Chapter 17: Airway Management/The Difficult Airway

The majority of patients admitted to the intensive care unit have varying degrees of respiratory failure. The intensivist must decide whether these patients would benefit from supplemental oxygen, noninvasive positive pressure ventilation (NIPPV), or endotracheal intubation with mechanical ventilation. The ability to secure an airway with a tracheal tube is therefore a necessity for all intensivists. As can be seen in Table 17–1, there are multiple indications for intubation. In this chapter, the airway management of critically ill patients will be discussed, including the preintubation assessment, the process of intubation, and weaning to extubation.

The upper airway (Figure 17–1) includes all of the structures externally from the nares and mouth to the vocal cords. The nasal passage way is highly vascular; disruption of Kiesselbach's plexus, located in the anterior aspect of the nasal septum, is the most common cause of epistaxis. The floor of the nasal passage way slopes very gently downward. As such, nasal airways, nasogastric tubes, and endotracheal tubes (ETTs) should be directed perpendicularly or with a slightly caudad tilt in the supine patient. The nasal passages open into the nasopharynx. Laceration of the mucosa and creation of a false passage during nasotracheal intubation most commonly occurs in the region of the eustachian tubes.

The larynx is continuous with the trachea inferiorly. The thyroid cartilage, cricoid cartilage, and the hyoid bone are the main components of the laryngeal skeleton. The epiglottis and arytenoids contribute to this structure. The piriform recesses, which reside on each side of the larynx, function to divert the food bolus laterally and away from the glottic opening. The thyroid cartilage supports most of the soft tissue structures in the larynx. The cricoid cartilage is the only complete ring in the larynx and serves to support the posterior laryngeal structures. During an emergent intubation in which the patient cannot be ventilated or intubated, the cricothyroid membrane, which is located between the thyroid and cricoid cartilages, may be surgically accessed to obtain an airway. The epiglottis projects posteriorly from the tongue and aids in airway protection during deglutition. Arytenoid cartilages articulate with the posterior parts of the cricoid cartilage. The muscles of voice control are attached to the two arytenoid cartilages, and the vocal cords project anteriorly from the arytenoid cartilages to the thyroid cartilage. Movement of the cricoarytenoid joints from the action of the laryngeal muscles causes a change in size of the opening between the vocal cords. The larynx is heavily innervated by the vagus nerve.¹

The trachea starts at the inferior border of the cricoid cartilage and extends 12 to 15 cm distally to the carina, the first branch point marking the start of the right and left main bronchus. The anterior portion of the trachea is composed of structured cartilaginous rings, while the posterior portion is membranous. The average internal diameter is 9 to 15 mm. Typically, an ETT with an internal diameter of 7 to 7.5 and 7.5 to 8.5 mm is used for intubation of adult female and male patients, respectively. If bronchoscopy is anticipated, the minimum ETT size is 7.5 mm to allow passage of the bronchoscope. In some circumstances, such as pregnancy, a smaller ETT could be required due to swelling and edema of the airway.²

Predictors of Difficult Ventilation

Bag valve mask (BVM) ventilation is a fundamental skill in airway management. Adequate ventilation is essential to provide oxygenation and removal of carbon dioxide. The American Society of Anesthesiologists (ASA) Task Force on Management of the Difficult Airway described difficult mask ventilation as a situation where a conventionally trained anesthesiologist experiences difficulty with ventilation due to one or more of the following problems: inadequate mask or supraglottic airway seal, excessive gas leak, or excessive resistance to the ingress or egress of gas.³ Fortunately, the incidence of difficult ventilation in the operating room is relatively low at 1.4% to 5%, with the incidence of impossible ventilation at 0.006% to 0.15%.^4,5,6 Likely, the incidence of both situations is higher outside of the controlled setting of the operating room.

The predictors of difficult mask ventilation include factors that prevent proper seal of the mask and that obstruct air flow to and from the lungs. Identified risk factors for difficult ventilation are of age greater than 55, body mass index greater than 26, edentulous, history of snoring, and the presence of a beard.⁷ The risk factors associated with impossible ventilation include a history of neck radiation, male sex, history of sleep apnea, Mallampati III or IV score, and the presence of a beard.⁴

Predictors of Difficult Tracheal Intubation

Tracheal intubation is the process by which the airway is secured by placing an ETT past the vocal cords and into the trachea to facilitate ventilation and oxygenation. The difficulty of intubation and the difficulty of ventilation should be considered as two distinct issues. In some instances, they may be concordant, such as with Ludwig's angina, where swelling of the posterior aspect of the tongue can interfere with both intubation and ventilation. However, in other situations, the ability to intubate and ventilate may be mutually exclusive. A male patient with a large bushy beard may be difficult to ventilate with a BVM, but easy to intubate. Conversely, a thin patient with severe rheumatoid arthritis affecting the cervical spine may be easy to ventilate, but extremely difficult to intubate due to limited range of motion of the neck.

The ASA has defined the difficult intubation as a situation involving “three or more laryngoscopic attempts to place the ETT into the trachea” or a situation where the attempts last for more than 10 minutes using conventional laryngoscopy.⁸ In the operating room, the incidence of difficult intubation is around 5.8%. In the nonoperative emergent setting, the incidence varies from 10% to 24%.^8,9,10 Patients who were difficult to intubate had a 51% incidence of life threatening complications defined by death, cardiac arrest, cardiovascular collapse, shock, or hypoxic injury.⁸ A difficult intubation increased the incidence of esophageal intubation, aspiration, pneumothorax, dental injury, and death in those requiring emergent intubation.¹² Identifying patients who may be a difficult intubation is essential. By anticipating a difficult intubation, advance airway equipment can be procured and additional personnel, such as a more experienced laryngoscopist or a surgeon experienced in emergent surgical airways can be assembled for help.

The probability of a difficult airway may be predicted by a thorough upper airway exam. “LEMON” is a mnemonic tool that can be used in assessing and predicting the difficult airway. The mnemonic stands for: L = look externally, E = evaluate using the 3-3-2 rule (a minimum of 3 finger breadths for the mouth opening measured as the distance between the top and bottom incisors, 3 finger breadths for the thyromental distance, and 2 finger breadths for the distance between the hyoid and thyroid), M = Mallampati score (Figure 17–2), O = obstruction (anything that might interfere with intubation, such as a foreign body), and N = neck mobility. Numerous prediction models and scores have been devised and validated for the prediction of a difficult intubation.¹¹ Most of these scores are comprised of a mix of patient characteristics, disease characteristics, and operator characteristics, and have been validated in the operating room. One multicenter study evaluated the usefulness of the MACOCHA score, which consists of several risk factors that can be easily evaluated in the intensive care unit (ICU), to predict difficult intubations (Table 17–2). A score of 0-3 is associated with a low incidence of difficult intubation, while a score of 10-12 is associated with a very high incidence of difficult intubation.⁸ An easy intubation is predicted by a large mouth opening, lack of top teeth, adequate thyromental distance, and adequate range of motion of the neck. A small mouth opening, protruding top teeth, short thyromental distance, and decreased range of motion of the neck conspire to prevent adequate alignment of the oral and pharyngeal axes during laryngoscopy and decreases the probability of visualization of the glottis (see “Visualizing the Larynx”). A normal airway can subsequently become difficult due to processes, such as angioedema.

History of a prior difficult intubation appears to be strongly predictive of a difficult airway.¹³ The association between obesity and difficult airway has been studied. Though a link between the two would be expected, Brodsky et al showed in their study that obesity as an isolated factor does not increase chance of difficult intubation.¹⁴ Ultrasound has been proposed as a method of identifying patients with difficult airway. The recommended technique is to image and measure the soft tissue thickness over the vocal cords. These studies have had variable outcomes; at this time, ultrasound cannot be recommended as a definite tool to identify a difficult airway.^15,16

A key step in securing an airway is to maintain adequate oxygenation in the peri-intubation period. Prolonged periods of hypoxia can result in significant hemodynamic instability and hypoxic brain injury. Preoxygenation increases the safety buffer time available during the process of intubation. The terms safe period of apnea and duration of apnea without desaturation (DAWD) have been defined as the peri-intubation period where the patient is apneic and still able to maintain saturation more than 90%.¹⁷

During an elective intubation for general anesthesia in the operating room, the patient is preoxygenated prior to induction of anesthesia. Following induction, the patient is ventilated until the optimal intubation conditions are achieved. For the period of time required for the actual intubation, the patient remains apneic; however, the aforementioned maneuvers typically prevent desaturation. Emergent intubations are not as slow and controlled as elective intubations. In addition, patients undergoing emergent intubations are typically presumed to be full stomachs, where the benefit of ventilation prior to intubation must be weighed against the risk of insufflating the stomach and increasing the risk of aspiration.

Oxygen can be delivered by a simple face mask providing a FiO₂ of 60% to 70% or a nonrebreather (NRB) face mask with a reservoir bag providing a FiO₂ of greater than 90%. A BVM may be required for preoxygenation prior to intubation; however, its use increases the risk of gastric insufflation and aspiration even if used properly.¹⁸ Irrespective of how the oxygen is delivered, the flow rate must be high enough to wash out all nitrogen from the circuit prior to delivery to the patient. This is typically accomplished by using high flow rates of 10 to 15 L/min. In patients who have failed a trial of NIPPV, preoxygenation may be more efficient using the NIPPV circuit than switching to NRB mask. NIPPV may also be considered for preoxygenation in morbidly obese patients and in those patients that cannot support a SaO₂ of greater than 95% by other means of oxygen delivery.¹⁹ Preoxygenation can be improved by having the patient in a sitting up or reverse Trendelenburg position so to increase their total respiratory system compliance and subsequently their functional residual capacity (FRC) and DAWD.²⁰ Failure of preoxygenation most commonly occurs due to low oxygen flow rates, inadequate preoxygenation time, air leak, and rebreathing.¹⁷

Intravenous anesthetics play an important role in airway management. They permit rapid induction of anesthesia and facilitate securing of the airway with an ETT with minimal stress to the patient. The ideal medication would provide amnesia, analgesia, and muscle relaxation. In addition, the ideal medication would have minimal effect on the patient's hemodynamics, preventing both hypotension due to sedation and analgesia and hypertension from the stimulation of laryngoscopy. Unfortunately, no one medication encompasses all of these properties. Instead, a balanced approach with a combination of medications is utilized. In elective intubations, the availability of time and the presence of an empty stomach allow for premedication; this is followed by an induction agent, which provides unconsciousness, and then a paralytic to help facilitate intubation. In emergent intubations, premedication may not be possible due to the lack of time or a contraindication to its use such as a full stomach. The hemodynamics of the patient may limit the amount of induction agent utilized; those who are particularly tenuous may not tolerate any induction agent. If paralysis is required, only fast acting paralytics will be of benefit during an emergent intubation. Common medication dosages are listed in Table 17–3.

Premedication

Midazolam

Midazolam is a fast-acting intravenous benzodiazepine that allows for anxiolysis and anterograde amnesia by crossing the brain blood barrier and activating the gamma-butyric acid (GABA) receptor complex. The acidic nature of the intravenous preparation keeps midazolam in solution; however, exposure to the pH of the blood causes a structural change making midazolam extremely lipophilic. Hepatic metabolism results in formation of an active metabolite (1-hydroxymidazolam), which is renally cleared and may contribute to prolonged sedation with extended infusions. The short duration of action is aided by its lipophilicity, which promotes rapid redistribution from the brain to inactive tissue sites, such as fat and skeletal muscle. Side effects include respiratory depression, especially when given in conjunction with an opioid.²¹

Fentanyl

Fentanyl is a synthetic centrally acting opioid agonist that is structurally related to meperidine. It is approximately 100 times more potent than morphine. The rapid onset of action is attributed to its high lipophilicity, which also leads to its short duration of action by redistribution. Hepatic metabolism results in an inactive metabolite. Side effects include respiratory depression, especially when given with a benzodiazepine, and chest wall rigidity at high doses.²¹

Lidocaine

Lidocaine is an amide local anesthetic that is capable of suppressing the sympathetic response to intubation by inhibiting the gag and cough reflex. Blunting the sympathetic response can limit the rise in blood pressure due to manipulation of the airway during intubation and can prevent an increase in intracranial pressure (ICP) in a patient with known or presumed head injury.²¹

Induction Agents

Propofol

Propofol is an alkylphenol compound that is insoluble in water, and therefore, must be administered as an emulsion, containing soybean oil, glycerol, and egg lecithin. The rapid onset of action is attributed to its high lipophilicity. The short duration of action is primarily due to redistribution from highly perfused compartments such as the brain to the poorly perfused compartments such as skeletal muscle. In addition to its hypnotic properties, propofol is both a respiratory and cardiac depressant. Hypotension following induction is primarily due to vasodilation; however, the baroreceptor response is also attenuated preventing an adequate increase in heart rate to compensate for the decrease in blood pressure.²² Hypotension following emergent intubation in critically ill patients is a strong predictor of mortality.²³ As a result, propofol should be used with caution if at all in hypotensive patients requiring emergent intubation. Hypotension should be immediately treated with a vasopressor, such as phenylephrine (IV dose 50-200 mcg).

Etomidate

Etomidate is a carboxylated imidazole that acts as a hypnotic by inhibiting the GABA receptors of the reticular activating system. Etomidate is poorly soluble in water and therefore is administered in a 35% propylene glycol solution. Onset of action is similar to propofol with a duration of effect of 3 to 12 minutes. Unlike propofol, etomidate has minimal effects on myocardial contractility and cardiac output resulting in less hypotension.²¹ Myoclonus caused by a disinhibitory effect on the central nervous system is characteristic. A single induction dose of etomidate causes adrenal suppression by inhibiting the activity of 11-β-hydroxylase, an enzyme required for cortisol and aldosterone synthesis.^24,25 Subgroup analysis in the Corticus trial indicated a decreased responsiveness to cortisol and increased mortality in septic patients who received etomidate.²⁶ In summary, etomidate is an attractive induction agent in hemodynamically unstable patients; however, recent studies demonstrating adrenal suppression and increased mortality in septic patients has called its use into question.

Ketamine

Ketamine is a phencyclidine derivative that causes a dose-dependent state of “dissociative anesthesia” that produces both amnesia and analgesia. Ketamine interacts with a variety of receptors. Its amnestic property is likely due to the inhibition of the N-methyl-d-aspartate (NMDA) complex. Stimulation of the sympathetic nervous system causes an increase in heart rate, myocardial contractility, and mean arterial blood pressure, making it an ideal agent in hemodynamically compromised patients.²⁷ The mechanism of this indirect stimulatory effect is via inhibition of norepinephrine reuptake. The direct myocardial depressant effect of ketamine may be unmasked in patients with poor catecholamine stores such as the chronically or severely critically ill patient. Stimulation of the sympathetic nervous system will also result in increased cerebral oxygen consumption, cerebral blood flow, and ICP making it a poor choice in patients with suspected head injury and elevated ICP. Ketamine is infamous for its ability to cause hallucinations; however, the coadministration of a benzodiazepine can usually attenuate this effect.²⁸

Paralytics

Succinylcholine

Succinylcholine (SCh) is a depolarizing neuromuscular blocking drug (NMBD) that has a fast onset time and short duration of action, making it ideal for rapid sequence induction (RSI). Structurally, SCh is related to acetylcholine (ACh) and functions by binding to the ACh receptor on the motor end plate, resulting in continued depolarization of the receptor and subsequent skeletal muscle paralysis. The onset of paralysis is typically preceded by fasciculations, which are caused by transient generalized skeletal muscle contractions. Normal muscle function does not return until the receptor is allowed to repolarize to its resting state, which occurs after metabolism of SCh by pseudocholinesterase. Pseudocholinesterase metabolizes SCh at such a high rate that only a small portion of the injected dose reaches the motor end plate. Altered pseudocholinesterase function can result in paralysis lasting 6 to 8 hours after a single dose. A relative decrease in pseudocholinesterase levels in patients with liver disease, kidney disease, anemia, pregnancy, and cocaine and amphetamine abuse has a minimal clinical effect and no alteration in the dose of SCh is required.²⁹

Certain adverse effects may prevent SCh from being utilized as a RSI paralytic. On average, the serum potassium level increases 0.5 mEq/L following administration of SCh. If a paralytic is required in a patient with symptomatic hyperkalemia, an alternative medication should be considered; however, renal failure in a patient with normal potassium levels is not a contraindication to the administration of SCh.³⁰ Severe hyperkalemia causing cardiac arrest has been documented in patients with disorders of the nervous system (eg, spinal cord transection, amyotrophic lateral sclerosis, Guillain-Barre, etc) or extensive burn injury. Approximately 5 to 15 days after an acute injury, the body produces extrajunctional ACh receptors (ie, outside of the motor end plate) that may cause excessive potassium release following administration of SCh. This phenomenon puts patients at increased risk of hyperkalemic cardiac arrest. Nonetheless, it is generally considered safe to administer SCh within 24 hours of the acute injury nerve injury or stroke.³¹ Hyperkalemia may also occur in patients with disorders of the musculoskeletal system, such as Duchenne muscular dystrophy, and SCh should be avoided in such patients. The increase in potassium level after SCh administration has been associated with length of stay in the ICU, which acts a surrogate marker for immobility. The risk of acute hyperkalemia (≥ 6.5 mEq/L) is highly significant after 16 days in the ICU.³²

The administration of SCh increases intraocular pressure (IOP) by 5 to 10 mm Hg for 2 to 6 minutes.³³ In patients with open eye injuries, the concern that increased IOP will cause extrusion of intraocular contents has limited the use of SCh in these patients. Nonetheless, there has never been a reported case of this occurring in the literature. The effect of SCh on ICP is controversial. Studies demonstrating a modest increase in ICP after SCh administration are inconsistent.³⁴ SCh may act as a trigger for malignant hyperthermia, a life-threatening complication resulting in high fever, muscle rigidity, rhabdomyolysis, and renal failure, which requires immediate treatment with dantrolene. Fortunately, the incidence of malignant hyperthermia after administration of SCh is rare. While SCh may cause a number of potential adverse side effects, no medication has been able to match its fast onset time and short duration to replace it as the paralytic of choice during an emergent intubation.

Rocuronium

Rocuronium is a nondepolarizing NMBD that causes paralysis by antagonism of the ACh receptor. The onset of action is slightly slower than that of SCh at a rapid sequence intubating dose of 1.2 mg/kg (normal intubating dose 0.6 mg/kg), but faster than the other nondepolarizing NMBDs. The major concern with rocuronium is its prolonged duration of action of approximately 1 hour. If a patient needs to be woken up due to difficulty in intubating or ventilating, they will not be able to regain spontaneous respiration for a prolonged period of time. Sugammadex is a specific reversal agent for rocuronium, preventing its interaction with the ACh receptor, and reverses paralysis within 3 minutes of administraion.³⁵ Sugammadex is currently not available in the United States due to Food and Drug Administration concerns regarding hypersensitivity and allergic reactions.

The ability to oxygenate and ventilate a patient is an essential skill for anyone involved in airway management. While much emphasis is often placed on the ability to intubate, the morbidity associated with emergent intubations is in fact more closely correlated to the inability to ventilate and provide gas exchange. When an unanticipated difficult airway is encountered, the ability to ventilate the patient allows the practitioner to breathe for the patient, allowing time to obtain help in the form of additional equipment or personnel. Many practitioners insist that the ability to ventilate a patient be demonstrated prior to the administration of a paralytic. Paradoxically, ventilation can be made easier by paralysis due to loss of muscular tone. In addition, the administration of a paralytic can be crucial in increasing the intubatability of a patient assessed to be a moderately difficult airway. Ventilation with a BVM is relatively contraindicated during rapid sequence intubation (RSI) as it can lead to insufflation of the stomach and increase the risk of aspiration.

Facemasks for BVM are available in multiple sizes. Clear masks have the benefit of allowing visualization of vapor condensation, skin color, and signs of regurgitation. The last three digits of the practitioner's left hand are utilized to open the patient's airway with either a chin lift or jaw thrust. Compression of the soft tissue below the mandible should be avoided to prevent obstruction of the airway. The thumb and index finger are utilized to hold the facemask in place. The bag mask ventilation is a self-inflating resuscitation bag with reservoir connected to a high-flow oxygen source.³⁶

Maintaining an open airway is one of the first steps required to insure adequate ventilation. Typically, a head tilt chin lift position is used. However, in patients with suspected spinal injuries, a jaw thrust maneuver is performed. Adjuncts, such as oral and nasal airways can be used to improve ventilation by moving an obstructing tongue from the posterior pharyngeal wall. In lightly anesthetized patients, an oral airway may trigger the gag reflex with a nasal airway being better tolerated. Nasal airways are relatively contraindicated in patient on blood thinners and antiplatelet therapy due to the risk of epistaxis and in any patient with a suspected basilar skull fracture. In a patient who is difficult to ventilate despite these maneuvers and adjuncts, a two person technique may be required. One person opens the airway using two hands and the other ventilates the patient.^37,38,39 The adequacy of BVM ventilation may be assessed by looking for chest rise and condensation on the mask due to the return of humidified exhaled air.

Laryngeal Mask Airways

The first supraglottic device was the laryngeal mask airway (LMA) classic invented by Dr Archie Brain in 1981. This first-generation LMA device was a reusable device, but a disposable version was soon developed. The LMA consists of a ventilation tube and an elliptical mask with a cuff, pilot tube, and balloon. Aperture bars in the mask prevent the epiglottis from obstructing ventilation. When inserted, the LMA moves along the hard then soft palate to the hypopharynx and then resides in the proximal esophagus, ultimately acting as a “mask” for the glottic opening with the distal tip located posterior to the cricoid cartilage and the proximal portion against the base of the tongue. Advantages of LMA use during general anesthesia include less irritation to the vocal cords and decreased hemodynamic changes upon awakening. Since its introduction, the use of LMAs has been extended from purely elective use to rescue situations where a patient cannot be ventilated. Other variations in the LMA have evolved and include the flexible LMA, ProSealLMA, LMA supreme, and LMA Excel. The Fastrach LMA was designed to aid with intubation as well as ventilation. Other supraglottic devices such as the Ambu laryngeal mask, Air Q intubating laryngeal airway, Cobra perilaryngeal airway, and others are based on the same concept as the LMA.

Combitube

The Combitube was developed for patients that could not be intubated by traditional laryngoscopy. The device is designed to be inserted blindly. Ventilation can be supported regardless of whether the distal portion of the tube is placed in the esophagus or trachea. The tube is a disposable, double lumen, and double cuffed device with separate pilot balloons for the proximal and distal cuffs. Approximately 95% of the time the distal tip of the tube enters the esophagus allowing the lungs to be ventilated via the proximal lumen. Less than 5% of the time, the tip of the tube enters the trachea and ventilation occurs via the distal lumen. Disadvantages of the Combitube include the inability to suction the trachea when the distal tip of the tube is in the esophagus. Contraindications for the use of the Combitube include patients with esophageal disease, suspected caustic substance ingestion, and history of esophageal varices.^41,42

Tracheal intubation consists of three steps: visualization of the larynx, placement of the ETT proximal to the larynx, and then advancement of the ETT distal to the glottis.

Visualizing the larynx

Laryngoscopic visualization of the vocal cords is made easier when the patient is placed in the sniffing position. The sniffing position, which is defined as flexion of the neck on the body and extension of the head on the neck, may be achieved by elevating the head in the extended position.⁴³ In the sniffing position, the oral and pharyngeal are placed into greater alignment facilitating direct visualization of the glottis (Figure 17–3). Modifications to patient positioning such as ramping up of the morbidly obese patient can lead to improved visualization as well (Figure 17–4).

Direct laryngoscopy involves using a laryngoscope, which contains a handle, blade and light source. Laryngoscope blades come in a variety of shapes and sizes. The most commonly used blades include the Macintosh and Miller blades. The curved Macintosh blade requires insertion in the right corner of the mouth, sweeping the tongue to the left with concurrent insertion, and placement of the tip into the vallecula, which indirectly elevates the epiglottis. The Miller, or straight blade, is inserted under and directly lifts the epiglottis. The choice of the blade is typically operator dependent; however, a straight blade that directly lifts the epiglottis may work better in certain situations such as with a patient with a large floppy epiglottis.⁴⁴

Confirmation of Endotracheal Placement of ETT

The best confirmation of tracheal placement of an ETT is by directly visualizing the tube passing through the vocal cords. Other confirmatory measures may be utilized including bilateral breath sounds, rise and fall of the chest wall, and condensation on the ETT. Capnography detects the presence of carbon dioxide in the exhaled air and is another positive indicator of tracheal intubation. False negatives may occur in the presence of decreased pulmonary blood flow, such as with a cardiac arrest or with airflow limiting processes, such as bronchospasm.

Rapid Sequence Intubation

RSI is typically utilized in situations where the risk of aspiration is deemed to be high due to known or suspected full stomach, physiological conditions, such as late-term pregnancy, and pathological conditions, such as bowel obstruction or severe ascites. The goal of RSI is to decrease the risk of aspiration by minimizing the time between loss of airway reflexes, which occurs upon induction of anesthesia, and protection of the airway by ETT placement. RSI is accomplished by preoxygenation, application of cricoid pressure, administration of an induction agent and paralytic, and then intubation when the paralytic has come into full effect. The goal of applying downward pressure at the cricoid cartilage is to prevent regurgitation of stomach contents by collapsing the esophagus between the trachea and spine. The utility of this maneuver has recently been called into question and may in the future be removed from the algorithm.⁴⁰ If cricoid pressure obscures the view of the vocal cords, it should be stopped immediately. In order to lessen the time between induction and intubation, only fast acting induction agent and paralytic should be chosen, and the two agents should be administered in rapid succession. As discussed in “Intravenous Anesthetics,” SCh provides paralysis with a fast onset and short duration. When a contraindication to SCh exists, “double-dose” rocuronium (twice the usual intubating dose of 0.6 mg/kg, ie, 1.2 mg/kg) provides an onset of paralysis nearly as fast as SCh, but with a prolonged duration of action. Ventilation with a BVM is typically avoided in order to prevent insufflation of the stomach with air, which increases the risk of aspiration. However, if the risk of injury from hypoxemia is judged to be greater than the risk of aspiration, manual ventilation should be utilized. In addition, injury from hypercarbia may also necessitate ventilation if the intubation is not immediately successful. Table 17–4 outlines various conditions where ventilation during RSI may need to be utilized. RSI is contraindicated in the setting of a known difficult airway. In this circumstance, an awake intubation will maintain protective airway reflexes while the airway is being secured.

Videolaryngoscopy

Recently, a number of devices that utilize a high-resolution camera and a small monitor screen have been developed to improve the view and the success rate of intubation over that of direct laryngoscopy. All of these devices allow a view of the glottis without requiring alignment of the oral and pharyngeal axes (Figure 17–3). The learning curve of these newer devices appears to be less than that of direct laryngoscopy, especially for novice laryngoscopists.⁴⁵

Fiberoptic Intubation

The gold standard for management of an anticipated difficult airway is fiberoptic intubation. Fiberoptic intubation via the nasopharynx as compared to the oropharynx is technically easier because of the gentler curve toward the glottis, but the high vascularity of the nasal passage increases the incidence of bleeding which can interfere with fiberoptic visualization of the glottis. Subsequently, an inherent weakness of the fiberoptic bronchoscope is that the camera view can easily be obscured by any contamination of the larynx. In addition, fiberoptic bronchoscopy is a modality that is not easily utilized during a truly emergent intubation.

In an awake, nonemergent fiberoptic intubation the patient is instructed about the procedure. The upper airway is topicalized with local anesthetic such as 4% lidocaine. Mild sedation typically aids in the patient tolerating the intubation; however, care must be taken not to over-sedate the patient, causing respiratory depression. Pretreatment with glycopyrrolate, a potent antisialagogue, decreases secretions. If time allows, a topical vasoconstrictor to the nares can help prevent bleeding. If the oral approach is attempted, a bite block will prevent damage to the fiberoptic scope. Once the tip of the scope is advanced past the glottis the ETT is advanced. Frequently, full advancement of the tip of the ETT into the trachea is hindered by the arytenoids. Gentle pressure and a clockwise rotation usually allow passage into the trachea. The correct position of the tube can then be confirmed by bronchoscopy.⁴⁶

Other Techniques

In patients that are spontaneously breathing and cannot be intubated orally or have failed traditional oral intubation, a blind nasal intubation can be utilized to secure the airway. This procedure became popular during pre-hospital intubations when the use of paralytics was restricted. Typically the ETT is lubricated to aid in insertion. If time allows, the nasal passage way can be pretreated with a vasoconstricting agent to minimize bleeding. As the ETT is advanced from the nasopharynx to the oropharynx, the transmission of breath sounds via the tube are used to guide advancement through the glottis. Complications of this procedure include epistaxis, sinusitis, turbinate injury, perforation of the pharynx, and failed intubation and loss of airway. Contraindications include patients who are not spontaneously breathing and suspected basilar skull^47,48 or neck injuries.

LMAs have officially been identified as a rescue device during difficult intubation as well as ventilation. The Fastrach LMA was specifically designed to allow ventilation as well as to facilitate intubation. It differs from the classic LMA by having an anatomically curved rigid airway tube, integrated guiding handle, epiglottic elevating bar, and a guiding ramp. A Fastrach LMA with an internal diameter of 13 mm can accommodate up to an 8.0 sized lubricated ETT. The insertion technique of the Fastrach LMA is similar to other LMAs; however, after insertion, it is recommended to first rotate the Fastrach LMA in the sagittal plane to align with the glottis opening and then to move the metal handle vertically to prevent pressure on the arytenoids. Once in place with the cuff inflated, this LMA can be used to ventilate the patient. The ETT is inserted blindly through the LMA, although this method can be modified by use of the fiberoptic bronchoscope if blind insertion is not successful. After insertion, the LMA must be removed very carefully with an obturator holding the tube in place. In certain circumstances, the LMA may be deflated and left in place if risk of tube displacement is high. Complications of the intubating LMA include esophageal intubation, injury to the larynx and teeth, and aspiration of gastric contents.^49,50

If a patient cannot be intubated, one of two options remains. The first is to continue ventilating the patient and to allow them to awaken from sedation. The second option is to proceed to a surgical airway, which may need to be performed emergently if the patient cannot be ventilated or intubated. Cricothyroidotomy involves placement of an airway device by piercing the cricothyroid membrane. The prevalence of out-of-hospital cricothyroidotomy placement by emergency services varies based on location—from 4/100,000 overall up to 10% in a single center study.^51,52 The cricothyroid membrane is located above the cricoid cartilage and 1 to 1.5 fingerbreadths below the thyroid cartilage in the midline of the neck. The airway can be entered by a surgical technique that involves a vertical skin incision, blunt dissection down to the trachea, and a horizontal incision in the trachea. Otherwise, the airway is entered by a percutaneous puncture technique. Confirmation of successful placement of the airway into the trachea involves capnography, chest auscultation and a confirmatory chest x-ray. Generally, the cricothyroidotomy is converted to a regular tracheostomy within 24 hours, provided the patient has stabilized.⁵³

Despite proper care and suctioning, an ETT may need to be exchanged for a host of reasons such as development of a pilot balloon leak or tube obstruction. One method to exchange the tube is to induce anesthesia with or without a paralytic, remove the ETT, and reintubate the patient. Even if the patient was originally an easy intubation, it is not uncommon for airway swelling and edema to make a subsequent reintubation more difficult. If the patient was originally a difficult airway or the patient's condition has changed and a difficult airway is now anticipated, a safer method of changing the ETT may involve using an airway exchange catheter (AEC). These catheters are thin, elongated, hollow tubes. Various sizes and diameters are available ranging from the Cook AEC (8-11 Fr) to the Aintree intubating catheter (14Fr). These tubes come with a ventilation port that can be used to oxygenate the patient if reintubation becomes prolonged.⁵⁴ The AEC is inserted through the ETT until resistance is encountered, usually between 30 and 40 cm. Some studies now suggest that the AEC should not be inserted more than 23 cm to prevent injury to the lung.⁵⁵ The indwelling ETT is removed and a new ETT is advanced over the catheter. Advancement of the tip of the ETT can be impeded by the posterior arytenoids. Gentle rotation in a clock-wise fashion will typically allow passage. The first pass success rate of reintubation over an AEC is 80% to 90%. If unable to pass the tube into the trachea, direct or indirect laryngoscopy may be required for reintubation. If that fails, a surgical airway may be emergently required. Complications associated with AEC use are direct injury to the airway by the catheter, which can on rare occasion lead to a pneumothorax.^56,57

Critically ill patients are intubated for a host of different reasons that range from respiratory failure to prophylactically securing the airway. Objective clinical criteria and weaning predictors help predict if a patient is ready to be extubated. The required and optional criteria are listed in Table 17–5.⁵⁸ The rapid shallow breathing index (RSBI), which compares the respiratory frequency to the tidal volume (f/V_T), is the most commonly used and best studied weaning parameter. A RSBI of more than 105 breaths/min/L suggests that that patient is not ready to be liberated from the ventilator. As a patient is being weaned from support on the ventilator, they can be placed on a spontaneous breathing trial (SBT) either with a progressive pressure support wean with or without PEEP or with a T-piece trial. The rationale for performing a SBT with decreasing/minimal pressure support rather than no support with a T-piece trial is that the lumen of the ETT tends to become occluded over time, and this occlusion can cause resistance to air flow movement that is equivalent to a tube that is 1 to 4 times smaller.⁵⁹ Additional support for using low-level pressure support comes from a study that took patients who were failing a T-piece trial; after being placed immediately on pressure support ventilation at 7 cm of H₂O for 30 minutes, 21 of 31 patients were successfully weaned.⁶⁰ Common causes of weaning failures are volume overload, cardiac dysfunction, neuromuscular weakness, delirium, anxiety, metabolic disturbances, and adrenal insufficiency, in addition to unresolved lung pathology.

Following a successful weaning trial and the achievement of minimal ventilator settings, the patient's ability to protect their airway and the patency of the airway must be assessed. The ability to protect the airway is dependent on the strength of the patient's cough and inversely correlated with the required frequency of suctioning.⁶¹ Airway patency is commonly demonstrated with a positive cuff leak prior to extubation. A cuff leak refers to airflow around the ETT after the cuff balloon has been deflated. The absence of a leak suggests that the space between the ETT and larynx is reduced, which places the patient at risk for postextubation stridor. A cuff leak can be quantitatively determined by placing the patient on full mechanical support and measuring the difference between the inspired tidal volume and the expired tidal volume. A difference less than 110 mL or 12% to 24% of delivered tidal volume suggests that airway patency is diminished.⁶²

Once the decision to extubate the patient has been made, the patient should be seated in an upright position. The oral cavity and ETT should be suctioned. The patient is instructed to inhale deeply and during exhalation the cuff balloon is deflated and the tube is removed. Typically, an orogastric tube is removed simultaneously with the ETT. The oral cavity is again suctioned after extubation and the patient is placed on supplemental oxygen. If the concern for postextubation failure/complication is high and the airway was not easily secured, an AEC can be left in place after extubation. Early aggressive management with bronchodilators, suctioning, diuresis, and/or NIPPV can prevent reintubation. The caveat is that NIPPV applied after the onset of postextubation complications appears to be ineffective and may be harmful; therefore, patients who are anticipated to need NIPPV after extubation should be extubated directly to NIPPV.⁶³

Severe Sepsis and Septic Shock

Indications for intubating patients with severe sepsis and septic shock are to reduce the work of breathing, treat hypoxic respiratory failure, and for airway protection in patients with septic encephalopathy. Typically, these patients must be assumed to be a full stomach with a RSI as the preferred method of intubation. Etomidate is typically used as the induction in hypotensive patients because it causes less vasodilation than propofol. However, a growing amount of evidence argues against its use in this patient population.^64,65,66

Head Injury

Head-injured patients are specifically at a higher risk of worsening neurologic injury with increasing ICP. The act of laryngoscopy, intubation, and tracheal suctioning has been shown to induce a sympathetic response,⁶⁷ which causes an increase in the systemic blood pressure and subsequently can lead to increased ICP. A balance must be obtained between RSI, which tends to elicit more hemodynamic lability, but may protect against aspiration, versus a slower and gentler induction and intubation, which may mitigate increases in blood pressure and ICP, but may increase the risk of aspiration. A complicating aspect is the fact that hypotension is also detrimental to these patients. Increasing the amount of narcotic administered during induction can block the sympathetic response with minimal effect on systemic blood pressure; however, the sedative effects will last longer than with propofol or etomidate and may interfere with the postintubation neurological exam. Esmolol is an ultrashort acting β-blocker that can block sympathetic simulation; however, care must be taken to not cause hypotension.⁶⁷

Lidocaine is a local anesthetic that can also block the sympathetic response to intubation for a few minutes with minimal effect on blood pressure and effect on the postintubation neurological exam.^68,69 Muscle relaxation with SCh might cause a temporary rise in ICP.⁶⁷ Pretreatment with a low dose of nondepolarizing neuromuscular blocker (NDMB) can attenuate this effect; however, this technique should not be used in an anticipated difficult airway because even that small dose of a NDMB can interfere with waking a patient up after a failed intubation attempt and may drive the algorithm toward a surgical airway. The patient with suspected head injury should be positioned with the head up at least 30° to facilitate venous drainage and mitigation of the ICP.⁷⁰ Hyperventilation should only be utilized for the emergent treatment of impending cerebral herniation.

Cervical Spine Injury

All trauma patients are assumed to have a cervical spine injury unless proven otherwise. Studies have estimated that all airway devices cause some degree of extension of the cervical spine. In a cadaver study, the McCoy laryngoscope caused maximal extension at the C₀-C₁ level and the Macintosh caused maximal extension at the C₂-C₃ level.⁷¹ While the least amount of cervical manipulation occurs with fiberoptic intubation, direct laryngoscopy may still be required in the patient with an unstable C-spine if the airway must be secured emergently.⁷² In order to secure the airway without causing movement at the C-spine, manual inline stabilization is mandatory. The C-collar is removed and an assistant stands at the head of the stretcher and gently, but firmly maintains the head in a neutral position during laryngoscopy.⁷³

Cardiac Arrest

The latest ACLS guidelines from the American Medical Association emphasize good chest compressions and ventilation with end-tidal carbon dioxide (EtCO₂) monitoring over placement of an advanced airway.⁷⁴ If BVM ventilation is ineffective, then a supraglottic devices, such as a LMA can be placed fairly quickly. If endotracheal intubation is to be attempted, it is usually done without any sedation or paralysis. Tube placement is confirmed by visualizing the ETT going through the vocal cords as well as with auscultation and positive capnography. The positivity of capnography may be diminished due to decreased pulmonary blood flow.

Airway management is the process by which differing degrees of respiratory failure are treated. Mild respiratory failure may only require supplemental oxygen via a nasal cannula, while more severe forms of respiratory failure may require endotracheal intubation. The intensivist must be facile with all aspects of securing the airway including assessment of the anticipated ease or difficulty of ventilation and intubation, the role of RSI, the anesthetic agents to be utilized, and the advanced equipment available for placing an ETT in the trachea. Of even greater importance is the ability to recognize the unanticipated difficult airway and to call for help immediately when difficulty with intubation or especially ventilation is encountered. In addition to securing the airway, the intensivist must be able to evaluate the safety and appropriateness of removing an ETT. These basic principles of airway management are frequently utilized in the intensive care and are foundational to the care of those critically ill with respiratory processes or in need of airway protection.