Critical care staff members require an understanding of structure and function in order to successfully manage the airway and the conditions that may affect it. The relevant information can be gained from a variety of sources.1–5 The airway begins at the nose and oral cavity and continues as the pharynx and larynx, which lead to the trachea (beginning at the lower edge of the cricoid cartilage) and then the bronchial tree. The airway¹ provides a pathway for airflow between the atmosphere and the lungs;² facilitates filtering, humidification, and heating of ambient air before it reaches the lower airway;³ prevents nongaseous material from entering the lower airway;⁶ and allows phonation by controlling the flow of air through the larynx and oropharynx.⁴

The larynx prevents food and other foreign bodies from entering the trachea. Reflex closure of the glottic inlet occurs during swallowing ⁶ and periods of increased intrathoracic (e.g., coughing, sneezing) or intra-abdominal (e.g., vomiting, micturition) pressure. In unconscious patients, these reflexes are lost, so glottic closure may not occur, increasing the risk of pulmonary aspiration.

Assessing Adequacy of the Airway

Adequacy of the airway should be considered in four aspects:

• Patency. Partial or complete obstruction will compromise ventilation of the lungs and likewise gas exchange.

• Protective reflexes. These reflexes help maintain patency and prevent aspiration of material into the lower airways.

• Inspired oxygen concentration. Gas entering the pulmonary alveoli must have an appropriate oxygen concentration.

• Respiratory drive. A patent, secure airway is of little benefit without the movement of gas between the atmosphere and the pulmonary alveoli effected through the processes of inspiration and expiration.

Patency

Airway obstruction most frequently is due to reduced muscle tone, allowing the tongue to fall backward against the postpharyngeal wall, thereby blocking the airway. Loss of patency by this mechanism often occurs in an obtunded or anesthetized patient lying supine. Other causes include the presence of blood, mucus, vomitus, or a foreign body in the lumen of the airway or edema, inflammation, swelling, or enlargement of the tissues lining or adjacent to the airway.

Upper airway obstruction has a characteristic presentation in the spontaneously breathing patient: noisy inspiration (stridor), poor expired airflow, intercostal retraction, increased respiratory distress, and paradoxical rocking movements of the thorax and abdomen.⁹ These resolve quickly if the obstruction is removed. In total airway obstruction, sounds of breathing are absent entirely, owing to complete lack of airflow through the larynx. Airway obstruction may occur in patients with an endotracheal tube (ET) or tracheostomy tube in situ due to mucous plugging or kinking of the tube or the patient’s biting down on a tube placed orally. If such patients are spontaneously breathing, they will have the same symptoms and signs just described. Patients on assisted (positive-pressure) breathing modes will have high inflation pressures, decreased tidal and minute volumes, increased end-tidal carbon dioxide levels, and decreased arterial oxygen saturation.

Protective Reflexes

The upper airway shares a common pathway with the upper gastrointestinal tract.⁶ Protective reflexes, which exist to safeguard airway patency and to prevent foreign material from entering the lower respiratory tract, involve the epiglottis, the vocal cords, and the sensory supply to the pharynx and larynx.¹⁰ Patients who can swallow normally have intact airway reflexes, and normal speech makes absence of such reflexes unlikely. Patients with a decreased level of consciousness (LOC) should be assumed to have inadequate protective reflexes.

Inspired Oxygen Concentration

Oxygen demand is elevated by the increased work of breathing associated with respiratory distress ¹¹ and by the increased metabolic demands in critically ill or injured patients. Often, higher inspired oxygen concentrations are required to satisfy tissue oxygen demand and to prevent critical desaturations during maneuvers for managing the airway. A cuffed ET, connected to a supply of oxygen, is a sealed system in which the delivered oxygen concentration also is the inspired concentration. A patient wearing a facemask, however, inspires gas from the mask and surrounding ambient air. Because the patient will generate an initial inspiratory flow in the region of 30 to 60 L per minute, and the fresh gas flow to a mask is on the order of 5 to 15 L per minute, much of the tidal inspiration will be “room air” entrained from around the mask. The entrained room air is likely to dilute the concentration of oxygen inspired to less than 50%, even when 100% oxygen is delivered to the mask.¹² This unwelcome reduction in inspired oxygen concentration can be mitigated by (1) using a mask with a reservoir bag, (2) ensuring that the mask is fitted firmly to the patient’s face, (3) using a high rate of oxygen flow to the mask (15 L per minute), and (4) supplying a higher oxygen concentration if not already using 100%.

Respiratory Drive

A patent, protected airway will not produce adequate oxygenation or excretion of carbon dioxide without adequate respiratory drive. Changing arterial carbon dioxide tension (PCO₂), by changing H⁺ concentration in cerebrospinal fluid (CSF), stimulates the respiratory center, which in turn controls minute volume and therefore arterial PCO₂ (negative feedback).11,13 This assumes that increased respiratory drive can produce an increase in minute ventilation (increased respiratory rate or tidal volume, or both, per breath), which may not occur if respiratory mechanics are disturbed. Brain injury and drugs such as opioids, sedatives, and alcohol are direct-acting respiratory center depressants.

Ventilation can be assessed qualitatively by looking, listening, and feeling. In a spontaneously breathing patient, listening to (and feeling) air movement while looking at the extent, nature, and frequency of thoracic movement will give an impression of ventilation. These parameters may be misleading, however. Objective assessment of minute ventilation requires PCO₂ measurement in arterial blood or monitoring of end-tidal carbon dioxide, which can be used as a real-time measure of the adequacy of minute ventilation.¹³ If respiratory drive or minute ventilation is inadequate, positive-pressure respiratory support may be required, and any underlying factors should be addressed if possible (e.g., depressant effect of sedatives or analgesics).

Management of the Airway

The aims of airway management are to provide an adequate inspired oxygen concentration; to establish a patent, secure airway; and to support ventilation if required.

Providing an Adequate Inspired Oxygen Concentration

Although oxygen can be administered via nasal cannula, this method does not ensure delivery of more than 30% to 40% oxygen (at most). Other disadvantages include lack of humidification of gases, patient discomfort with use of flow rates greater than 4 to 6 L per minute, and predisposition to nasal mucosal irritation and potential bleeding.¹⁴ Therefore, despite being more intrusive for patients, facemasks are superior for oxygen administration. The three main types of facemasks are shown in Figure 2.1:

Figure 2.1 Facemasks: anesthesia mask (A); simple facemask (B1); simple facemask with reservoir bag (B2); Venturi mask (C).

• The anesthesia-type facemask (mask A in Fig. 2.1) is a solid mask (with no vents) with a cushioned collar to provide a good seal. It is suitable for providing very high oxygen concentrations (approaching 100%) because entrainment is minimized and the anesthetic circuit normally includes a reservoir of gas. These masks become unacceptable for many awake patients within a few minutes because of the association with heat, moisture, and claustrophobia.

• The simple facemask has vents that allow heat or humidity out but that also entrain room air. These masks have no seal and are relatively loose-fitting. Such masks may have a reservoir bag (approximately 500 mL) sitting inferior to the mask (B2 in Fig. 2.1), or may have no reservoir (B1 in Fig. 2.1). Using a simple facemask, without a reservoir bag, it is difficult to deliver an inspired oxygen concentration in excess of 50% even with tight application and 100% oxygen flow to the mask. Under the same conditions, a simple mask with a reservoir bag can produce an inspired oxygen concentration of about 80%.

• The Venturi mask (C in Fig. 2.1) has vents that entrain a known proportion of ambient air when a set flow of 100% oxygen passes through a Venturi device.¹⁴ Thus, the inspired oxygen concentration (usually 24% to 35%) is known.

Establishing a Patent and Secure Airway

Establishing a patent and secure airway can be achieved using simple airway maneuvers, further airway adjuncts, tracheal intubation, or a surgical airway.

Airway Maneuvers

Simple airway maneuvers involve appropriate positioning, opening the airway, and keeping it open using artificial airways if needed.

Positioning for Airway Management

In the absence of any concerns about cervical spine stability (e.g., with trauma, rheumatoid arthritis, or severe osteoporosis), raising the patient’s head slightly (5 to 10 cm) by means of a small pillow under the occiput can help in airway management. This adjustment extends the atlanto-occipital joint and moves the oral, pharyngeal, and laryngeal axes into better alignment, providing the best straight line to the glottis (“sniffing” position).15,16

Clearing the Airway

Acute airway obstruction in the obtunded patient often due to the tongue or extraneous material—liquid (saliva, blood, gastric contents) or solid (teeth, broken dentures, food)—in the pharynx. In the supine position, secretions usually are cleared under direct vision using a laryngoscope and a rigid suction catheter.¹⁷ In some cases, a flexible suction catheter, introduced through the nose and nasopharynx, may be the best means of clearing the airway. A finger sweep of the pharynx may be used to detect and remove larger solid material in unconscious patients without an intact gag reflex. During all airway interventions, if cervical spine instability cannot be ruled out, relative movement of the cervical vertebrae must be prevented—most often by manual inline immobilization.^17,18

Triple Airway Maneuver

The triple airway maneuver often is beneficial in obtunded patients if it is not contraindicated by concerns about cervical spine instability. As indicated by its name, this maneuver has three components: head tilt (neck extension), jaw thrust (pulling the mandible forward), and mouth opening.¹⁹ The operator stands behind and above the patient’s head. Then the maneuver is performed as follows:

• Extend the patient’s neck with the operator’s hands on both sides of the mandible.

• Elevate the mandible with the fingers of both hands, thereby lifting the base of the tongue away from the posterior pharyngeal wall.

• Open the mouth by pressing caudally on the anterior mandible with the thumbs or forefingers.

Artificial Airways

If the triple airway maneuver or any of its elements reduces airway obstruction, the benefit can be maintained for a prolonged period by introducing an artificial airway into the pharynx between the tongue and the posterior pharyngeal wall (Fig. 2.2).

Figure 2.2 Artificial airways: oropharyngeal airway (OPA); nasopharyngeal airway (NPA); laryngeal mask airway (LMA).

The oropharyngeal airway (OPA) is the most commonly used artificial airway. Simple to insert, it is used temporarily to help facilitate oxygenation or ventilation before tracheal intubation. The OPA should be inserted with the convex side toward the tongue and then rotated through 180 degrees. Care must be taken to avoid pushing the tongue posteriorly, thereby worsening the obstruction. The nasopharyngeal airway (NPA) has the same indications as for the OPA but significantly more contraindications ²⁰ (Box 2.1). It is better tolerated than the OPA, making it useful in semiconscious patients in whom the gag reflex is partially preserved. These artificial airways should be considered to be a temporary adjunct—to be replaced with a more secure airway if the patient fails to improve rapidly to the point at which an artificial airway no longer is needed. Such airways should not be used in association with prolonged positive-pressure ventilation.

Advanced Airway Adjuncts

Advanced airway adjuncts fill the gap between simple airway maneuvers and the insertion of a tracheal tube or surgical airway. These devices can be used to facilitate safe reliable airway management and manual ventilation in the prehospital or emergency resuscitation setting, often without expert medical presence.

The laryngeal mask airway (LMA) is a small latex mask mounted on a hollow plastic tube.21–26 It is placed “blindly” in the lower pharynx overlying the glottis. The inflatable cuff helps wedge the mask in the hypopharynx, sitting obliquely over the laryngeal inlet. Although the LMA produces a seal that will allow ventilation with gentle positive pressure, it does not definitively protect the airway from aspiration. Indications for use of the LMA in critical care are (1) as an alternative to other artificial airways, (2) the difficult airway, particularly the “can’t intubate–can’t ventilate” scenario, and (3) as a conduit for bronchoscopy. It is possible to pass a 6.0-mm ET through a standard LMA into the trachea, but the LMA must be left in situ. The intubating LMA (ILMA), which was developed specifically to aid intubation with a tracheal tube, has a shorter steel tube with a wider bore, a tighter curve, and a distal silicone laryngeal cuff.^27–30 A bar present near the laryngeal opening is designed to lift the epiglottis anteriorly. The ILMA allows the passage of a specially designed size 8.0 ET.

In recent years, many modified LMAs have reached clinical practice. They have been designed with the intention of promoting easier insertion, improving reliability of the laryngeal seal, and allowing safe gastric drainage of gastric fluid.

The Combitube (esophageal-tracheal double-lumen airway) is a combined esophageal obturator and tracheal tube, usually inserted blindly.31–35 Whether the “tracheal” lumen is placed in the trachea or esophagus, the Combitube will allow ventilation of the lungs and give partial protection against aspiration. The Combitube also is a potential adjunct in the “cannot intubate–cannot ventilate” situation. Disadvantages include the inability to suction the trachea when the device is sitting in its most common position (in the esophagus). Insertion also may cause trauma, and the Combitube is contraindicated in patients with known esophageal disease or injury or intact laryngeal reflexes and in persons who have ingested caustic substances.

Tracheal Intubation

If the foregoing interventions are not effective or are contraindicated, tracheal intubation is required. This modality will provide (1) a secure, potentially long-term airway; (2) a safe route to deliver positive-pressure ventilation if required; and (3) significant protection against pulmonary aspiration. Orotracheal intubation is the most widely used technique for clinicians practiced in direct laryngoscopy (indications and contraindications in Box 2.2). Normally, anesthesia with or without neuromuscular blockade is necessary for this procedure, which is summarized in Box 2.3.

Box 2.3 Procedure

Orotracheal Intubation

• Position patient and induce anesthesia ± neuromuscular blockade (if needed).

• Perform manual ventilation using triple airway maneuver and oropharyngeal airway

• Hold laryngoscope handle (left hand) near the junction with blade.

• Insert the blade along the right side of the tongue—moving tongue to the left.

• Advance tip of the blade in the midline between tongue and epiglottis.

• Pull upward and along the line of the handle of the laryngoscope.

• Lift the epiglottis upward and visualize the vocal cords.

• Do not use the patient’s teeth as a fulcrum when attempting to visualize the glottis.

• Stop advancing tube when cuff is 2 to 3 cm beyond the cords.

• Connect to a bag-valve system and pressurize it by squeezing bag.

• Inflate cuff until audible leak around tube stops.

• Check correct tube position (auscultation) and assess cuff pressure.

• Check end-tidal CO₂ trace.

Tracheal intubation requires lack of patient awareness (as in the unconscious state or with general anesthesia) and the abolition of protective laryngeal and pharyngeal reflexes. The drugs commonly used to achieve these states are shown in Table 2.1. Anesthesia is achieved using an intravenous induction agent, although intravenous sedatives (e.g., midazolam) theoretically may be used. Opioids often are used in conjunction with induction agents because they may reduce the cardiovascular sequelae of laryngoscopy and intubation (tachycardia and hypertension) and may contribute to the patient’s unconsciousness.

Table 2.1

Drugs Used to Facilitate Tracheal Intubation

Drug	Dose (Intravenous)
Induction Agent
Propofol	1-2.5 mg/kg
Opioids
Fentanyl	1.0-1.5 µg/kg
Morphine	0.15 mg/kg
Nondepolarizing Agents
Atracurium	0.4-0.5 mg/kg
Vecuronium	0.1 mg/kg
Rocuronium	0.45-0.6 mg/kg
Depolarizing Agent
Succinylcholine (suxamethonium)	1.0-1.5 mg/kg

Abolition of protective laryngeal and pharyngeal reflexes sometimes is achieved by inducing a deep level of unconsciousness using one or more of the aforementioned agents, followed by inhalation of high concentrations of a volatile anesthetic agent (e.g., sevoflurane, isoflurane). This technique sometimes is used in the difficult airway scenario to obtain conditions suitable for tracheal intubation in a patient who is still breathing spontaneously.

More often, a muscle relaxant is used to abolish the protective reflexes, abduct the vocal cords, and facilitate tracheal intubation. In the elective situation, nondepolarizing neuromuscular blocking agents are used. These drugs have the disadvantage of requiring several minutes to exert their effect, during which the patient must receive ventilation via a mask, thus allowing the possibility of gastric dilation and pulmonary aspiration. In patients at high risk of the latter (e.g., nonfasting patients), a depolarizing muscle relaxant (succinylcholine) is used because it produces suitable conditions for intubation within 15 to 20 seconds, and mask ventilation is not required. Succinylcholine has several side effects—among them hyperkalemia, muscle pains, and (rarely) malignant hyperpyrexia.

Nasotracheal intubation shares the problems and contraindications associated with the NPA.²⁰ The technique usually is employed when there are relative contraindications to the oral route (e.g., anatomic abnormalities, cervical spine instability). Nasotracheal intubation may be achieved under direct vision or with use of a blind technique, either with the patient under general anesthesia or in the awake or lightly sedated patient with appropriate local anesthesia (Box 2.4). If orotracheal or nasotracheal intubation is required but cannot be achieved, then a surgical airway is required (see later).

Box 2.4 Procedure

Nasotracheal Intubation (Blind and Under Direct Vision)

Preparation and Assessment of the Patient

1. Use a nasal decongestant such as phenylephrine to reduce bleeding.

2. Provide local anesthesia to the nasal mucosa.

3. Examine each nostril for patency and deformity.

4. Choose the most patent nostril, and use an appropriate-size ET.

5. After induction of anesthesia, position the head and neck as for oral intubation.

ET, endotracheal tube.

With a need for isolation of one lung from another, a double-lumen tube (having one cuffed tracheal lumen and one cuffed bronchial lumen fused longitudinally) can be used.³⁶ The main indications are (1) to facilitate some pulmonary or thoracic surgical procedures; (2) to isolate a lung containing contaminated fluid (e.g., in lung abscess) or blood, thereby preventing contralateral spread; and (3) to enable differential or independent lung ventilation (ILV). ILV allows each lung to be treated separately—for example, to deliver positive-pressure ventilation with high positive end-expiratory pressure (PEEP) to one lung while applying low levels of continuous positive airway pressure (CPAP) only to the other. Such a strategy may be advantageous in cases of pulmonary air leak (bronchopleural fistula, bronchial tear, or severe lung trauma) or in severe unilateral lung disease requiring ventilatory support.^37,38