Predictors of Difficult Mask Ventilation

Difficult mask ventilation is encountered in 8% of patients. Patients at risk for such difficulty include those with preexisting partial airway obstruction, snorers, those with facial asymmetry, the obese, persons with Mallampati class IV airway anatomy (complete lack of epiglottis visualization), and the edentulous. Management of patients with facial hair, which often hides a receding mandible and in itself makes a seal for mask ventilation difficult, can present significant problems.

Predictors of Difficult Intubation

Difficult intubation is common in the ICU population, with frequency of reported difficulty between 6.6% and 22%. A history of airway problems should be sought—for example, snoring, sleep apnea, congenital diseases (such as Down or Pierre-Robin syndrome), and previous anesthetic problems.

Examination of the patient should determine the following: ability to protrude the mandible, range of neck movement, atlantooccipital flexion and extension, interincisor distance (less than 3 cm indicates a high likelihood of difficulty), and modified Mallampati test (Figure 34-1). Other predictors of difficult intubation include thyromental distance of less than 7 cm (Patil’s test) and obesity. Further investigations when indicated could include the view of the larynx obtained at nasal endoscopy, which may predict the view at laryngoscopy; chest radiographs, which may show tracheal deviation or mediastinal masses; and CT scans, which may be useful when abnormal anatomy is suspected—for example, in association with tracheal stenosis.

Figure 34-1 Mallampati test to classify view of the pharynx. For this test, the patient is asked to protrude the tongue fully while opening the mouth maximally, with the head in the neutral position. Class I: The pharyngeal pillars, soft palate, and uvula are visible. Class II: Only the soft palate and uvula are visible. Class III: Only the soft palate is seen. Class IV: Only the hard palate is seen. Mallampati class correlates with Cormack and Lehane grade for the view at laryngoscopy (see Figure 34-5). Increasing class suggests more difficult laryngoscopy.

Aspiration Risk

Aspiration of gastric contents can cause significant morbidity and mortality. Evidence suggests that reducing gastric volume and increasing pH of gastric contents will limit the risk of disorders associated with aspiration. Acid aspiration may lead to a chemical pneumonitis, but aspiration of food particles can result in physical obstruction of the bronchial tree with secondary bacterial pneumonia (Box 34-1 and Table 34-1).

Table 34-1 Drugs to Reduce Aspiration Risk

Positioning

Putting the patient in the optimal position is the first and most important step. Appropriate positioning allows good access to the airway and allows efficient preoxygenation. Other advantages include improvement of the laryngoscopic view and, with the head held slightly up, a reduced risk for aspiration of gastric contents. The “head-up” position also increases functional residual capacity by allowing better diaphragmatic excursion, thereby increasing oxygen reserves. The patient should be positioned with the neck flexed on a pillow, but with the head extended, so long as cervical spine injury is not suspected. Positioning is of particular importance in the obese patient (Figure 34-2).

Figure 34-2 Positioning the obese patient. Correct positioning is important to optimize the view during laryngoscopy. Flexion of the lower cervical spine brings the trachea in line with the pharynx, and extension at the atlantooccipital joint aligns the trachea with the oral cavity. With the obese patient in the supine position, neck movement and access with a laryngoscope are hindered by fat. When the patient is repositioned with the shoulders elevated and the occiput further elevated so that the head assumes a “sniffing” position, access to the airway is facilitated.

(From Wiener-Kronish JP, Shimabukuro DW: Airway management. In Albert RK, Spiro SG, Jett JR, editors: Clinical respiratory medicine, ed 3, Philadelphia, 2008, Mosby.)

Bag Mask Ventilation

In an elective setting, patients who are adequately fasted may be anesthetized and administered a nondepolarizing muscle relaxant. While awaiting the onset of muscle relaxation, the patient is oxygenated by positive-pressure bag mask ventilation before endotracheal intubation. In other circumstances, a profoundly hypoxic patient may need bag mask ventilation before intubation—for example, in a cardiac arrest scenario. Implementation of this ventilatory mode is therefore a key, lifesaving skill for all physicians to possess. Successful bag mask ventilation requires an open airway, a good seal with the face mask, and the application of positive pressure achieved by compressing the reservoir bag connected to high-flow oxygen.

In patients at risk for aspiration, rapid sequence induction can be performed. With the classic technique, the initial step is formal preoxygenation of the patient, followed by the administration of a predetermined dose of rapid-onset general anesthetic agent and then a rapid-onset muscle relaxant. Cricoid pressure is applied, and the patient is intubated as soon as muscle relaxation has occurred.

Preoxygenation

Before induction of anesthesia, the patient should be fully preoxygenated. The patient breathes 100% oxygen through a tight-fitting face mask for 3 minutes. In an emergency, five vital capacity breaths of 100% oxygen can be used in an attempt to fill the functional residual capacity lung compartment with oxygen, thereby significantly raising the alveolar PO₂. This step prolongs the time to desaturation after the onset of apnea, affording the operator significant time to manage the airway safely (Table 34-2).

Cricoid Pressure

A major concern with an emergency intubation is the risk of aspiration of gastric contents. Cricoid pressure can be applied to prevent passive regurgitation of stomach contents, with subsequent airway soiling. The assistant exerts firm pressure (equivalent to 30 newtons [N] of backward pressure) on the cricoid cartilage to compress the esophagus against the body of the sixth cervical vertebra as the level of consciousness diminishes (Figure 34-3). Cricoid pressure should be removed only if the patient actively vomits (associated with risk for esophageal rupture) or in specific instances of inability to oxygenate (see the difficult intubation algorithm presented in Figure 34-7).

Figure 34-3 Cricoid pressure. The cricoid cartilage is identified by palpation below the thyroid cartilage. Firm pressure is placed on this structure to occlude the esophagus against the body of the sixth cervical vertebra. Pressure is maintained until after intubation is accomplished and airway control is achieved.

(From Wiener-Kronish JP, Shimabukuro DW: Airway management. In Albert RK, Spiro SG, Jett JR, editors: Clinical respiratory medicine, ed 3, Philadelphia, 2008, Mosby.)

Equipment

The most common laryngoscope blade used for intubation in adults is the curved Macintosh blade (Figure 34-4). This is inserted into the right side of the mouth to displace the tongue laterally. The tip of the blade sits in the vallecula and is lifted forward to elevate the epiglottis and expose the laryngeal inlet. The McCoy blade is a variant of the Macintosh and has a hinged tip to further lift the epiglottis. A straight Miller blade can be useful in adults in whom the epiglottis is difficult to displace. This blade is inserted further to directly lift the epiglottis. Where laryngoscopy may be difficult a video laryngoscope can be used. There are now a variety of different models available on the market (Figure 34-5). They require less manipulation of the cervical spine and allow intubation in a seated position, reducing the aspiration risk (Videos 1-4). Awake fiberoptic intubation may be used were visualization of the larynx may be difficult by other means or if access to the mouth is restricted.

Figure 34-4 Airway management equipment. A, Laryngoscope blades: (i), The Macintosh blade size 3, the most commonly used blade for intubating an adult. (ii), The McCoy blade size 3. Note the hinged tip, which can be activated by depressing the lever; this can be used to elevate the epiglottis when the view of the laryngeal inlet is obscured. (iii), The Miller blade size 3, a straight blade that is used to lift the epiglottis directly with the tip, to obtain a view of the larynx. B, Oropharyngeal airways in the most common sizes used in adult patients: size 4 (red), 3 (orange), and 2 (green). C, Nasopharyngeal airways in different sizes, from top: 8.5F, 7.5F, and 6.5F. The size chosen will depend on the size of the patient and appropriate length.

Figure 34-5 Video laryngoscopes. Indirect laryngoscopy performed using a video laryngoscope allows intubation with less manipulation of the cervical spine and often provides an improved view of the laryngeal inlet when problems have been encountered with direct laryngoscopy. A, The Venner AP Advance, a handheld laryngoscope with a screen built into the handle. B, The Storz C-Mac scope. C, The Glidescope. The instruments shown in B and C both are handheld laryngoscopes with a separate video display screen.

Laryngeal Anatomy

The view of the laryngeal inlet obtained at direct laryngoscopy can be graded in accordance with the Cormack and Lehane classification (Figure 34-6). Grade I and grade II views indicate straightforward intubation. A grade III view frequently necessitates the use of a gum elastic bougee to “railroad” the endotracheal tube (ETT) into position, whereas a grade IV view often will mandate a different strategy to achieve successful intubation.

Figure 34-6 Cormack and Lehane grading of laryngoscopic views: Grade I: The entire laryngeal inlet is visible. Grade II: The arytenoids and posterior vocal cords are seen. Grade III: Only the epiglottis is visible, Grade IV: The epiglottis is not visualized. Grade III and grade IV views suggest difficulty in passing an endotracheal tube. They often will mandate the use of a bougee or an alternative intubation technique.

Rescue Techniques

Whenever the decision is taken to intubate, the physician should always have an alternative plan to use if any difficulties are encountered. In the United Kingdom, the difficult intubation algorithm produced by the Difficult Airway Society is used (Figure 34-7).

Figure 34-7 Difficult Airway Society guidelines for airway management. If the initial intubation plan (Plan A) fails, no more than three attempts at direct laryngoscopy should be performed; an alternative method should be used to facilitate tracheal intubation and maintain oxygenation. Should these techniques fail, then oxygenation is the main priority while the patient is woken up, if appropriate (Plan C). Should it be impossible to intubate or ventilate the patient using any of these methods, emergency transtracheal ventilation or cricothyroidotomy should be performed (Plan D).

(Courtesy Difficult Airway Society, Association of Anaesthetists of Great Britain and Ireland, London.)

Mask Ventilation

When intubation is difficult or impossible, oxygenation of the patient remains the main priority. Mask ventilation (with application of cricoid pressure if aspiration is a risk) with 100% oxygen should be attempted. If this proves to be difficult, an airway adjunct, such as a Guedel oropharyngeal airway, should be inserted. A four-handed technique may be used, with two hands holding the mask and maintaining the patient’s airway while an assistant operates the bag used to ventilate the patient.

Laryngeal Mask Airway

The laryngeal mask is a supraglottic device that has revolutionized anesthesia in the past 25 years. It is inserted orally and when inflated sits in the posterior hypopharynx, pushing the tongue base anteriorly and keeping the glottis open. Its design has been modified and improved over time. Using the laryngeal mask airway (LMA) to ventilate a paralyzed patient in the operating room is now considered acceptable, particularly since the advent of such masks with a gastric port to allow venting of any refluxed gastric secretion. The use of LMAs in resuscitation is promoted in certain practice areas owing to its ease of insertion.

The LMA is extremely useful in clinical situations in which mask ventilation would be difficult or when intubation is impossible. It can be used to maintain oxygenation while a plan for definitive airway control is made, or it can be used as a conduit for access to the larynx. A fiberoptic scope can be inserted through the LMA and an ETT passed over the scope into the trachea. The intubating LMA is specifically designed to facilitate tracheal intubation; special features include an epiglottic elevating bar facilitating access to the trachea. It can be useful in patients in whom neck movement is limited (Figure 34-8).

Figure 34-8 Laryngeal mask airway (LMA), available in sizes 1 to 5. Usually, the LMA is inserted fully deflated, with the tongue and mandible gently pulled anteriorly, with continuous pressure posteriorly until a firm end point is reached. Position is confirmed after cuff inflation by the ability to ventilate the patient, as noted by chest rise and auscultation. Most adults will accept a size 4 or 5 airway. The LMA allows ventilation and placement of an endotracheal tube by means of a fiberoptic bronchoscope if direct laryngoscopy has failed. A, Size 3, partially inflated. B, Size 4, completely deflated. C, Size 5, completely inflated.

(From Wiener-Kronish JP, Shimabukuro DW: Airway management. In Albert RK, Spiro SG, Jett JR, editors: Clinical respiratory medicine, ed 3, Philadelphia, 2008, Mosby.)

Cricothyroidotomy

In situations in which it is not possible to either intubate or ventilate the patient, an emergency cricothyroidotomy should be performed (Figure 34-9). The cricothyroid membrane is an avascular membrane joining the thyroid cartilage to the cricoid cartilage. A purpose made kink-resistant cannula can be inserted through the cricothyroid membrane into the trachea, and then the patient is jet ventilated through this tube. Larger cricothyroidotomy devices can be used that allow the patient to be ventilated in a conventional fashion. Surgical cricothyroidotomy requires more skill and involves dissection down to the cricothyroid membrane, followed by insertion of an appropriately sized endotracheal tube (6-mm inner diameter) into the trachea. Cricothyroidotomy is associated with multiple complications, including airway loss, esophageal trauma, bleeding, false passage formation, pneumothorax, and surgical emphysema.

Figure 34-9 A, Transtracheal ventilation. The cricothyroid membrane is palpated below the laryngeal prominence of the thyroid cartilage. The membrane is punctured with a catheter while aspirating for air. The catheter is advanced caudally into the trachea after the needle is removed. The catheter is then attached to a high-pressure oxygen delivery system. Exhalation is passive, relying on a patent upper airway. B, To perform a cricothyroidotomy, a 3-cm vertical skin incision is made below the thyroid notch; then the cricothyroid membrane is incised with a scalpel blade and the opening dilated with use of the handle, and a small endotracheal tube is placed in the airway.

(From Wiener-Kronish JP, Shimabukuro DW: Airway management. In Albert RK, Spiro SG, Jett JR, editors: Clinical respiratory medicine, ed 3, Philadelphia, 2008, Mosby.)

Tracheostomy

Surgical tracheostomy requires a skilled surgeon. It can be performed to sidestep impending upper airway loss in the awake patient with use of local anesthesia. It is not suitable as a rescue technique because it is time-consuming. Boxes 34-2 and 34-3 lists the indications for and possible complications of surgical tracheostomy.

Percutaneous Tracheostomy

Percutaneous tracheostomy is favored by many intensive care physicians because it can be performed at the bedside—useful when an unstable patient is too ill for transfer to the operating room—and because the tracheostomy cannula can be inserted by physicians, thus avoiding the logistics of arranging for a surgeon. Other advantages include less tissue damage, lower risk of bleeding, and lower infection rates. Factors making percutaneous tracheostomy an unsuitable choice include difficult anatomy, short neck, and abnormal vascular anatomy.

The intubated patient is positioned with the neck extended while being preoxygenated with 100% oxygen. Two skilled operators are required, one to perform the procedure and another to manage the airway. Local anesthetic is infiltrated and a small incision is made below the cricoid cartilage. Blunt dissection is performed until the trachea is easily palpable to the operator. A fiberoptic bronchoscope is inserted down the ETT, and a needle attached to a syringe is inserted into the trachea between the second and third tracheal rings under direct vision. Air is aspirated through the syringe, and a wire is threaded through the needle into the trachea. A dilator is passed over the wire to enlarge the tract. The tracheostomy tube is then inserted over the wire.