Anesthesia: techniques and agents

Loss of memory.

Relief of pain by altering perception of painful stimuli; acts on specific receptors in the nervous system. Does not alter consciousness.

Anesthesia

Loss of feeling or sensation, especially loss of the sensation of pain with loss of protective reflexes.

Anoxia

Absence of oxygen.

Anticholinergic

Antagonist to action of parasympathetic and other cholinergic nerve fibers.

Apnea

Suspension or cessation of breathing.

Conduction anesthesia

Loss of sensation in a region of the body produced by injecting an anesthetic drug along the course of a nerve or a group of nerves to inhibit conduction of impulses to and from the area supplied by that nerve or nerves (block anesthesia, nerve block anesthesia).

Depolarization

Neutralization of polarity; reduction of differentials of ion distribution across polarized semipermeable membranes, as in nerve or muscle cells in the conduction of impulses; to make electrically negative.

Dysrhythmia

Ineffective rhythm, as of heart rate or brain waves; term used interchangeably with arrhythmia.

Emergence

Return of consciousness, sensation, and reflexes after general anesthesia.

Endotracheal

Within the trachea. An endotracheal tube may be placed in the trachea to maintain a patent airway during loss of consciousness.

Epidural anesthesia

Loss of sensation below the level of peridural injection of an anesthetic drug into the epidural space in the spinal canal for relief of pain in the lower extremities, abdomen, and pelvis without loss of consciousness.

Extubation

Removal of an endotracheal tube.

Fasciculation

Abnormal skeletal muscle contraction in which groups of muscle fibers innervated by the same neuron contract together.

Hypercapnia

Excessive amount of carbon dioxide in the blood; may also be termed hypercarbia.

Hypnotic

Drug or verbal suggestion that induces sleep.

Hypothermia

State in which body temperature is lower than the physiologic normal (i.e., below 95° F [35° C]).

Hypoxia, hypoxemia

Oxygen deficiency; state in which an inadequate amount of oxygen is available to or used by tissue; inadequate tissue oxygenation.

Induction

Period from the beginning of administration of an anesthetic until the patient loses consciousness and is stabilized in the desired plane of anesthesia.

Intrathecal injection

Instillation of solution, such as an anesthetic drug, into the subarachnoid space for diffusion in spinal fluid, as for spinal anesthesia.

Intubation

Insertion of an endotracheal tube.

Laryngospasm

Involuntary spasmodic reflex action that partially or completely closes the vocal cords of the larynx.

Local anesthesia

Loss of sensation along specific nerve pathways produced by blocking transmissions of impulses to receptor fibers. The anesthetic drug injected depresses sensory nerves and blocks conduction of pain impulses from their site of origin. The patient remains conscious, with or without IV sedation.

Moderate sedation

(formerly known as intravenous conscious sedation [IVCS]). Depressed level of consciousness produced by IV administration of pharmacologic agents. The patient retains the ability to continuously maintain a patent airway independently and respond to physical or verbal stimulation. Sedation may relieve anxiety and produce amnesia.

Narcosis

State of arrested consciousness, sensation, motor activity, and reflex action produced by drugs.

Narcotic

Drug derived from opium or opium-like compounds, with potent analgesic effects associated with significant alteration of mood and behavior.

Nerve block

Loss of sensation produced by injecting an anesthetic drug around a specific nerve or nerve plexus to interrupt sensory, motor, or sympathetic transmission of impulses.

Paco₂

Arterial carbon dioxide tension (partial pressure of carbon dioxide in arterial blood). Normal 35 to 45 torr.

Pao₂

Arterial oxygen tension (partial pressure of oxygen in the arterial blood); degree of oxygen transported in the circulating blood. Normal 80 to 100 torr.

Expression for hydrogen ion concentration or acidity. In blood, alkalemia: values above 7.45; acidemia: values below 7.35; normal 7.4.

Regional anesthesia

Loss of sensation in a specific body part or region produced by blocking conductivity of sensory nerves supplying that area. The anesthetic drug is injected around a specific nerve or group of nerves to interrupt pain impulses. The patient remains conscious, with or without IV sedation. Regional anesthetic techniques include nerve, intrathecal, peridural, and epidural blocks.

Sedative

Pharmacologic agent (drug) that suppresses nervous excitement, allays anxiety, and produces a calming effect. Benzodiazepines, barbiturates, and opioids (narcotics) are the most commonly used drugs for conscious sedation.

Spinal anesthesia

Loss of sensation below the level of the diaphragm produced by intrathecal injection of an anesthetic drug into the subarachnoid space without loss of consciousness.

Tachycardia

Excessive rapidity of heart action, heartbeat. The pulse rate is higher than 100 beats per minute.

Tachypnea

Abnormally rapid rate of breathing.

Topical anesthesia

Depression of sensation in superficial peripheral nerves by application of an anesthetic agent directly to the mucous membrane, skin, or cornea.

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The art and science of anesthesia

Anesthesiology is the branch of medicine and nursing that is concerned with the administration of medication or anesthetic agents to relieve pain and support physiologic functions during a surgical procedure. It is a specialty that requires knowledge of biochemistry, clinical pharmacology, cardiology, and respiratory physiology. The American Society of Anesthesiologists (ASA), founded in 1905 and incorporated in 1936, has defined anesthesiology as the practice of medicine dealing with the management of procedures for rendering a patient insensible to pain during surgical procedures and with the support of life functions under the stress of anesthetic and surgical manipulations.

The purpose of this chapter is to acquaint the surgical team with several processes associated with the delivery and maintenance of anesthesia and how the team works together to provide a safe surgical procedure for the patient. All perioperative team members should be readily available to assist the anesthesia provider as needed. Continuing education is advised for the entire team. (Internet websites included with links to educational information, such as Gas Net, http://anestit.unipa.it/HomePage.htmland The Virtual Anesthesia Machine http://vam.anest.ufl.edu, are excellent free resources.) Links on the Virtual Anesthesia Machine website offer other forms of simulated learning such as difficult airway management for a signup fee.

Choice of anesthesia

Selection of anesthesia is made by the anesthesia provider in consultation with the surgeon and the patient. The primary consideration with any anesthetic is that it should be associated with low morbidity and mortality. Choosing the safest agent and technique is a decision predicated on thorough knowledge, sound judgment, and evaluation of each individual situation.

The anesthesia provider uses the lowest concentration of anesthetic agents compatible with patient analgesia, relaxation, and facilitation of the surgical procedure. An ideal anesthetic agent or technique suitable for all patients does not exist, but the one selected should include the following characteristics:

• Provide maximum safety for the patient

• Provide optimal operating conditions for the surgeon

• Provide patient comfort

• Have a low index of toxicity

• Provide potent, predictable analgesia extending into the postoperative period

• Produce adequate muscle relaxation

• Provide amnesia

• Have a rapid onset and easy reversibility

• Produce minimum side effects

The patient’s ability to tolerate stress and adverse effects of anesthesia and the surgical procedure depends on respiration; circulation; and function of the liver, kidneys, endocrine system, and central nervous system (CNS). The following factors are important:

• Age, size, and weight of the patient

• Physical, mental, and emotional status of the patient

• Presence of complicating systemic disease or concurrent drug therapy

• Presence of infection at the site of the surgical procedure

• Previous anesthesia experience

• Anticipated procedure

• Position required for the procedure

• Type and expected length of the procedure

• Local or systemic toxicity of the agent

• Expertise of the anesthesia provider

• Preference of the surgeon and patient

Anesthesia state

Both the central and the autonomic nervous systems play essential roles in clinical anesthesia. The CNS exerts powerful control throughout the body. The effect of anesthetic drugs is one of progressive depression of the CNS, beginning with the higher centers of the cerebral cortex and ending with the vital centers in the medulla. The cerebral cortex is not inactive during deep anesthesia. Afferent impulses continue to flow into the cortex along primary pathways and excite cells in appropriate sensory areas. Also, the cerebral cortex is integrated with the reticular system.

The brain represents approximately 2% of body weight but receives about 15% of cardiac output. Various factors cause alterations in cerebral blood flow and are of considerable importance in anesthesia.⁸ These factors are oxygen, carbon dioxide, temperature, arterial blood pressure, drugs, the age of the patient, anesthetic techniques, and neurogenic factors.⁸

The autonomic nervous system is equally important because of its role in the physiology of the cardiovascular system, the anesthesia provider’s ability to block certain autonomic pathways with local analgesic agents, specific blocking effects of certain drugs, and the sympathomimetic and parasympathomimetic effects of many anesthetic agents.

The anesthesia state involves control of motor, sensory, mental, and reflex functions. The anesthesia provider constantly assesses the patient’s response to stimuli to evaluate specific anesthetic requirements. Specific drugs are used to achieve the desired results: amnesia, analgesia, and muscle relaxation. ASA and AANA have established guidelines and standards for safely administering and monitoring anesthesia care. ASA also developed the taxonomy for classifying patients by physical status from class I, the lowest risk, to class VI, the highest risk (Box 24-1).

BOX 24-1 ASA Classifications

• Class I theoretically includes relatively healthy patients with localized pathologic processes. An emergency surgical procedure, designated E, signifies additional risk. For example, a hernia that becomes incarcerated changes the patient’s status to class I-E.

• Class II includes patients with mild systemic disease (e.g., diabetes mellitus controlled by oral hypoglycemic agents or diet).

• Class III includes patients with severe systemic disease that limits activity but is not totally incapacitating (e.g., chronic obstructive pulmonary disease or severe hypertension).

• Class IV includes patients with an incapacitating disease that is a constant threat to life (e.g., cardiovascular or renal disease).

• Class V includes moribund patients who are not expected to survive 24 hours with or without the surgical procedure. They are operated on in an attempt to save their life; the surgical procedure is a resuscitative measure, as in a massive pulmonary embolus. The patient may or may not survive the surgical procedure.

• Class VI includes patients who have been declared brain dead but whose organs will be removed for donor purposes. Mechanical ventilation and life support systems are maintained until the organs are procured.

Knowledge of anesthetics

Anesthesia involves the administration of potentially lethal drugs and gases. Interactions of these with human physiology can be profound. Using discerning observation, astute deduction, and meticulous attention to the minutiae, the anesthesia provider delivers skilled induction, careful maintenance of anesthesia, and prophylaxis to avoid postoperative complications.

Being responsible for vital functions of the patient, the anesthesia provider must know physical and chemical properties of all gases and liquids used in anesthesia. These properties determine how agents are supplied, their stability, systems used for their administration, and their uptake and distribution in the body.⁵ Important factors are diffusion, solubility in body fluids, and relationships of pressure, volume, and temperature. The synthesized general anesthetic agents are nonflammable, in contrast with the older agents.

Perioperative and PACU nurses need to be cognizant of the pharmacologic characteristics of the most commonly used anesthetics and techniques. Anesthesia and surgical trauma produce multiple systemic effects, which are continually monitored throughout the perioperative care period. The type and level of anesthesia will vary according to the type of surgery being performed. These types are described as follows:

1. Minimal or light sedation (anxiolysis): A drug-induced state during which patients respond normally to verbal command. Although cognitive function and coordination may be impaired, ventilatory and cardiovascular functions are unaffected.

2. Moderate sedation/analgesia (formerly known as conscious sedation): A drug-induced depression of consciousness during which patients can respond purposefully to verbal commands, either alone or accompanied by light tactile stimulation. No interventions are required to maintain a patent airway, and spontaneous ventilation is adequate.

3. Deep sedation/analgesia: A drug-induced depression of consciousness during which patients cannot be easily aroused but respond purposefully after repeated or painful stimulation. The ability to independently maintain ventilatory function may be impaired. Patients may require assistance to maintain a patent airway, and spontaneous ventilation may be inadequate.⁵

4. Full anesthesia: General anesthesia and regional anesthesia. General anesthesia is a drug-induced loss of consciousness during which patients cannot be roused, even by painful stimulation. The ability to independently maintain ventilatory function is often impaired. Patients often require assistance to maintain a patent airway, and positive pressure ventilation may be required because of depressed spontaneous ventilation or drug-induced depression of neuromuscular function. Cardiovascular function may be impaired.

Types of anesthesia

Anesthesia may be produced in a number of ways:

• General anesthesia: Pain is controlled by general insensibility. Basic elements include loss of consciousness, analgesia, interference with undesirable reflexes, and muscle relaxation.

• Balanced anesthesia: The properties of general anesthesia (i.e., hypnosis, analgesia, and muscle relaxation) are produced, in varying degrees, by a combination of agents. Each agent has a specific purpose. This often is referred to as neuroleptanesthesia.

• Local or regional block anesthesia: Pain is controlled without loss of consciousness. The sensory nerves in one area or region of the body are anesthetized. This is sometimes called conduction anesthesia. Acupuncture is sometimes used.

• Spinal or epidural anesthesia: Sensation of pain is blocked at a level below the diaphragm without loss of consciousness. The agent is injected in the spinal canal.

General anesthesia

Anesthesia is produced as the CNS is affected. Association pathways are broken in the cerebral cortex to produce more or less complete lack of sensory perception and motor discharge. Unconsciousness is produced when blood circulating to the brain contains an adequate amount of the anesthetic agent. General anesthesia results in an unconscious, immobile, quiet patient who does not recall the surgical procedure.

Most anesthetic agents are potentially lethal. The anesthesia provider must constantly observe the body’s reflex responses to stimuli and other guides to determine the degree of CNS, respiratory, and circulatory depression during induction and the surgical procedure. No one clinical sign can be used as a reliable indication of anesthesia depth. Continuous watching and appraisal of all clinical signs, in addition to other available objective measurements, are necessary. In this way the anesthesia provider judges the level of anesthesia, referred to as light, moderate, or deep, and provides the patient with optimal care (Table 24-1).

TABLE 24-1

Depth of General Anesthesia

From	To	Patient’s Responses	Patient Care Considerations
Induction of general anesthesia and beginning and/or IV drug of inhalant	Begins to lose consciousness; will have recall Bispectral state 100	Drowsy, dizzy, amnesic	Close OR doors. Keep room quiet. Stand by to assist. Initiate cricoid pressure if requested.
Loss of consciousness: excitement phase	Relaxation, light hypnosis; low probability of recall Bispectral state 70 to 50	May be excited, with irregular breathing and movements of extremities; susceptible to external stimuli (e.g., noise, touch)	Restrain patient. Remain at patient’s side, quietly, but ready to assist anesthesia provider as needed.
Surgical anesthesia stage of relaxation	Loss of reflexes: depression of vital functions Bispectral state 40: maintenance range	Regular respiration; contracted pupils; reflexes disappear; muscles relax; auditory sensation lost	Position patient and prepare skin only when anesthesia provider indicates this stage is reached and under control.
Danger stage: vital functions too depressed	Respiratory failure; possible cardiac arrest Bispectral state 0	Not breathing; little or no pulse or heartbeat	Prepare for cardiopulmonary resuscitation.

IV, Intravenous; OR, operating room.

The three methods of administering general anesthetic are inhalation, IV injection, and rectal instillation. The latter method is not commonly used, except occasionally in pediatrics, because retention and absorption in the colon are unpredictable. Control of each method varies.

Induction of general anesthesia

Induction involves putting the patient safely into a state of unconsciousness. Figure 24-1 depicts the levels of unconsciousness associated with general anesthesia. A patent airway and adequate ventilation must be ensured. If one is not already running, an intravenous (IV) infusion is started. The anesthesia provider should wear gloves for venipuncture. A nasogastric tube may be inserted to decompress the gastrointestinal tract and evacuate stomach contents.

FIG. 24-1 Levels of unconsciousness associated with general anesthesia.

A patent airway must be established to provide adequate oxygenation and control breathing of the unconscious patient. The patient’s tongue and secretions can obstruct respiration in the absence of protective reflexes. The anesthesia provider evaluates the airway for the risk of difficult intubation using the Mallampati classification chart (Fig. 24-2). Other measurements include thyromental distance, neck flexion/extension range, and the ability to prognath (protrude the mandible). An oropharyngeal airway, nasopharyngeal airway, laryngeal mask, endotracheal tube, or endobronchial tube (for lung procedures) may be inserted.

FIG. 24-2 Mallampati classification for difficult intubation.

Physiologic indicators of a difficult airway include the following:

• Inability to open the mouth. Patients with previous jaw surgery may have jaw wires in place. Wire cutters should be immediately available in the event of a return to surgery.

• Immobility of the cervical spine. Patients with vertebral disease or injury may not have full range of motion necessary for intubation.

• Chin or jaw deformities. Patients with small jaws or chin may have a difficult airway. Edentulous patients commonly have some bone loss that alters facial contours.

• Dentition can be an issue if the patient has loose teeth or periodontal disease. A tooth can be aspirated during the airway maintenance process. Children between the ages of 6 and 8 commonly have loose baby teeth.

• Short neck or morbid obesity (Fig. 24-3).

FIG. 24-3 Short neck or morbid obesity. A, Supine position. B, Sniffing position.

• Pathology of the head and neck such as tumors or deformity. An enlarged tongue can be an obstruction to a full view of the glottis.

• Previous tracheostomy scar, which can cause a stricture.

• Trauma.¹²

Intubation is insertion of an endotracheal tube between vocal cords, usually with an oral tube by direct laryngoscopy. A nasotracheal tube may be inserted by blind intubation or with a direct approach using Magill forceps to guide the tube through the pharynx (Fig. 24-4). Epistaxis can be a complication of nasal airway use. This method is contraindicated in anticoagulated patients. Tubes may be made of metal, plastic, silicone, or rubber. Most styles for adult sizes have a built-in cuff that is inflated with a measured amount of air, water, or saline after insertion, to completely occlude the trachea.

FIG. 24-4 Nasotracheal intubation with a Magill forceps.

The anesthesia provider is informed if a laser will be used in the mouth or throat so that a laser-resistant endotracheal tube can be inserted. The endotracheal tube must be securely fixed in place to prevent irritation of the trachea and maintain ventilation. The anesthesia provider should wear a mask and protective eyewear to prevent secretions from splashing in the eyes during intubation. An oropharyngeal suction tip and tubing should be kept close at hand.

Neuromuscular blocking agents are given before intubation to relax the jaw and larynx. Pediatric patients and patients susceptible to malignant hyperthermia may experience jaw tightness, which is referred to as masseter muscle rigidity (MMR) or trismus. Intubation during induction and extubation during emergence from anesthesia are precarious times for the patient. The patient may cough, jerk, or experience laryngospasm from tracheal stimulation. Cardiac dysrhythmias may occur. Hypoxia is a potential complication. Hypoxia commonly precedes dysrhythmia.

Aspiration is also a hazard, particularly in a patient with a full stomach or with increased intraabdominal or intracranial pressure. Any patient who arrives in the OR unconscious or who is a victim of trauma should be treated as though he or she has a full stomach.¹² Pregnant and obese patients should also be considered in this category because of increased intraabdominal pressure in the supine position and possible decreased gastric motility.

1. Sources of oxygen and compressed gases (Fig. 24-7). These may come from piped-in systems, but mounted oxygen tanks are necessary in the event of failure of systems.

FIG. 24-7 Minimum number of components needed for an anesthesia gas machine.

2. Means for measuring (flowmeters) and controlling (reservoir bag) delivery of gases.

3. Means to volatilize liquid (vaporizer) and deliver (breathing tubes) anesthetic vapor or gas.

4. Device for disposal of carbon dioxide (carbon dioxide absorption canister).

5. Safety devices:

a. Oxygen analyzers

b. Oxygen pressure interlock system or equivalent to automatically shut off the flow of gases in the absence of oxygen pressure

c. End-tidal carbon dioxide monitors

d. Pressure and disconnect alarms to notify the anesthesia provider if the flow of oxygen and gases becomes disproportional

e. Gas scavenger system to collect exhaled gases

Waste gases.

The elimination of waste gas, vented through an exhaust valve into a waste gas scavenger system, controls pollution of the room air.¹³ Nitrous oxide and halogenated agents can escape into room air if they are not directed through the scavenger system. Substantial amounts may be an occupational health hazard to OR team members. Prolonged exposure to high concentrations of waste gases (1000 ppm) can cause reproductive abnormalities.¹² Halogenated waste gases should be lower than 2 ppm and nitrous oxide (N₂O) waste gas should be lower than 25 ppm according to safety regulations set by OSHA.¹²

Valves on the machine and tubing connections should be checked daily and must be secure for the system to work properly. Room air should be monitored. This may be done by an infrared spectrophotometer, for example, to monitor the escape of gases from the patient’s exhalations and from the anesthetic delivery system. Patients continue to exhale retained gases when they arrive in the PACU. Passive dosimeters may be used to monitor air in team members’ personal breathing spaces.

Inhalation systems.

The method for administration of inhalation anesthetics through the anesthesia machine can be classified as semiclosed, closed, semiopen, or open (Fig. 24-8):

FIG. 24-8 Complete anesthesia breathing circuit.

• Semiclosed system: The most widely used system, a semiclosed system permits exhaled gases to pass into the atmosphere so that they will not mix with fresh gases and be rebreathed. A chemical absorber for carbon dioxide is placed in the breathing circuit. This reduces carbon dioxide accumulation in blood. Induction is slower but with less loss of heat and water vapor than with open methods.

• Closed system: A closed system allows complete rebreathing of expired gases. Exhaled carbon dioxide is absorbed by soda lime or a mixture of barium and calcium hydroxide (Baralyme) in the absorber on the machine. The body’s metabolic demand for oxygen is met by adding oxygen to the inspired mixture of gases or vapors. This system provides maximal conservation of heat and moisture. It reduces the amount and therefore the cost of agents and reduces environmental contamination.

• Semiopen system: With the semiopen system some exhaled gas can pass into surrounding air but some returns to the inspiratory part of the circuit for rebreathing. The degree of rebreathing is determined by the volume of flow of fresh gas. Expired carbon dioxide is not chemically absorbed.

• Open system: In an open system, valves direct expired gases into the lower portion of the canister, where they are removed by vacuum. The patient inhales only the anesthetic mixture delivered by the anesthesia machine. The composition of the inspired mixture can be accurately determined. However, anesthetic gases are not confined to the breathing system. High flows of gases are necessary, because resistance to breathing varies. Water vapor and heat are lost. Inspired gases should be humidified for respiratory mucosa to function properly, especially for children and during long surgical procedures.

Administration techniques.

Inhalation gases and vapors can be delivered from an anesthesia machine via a facemask, laryngeal mask, or endotracheal tube. Respirations must be assisted or controlled.

Mask inhalation.

Anesthetic gas or vapor of a volatile liquid is inhaled through a facemask attached to the anesthesia machine by breathing tubes. The mask must fit the face tightly to minimize escape of gases into room air. Significant leakage occurs around an ill-fitting mask, particularly in the area above the nose. Several sizes should be available.

Laryngeal mask.

An airway can be maintained by inserting a laryngeal mask airway (LMA) into the larynx. This flexible tube has an inflatable silicone ring and cuff. When the cuff is inflated, the mask fills the space around and behind the larynx to form a seal between the tube and the trachea.⁹ All types do not protect against regurgitation and aspiration. Newer types (i.e., ProSeal) have a passage for gastric tube placement which provides better protection against regurgitation. Adult and pediatric styles are available and can be useful when intubation and mask ventilation are complex.⁹

Reusable and disposable styles are commercially available (Fig. 24-9). The mask is selected for use by size of the patient as follows:

• Size 1: 0 to 6.5 kg

• Size 2: 6.5 to 20 kg

• Size 2.5: 20 to 30 kg

• Size 3: 30 to 70 kg

• Size 4: 70 to 80 kg

• Size 5: 80 kg or greater

Endotracheal administration.

Anesthetic vapor or gas is inhaled directly into the trachea through a nasal or oral tube inserted between the vocal cords by direct laryngoscopy. The tube is securely fixed in place to minimize tissue trauma. The patient is given oxygen before and after suctioning of a tracheal tube. Advantages of endotracheal administration are the following:

• It ensures a patent airway and control of respiration. Secretions are easily removed from the trachea by suctioning. Positive pressure can be given immediately by pressing the reservoir bag on the machine without danger of dilating the stomach.

• It protects the lungs from aspiration of blood, vomitus of gastric contents, or foreign material.

• It preserves the airway regardless of the patient’s position during the surgical procedure.

• It interferes minimally with the surgical field during head and neck procedures.

• It helps minimize the escape of vapors or gases into the room atmosphere.

Intubation and extubation can cause tracheal stimulation. The patient may cough, jerk, or develop spasms of the larynx (laryngospasm). Other potential complications of endotracheal administration include the following:

• Trauma to the teeth, pharynx, vocal cords, or trachea: Postoperatively the patient may experience sore throat, hoarseness, laryngitis, and/or tracheitis. Laryngeal edema is more common in children than in adults. Ulceration of the tracheal mucosa or vocal cords may cause granuloma.

• Cardiac dysrhythmias: Cardiac dysrhythmias may occur in light anesthesia or be caused by suctioning through the endotracheal tube.

• Hypoxia and hypoxemia: Hypoxia is a common complication during intubation and extubation. Endotracheal tube suctioning can cause hypoxemia.

• Accidental esophageal or endobronchial intubation: The latter results in ventilation of only one lung.

• Aspiration of gastrointestinal contents: This is a hazard in a patient with a full stomach or a patient who has increased intraabdominal or intracranial pressure. It can also occur in a patient with intestinal obstruction who is extubated before protective reflexes return.

Controlled respiration.

Respirations may be assisted or controlled. Assistance, to improve ventilation, may easily be given by manual pressure on the reservoir (breathing) bag of the anesthesia machine. Assisted respiration implies that the patient’s own respiratory effort initiates the cycle. Controlled respiration may be defined as the completely controlled rate and volume of respirations. The latter is best accomplished by means of a mechanical device that automatically and rhythmically inflates the lungs with intermittent positive pressure, requiring no effort by the patient. Gas moves in and out of the lungs.

The combination of a volume preset ventilator with an assist mechanism maintains the integrity of the respiratory center. Controlled respiration is initiated after the anesthesia provider has produced apnea by hyperventilation or administration of respiratory depressant drugs or a neuromuscular blocker.

Controlled ventilation is used in all types of surgical procedures, especially in lengthy ones. The anesthesia provider’s artificial control of respiration or the patient’s respiratory efforts influence the minute-to-minute level of anesthesia. Advantages of controlled respiration are the following:

• It provides for optimal ventilation.

• It allows for selective lung deflation for thoracic procedures.

• It provides access to deep regions of the thorax and upper abdomen.

• It permits deliberate production of apnea to facilitate surgical manipulation below the diaphragm, ligation of deep vessels, or obtaining radiographic films.

The patient is taken off the respirator gradually near the end of the surgical procedure, and spontaneous respiration resumes. Assisted ventilation may be continued postoperatively after lengthy procedures until reflexes and spontaneous respirations return.

Inhalation anesthetic agents

Inhalation is a controllable method of administration because uptake and elimination of anesthetic agents are accomplished mainly by pulmonary ventilation and selective organ metabolism. The anesthetic vapor of a volatile liquid or an anesthetic gas is inhaled and carried into the bloodstream by passing across the alveolar membrane into the general circulation and on to the tissues.

Ventilation and pulmonary circulation are two critical factors involved in the process. Each can be affected by components of the anesthetic experience, such as a change in body position, preanesthetic medication, alteration in body temperature, or respiratory gas tensions.

In inhalation anesthesia, the aim is to establish balance between the content of the anesthetic vapor or gas inhaled and that of body tissues. The blood and lungs function as the transport system. Anesthesia is produced by the development of an anesthetizing concentration of anesthetic in the brain. The depth of anesthesia is related to concentration and biotransformation (see Table 24-1).

Pulmonary blood-gas exchange is important to tissue perfusion. Defective gas exchange can cause hypoxemia and respiratory failure. It also interferes with delivery of the anesthetic. Potent inhalation agents, such as myocardial depressants, affect oxygenation. Most of them induce a dose-related hypoventilation. The deeper the anesthesia, the more depressed ventilation becomes. Surgical stimulation partially corrects depression, but respiration must be controlled to keep oxygen and carbon dioxide exchange constant to prevent hypoventilation and cardiac depression.

Although the respiratory system is employed for distribution of the anesthetic, it also must carry on its normal function of ventilation (i.e., meeting tissue demands for adequate oxygenation and elimination of carbon dioxide and helping maintain normal acid-base balance). The amount of anesthetic vapor inspired is influenced by the volume and rate of respirations. Gas or vapor concentration and the rate of delivery are also significant.

Pulmonary circulation is the vehicle for oxygen and anesthetic transport to the general circulation. The large absorptive surface of the lungs and their extensive microcirculation provide a large gas-exchanging surface. In optimal gas exchange, all alveoli share inspired gas and cardiac output equally (ventilation-perfusion match). Because respiratory and anesthetic gases interact with pulmonary circulation, alveolar anesthetic concentrations are rapidly reflected in circulating blood.

Alveolar concentration results from a balance between two forces: ventilation that delivers the anesthetic to the alveoli and uptake that removes the anesthetic from the alveoli. Certain factors influence uptake of the anesthetic and thus induction and recovery. Uptake has two phases:

1. Transfer of anesthetic from alveoli to blood: The rate of transfer is determined by the solubility of the agent in the blood, the rate of pulmonary blood flow (related to cardiac output), and the partial pressure of the anesthetic in arterial and mixed venous blood.

2. Transfer of anesthetic from blood to tissues: Factors influencing uptake by individual tissues are similar to those for uptake by blood. They are the solubility of the gas in tissues, the tissue volume relative to the blood flow (flow rate), and the partial pressure of the anesthetic in arterial blood and tissues. Tissues differ; thus uptake of the anesthetic differs. Highly perfused tissues (heart) equilibrate more rapidly with arterial tension than does poorly perfused tissue (fat), which has a slow rise to equilibrium and retains anesthetic longer.

Elimination of the anesthetic is affected by the same factors that affected uptake. As an anesthetic is eliminated, its partial pressure in arterial blood drops first, followed by that in tissues.

The most important factors influencing safe administration of any anesthetic are the knowledge and skill of the anesthesia provider. A perfect agent has not been found, and no agent is entirely safe. Commonly used agents are listed in Table 24-2. Advantages and disadvantages are relative.

TABLE 24-2

Most Commonly Used General Anesthetic Agents

Generic Name	Trade Name	Administration	Characteristics	Uses
INHALATION AGENTS
Nitrous oxide	None	Inhalation	Inorganic nonvolatile gas; slight potency; pleasant, fruitlike odor; nonirritating; nonflammable but supports combustion; poor muscle relaxation	Rapid induction and recovery; short procedures when muscle relaxation unimportant; adjunct to potent agents. Should be mixed with 30% oxygen to prevent hypoxia
Halothane	Fluothane	Inhalation	Halogenated volatile liquid; potent; pleasant odor; nonirritating; cardiovascular and respiratory depressant; incomplete muscle relaxation; potentially toxic to liver	Rapid induction; wide spectrum for maintenance; depth of anesthesia easily altered; rapid reversal Rarely used
Enflurane	Ethrane	Inhalation	Halogenated ether; potent; some muscle relaxation; respiratory depressant	Rapid induction and recovery; wide spectrum for maintenance Rarely used
Desflurane	Suprane	Inhalation	Halogenated liquid with low solubility, desflurane has faster uptake by inhalation and elimination	Not used for induction with children. Can be used for maintenance in adults and children
Sevoflurane	Ultane	Inhalation	Volatile liquid form, nonflammable and nonexplosive; noted for its rapid induction and rapid emergence qualities	Used for adults and children Rapid elimination
Isoflurane	Forane	Inhalation	Halogenated methyl ether; potent; muscle relaxant; profound respiratory depressant; metabolized in liver	Rapid induction and recovery with minimal aftereffects; wide spectrum for maintenance
INTRAVENOUS AGENTS
Thiopental sodium	Pentothal sodium	Intravenous	Barbiturate; potent; short acting with cumulative effect; rapid uptake by circulatory system; no muscle relaxation; respiratory depressant	Rapid induction and recovery; short procedures when muscle relaxation not needed; basal anesthetic
Methohexital	Brevital	Intravenous	Barbiturate; potent; circulatory and respiratory depressant	Rapid induction; brief anesthesia
Propofol	Diprivan	Intravenous	Alkyl phenol; potent short-acting sedative-hypnotic; cardiovascular depressant	Rapid induction and recovery; short procedures alone; prolonged anesthesia in combination with inhalation agents or opioids
Ketamine	Ketaject, Ketalar	Intravenous, intramuscular	Dissociative drug; profound amnesia and analgesia; may cause psychologic problems during emergence	Rapid induction; short procedures when muscle relaxation not needed; children and young adults
Fentanyl	Sublimaze	Intravenous	Opioid; potent narcotic; metabolizes slowly; respiratory depressant	High-dose narcotic anesthesia in combination with oxygen
Sufentanil	Sufenta	Intravenous	Opioid; potent narcotic, respiratory depressant	Premedication; high-dose narcotic anesthesia in combination with oxygen
Fentanyl and droperidol	Innovar	Intravenous	Combination narcotic and tranquilizer; potent; long acting	Neuroleptanalgesia
Diazepam	Valium	Intravenous, intramuscular	Benzodiazepine; tranquilizer; produces amnesia, sedation, and muscle relaxation	Premedication; awake intubation; induction
Midazolam	Versed	Intravenous, intramuscular	Benzodiazepine; sedative; short-acting amnesic; central nervous system and respiratory depressant	Premedication; conscious sedation; induction in children