Chapter 97: Airway Management in the Critically Ill Patient

Endotracheal intubation in the ICU differs significantly from when performed by anesthesiologists in the operating room, with availability of special airway equipment, and trained staff support.¹ In addition, ICU endotracheal intubation is usually marked by an urgent need in face of cardiorespiratory instability, poor physiologic reserve, and an unknown airway history.²

The incidence of difficult intubation in the ICU is 12% to 22%, considerably higher than reported in operating room settings.^3,4,5 The rate of severe complications is very high, including severe hypoxemia (26%), hemodynamic collapse (25%), cardiac arrest (1.6%), and death (0.8%).² The Fourth National Audit Project Report (NAP4), a review of major airway-related events occurring in the United Kingdom over a period of a year, revealed that 61% of the airway events that occurred in the ICU resulted in death or brain damage. More concerning is that after qualitative analysis of the events, reviewers assessed airway management as good in only 11%.³

Airway assessment must be performed before any procedure. The purpose of this evaluation is to identify possible difficulty with bag-mask ventilation, intubation, supraglottic device placement, or cricothyroidotomy/tracheostomy. Even in the ICU, where most intubations are performed emergently, an abbreviated assessment is warranted.^6,7,8

Predicting Difficult Bag-Mask Ventilation

If endotracheal intubation is difficult or impossible, ventilation with a bag-mask will maintain oxygenation until the airway is secured. Difficult ventilation is a serious problem, and every effort should be made to anticipate this complication.⁹ Five independent predictors have been identified: age greater than 55, body mass index greater than 26 kg/m², lack of teeth, history of snoring, male gender, and presence of beard.^7,9

Predicting Difficult Direct Laryngoscopy

Indicators of potentially difficult direct laryngoscopy include inability to prognath (move the lower teeth in front of the upper teeth), high Mallampati-Samsoon classification score (Figure 97–1), interincisor distance of less than 4 cm (~2 fingerbreadths), short thyromental distance (< 6.5 cm or 3 fingerbreadths) measured from the top of the thyroid cartilage to the anterior border of the mandible with the head in full extension, range of neck extension less than 35°, and previous history of a difficult intubation.^7,8,9,10,11

It is important to note that all these indicators have poor sensitivity and specificity and that no single factor reliably predicts difficulty. The positive predictive value of any of these tests alone is low, but prediction is improved with the presence of multiple concerning factors.^6,10,12

Predicting a Difficult Surgical Airway

The techniques of needle or surgical cricothyroidotomy depend on the cricothyroid membrane being accessible, which may not always be the case. Some features that may cause difficulty in accessing this membrane include obesity, neck immobility, and local blunt, or penetrating trauma.^6,9

Indications for Intubation

Indications for intubation include airway obstruction, airway protection (any cause of depressed level of consciousness, eg, stroke, trauma, or intoxication), facilitation of mechanical support for respiratory failure (hypoxemic or hypercapnic), and circulatory failure (shock).

Equipment

All necessary equipment should be available and checked prior to any intubation attempt regardless of predicted difficulty^7,8 (Table 97–1). The patient should be monitored with at least pulse oximetry, blood pressure cuff, and continuous electrocardiogram. Intravenous access should be secured.

Positioning

The airway contains three visual axes: (1) mouth, (2) oropharynx, and (3) larynx. In the neutral position, these axes form acute and obtuse angles with one another and the glottic opening is not visualized.¹¹ The traditional teaching is that the “sniffing the morning air” position (Figure 97–2) helps align the oral, pharyngeal, and laryngeal axes.¹³ This is achieved by flexing the neck and extending the head at the atlantooccipital joint.^7,8 Cervical flexion approximates the pharyngeal and laryngeal axes, and extension at the atlantooccipital joint brings the oral axis into better alignment with the other two.¹¹ However, over the past decade, several authors have controversially challenged this issue.^14,15,16 Adnet et al studied magnetic resonance imaging (MRI) scans of healthy volunteers in 3 anatomic positions (neutral, simple extension, and in the sniffing position) and concluded that the “sniffing position” does not achieve alignment of the 3 important axes.¹⁵ Despite this, we consider that the sniffing position is probably the best starting position for direct laryngoscopy.

In patients at risk of aspiration, compressing the cricoid cartilage posteriorly against the vertebral body (ie, Sellick maneuver) may reduce the diameter of the upper esophageal sphincter and prevent regurgitation of stomach content into the trachea during intubation, but may also interfere with laryngoscopy and endotracheal tube insertion.

Preoxygenation

Preoxygenation should take place prior to any airway intervention. The two primary goals of preoxygenation are (1) maximization of the arterial PaO₂ and (2) denitrogenation of the functional residual capacity. A beneficial secondary goal of the process is hypocarbia.

Preoxygenation can temporarily raise the SpO₂ and PaO₂, maximizing blood oxygen content. Although dissolved oxygen adds little to the oxygen content of blood as the SpO₂ nears 100%, critically ill patients often suffer from anemia, increased oxygen consumption and regional hypoperfusion, so any increase in dissolved oxygen (eg, 1.5 mL O₂/100 mL of blood) is desirable. More importantly, preoxygenation replaces the nitrogen in the functional residual capacity with oxygen. The functional residual capacity is approximately 30 mL/kg, oxygen consumption is 3 to 4 mL/kg/min, and in healthy patients this “oxygen reserve” may allow up to 8 minutes of apnea before the SpO₂ decreases below 90%. However, efficiency of denitrogenation and the time to critical SpO₂ decrease is markedly reduced in critically ill patients and apnea rarely tolerated for more than 60 to 120 seconds; but, any increase in the time to desaturation will decrease morbidity and mortality.

Encouragement of hyperventilation during preoxygenation may result in hypocarbia that is desirable in the soon to be apneic patient. During the first minute of apnea the Pco₂ will increase by 6 mm Hg and afterward by 3 mm Hg per minute. Hypercarbia will cause hemodynamic instability, cardiac arrhythmias, and increase the intracranial pressure, even while the SpO₂ remains in a safe range.

Preoxygenation may be achieved by administration of 100% oxygen with a tight-fitting face mask, application of continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP), or manually assisted bag-mask ventilation. The choice of method is highly dependent on personal preference or unit policy and the emergent nature of the procedure. The authors of this chapter recommend that while setting up for intubation at minimum a tight-fitting Fio₂ 1.0 face mask should be applied and if time allows BiPAP be initiated.

Face Mask and Bag-Valve Device

Face masks are designed to form a seal around the mouth and nose and connect to a bag-valve device.⁶ The operator stands at the patient’s head and presses the mask onto the patient’s face with the left hand. The thumb should be on the nasal portion of the mask, the index finger near the oral portion, and the rest of the fingers spread on the left side of the patient’s mandible so as to pull slightly anterior.⁸ The 2 key elements are to establish a tight fit with the mask, covering the patient’s mouth and nose to prevent air leaks, and an unobstructed airway.^6,8 The minimum effective insufflation pressure should be used to decrease the risk of insufflating the stomach and increasing the risk of aspiration.⁸

If ventilation is not effective, 2 maneuvers can be performed to improve airflow obstruction. Head extension, by stretching the anterior neck structures and moving the hyoid bone and attached structures anteriorly, is probably the most important single maneuver for maintaining space between the pharyngeal soft tissues (caution is advised in patients with unstable cervical spine). Jaw thrust, achieved by exerting anterior pressure behind the angles of the mandible, can also reduce the airway obstruction.⁶

Airway Adjuncts

If ventilation is not adequate despite proper positioning of the patient and proper use of the bag-valve device, several airway adjuncts may be helpful.⁸

An oropharyngeal airway (Figure 97–3) is semicircular and made of plastic. The 2 types are the Guedel airway, with a hollow tubular design, and the Berman airway, with airway channels along the sides.⁷ Both are inserted with the curved portion toward the mouth floor with the help of a tongue depressor to lower the tongue if needed. It should be inserted only when the pharyngeal reflexes are depressed, to minimize the risk of coughing and laryngospasm.⁶

The nasopharyngeal airway is a soft tube approximately 15-cm long that is inserted through the nostril into the posterior pharynx. This device may be preferable in patients with limited mouth opening and poor dentition. It is also better tolerated in conscious patients. The airway should be lubricated and a vasoconstrictor should be applied before insertion. Its use is contraindicated in patients with skull base trauma, rhinorrhea, and severe coagulopathy.^6,8

Induction Agents/Muscle Relaxants

Endotracheal intubation is safer and the success rate is higher in the sedated paralyzed state. Given the frequency of difficulty encountered, persons who are not experts and not comfortable with the administration of hypnotics and paralytics should defer to experts.

The choice of sedative hypnotic agent should emphasize hemodynamic stability and short duration of action. Hypotension is best avoided if etomidate (2-3 mg/kg) or ketamine (1-5 mg/kg) are used. Etomidate may cause myoclonus, does not promote muscle relaxation, burns on injection, and is associated with decreased endogenous steroid production, but its hemodynamic stability is unmatched. The administration of propofol to critically ill patients, even in low doses (eg, 1-1.5 mg/kg), almost always causes hypotension.

The choice of paralytic should emphasize short onset of action (ie, “rapid-sequence intubation”, ~ 60-75 seconds) and limits the choice to the depolarizing agent succinylcholine, and the nondepolarizing agent rocuronium. Succinylcholine (1-1.5 mg/kg) is the most rapid in onset (30-45 seconds) and shortest in duration (5-10 minutes), but in normal patients may raise the potassium level, and in certain conditions (eg, paralytic stroke, major burns) this rise may be quite profound and result in a hyperkalemic arrest. In these conditions, rocuronium (0.6-1.2 mg/kg) is preferable, although its duration of action is longer than succinylcholine (30-50 minutes).

Given the frequency of hemodynamic instability accompanying intubation in the ICU, preparation and administration of vasoactive medications should occur simultaneously with preparation and probably should precede administration of sedative hypnotics and paralytics. Patients who are already hypotensive prior to endotracheal intubation need reliable venous access through which a vasopressor can be infused. If the mean arterial pressure (MAP) is less than 60 mm Hg the vasopressor infusion should be increased, or alternatively a bolus of vasopressor can be administered. Ephedrine (5-10 mg), phenylephrine (100 μg), and vasopressin (0.5-1 units) have all been frequently used by the authors, usually determined by availability and heart rate. Bradycardic patients should all have at minimum atropine 0.4 mg or glycopyrolate 0.2 mg administered. Hypertensive patients, especially those with intracranial lesions, may benefit from titrated doses of short-acting hypotensive agents such as nitroglycerin (40-100 μg) and esmolol (0.5 mg/kg).

Direct Laryngoscopy and Endotracheal Tubes

Direct laryngoscopy is performed to visualize the laryngeal opening (Figure 97–4). To see through the airway, light must travel from the glottic opening to the laryngoscopist’s eye. Since light travels in a straight line, the technique requires an uninterrupted linear path between the larynx and the observer.¹¹ The tongue and the epiglottis are the anatomic structures that intrude into this line of sight.⁶ One of the main objectives of direct laryngoscopy is to move the tongue anteriorly into the mandibular space, allowing direct view of the glottic opening.

The laryngoscope has a handle and a blade that snaps securely into the top of the handle. Many blades’ shapes and sizes are available, however, 2 types are commonly used (Figure 97–5). The Macintosh blade is curved along its long axis, has a broad, flat surface, and has a right-angled Z-shaped cross-section. The straight blades (Cranwall, Miller, Phillips, Wisconsin) tend to be narrower than the curve blades and have a D-shaped flange.^6,7,8 Which blade to use is in many cases a matter of personal preference, but the straight blade could be of special use in patients with a larger than normal epiglottis, or with a larynx that is positioned more anterior.

The laryngoscope is held in the left hand and introduced into the right side of the mouth. While advancing the blade to the base of the tongue, the blade is simultaneously moved to the midline sweeping the tongue to the left. The tip is advanced and the epiglottis is lifted away from the glottic opening.⁷ With a curved blade the tip rests and applies upward traction to the base of the tongue at the vallecula; the straight blade tip rests in the posterior surface of the epiglottis and lifts it directly.^7,8 Elevating the epiglottis through a lifting motion at 45° from the horizontal using the arm and shoulder exposes the vocal cords. Keeping the wrist stiff to avoid a prying motion that uses the teeth as fulcrum will prevent dental injury.^7,8 Once the vocal cords are visualized, the endotracheal tube is advanced from the right corner of the mouth and into the trachea. The cuff is inflated with enough air to prevent a leak with positive pressure ventilation.

Correct placement is confirmed with visualization of symmetric chest expansion, auscultation over epigastrium and lung fields, and an end-tidal CO₂ detection device (capnography or calorimetric chemical detection of CO₂). After confirmation, the tube is secured in place. A chest radiograph should be ordered in order to confirm correct placement and position of endotracheal tube tip above carina.

Endotracheal tubes are designed to provide a secure channel through the upper airway. The size of the endotracheal tubes is described as the internal diameter in millimeters.⁶ Tubes are available in 0.5 mm increments, starting at 2.5 mm. Selection of the proper diameter is important because small-diameter tubes increase airway resistance and work of breathing; moreover, certain procedures like bronchoscopy require large tubes. Common sized tracheal tubes are 8 mm for males and 7.5 mm for females. In general, the larger the patient, the larger the endotracheal tube that should be used.^6,8 For oral intubation, an endotracheal tube insertion depth of 21 cm from the incisors in females and 23 cm from the incisors in males results in proper positioning about 4 cm above the carina in the majority of patients. The endotracheal tubes have a cuff near the distal end that is inflated to provide a seal to protect from aspiration and to allow positive pressure ventilation. Prevention of excessive cuff pressure may reduce the incidence of tracheal damage. Cuff pressure monitoring should be used whenever possible to maintain a pressure in the range of 25 to 30 cm H₂O.⁶

In addition to conventional laryngoscopes, many airway devices and techniques have been developed to manage difficult ventilation or intubation. However, no device or technique is universally successful in all situations.^2,17

Supraglottic Devices

The laryngeal mask airway (LMA) is a plastic tube attached to a shallow mask with an inflatable ring designed for blind insertion into the pharynx to fit the laryngeal inlet and form a seal around it.^8,17 It was introduced in 1988 and is widely used in the operating room for elective cases. It has also been used as a rescue device and is the key in the adaptive simulated annealing (ASA) algorithm for difficult airway used as a ventilatory device in patients that are difficult to ventilate and intubate, or as a conduit for fiberoptic bronchoscopic intubation in patients that can be ventilated but not intubated.^6,17

Rigid Indirect Laryngoscopy

Indirect laryngoscopy is a technique in which the glottic view is transmitted through fiberoptic bundles or video technology to the eyepiece of the instrument, or a monitor screen.¹⁸ There are several available devices differing in their design and technical features.

In recent years, the use of bladed indirect laryngoscopes (eg, Glidescope, C-MAC, McGrath) has increased, not only in the operating room, but also in the emergency department and in the ICU (Figure 97–6). Videolaryngoscopes consist of an angled blade with a camera at the tip. The image captured at the distal lens is transmitted to the proximal end of the device into a small screen. It is important to note that the Glidescope and the McGrath have an angulated blade, and provide a view of the glottic opening on the screen while the actual glottis is not directly in the operator’s view line. Because of this the endotracheal tube requires the use of a curved stylet to be inserted around and behind the tongue.¹⁸ For this same reason, in some occasions, despite an improved visualization, it is difficult to advance the endotracheal tube into the trachea.

Flexible Fiberoptic Bronchoscope Intubation

Fiberoptic bronchoscopes use flexible optical fibers to transmit images from a distal lens as it is steered under vision toward the larynx and into the trachea. The image transmitted is displayed on a monitor by attaching a camera to the eyepiece, or by using a bronchoscope with an integral camera. This device is the most versatile laryngoscope for tracheal intubation and can facilitate intubation that is otherwise impossible.⁶

Unfortunately, many of the relative contraindications for its use are encountered in the ICU, including poor patient cooperation, blood in the airway, and time constraints.

For clinical or anatomic reasons, airway management may be difficult without specialized expertise or tools. One significant advance in the management of patients with difficult airways is the development of algorithms to standardize its approach.¹⁹

The American Society of Anesthesiologists recently published an update of the Practice Guidelines for Management of Difficult Airway. Although these guidelines were designed for its use in the operating room, and in the majority of cases in the ICU the elective pathway is not applicable, they offer a useful stepwise approach to anticipated and unanticipated airway problems (Figure 97–7).^19,20

During low-blood-flow states such as patients in cardiac arrest, oxygen delivery is limited by flow rather than by arterial oxygen content. In these circumstances, rescue breaths are less important than chest compressions during the first few minutes of resuscitation. Moreover, rescue breaths could reduce cardiopulmonary resuscitation (CPR) efficacy due to interruption of chest compressions and increases in intrathoracic pressure. Advanced airway placement in cardiac arrest should not delay initial CPR and defibrillation.²¹

The current American Heart Association guidelines for CPR included several changes related to airway management.^2,21 The traditional airway, breathing, circulation (ABC) was changed to compressions, airway, breathing (CAB).

Both bag-mask ventilation and ventilation with a bag through an advanced airway are acceptable methods of providing ventilation and oxygenation during CPR. The number and the duration of intubation attempts should be minimized, with a goal of no more than 10 seconds to minimize chest compression interruptions. Supraglottic devices may be reasonable alternatives to tracheal tubes.²¹

Extubation can be as challenging as intubation and is also fraught with potential complications. Reintubation can be complicated by the urgency of the situation, changes in airway anatomy (edema), and hemodynamic instability, among other factors already discussed.²

Planning for extubation should be performed immediately after intubation. The Difficult Airway Society Extubation guidelines published in 2012 suggest that patients should be approached as “low risk” and “high risk” for extubation. The high-risk patients with history of difficult intubation may be extubated using a supraglottic airway or an airway exchange catheter as a bridge to extubation, but more importantly a highly competent airway person should be at the bedside. Airway exchange catheters can be left in place as needed, are well tolerated by patients, and can be used as a conduit for reintubation.^2,22