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.
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 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.
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.
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).
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.
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.
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.
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.
(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.
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.
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.
(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.)
(From McSwain N: The basic EMT: Comprehensive prehospital care, ed 2, St Louis, 2003, Mosby.)
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.
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.
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 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:
(Courtesy Drs. H. William Gottschalk and Kenneth Lee.)
Figure 31-16 Esophageal stethoscope aids in monitoring both heart and lung sounds. A, Distal end of esophageal stethoscope has multiple perforations that aid in picking up sounds in thorax. B, Esophageal stethoscope is inserted into esophagus to the level of the heart, thereby maximizing sound amplification.
(Courtesy Criticare Systems, Inc.)
An invasive procedure, CVP monitoring is recommended in older patients, in patients in whom considerable blood or fluid loss is expected, during major traumatic surgery, in cases in which multiple transfusions are given, and during open heart surgery, among other indications. For the typical American Society of Anesthesiologists ASA 1 or 2 ambulatory dental patient undergoing general anesthesia, routine use of CVP is not necessary.
Complications occur during the administration of general anesthesia. Among the more frequently observed complications are hypotension and cardiac dysrhythmias. Monitoring of the anesthetized patient enables the entire anesthesia team to be aware of the presence of these and other potentially lethal problems and to initiate appropriate corrective treatment. The anesthesiologist will have available a supply of emergency drugs and equipment for use in these circumstances. The emergency drugs required by the board of dental examiners in the state of California4 for dentists using general anesthesia are listed here. Suggested emergency drugs and equipment from the American Association of Oral and Maxillofacial Surgeons may be found in Box 33-2.5 A more thorough discussion of emergency drugs recommended for outpatient facilities is presented in Chapter 33.
Several of the drugs recommended for the emergency tray are also commonly used during the routine administration of general anesthesia. These include succinylcholine, IV replacement fluid, and opioid antagonists.
An array of drugs may be used during the administration of general anesthesia. Many of these drugs have been discussed in other sections of this book and are listed here with minimal discussion. Other drugs make their first appearance at this time; however, they too will receive only a brief review because it is not the goal of this textbook to provide the reader with an in-depth knowledge of general anesthesia.