The use of tracheal cannulation for the administration of anesthetics and provision of a patent airway was first reported in 1858 by John Snow in On Chloroform and Other Anaesthetics,² in which he described a tracheostomy and cannulation for the administration of chloroform in a spontaneously breathing rabbit. The first human use of tracheostomy for anesthesia and protection against aspiration was reported by Trendelenburg in 1869, and it also was accompanied by spontaneous ventilation.³ In the next 10 years, numerous investigators developed nonsurgical techniques and apparatus for cannulation of the trachea for surgical (ear, nose, throat) or medical (diphtheria) indications. Matas was among the first to advocate the use of PPV through a tracheal cannula to avoid the catastrophic consequences of pneumothorax for a spontaneously ventilating patient during thoracotomy.⁵

Endotracheal anesthesia came into its own during and immediately after World War I because of the volume of facial and mandibular injuries treated in England, especially at the hospital in Sidcup. In 1936, I.W. Magill wrote one of many descriptive treatises on intubation in anesthesia ⁵:

The maintenance of a free airway has long been recognized as a first principle in general anesthesia and the danger of complete laryngeal obstruction has always been obvious. On the other hand, the cumulative effects of partial respiratory obstruction have, in the past, been frequently overlooked and it is not improbable that many of the surgical difficulties, postoperative complications, and even fatalities attributed to the anesthetic agent have been primarily due to an imperfect airway. It may be said without exaggeration that in remedying this defect endotracheal anesthesia has proved as great a factor in the advances of anesthesia as the discovery of new drugs or the development of improved apparatus.

Immediately thereafter, Magill inserts a caveat:

… owing to the ease of control it affords, there is a tendency towards its employment in every operation, regardless of other considerations. This tendency is to be deprecated, especially in the teaching of students. The novice should learn airway control by simple methods in the first instance, for he may be called to administer an anesthetic in circumstances in which artificial devices are not available. Moreover, as the method involves instrumentation, which is not devoid of the risk of trauma, even though it may be slight, intubation should only be attempted when the necessity for it has been considered carefully.

The historical lesson here is that, no matter how routine endotracheal intubation becomes, it is still an invasive procedure with nontrivial risks and significant complications. It should be used for specific indications and only after careful consideration of the balance of risks to and benefits for the patient.

II Laryngoscopic Orotracheal Intubation

The conventional orotracheal route is the simplest and most direct approach to tracheal cannulation. Done under direct laryngoscopic vision, this technique is the easiest and most straightforward for the purposes of administering general anesthesia, ventilation of critically ill patients, and cardiopulmonary resuscitation. The vocal cords are visualized with the aid of a handheld laryngoscope, and the endotracheal tube (ETT) is introduced and positioned in the trachea under continuous direct observation. After confirmation of correct placement, the tube is secured in place and ventilation assisted or controlled as indicated.

A Preparation and Positioning

Box 17-1 lists the basic materials required for conventional orotracheal intubation. The materials are grouped according to the temporal sequence of events. All items are required for routine intubation, dealing with common difficulties, or preventing complications. Redundancy is the key in preparing for a critical event, such as endotracheal intubation. All essential equipment (e.g., laryngoscope handles, ETTs) should have readily available back-up counterparts in case of unexpected failure. An assortment of laryngoscope blades, both straight (Miller) and curved (Macintosh), should be available.

Box 17-1 Basic Equipment for Endotracheal Intubation

Preoxygenation and Ventilation

1. Oxygen (O₂) source

2. Ventilation bag or anesthesia circuit (for positive-pressure ventilation)

3. Appropriately sized face mask

4. Appropriately sized oropharyngeal and nasopharyngeal airways

5. Tongue blade

Endotracheal Tubes

6. Appropriately sized endotracheal tubes (at least twp)

7. Malleable stylet

8. Syringe for tube cuff, 10 mL

9. Jelly and/or ointment, 4% lidocaine (Xylocaine)

Drugs

10. Intravenous anesthetics and muscle relaxants (ready to administer)

11. Reliable, free-flowing intravenous infusion (some pediatric exceptions)

12. Topical anesthetics and vasoconstrictors (for nasotracheal intubation)

Laryngoscopy

13. Working suction apparatus with tonsil tip

14. Assortment of Miller blades with functioning battery handle

15. Assortment of Macintosh blades with functioning battery handle

16. Bolsters (folded sheets, towels) for positioning of head and shoulders

Fixation of the Endotracheal Tube

17. Tincture of benzoin

18. Appropriate tape or tie

19. Stethoscope

20. End-tidal carbon monoxide (ETCO₂) monitor

21. Pulse oximeter

The proper sequence of events before laryngoscopy should be followed:

1. Adequate access to the head of the bed or table is essential. Removal of side rails and headboard (if outside the operating room) ensures freedom of movement; confirming that the bed or table is locked in position prevents unnecessary and stress-inducing pursuit of the patient around the room. The height of the surface should be adjusted to the level of the laryngoscopist’s chest. An experienced aide should be in constant attendance to provide items such as suction lines, airways, tubes, and drugs to the primary laryngoscopist, as well as to apply optimal external laryngeal manipulation (OELM), as needed.

2. The patient must be properly positioned before laryngoscopy. Patients who are uncooperative, agitated, or otherwise mobile may require rapid and efficient positioning after sedation. Pads or rolls should be prepared in advance and be readily at hand.

The earliest attempts at laryngoscopy used the classic positioning of full extension. Described by Jackson in 1913, this position required full extension of the head and neck on a flat surface.⁶ After 20 years, he amended his view to one that supported the contemporary sniffing position of flexion at the neck and extension at the head.⁷ This was accomplished by supporting the head on a pillow that was at least 10 cm thick. Numerous investigators have examined radiographs of subjects to determine the optimal positioning for orotracheal access. Various theoretical models of positioning for intubation have been proposed. For the past 60 years, the three-axis theory has proposed that the oral, pharyngeal, and laryngeal axes should be brought into approximate alignment to best facilitate orotracheal visualization and intubation (Fig. 17-1). Proposed by Bannister and MacBeth in 1944, this model presumes that laryngoscopy is done in the midline (two-dimensional model) and that laryngeal axis alignment is necessary for proper intubation.⁸ This idea has been challenged by the work of Adnet and colleagues in imaging studies and clinical comparisons.⁹^–¹¹

Figure 17-1 Schematic diagrams show the alignment of the oral axis (OA), pharyngeal axis (PA), and laryngeal axis (LA) in four different head positions. Each head position is accompanied by an inset that magnifies the upper airway (oral cavity, pharynx, and larynx) and superimposes (bent bold line) the continuity of these three axes within the upper airway. A, The head is in the neutral position with a marked degree of nonalignment of the LA, PA, and OA. B, The head is resting on a large pad that flexes the neck on the chest and aligns the LA with the PA. C, The head is resting on a pad (which flexes the neck on the chest). Concomitant extension of the head on the neck brings all three axes into alignment (sniffing position). D, Extension of the head on the neck without concomitant elevation of the head on a pad, which results in nonalignment of the PA and LA with the OA.

(From Benumof JL, editor: Airway management: principles and practice, St. Louis, 1996, Mosby, p 263.)

Adnet’s conclusion, however, has been questioned at length.¹² Greenland and colleagues reexamined the issue, finding “the sniffing position the most favorable for direct laryngoscopy” as determined by magnetic resonance imaging (MRI).¹³ This perspective has been corroborated by evidence indicating 9 cm as the optimal pillow height.¹⁴ Others have advocated for an extension-extension position, in which the head and neck are extended by lowering the head of the table 30 degrees, proposing that direct laryngoscopy requires less axial force in this position than in the sniffing position.¹⁵ Whether the lower cervical spine is flexed, extended, or neutral, the extension of the atlanto-occipital joint remains the critical factor for optimal positioning. The reasonable option in view of conflicting evidence (and patients’ variability) is to position the patient with the occiput on a pad (traditional sniffing position) and be prepared to remove the pad (convert to simple extension) if the initial laryngoscopy becomes inadequate (Fig. 17-2).

Figure 17-2 A, In some obese patients, placing the head on a pillow does not result in the sniffing position; in the obese patient shown and as illustrated by the overlying bold black line, the oral and laryngeal axes are perpendicular to one another, the neck is not flexed on the chest, and the head is not extended on the neck at the atlanto-occipital joint. B, In the same patient, placing support (e.g., blankets, towels) under the scapula, shoulders, nape of the neck, and head results in a much better sniffing position; the oral, pharyngeal, and laryngeal axes form only a slightly bent curve, the neck is flexed on the chest, and the head is extended on the neck at the atlanto-occipital joint.

(From Benumof JL, editor: Airway management: Principles and practice, St. Louis, 1996, Mosby, p 264.)

Obese patients often require more extensive padding (planking) starting at the midpoint of the back to the head to assume an optimal position for laryngoscopy. Occasionally, it is necessary to place towels and blankets under the scapula, shoulders, nape of the neck, and head to flex the neck on the chest (see Figs. 17-1B and 17-2) and extend the head on the neck (see Figs. 17-1C and 17-2). In this instance, the purpose of the scapula, shoulder, and neck support is to give the head room so that it may be extended on the neck. When in doubt, the final assessment of the position should be from a lateral view of the patient, because only a lateral view enables precise assessment of the chest, neck, face, and head axes (see Figs. 17-1C and 17-2).

B Preoxygenation

Instrumentation of the airway may be done on an awake, spontaneously breathing patient. This sometimes is the safest and most prudent approach, but most commonly, laryngoscopy and intubation are performed on an anesthetized and (usually) apneic patient. Because this requires some finite period of time, the patient is at risk for arterial desaturation and hypoxic injury. Preoxygenation is done before laryngoscopy to minimize this risk.

Administration of 100% oxygen (O₂) through a tight-fitting face mask may occur by means of the patient’s spontaneous respirations or by PPV by a bag-mask unit. In either case, adequate ventilation must occur to “wash out” alveolar nitrogen (N₂) and fill the lungs with O₂. The goal is to fill the alveoli used in normal tidal breathing and the remaining alveoli and airways constituting the functional residual capacity (FRC). This additional O₂ serves as a reservoir to delay the onset of arterial hypoxia for as long as 5 minutes. A number of guidelines have been proposed to accomplish this potentially lifesaving goal.

Depending on the minute ventilation of the patient (spontaneous or assisted), the time to complete effective preoxygenation varies from 1 to 5 minutes; although in an awake and cooperative patient, this may be mostly accomplished with three or four full, vital capacity (VC) breaths.^16,¹⁷ Work has documented the increased efficacy of eight full breaths in about 60 seconds, with times to desaturation approaching those of the more traditional 3- to 5-minute preoxygenation.¹⁸ A higher minute ventilation level leads to more rapid and complete preoxygenation. Measures of the adequacy of preoxygenation include real-time gas analysis of expired O₂ concentration (goal = 95%) and analysis of expired N₂ (goal < 5%). Essential to either of these measurements is the presence of a capnograph waveform with a plateau reflecting the expected alveolar carbon dioxide (CO₂) concentration. This documents the presence of an effective seal of the circuit-bag system to the patient’s airway and the effective delivery of 100% O₂. The use of an air-mask-bag unit (AMBU) without an expiratory valve may not provide optimal preoxygenation.¹⁹

The effectiveness of preoxygenation in preventing hypoxia during laryngoscopy is significantly reduced in the morbidly obese patient. Even with the most careful preoxygenation, the duration of apnea before the onset of hypoxia is one half of the duration seen in patients with normal body weight. This situation is attributed to the considerable reduction in FRC and VC in the obese patient and to the additional reduction attributable to the cephalad diaphragmatic shift related to supine positioning.²⁰ This places morbidly obese patients at significantly increased risk for injury if any difficulty with ventilation or intubation is encountered.

Pharyngeal insufflation of O₂ can significantly prolong the safe duration of apnea. In a typical adult, approximately 250 mL/min of O₂ is transferred from the lungs into the bloodstream, while only 200 mL/min of CO₂ enters the lungs from the bloodstream (respiratory quotient = 0.8). This alveolar gas deficit causes alveolar pressures to become slightly subatmospheric. If the airway is patent, there is a net flow of gas from the pharynx into the alveoli (apneic oxygenation). If, after adequate preoxygenation, the pharynx is filled with O₂, the onset of hypoxia is delayed because O₂, rather than air, is drawn into the lungs by this mechanism. Pharyngeal insufflation may be conveniently achieved by passing a catheter into the pharynx through a nasopharyngeal airway and attaching an O₂ source at 2 to 3 L/min. Alternatively, some laryngoscopes have a side port suitable for attachment of O₂ tubing. When preoxygenation is followed by pharyngeal insufflation as previously described, in normal but apneic patients, the O₂ saturation from pulse oximetry [SpO₂] remains equal to 98% for 10 or more minutes (although at the end of 10 minutes the arterial carbon dioxide tension [PaCO₂] may be expected to be about 80 mm Hg).²¹

C Laryngoscopy

The purpose of direct laryngoscopy is to provide adequate visualization of the glottis to allow correct placement of the ETT with the minimum effort, elapsed time, and potential for injury to the patient. Considerable effort has been expended to develop laryngoscopic techniques and equipment to facilitate this important procedure. Although this textbook outlines numerous techniques for tracheal cannulation, the facilitative use of direct laryngoscopy is by far the most common technique.

Two basic types of laryngoscope blades are the curved blade (Macintosh) and the straight blade with a curved tip (Miller).^22,²³ They are designed for right hand–dominant use; the laryngoscope is held in the left hand while the right hand manipulates the ETT. Historically, either hand (or both) could be initially used, shifting the laryngoscope to the left hand while the right hand manipulated the tube. Both blade styles include a flange on the left side of the blade for lateral retraction of the tongue and contain a light-emitting area (bulb or fiberoptic tip). Each blade has a channel with an open right side for visualization of the larynx and for insertion of the ETT (Figs. 17-3 to 17-10). Despite numerous modifications and variations, they are all lighted, handheld retractors for oropharyngeal soft tissues.