Emergency Airway Management

Respiratory distress is a common complication of acute illness. It may result from a primary pulmonary disorder, such as acute respiratory distress syndrome, or be a sequela of another disease, such as volume overload owing to acute renal insufficiency or sepsis. Respiratory distress may necessitate support with control of the airway and positive pressure ventilation. Failure to maintain adequate oxygenation and ventilation can lead to brain injury and death.

Requirements for Safe Airway Management

Regardless of the nature of the respiratory distress, a few components to airway management are essential to safe and effective practice. A checklist should be provided at the bedside of critically ill patients so that items are not missed in acute scenarios ( Box 49.1 ). It is also recommended that a time-out be performed prior to securing a patient's airway and essential equipment named and located prior to induction of anesthesia. This will ensure that the operator is prepared and familiar with the surroundings and equipment and will allow safe airway management for the patient.

Box 49.1

Checklist Prior to Safe Emergency Airway Management

Supplemental oxygen applied to patient
Suction
Direct laryngoscopy blade
Videoscope (if indicated)
Endotracheal tube (multiple sizes available)
End-tidal CO ₂ analyzer
Bag-valve-mask
Pulse oximetry
Blood pressure monitoring
Functional intravenous line
Induction drugs
Vasopressors

Supplemental Oxygen

Preoxygenation should be performed on any patient requiring invasive airway management. For any patient who has been rendered apneic, there is a finite period before oxygen desaturation occurs. Preoxygenation is defined as providing supplemental oxygen (inspired fraction of oxygen, FiO _2, approximately 1.0) prior to induction of anesthesia and intubation, allowing for an increased time of apnea before the patient desaturates. It is imperative to perform preoxygenation, allowing safe airway management and preventing hypoxemia; however, preoxygenation of critically ill patients has been found to be less effective than preoxygenation of healthy patients. Preoxygenation can be performed by applying a bag-valve-mask with a one-way inhalation and exhalation port (FiO ₂ 1.0), face mask at maximal flow rate (FiO ₂ 0.9), or bilevel positive airway pressure (BiPAP) with FiO ₂ set to 1.0. A high-flow nasal cannula has been advocated to preoxygenate healthy patients or patients with mild to moderate hypoxemia but has not been shown to be effective at preoxygenating patients who are critically ill with severe hypoxemia. The supine position and obesity both decrease functional residual capacity (FRC), leading to desaturation during apnea. Studies have demonstrated that preoxygenation in the head-up or sitting position improves preoxygenation and lengthens the time before desaturation during intubation in patients. If a patient is being supported with BiPAP prior to intubation, it is advised to continue BiPAP as the method of preoxygenation, as derecruitment and alveolar collapse can occur, resulting in shunting and ineffective preoxygenation with its discontinuation. Noninvasive ventilation may also lead to improved preoxygenation with less time to reach an expired fraction of oxygen (FeO ₂ ) greater than 0.9 in patients not previously supported with BiPAP. With adequate preoxygenation, up to 10 minutes of apnea can be maintained prior to desaturation. However, the amount of time before a patient desaturates is also dependent on delivery of oxygen, oxygen demand, oxygen extraction, and functional residual capacity. Apneic oxygenation was first described by Frumin et al. in 1959 and can allow up to 30 to 55 minutes of adequate oxygenation during apneic periods. Because of the favorable benefits of apneic oxygenation during difficult intubations, a high-flow nasal cannula at 60 L/min has recently been advocated during intubation and in one study has shown to lengthen the time before desaturation during periods of apnea, prior to securing the airway. However, other studies have not shown a difference. More clinical trials are needed before this technique becomes standard practice during intubation.

Functioning Intravenous Line

A functioning intravenous (IV) catheter is mandatory during airway control in order to administer induction agents and emergent vasoactive medications. In the event of cardiac arrest with an endotracheal tube in place and loss of a functioning IV, some medications can be administered directly through the endotracheal tube, including naloxone, atropine, vasopressin, epinephrine, and lidocaine (mnemonic: NAVEL).

Monitoring

Airway management should occur in a monitored environment, with continuous blood pressure monitoring (invasive or noninvasive), electrocardiogram (ECG), and pulse oximetry. Ideally, hemodynamic stability should ensue prior to induction of anesthesia and invasive airway management, as induction of anesthesia and institution of positive pressure ventilation can worsen the hemodynamic profile of the patient.

Suction

During airway management, continuous suction should be available and near the patient. After induction of anesthesia, gastric contents can arise from the stomach and suction may be immediately required to withdraw the contents from the oropharynx to prevent aspiration of the gastric contents. Additionally, patients may have secretions obscuring the view of the laryngeal anatomy and preventing instrumentation of the airway. A rigid suction tip (Yankauer) should be used during induction of anesthesia and instrumentation of the airway. If thick secretions are found after endotracheal intubation, a soft endotracheal suction catheter may be used to clear material from the airways.

Vasoactive Medications

Vasopressors and inotropes should be readily available, as hypotension is a common occurrence after induction of anesthesia. Hypotension after airway management is often multifactorial in a patient with respiratory distress and may be related to vasodilatory properties of induction agents, institution of positive pressure ventilation, or reduction in catecholamines after relief from hypoxemia or hypercarbia. Hypotension may be short lived, or may require institution of vasoactive infusions after airway management.

Airway Evaluation and Preparation

When a patient is going to require invasive mechanical ventilation and airway management, the clinician should assess difficulty of securing the airway. Difficulty can be encountered during any part of the intubation process, including bag-mask ventilation, direct or indirect laryngoscopy, or tracheal intubation. It is important to note that increased difficulty with airway management is encountered in the intensive care unit (ICU) more than in the operating rooms, with three or more attempts at tracheal intubation increasing from 0.9% to 1.9% in the operating room to 6.6% to 9.0% in the ICU.

Definition of a Difficult Airway

Difficult Bag-Mask Ventilation.

Unless rapid-sequence intubation is being performed, bag-mask ventilation is often performed prior to intubation in order to maintain adequate oxygenation and ventilation prior to airway instrumentation. Difficulty may be encountered during bag-mask ventilation due to anatomic or operator features and may range along a continuum from no difficulty to impossible. Difficult bag-mask ventilation is signified by manipulations required for effective gas exchange, including adjustments of the head and neck, the use of adjuvants (oral and nasal airways), use of exaggerated jaw lift, and two-handed face-mask application with assistance of a second operator. Impossible bag-mask ventilation may be encountered if no gas exchange exists despite all additional manipulations.

Difficult Laryngoscopy.

Laryngoscopic exposure using direct or indirect laryngoscopy is typically graded using the Cormack-Lehane scale ( Fig. 49.1 ). Similar to bag-mask ventilation, difficulty encountered during laryngoscopy proceeds along a continuum from no difficulty to impossible. Generally, a grade 1 view of laryngeal structures indicates no difficulty in laryngoscopy, while a grade 3 or grade 4 view represents difficult or failed laryngoscopy, respectively. It is important to note whether any maneuvers were required to obtain the view (Sellick maneuver, head and neck positioning), as all laryngeal views are vulnerable to nonreproducibility and worsening if the patient's positioning and optimization maneuvers are not reproduced, a circumstance in which the critically ill patient in extremis requiring airway instrumentation may be most at risk. Notably, the incidence of grade 3 and grade 4 laryngeal views increases from 0.8% to 7% in the operating room to 11% in ICUs.

Fig. 49.1, Four grades of laryngoscopic view. The grading of laryngoscopic view is based on the anatomic features that are visualized during the performance of direct laryngoscopy.

Difficult Tracheal Intubation.

Despite a laryngoscopic view that appears favorable to tracheal intubation (i.e., grade 1 or grade 2), difficulty in subsequent tracheal intubation may be encountered. For this reason, difficulty in laryngoscopy and difficulty in tracheal intubation should be assessed independently. Difficulty in tracheal intubation can be defined as one of the following: multiple attempts or more than one operator required; or use of an adjunct, such as a tracheal tube introducer or alternative intubation device required after unsuccessful use of the primary device (i.e., changing from direct to indirect laryngoscopy after unsuccessful attempt).

Failed Airway.

A failed tracheal intubation is defined as failure to achieve successful tracheal intubation in a maximum of three attempts , regardless of the technique(s) used. In the most extreme circumstances, a failed airway may be encountered where failed oxygenation has occurred (cannot intubate, cannot ventilate) in which the patient cannot be successfully ventilated (by bag-mask ventilation or a supraglottic airway) and cannot be intubated despite multiple techniques.

History

If the patient is able to communicate or there is a family member available to communicate with, it is important to take a history relevant to the airway prior to induction of anesthesia. This focused history will include a history of prior intubations, difficulty with prior intubations, and a history of prior tracheostomies. If the patient has been intubated previously, any available records of that intubation should be reviewed, including equipment used and laryngeal view during intubation. Additional topics pertinent to the airway history include large weight loss or weight gain since prior intubation (large weight gain since last intubation may reflect more difficulty with the current intubation attempt), history of obstructive sleep apnea, radiation therapy to the head and neck (impairing neck mobility and mouth opening), and cervical spine abnormalities (impairing neck mobility).

Physical Examination

A physical examination directed to airway management should always be performed. Three easy-to-perform tests have emerged as highly predictive indicators of intubation difficulty: Mallampati class, thyromental distance, and atlanto-occipital extension. The Mallampati classification evaluates the size of the tongue in relation to the size of the oral cavity ( Fig. 49.2 ). The Mallampati score exists on a continuum; in general, a Mallampati I airway is favorable, whereas a Mallampati IV is less favorable. The thyromental distance—the distance from the anterior larynx (neck) to the mandible (chin)—is a predictor of difficult intubation. Generally, a thyromental distance of greater than or equal to 3 cm or the width of 3 fingerbreaths is acceptable. A thyromental distance that is less than or equal to 3 cm or less than 3 fingerbreaths is a predictor of a difficult intubation. Last, the atlanto-occipital joint extension is an important predictor. Normally, 35 degrees of atlanto-occipital joint extension is possible. If restricted extension is encountered, difficulty with intubation may be predicted. While the presence of one of these factors may indicate some difficulty with intubation, the presence of multiple factors (e.g., Mallampati IV, short thryomental distance, reduced atlanto-occipital extension) is highly predictive of difficulty with airway management.

Fig. 49.2, Mallampati classification. Classification of the upper airway relating to the size of the tongue to the pharyngeal space based on the anatomic features seen with the mouth open and the tongue extended.

The history and physical should be performed together to anticipate the difficulty of bag-mask ventilation, laryngoscopy, or tracheal intubation. Predictors of difficult bag-mask ventilation are described in Box 49.2 , and predictors of difficult laryngoscopy are described in Box 49.3 .

Box 49.2

Predictors of Difficult Bag-Mask Ventilation

Obesity
Older age
Male gender
Limited mandibular protrusion
Reduced thyromental distance
Mallampati class III or class IV
Presence of a beard
Edentulousness
History of snoring or obstructive sleep apnea
History of neck radiation
Limited head and neck extension

Box 49.3

Predictors of Difficult Laryngoscopy

Limited mouth opening
Limited mandibular protrusion
Reduced thryomental distance
Mallampati class III or class IV
Limited head and neck extension

Anesthesia Induction

For most advanced airway management techniques, rendering a patient unconscious with the induction of anesthesia is desired. There are numerous medications to facilitate induction of anesthesia, which will be reviewed here. First and foremost, hemodynamic stability should be achieved prior to induction of anesthesia, as this process can result in further hypotension and hemodynamic compromise owing to the vasodilatory effects of anesthetic agents and loss of sympathetic drive following withdrawal of endogenous catecholamines.

Induction Agents

Propofol.

Propofol is a potent hypnotic agent that is commonly used for the induction and maintenance of anesthesia in the operating room and ICU. Propofol directly activates GABA _A receptors, inhibits the N-methyl-D-aspartate (NMDA) receptor and modulates calcium influx through slow calcium ion channels. Propofol has a rapid onset of action and rapid recovery. The hypnotic effect of propofol is dose related, with small doses causing sedation and increased doses causing unconsciousness and apnea. Propofol decreases cerebral oxygen consumption, reduces intracranial pressure, and has anticonvulsant and bronchodilatory properties. The most common complications of propofol include irritation and discomfort during IV administration and dose-dependent hypotension. Rarely, allergic complications have been reported and are attributed to the metabisulfite found in some formulations, added to retard bacterial and fungal growth. The propofol formulation also includes soybean oil, glycerol, and egg phosphatide. Concern exists for cross-reactivity in patients allergic to egg and/or soybean oil. However, current evidence does not support a direct link and the use of propofol should be used with caution but is not contraindicated in these patients. Propofol is metabolized in the liver by conjugation to glucoronide and sulfate and is eventually excreted via the kidneys. The induction dose of propofol is typically 1 to 2 mg/kg. However, in critically ill patients, this dose may be excessive and 0.1 to 0.4 mg/kg may be effective and safer.

Etomidate.

Etomidate is a hypnotic agent first introduced in 1972 after being initially developed as an antifungal agent. Etomidate binds to GABA _A receptors, potentiating the effects of GABA. In addition, etomidate is an agonist at central α ₂ -receptors, thereby maintaining vascular tone and myocardial contractility following an induction dose. Etomidate maintains cardiovascular stability during induction of anesthesia and may be selected for this profile. Etomidate is metabolized by hepatic esterases and excreted via the kidney. Etomidate has also been found to cause adrenal suppression by 11-β-hydroxylase inhibition even after single-dose administration, but whether this increases mortality in critically ill patients remains unknown. A systematic review and meta-analysis after single-dose administration of etomidate in septic patients suggested that mortality is not increased but was largely based on potentially biased observational data. The standard induction dose of etomidate is 0.2 to 0.3 mg/kg and, while hemodynamic stability is favorable, etomidate can lead to hypotension in any patient maintaining one's cardiac output (CO) with a high level of endogenous catecholamines.

Ketamine.

Ketamine is a unique dissociative anesthetic that produces amnesia and analgesia. Ketamine acts by noncompetitive antagonism of the NMDA receptor. Ketamine also acts via a complex interaction with µ- and κ-opioid receptors, is an agonist of dopamine-D ₂ receptors, interacts with 5-hydroxytryptamine (5-HT ₂ ) receptors, and is an antagonist of muscarinic and nicotinic receptors. Ketamine has sympathomimetic effects and can cause hypertension and tachycardia. In a patient maintaining hemodynamic stability with maximal endogenous sympathetic tone, ketamine can lead to hypotension owing to depletion of catecholamines. The standard induction dose of ketamine is 2 to 5 mg/kg IV or 1 to 5 mg/kg intramuscularly (IM). With small doses of ketamine (0.5–2 mg/kg), respiration and airway reflexes remain intact, which may be advantageous in a difficult airway, as spontaneous ventilation may be maintained. Ketamine is a bronchial smooth muscle relaxant and therefore may be beneficial in asthmatic patients. Ketamine is metabolized in the liver by microsomal enzymes to metabolites that are then altered to the glucoronide form and excreted into the urine. Adverse reactions of ketamine include perceptual abnormalities, disruption of some cognitive and sensorimotor function, mood changes, and delirium/psychosis.

Rapid Sequence Induction

For patients at high risk of aspiration (such as inadequate nothing-by-mouth [NPO] status, esophageal pathology), a rapid-sequence intubation (RSI) should be performed. All other patients should undergo anesthetic induction with bag-mask ventilation attempted, as it is the safest technique and allows maintenance of hemodynamic stability, oxygenation, and ventilation that results in a controlled environment for reliable and safe intubation. RSI differs from other anesthetic induction, as the anesthetic agent and paralytic are given in immediate succession, without attempts at bag-mask ventilation to avoid inflation of the stomach and subsequent aspiration of gastric contents into the trachea. RSI is not well suited for anticipated difficult airways or hemodynamically unstable patients.

Induction Agents.

Any of the previously discussed induction agents can be used in RSI. Each will produce a dose-dependent hypnotic state; however, hemodynamic profiles vary. In a multicenter randomized controlled trial, ketamine was found to have a similar hemodynamic profile but less adrenal suppression than etomidate in RSI. Despite the choice of induction agents, a hypnotic state should be obtained after the initial dose of induction agent, as the paralytic will follow in immediate succession and the patient should be rendered unconscious by the time paralysis occurs.

Neuromuscular Blocking Agents.

For RSI, a neuromuscular blocking agent with rapid onset is desired. This can be achieved with either a depolarizing neuromuscular blocker, succinylcholine, or a nondepolarizing neuromuscular blocking agent, rocuronium. Succinylcholine is the most commonly used neuromuscular blocker in RSI. It has a rapid onset of action (40–60 seconds) and a short duration, lasting 6 to 10 minutes. Succinylcholine is degraded by pseudocholinesterase. Adverse effects of succinylcholine include hyperkalemia; it is contraindicated in major burns (starting 48 hours after the burn), major crush injuries (starting 48 hours after injury), severe abdominal sepsis, denervation syndromes, muscular dystrophy, and spinal cord injuries. It is also contraindicated in known hyperkalemia and malignant hyperthermia. The intubating dose (for RSI or otherwise) of succinylcholine is 1 to 2 mg/kg. Alternatively, rocuronium can be used to create similar intubating conditions. A standard intubating dose of rocuronium is 0.6 mg/kg; however, when larger doses are used (1 to 1.2 mg/kg), an onset of action similar to succinylcholine can be achieved (40 to 60 seconds). Unfortunately, the duration of action of rocuronium is longer (37 to 72 minutes) and can be prolonged in myasthenia gravis, hepatic disease, kidney disease, and neuromuscular disease. In a systematic review and meta-analysis, succinylcholine was found to be a superior neuromuscular blocking agent for RSI, leading to superior intubating conditions.

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