pulmonary complications.

Fluid therapy during the operative procedure is a joint responsibility of the surgeon and the anesthesiologist. Except in unusual circumstances, anemia, hemorrhage, and shock should be treated preoperatively. During the operation transfusions should be used with caution as there can be significant risks associated with transfusions. Most patients can withstand up to 500 mL of blood loss without difficulty. However, in operative procedures known to require several units of blood, the blood should be replaced as lost as estimated from the quantity of blood within the operative field, the operative drapes, and the measured sponges and suction bottles. The intravascular volume can be expanded by cross matched packed red blood cells, specifically indicated for their oxygen-carrying capacity, when the hematocrit (Hct) is ≤23 to 25 percent or the hemoglobin (Hb) is ≤7 g/dL. In emergency situations when blood is not available, synthetic colloids (dextran or hydroxyethyl starch solutions), albumin, or plasma may be administered to maintain an adequate expansion of blood volume. All blood products are used with caution because of the possibility of transmitting homologous viral diseases. Infusions of Ringer's lactate (a balanced electrolyte solution), via a secure and accessible intravenous catheter, should be used during all operative procedures, including those in pediatrics. Such an arrangement allows the anesthesiologist to have ready access to the cardiovascular system, and thereby a means of administering drugs or treating hypotension promptly. Additionally, large centrally placed catheters may be used to monitor central venous pressure or even cardiac performance if a pulmonary artery catheter is placed into the pulmonary vasculature. As many modern anesthetic agents may produce vasodilation or depression of myocardial contractility, anesthesiologists may volume load patients with crystalloid solutions. This maintains normal hemodynamic parameters and a good urine output. However, this fluid loading may have serious after effects in some patients; thus the anesthesiologist must monitor the type and volume of fluids given to the patient during the operation and communicate this to the surgeon.

The patient's body position is an important factor both during and after the operation. The patient should be placed in a position that allows gravity to aid in obtaining optimum exposure. The most effective position for any procedure is the one that causes the viscera to gravitate away from the operative field. Proper position on the table allows adequate anatomic exposure with less traumatic retraction. With good muscle relaxation and an unobstructed airway, exaggerated positions and prolonged elevations become unnecessary. The surgeon should bear in mind that extreme positions result in embarrassed respiration, in harmful circulatory responses, and in nerve palsies. When the surgical procedure is concluded, the patient should be returned gradually to the horizontal supine position, and sufficient time should be allowed for the circulatory system to become stabilized. When an extreme position is used, extremity wrappings should be applied and the patient should be returned to the normal position in several stages, with a rest period between each one. Abrupt changes in position or rough handling of the patient may result in unexpected circulatory collapse. After being returned to bed, the patient should be positioned for safe respiration. The patient is observed for unobstructed breathing and stable hemodynamic parameters until he or she is sufficiently alert.

Anesthesia in the aged patient is associated with an increased morbidity and mortality. Degenerative diseases of the pulmonary and cardiovascular systems are prominent, with the individual being less likely to withstand minor insults to either system. Sedatives and narcotics should be used sparingly in both the preoperative and postoperative periods. Regional or local anesthesia should be employed in this age group whenever feasible. This form of anesthesia decreases the possibility of serious pulmonary and cardiovascular system complications and at the same time decreases the possibility of serious mental disturbance that can occur following general anesthesia. Induction and maintenance of anesthesia can be made smoother by good preoperative preparation of the respiratory tract. This begins with cessation of smoking prior to admission and continues with vigorous pulmonary care that may involve positive-pressure aerosol therapy and bronchodilators. A detailed cardiac history in preoperative workup will uncover patients with borderline cardiac failure, coronary insufficiency, or valvular disease, who require specialized drug treatment and monitoring.

As most patients are anxious in the preoperative period, premedication with an anxiolytic agent is often given in the preoperative holding area. Once upon the operating table, the patient is preoxygenated before being induced rapidly and smoothly with an intravenous hypnotic and narcotic.

Induction of a full general anesthesia requires airway control with either a laryngeal mask airway (LMA) or an endotracheal tube, whose placement may require transient muscle paralysis.

Muscle relaxants such as succinylcholine or nondepolarizing neuromuscular blocking agents should be used for those operations requiring muscular looseness if it is not provided by the anesthetic agent. By the use of these drugs, adequate muscular relaxation can be obtained in a lighter plane of anesthesia, thereby reducing the myocardial and peripheral circulatory depression observed in the deeper planes of anesthesia. In addition, the protective reflexes, such as coughing, return more quickly if light planes of anesthesia are maintained. Finally, however, it is important to note that the mycin-derivative antibiotics may interact with curare-like drugs so as to prolong their effect with inadequate spontaneous respiration in the recovery area and may lead to extended respiratory support.

When the maximum safe dosages of local anesthetic agents are exceeded, the incidence of toxic reactions increases. These reactions, which are related to the concentration of the local anesthetic agent in the blood, may be classified as either central nervous system stimulation (i.e., nervousness, sweating, and convulsions) or central nervous system depression (i.e., drowsiness and coma). Either type of reaction may lead to circulatory collapse and respiratory failure. Resuscitative equipment consisting of positive-pressure oxygen, intravenous fluids, vasopressors, and an intravenous barbiturate should be readily available during all major operative procedures using large quantities of local anesthesia. The intensity of anesthesia produced by the local anesthetic agents depends on the concentration of the agent and on the size of the nerve. As the size of the nerve to be anesthetized increases, a higher concentration of anesthetic agent is utilized. Since the maximum safe dose of lidocaine (Xylocaine) is 300 mg, it is wise to use 0.5% lidocaine when large volumes are needed.

The duration of anesthesia can be prolonged by the addition of epinephrine to the local anesthetic solution. Although this prolongs the anesthetic effect and reduces the incidence of toxic reactions, the use of epinephrine is not without danger. Its concentration should not exceed 1:100,000; that is, 1 mL of 1:1,000 solution in 100 mL of local anesthetic agent. After the operative procedure has been completed and the vasoconstrictive effect of the epinephrine has worn off, bleeding may occur in the wound if meticulous attention to hemostasis has not been given. If the anesthetic is to be injected into the digits, epinephrine should not be added because of the possibility of producing gangrene by occlusive spasm of these end arteries, which do not have collaterals. Epinephrine is also contraindicated if the patient has hypertension, arteriosclerosis, and coronary or myocardial disease.

In any surgical practice, occasions arise when the anesthesiologist should refuse or postpone the administration of anesthesia. Serious thought should be given before anesthesia is commenced in cases of severe pulmonary insufficiency; with elective surgery in the patient with myocardial infarction less than 6 months prior; severe unexplained anemia; with inadequately treated shock; in patients who recently have been or are still on certain drugs such as monoamine oxidase (MAO) inhibitors and certain tricyclic antidepressants that may compromise safe anesthesia; and, finally, in any case in which the anesthesiologist feels he or she will be unable to manage the patient's airway, such as Ludwig's angina, or when there are large masses in the throat, neck, or mediastinum that compress the trachea.

Cessation of effective cardiac activity may occur at any time during an anesthetic or operative procedure performed under local or general anesthesia. Many etiologic factors have been cited as producing cardiac dysfunction; however, acute or prolonged hypoxia is undoubtedly the most common cause. In a few instances undiagnosed cardiovascular disease, such as severe aortic stenosis or myocardial infarction, has been the cause of cardiac standstill. Many sudden cardiac complications relate to anesthetic technique and they are often preceded by warning signs long before the castastrophe actually occurs. Common anesthetic factors include overdosage of anesthetic agents, either in total amount of drug or in speed of administration; prolonged and unrecognized partial respiratory obstruction; inadequate blood replacement with delay in treating hypotension; aspiration of stomach contents; and failure to maintain constant vigilance over the anesthetized patient's cardiovascular system. The last factor is minimized by the use of the precordial or intraesophageal stethoscope, a continuous electrocardiogram, end-tidal CO2, and oxygen saturation monitoring.

Mortality and morbidity from cardiac events can be minimized further by having all members of the surgical team trained in the immediate treatment of sudden cardiac collapse. Successful treatment of sudden cardiac collapse depends upon immediate diagnosis and the institution of therapy without hesitation. Diagnosis is established tentatively by the absence of the pulse and blood pressure as recognized by the anesthesiologist and confirmed by the surgeon's palpation of the arteries or observation of the absence of bleeding in the operative field. The Advanced Cardiac Life Support protocols developed by the American College of Cardiology provide a reasonable guide to resuscitation. It is imperative that external cardiac compression and the establishment of a clear and unobstructed airway be instituted immediately. Intravenous administration of epinephrine is appropriate. If adequate circulation is being produced, a pulse should be palpable in the carotid and brachial arteries. Many times, oxygenated blood being circulated through the coronary arteries by external compression will be sufficient to start a heart in asystole. If the heart is fibrillating, it should be defibrillated. Defibrillation may be accomplished by electrical direct current, which is the preferred method. If all of these resuscitative measures are unsuccessful, then thoracotomy with direct cardiac compression or defibrillation may be considered in an equipped and staffed operating room setting.

The treatment of a patient revived after a cardiopulmonary arrest is directed toward maintaining adequate cardiopulmonary ventilation and perfusion and preventing specific organ injuries such as acute renal tubular necrosis or cerebral edema. This may involve vasoactive drugs, steroids, diuretics, or hypothermia.

The anesthesiologist's skill is the most important factor in the choice of anesthesia. The anesthesiologist should select the drugs and methods with which he or she has had the greatest experience. The effects of the drugs are modified by speed of administration, total dose, the interaction of various drugs used, and the technique of the individual anesthesiologist. These factors are far more important than the theoretical effects of the drugs based on responses elicited in animals. With anesthetic agents reported to have produced hepatocellular damage, certain precautions should be observed. This is particularly important in patients who have been administered halogenated anesthetic agents in the recent past or who give a history suggestive of hepatic dysfunction following a previous anesthetic exposure. Further, the halogenated anesthetic agents should be used cautiously in patients whose occupations expose them to hepatocellular toxins or who are having biliary tract surgery.

The following factors about the proposed operation must be considered: its site, magnitude, and duration; the amount of blood loss to be expected; and the position of the patient on the operating table. The patient should then be studied to ascertain his or her ability to tolerate the surgical procedure and the anesthetic. Important factors are the patient's age, weight, and general condition as well as the presence of acute infection, toxemia, dehydration, and hypovolemia. Hence, there is a dual evaluation: first, of the overall state of the patients' vital organ systems and, second, of the superimposed hazards of the disease.

The patient's previous experience and prejudices regarding anesthesia should be considered. Some patients dread losing consciousness, fearing loss of control; others wish for oblivion. Some patients, or their friends, have had unfortunate experiences with spinal anesthesia and are violently opposed to it. An occasional individual may be sensitive to local anesthetics or may have had a prolonged bout of vomiting following inhalation anesthesia. Whenever possible, the patient's preference regarding the choice of anesthesia should be followed. If that choice is contraindicated, the reason should be explained carefully and the preferred procedure should be outlined in such a way as to remove the patient's fears. If local or spinal anesthesia is selected, psychic disturbance will be minimized and the anesthetic made more effective if it is preceded by adequate premedication.

If possible, the patient should be visited by the anesthesiologist prior to the operation. The anesthesiologist should have become acquainted with the patient's condition and the proposed operation. He or she must evaluate personally the patient's physical and psychic state and, at this time, should inquire about the patient's previous anesthetic experience and about drug sensitivity. The anesthesiologist should question the patient about drugs taken at home and be sure that medicines requiring continued administration, such as beta-blockers or insulin, are continued. Further inquiry should be made concerning drugs (such as corticosteroid drugs, antihypertensive drugs, MAO inhibitors, and tranquilizers) that may have an interaction with the planned anesthesia. If the patient is taking any of these drugs, proper precautions should be taken to prevent an unsatisfactory anesthetic and surgical procedure.

Preoperative medication is frequently part of the anesthetic procedure. The choice of premedication depends on the anesthetic to be used. Dosage should vary with the patient's age, physical state, and psychic condition. Premedication should remove apprehension, reduce the metabolic rate, and raise the threshold to pain. Upon arriving in the operating suite, the patient should be unconcerned and placid.