chapter 31 Armamentarium, Drugs, and Techniques

This chapter presents an overview of the equipment, drugs, and techniques that are important to the success of general anesthesia. Many of the drugs discussed have been reviewed in depth elsewhere in this book and receive only brief mention here. Other drugs are mentioned for the first time; however, their pharmacology is also reviewed briefly because it is not the purpose of this chapter to provide the reader with a belief that he or she is able to use these drugs safely after having read about them. As discussed in Chapter 30, this section is meant as an introduction to the vast subject of general anesthesia, not as a complete text in that area.

Following a review of the armamentarium and drugs, each of the major techniques of general anesthesia is discussed.

ARMAMENTARIUM

The equipment required for the administration of general anesthesia may vary according to the type of anesthesia delivered. In general, the equipment for TIVA (total intravenous anesthesia) anesthesia varies somewhat from that required for other types of anesthesia.

The armamentarium for general anesthesia may be divided into the following five groups:

1. Anesthesia machine

2. IV equipment

3. Ancillary anesthesia equipment

4. Monitoring equipment

5. Emergency equipment and drugs

Anesthesia Machine

The anesthesia machine is able to deliver oxygen (O₂) and inhalation anesthetics to the patient. The inhalation sedation unit used in dentistry to deliver nitrous oxide-oxygen (N₂O-O₂) is a modification of the anesthesia machine used in the operating room. The primary difference between the two is the number of inhalation anesthetics that the operating room unit is capable of delivering. As seen in Figure 31-1, the anesthesia machine can deliver many gases: N₂O, O₂, sevoflurane, desflurane, and isoflurane. Flowmeters and devices called vaporizers that contain the various volatile anesthetics and permit their concentrations to be controlled are integral parts of the anesthesia machine.

Figure 31-1 A, A general anesthesia machine can deliver a (B) variety of volatile anesthetics.

The anesthesia machine is capable of operating with O₂ and N₂O supplied from either a central cylinder system or portable cylinders mounted on the sides of the unit. In the operating room, some of the fail-safe devices used in dental inhalation units may not be present. However, most anesthesia machines have O₂ monitors that sound an alarm if the unit fails to provide a preset minimum percentage of O₂ (i.e., 25%).

During TIVA general anesthesia, an inhalation sedation unit, as discussed in Chapter 14, is used to supplement the patient’s ventilation with O₂ and perhaps N₂O. In the other forms of anesthesia, a unit similar to that shown in Figure 31-1 is used.

The modern anesthesia machine also contains a number of important devices for monitoring patients receiving these agents. Attached to the anesthesia machine shown in Figure 31-1 are a number of monitors, including a blood pressure monitor, electrocardiograph (ECG), pulse oximeter, capnograph (end-tidal carbon dioxide [ETCO₂] monitor), and a bispectral (BIS) electroencephalograph (EEG) monitor. Attached to the right side of the unit is a ventilator, a device used to control or assist the ventilation of a patient during anesthesia.

Intravenous Equipment

A supply of equipment for venipuncture and IV drug administration is required during general anesthesia. A continuous IV infusion is commonly used for all general anesthetic procedures. Winged needles are rarely used during general anesthesia. Indwelling catheters are much preferred. The gauge of the venipuncture needle or indwelling catheter should not be smaller than 21 (for short procedures), with the 18-gauge needle most commonly used for routine general anesthetic procedures and 16-gauge needles used when blood transfusion may be required. An assortment of needles should be available.

Tubing and bags of IV solution are also required. During short procedures a 250-ml bag may be adequate; however, a 1000-ml size is usually recommended for all procedures lasting more than 30 minutes because of the possibility of the patient becoming hypovolemic as a result of a combination of having been NPO (nothing by mouth) before surgery, blood loss during surgery, and evaporation of fluids during surgery (especially where the abdominal or thoracic cavity is exposed). A variety of disposable syringes and needles should also be available in addition to various adhesive tapes (e.g., paper, hypoallergenic).

Ancillary Anesthesia Equipment

The following items must also be available whenever general anesthesia is administered:

• Full-face masks in child and adult sizes and appropriate connectors

• Laryngoscope, complete with adequate selection of blades and spare batteries and bulb

• Adequate selection of endotracheal tubes and appropriate connectors

• Laryngeal mask airway (LMA)

• Adequate selection of oropharyngeal and nasopharyngeal airways

• Tonsillar suction tips

• Magill intubation forceps

• Child- and adult-size sphygmomanometers and stethoscopes

Face Masks

Face masks (Figure 31-2) are rubber or silicone masks that cover both the mouth and nose of the patient. Face masks are used to deliver O₂, N₂O-O₂, and/or other inhalation anesthetics before, during, and after the anesthetic procedure. Because of the variations in the size and shape of faces, several different sizes of full-face masks should always be available.

Figure 31-2 Full-face mask covers both the mouth and nose of the patient.

Face masks are made from a clear plastic or rubber that allows the patient’s mouth and nose to be seen so that foreign material (e.g., vomitus, blood) may be observed and removed.

Metal connectors that attach the face mask to the tubing of the anesthesia machine are required. These connectors come in a variety of sizes and shapes.

Laryngoscopes

The laryngoscope (Figure 31-3) is a device designed to assist in the visualization of the trachea during intubation. It consists of two parts: a handle and battery holder and a blade. The handle is usually made of metal (although some are made of plastic) and contains batteries that are used to operate the light bulb found in the blade.

Figure 31-3 Laryngoscope handle and several sizes of curved blades.

The blade of the laryngoscope is also usually made of metal, although plastic is also used today. The laryngoscope blade is designed to be placed into the patient’s mouth to aid in visualization of the larynx. A small light bulb that illuminates the laryngeal area is attached to the blade. There are two basic types of laryngoscope blades: the curved (Macintosh) and the straight (Miller) blade. Each of these blades is available in a variety of sizes. The technique for using these blades differs.

The Macintosh blade is more commonly used. The tip of the curved blade is inserted into the vallecula, the cul-de-sac between the base of the tongue and the epiglottis (Figure 31-4). The handle of the laryngoscope is then lifted straight up and slightly forward, a movement that visualizes the vocal cords. When a straight blade is used, its tip is placed underneath the laryngeal surface of the epiglottis (Figure 31-5), and the larynx is exposed by an upward and forward lift of the blade.

Figure 31-4 Curved blade is placed between the base of the tongue and the epiglottis. Laryngoscope is then lifted, elevating tongue and exposing larynx.

Figure 31-5 Straight laryngoscope blade is placed beneath epiglottis and lifted, thereby exposing larynx.

Most laryngoscopes and blades are designed to be held in the operator’s left hand, with the endotracheal tube held in the right. Special laryngoscope blades are available for left-handed operators.

Endotracheal Tubes and Connectors

Endotracheal tubes and connectors (Figure 31-6) are rubber tubes designed to be placed from the mouth (oroendotracheal) or nose (nasoendotracheal) into the patient’s trachea. Reusable and disposable endotracheal tubes are available, with disposables more popular today. Because the diameter of the laryngeal opening and the trachea varies from patient to patient, endotracheal tubes are manufactured in a variety of diameters. Endotracheal tubes are commonly referred to by their size (e.g., a No. 38 tube has an external diameter of 38 mm). For adult patients, a No. 36 tube is usually appropriate for an adult male and a No. 34 tube for an adult woman. Smaller and larger tubes are available to accommodate the child or larger patient.

Figure 31-6 A, Laryngoscope and endotracheal tube. Distal end of tube has inflatable cuff (arrow) designed to provide an airtight seal, preventing entry of substances into the trachea.

(B From Malamed SF: Medical emergencies in the dental office, ed 6, St Louis, 2007, Mosby.)

Endotracheal tubes normally have an inflatable cuff (see Figure 31-6, A) located near their distal ends. When a patient is intubated, the endotracheal tube is inserted into the trachea so that the uninflated cuff disappears just beyond the level of the larynx. Air is then injected into a tube that connects with the cuff to inflate it. Enough air is injected into the cuff to seal the trachea off from the pharynx, thereby preventing foreign material, such as blood, saliva, or vomitus, from entering the trachea and bronchi.

Connectors for endotracheal tubes are the same as those used for full-face masks. They are used to connect the endotracheal tube to the anesthesia machine.

Laryngeal Mask Airway (LMA)

Introduced in 1988, the laryngeal mask airway is a supraglottic device designed to provide and maintain airway integrity and to permit either spontaneous, assisted, or controlled ventilation in clinical situations in which intubation is either not possible or not practicable. In cases of general anesthesia involving dentistry, the LMA is used as an alternative to orotracheal or nasotracheal intubation. Orotracheal intubation is not indicated for most dental procedures because the tube will compete for space in the oral cavity with both the dental team and dental instruments used. Nasotracheal intubation offered the dental team greater access to the surgical site, but the presence of a tube within the trachea is a potent stimulus to the patient requiring a more profound level of CNS depression for the tube to be tolerated. The flexible LMA is commonly used in dental procedures because it is wire reinforced, permitting it to be repositioned from side to side during surgical procedures without loss of seal of the cuff against the larynx. Additionally the wire-reinforced flexible LMA resists kinking when it is flexed or compressed against a rigid mouth prop. However, the reinforced airway tube does not offer resistance to occlusion by biting.

LMAs are available in seven sizes (neonate; infant; young children; older children; and small, normal, and large adult) (Figure 31-7). The LMA is inserted with the tip of the cuff pressed upward against the hard palate by the index finger while the middle finger opens the mouth (Figure 31-8, A). The LMA is pressed backward in a smooth movement (using the opposite hand to extend the airway) (Figure 31-8, B). The LMA is advanced until definite resistance is felt (Figure 31-8, C), and before the index finger is removed, the opposite hand presses down on the LMA to prevent dislodgment during removal of the index finger (Figure 31-8, D). Following removal of the finger, the cuff is inflated with air.

Figure 31-7 LMA (various sizes).

(Courtesy LMA North America, Inc., San Diego, CA.)

Figure 31-8 A, LMA is inserted by pressing tip of cuff upward against hard palate. B, LMA is advanced backward in smooth movement until definite resistance is felt. C, Seated LMA. D, LMA cuff is inflated with air to prevent dislodgement.

(Courtesy Drs. H. William Gottschalk and Kenneth Lee.)

Oropharyngeal and Nasopharyngeal Airways

Oropharyngeal (Figure 31-10) and nasopharyngeal airways (Figure 31-11) are used to assist in maintaining a patent airway during and after the anesthetic procedure. Oropharyngeal airways are plastic, rubber, or metallic devices designed to lie between the base of the tongue and the posterior pharyngeal wall (Figure 31-12). The nasopharyngeal airway (also known as a nasal trumpet) is a thin, flexible rubber tube designed to be inserted through the nares and to rest between the base of the tongue and posterior pharynx (Figure 31-13). The purpose of both of these devices is to displace the tongue from the pharynx and thereby permit the patient to exchange air either around or through the airway. The nasopharyngeal airway is better tolerated by the conscious or sedated patient, thereby minimizing the occurrence of gagging and vomiting. Nasal airways should be lubricated before their insertion to rase their placement.

Figure 31-10 Oropharyngeal airways are available in a variety of sizes.

(Courtesy Sedation Resource, One Oak, Tex.)

Figure 31-11 Nasopharyngeal airways.

(From McSwain N: The basic EMT: Comprehensive prehospital care, ed 2, St Louis, 2003, Mosby.)

Figure 31-12 Oropharyngeal airway is designed to lift the tongue off the posterior wall of the pharynx.

(From McSwain N: The basic EMT: Comprehensive prehospital care, ed 2, St Louis, 2003, Mosby.)

Figure 31-13 Nasopharyngeal airway is designed to rest between the base of the tongue and pharyngeal wall, thus permitting air to pass between the lungs and the nose.

(From McSwain N: The basic EMT: Comprehensive prehospital care, ed 2, St Louis, 2003, Mosby.)

Tonsillar Suction Tips

The immediate availability of suction devices is absolutely essential before general anesthesia is started. Excessive salivation, bleeding in the mouth or pharynx, or vomiting can produce airway obstruction, laryngospasm, or possible infection of the trachea or bronchi. Tonsillar suction tips are recommended because they can be inserted blindly into the posterior pharynx of the patient with minimal risk of producing bleeding. The end of the tonsillar suction tip is rounded, making this device preferable to others that have sharper tips. Several tonsillar suction tips should be available in the event that one becomes clogged.

Magill Intubation Forceps

A Magill intubation forceps (Figure 31-14) is designed to assist in placing the endotracheal tube. It is most frequently used during nasoendotracheal intubation and is therefore a very important item in the armamentarium for general anesthesia for dental procedures.

Figure 31-14 Magill intubation forceps are designed to assist in passage of an endotracheal tube, especially during nasal intubation.

Sphygmomanometers and Stethoscopes

Sphygmomanometers and stethoscopes must also be available during general anesthetic procedures. They will be used for the monitoring of vital signs, specifically blood pressure, heart rate and rhythm, heart sounds, and breath sounds. Appropriate-size sphygmomanometers must be available if accurate blood pressure values are desired.

Monitoring Equipment

Monitoring of the patient during sedation or general anesthesia is essential to the overall safety of the procedure. During sedation procedures, monitoring of the central nervous system (CNS) via direct communication with the patient is of primary importance. Because the patient is able to respond appropriately to verbal command, other, more complex monitoring devices need not be used routinely.¹ However, once consciousness is lost (increased CNS depression), patients are unable to respond to command, and other means of determining their status during anesthesia must be used. For this reason, the level of monitoring during general anesthesia is greater than that required for sedative procedures. A monitor is a device that reminds and warns. The Department of Anesthesiology at the Harvard University School of Medicine has designed monitoring guidelines for use during general anesthesia.² The recommendations in these guidelines have been well received and widely implemented. The following are some of the methods and devices used to monitor patients during general anesthesia:

1. The stethoscope is used with auscultation to monitor the heart rate, heart rhythm, and/or breath sounds. Taped to the chest in the precordial region, the precordial stethoscope (see Figure 31-9) provides continuous monitoring of heart sounds, but when placed on the neck directly over the trachea, the pretracheal stethoscope (Figure 31-15) permits monitoring of respiration. The pretracheal stethoscope is recommended for use during IV sedation procedures and during all forms of general anesthesia. An alternative to the precordial stethoscope during general anesthesia is the esophageal stethoscope (Figure 31-16), a rubber tube inserted into the patient’s esophagus after intubation. This device provides continuous monitoring similar to that provided by the precordial stethoscope, but is more effective because of its closer proximity to the heart and lungs. Breath and heart sounds can usually be heard more distinctly. Esophageal stethoscopes are not used during sedation procedures and brief outpatient general anesthetics.

2. The pulse oximeter (Figures 31-17 and 31-18) provides a noninvasive means of monitoring the degree of O₂ saturation of hemoglobin in peripheral blood vessels. Pulse oximeters provide continuous monitoring of oxygenation and of the heart rate, permitting a more rapid detection of potential airway problems (there is a time lag of about 20 seconds). The use of pulse oximetry is considered to be a standard of care during general anesthesia.

3. End-tidal carbon dioxide (ETCO₂) monitors also represent standard of care in monitoring during general anesthesia. They evaluate the effectiveness of ventilation. Because the ETCO₂ monitor evaluates every breath, airway problems may be detected almost instantaneously (a lag time of several seconds exists), permitting correction before they become significant.

4. The sphygmomanometer, or blood pressure cuff, is used to monitor blood pressure by indirect determination. During general anesthesia, blood pressure, heart rate and rhythm, and respiratory rate are monitored continuously and recorded every 5 minutes. If events warrant, more frequent determinations are obtained.

5. The electrocardiograph (ECG) provides a means of monitoring the electrical activity and rhythm of the heart. The ECG permits continuous observation of the rate and rhythm of the heart during general anesthesia. Use of the ECG is considered a standard of care during all forms of general anesthesia.

6. Continuous temperature monitoring by rectal or esophageal probe has become increasingly common since the 1980s with the recognition of malignant hyperthermia. Although not used for all patients undergoing general anesthesia, temperature monitoring is considered a standard of care in children, young adults, patients with fever, and patients undergoing procedures involving induced hypothermia.²

7. Bispectral electroencephalographic monitoring (BIS monitoring). BIS monitoring is described more completely in Chapter 5. The bispectral (BIS) index is a continuous EEG parameter that ranges from an awake, no-drug effect value of 95-100 to zero with no detectable EEG activity.³ The BIS index appears to be a valuable adjunct during the administration of general anesthesia. It primarily measures the effects of hypnotics on the EEG. It is most accurate when used with anesthetic techniques consisting of a low or moderate dose of opioid analgesic and a hypnotic drug (volatile inhaled anesthetic, IV anesthetic) titrated to the BIS response. Low opioid doses enable the BIS index to accurately reflect the pharmacodynamics of the hypnotic drugs on the CNS. BIS monitoring provides an important new dimension to the ability to adjust the components of a general anesthetic in a logical manner.

8. Although not used routinely, direct measurement of arterial blood pressure is frequently of value in the critically ill patient and during cardiopulmonary bypass, major traumatic surgery, and hypotensive or hypothermic anesthesia. Its major advantage over indirect blood pressure methods is that it provides accurate values of intraarterial or intracardiac blood pressure on a continuous basis.

9. Collection and measurement of urine output are easily obtained in the anesthetized patient whose bladder has been catheterized. Urine output is a simple method of determining the degree of hydration of the body. During general anesthesia, the patient should produce urine at a rate approaching the normal rate of 40 to 60 ml/hr. Volumes below this may signify dehydration and indicate the need for additional fluid replacement. For routine general anesthetic procedures, the monitoring of urine output is not required.

10. Central venous pressure (CVP) measures the pressure exerted by blood returning to the right side of the heart and the ability of the right heart to manage it effectively. Monitoring CVP enables the doctor to distinguish between hemorrhage and congestive heart failure. With extensive blood loss, the CVP will fall, whereas in congestive heart failure or overhydration, CVP is elevated.