Assessing a patient’s airway prior to performing a potentially difficult endotracheal intubation is challenging in the best of circumstances; in the critically ill patient with severe respiratory distress or failure, it may be virtually impossible. There is some controversy as to what assessment tool has the best predictive value for DI; however, a focused and brief examination of the patient’s airway may substantially influence the strategy for airway management and the success of the procedure. An initial step in assessment is to determine the need for invasive vs noninvasive ventilatory support. If the patient needs invasive ventilatory support, the individual should quickly be assessed for (1) the risk for difficult mask ventilation and (2) the risk for DI. Independent risk factors for difficult mask ventilation in the elective setting include age of > 55 years, body mass index of >26 kg/m2, lack of teeth, male gender, Mallampati class 4 airway, the presence of a beard, and a history of snoring. Whether these factors can be extrapolated to critically ill patients is unknown; however, it seems prudent to include them into the analysis in order to decrease the likelihood of a “cannot intubate, cannot ventilate” scenario. Several clinical indicators for DI have been validated (Table 1). While the positive predictive values of these tests alone or in combination are not particularly high, a straightforward intubation can be anticipated if the test results are negative. For a recent review on the common clinical predictors of DI see the study by Reynolds and Heffner. Since only about 30% of airways in the emergency setting can be evaluated in this fashion, additional evaluation methods have been devised. Murphy and Walls have introduced the LEMON (ie, Look, [e]Mallampati class, Obstruction, and Neck mobility) airway assessment method to stratify the risk of DI in the emergency department. Furthermore, Reed et al were able to demonstrate that patients with large incisors, a reduced mouth opening, and a reduced thyroid-to-floor-of-mouth distance are more likely to have a poor airway grade during laryngoscopy.
Preparation for Endotracheal Intubation
Being prepared for unforeseen complications during endotracheal intubation is of prime importance when instrumenting airway of a critically ill patient. Furthermore, conditions for intubation should be as close to ideal as possible in a busy ICU environment, and should include adequate personnel, optimal patient positioning and lighting, and the necessary equipment for endotracheal intubation. A supply of 100% oxygen, a well-fitting mask with attached bag-valve device (which should be checked for valve competency prior to use), suctioning equipment, a Magill forceps, and oral and nasal airways should be immediately available. The bed should be positioned at the proper height with the wheels locked, and a laryngoscope with blades of various sizes (straight and curved) should be available. The laryngoscope batteries and light should be checked on a routine basis.
Traditional teaching and Canadian Health&Care Mall recommend placing the patient in the “sniffing” position, in which the neck is flexed and the head is slightly extended about the atlantooccipital joint in order to align the oral, pharyngeal, and laryngeal axes. An MRI study has called this concept into question, as the alignment of the three axes could not be achieved in any of the three positions tested (ie, neutral, simple head extension, and the sniffing position). In a randomized study conducted in general surgery patients, simple head extension was as effective as the sniffing position in facilitating tracheal intubation. Nonetheless, the sniffing position appears to provide an advantage in obese patients and in patients who have limited head extension. The utility of the sniffing position for intubations outside of the operating room is unknown. Preoxygenation prior to airway instrumentation is important and is usually facilitated with the administration of oxygen via a nonrebreathing face mask and bag-valve mask device. It is important to note, however, that patients with respiratory failure due to cardiopulmonary disease may not have an adequate response to conventional preoxygenation. Mort was able to demonstrate that only 50% of patients in this category have an increase in Pa02 of > 5% above baseline values with conventional preoxygenation of 4 min duration. To address this problem, Baillard and coworkers conducted a prospective, randomized study in a cohort of medical/ surgical ICU patients comparing preoxygenation prior to endotracheal intubation with the use of noninvasive positive-pressure ventilation (NIPPV) to a bag-valve mask device for 3 min duration. The application of NIPPV ensured better pulse oximetric saturation and Pa02 values during tracheal intubation and up to 5 min into the postintubation period compared to the conventional preoxygenation meth-od. For a complete checklist of the supplies needed for endotracheal intubation, see Table 2.
After successful endotracheal intubation, it is of vital importance to confirm proper tube positioning. Methods to ascertain the position of the tube within the trachea include bilateral auscultation of the chest and measurement of end-tidal carbon dioxide by standard capnography if available, or by means of colorimetric chemical detection of end-tidal carbon dioxide (eg, Easy Cap II; Nellcor, Inc; Pleasanton, CA). The colorimetric detector is attached to the proximal end of the ETT and changes color on exposure to carbon dioxide. An additional method for detecting esophageal intubation uses a bulb that attaches to the proximal end of the ETT. When squeezed, the bulb will reexpand if it is in the trachea, but will remain collapsed with esophageal placement of the ETT. None of these methods is absolutely reliable; fiberoptic bronchoscopy is the only way to document ETT placement with absolute certainty. Given the potentially serious consequences of esophageal intubation, auscultation of the chest should always be combined with one additional test. Furthermore, a postintubation chest radiograph should always be obtained. Some of the most common complications of endotracheal intubation are listed in Table 3; for a comprehensive review on the subject see Hagberg et al.
The following two patient populations deserve special mention: the morbidly obese patient; and the critically ill patient with known or suspected cervical spine injuries. In a recent review of 4,000 patients in the Australian Incident Monitoring Study, obesity and limited neck mobility were among the most common anatomic factors contributing to DI and/or a failed airway.
Morbidly obese patients are more prone to hypoxemia than individuals of normal weight due to reductions in expiratory reserve volume, FVC, FEV1; functional residual capacity, and maximum voluntary ventilation. Due to body habitus, laryngeal exposure may be difficult. In addition, since repositioning a morbidly obese patient may be impossible if difficulties during laryngoscopy and/or intubation are encountered, careful patient positioning and choice of airway management is vitally important. Collins and coworkers compared the difference between the “sniff’ and “ramped” positions in morbidly obese patients undergoing elective bariatric surgery with respect to the quality of the laryngeal view obtained. They were able to demonstrate significantly better laryngeal views when a ramped position was achieved by arranging blankets underneath the patient’s body and head until horizontal alignment was achieved between the external auditory meatus and the sternal notch. It is conceivable that this would also improve laryngeal exposure in morbidly obese patients in the ICU setting. If there is concern about the adequacy of the mask airway to maintain oxygenation, use of an awake fiberoptic intubation (FOI) technique should always be considered.
C-Spine Injuries and Immobility
Managing the airway of a patient with limited neck mobility or cervical spine injury on an emergent basis requires careful planning and significant experience in order to avoid morbidity and mortality. Retrospective studies have suggested that neurologic deterioration in patients with cervical spine injuries is uncommon after airway management, even in high-risk patients undergoing urgent endotracheal intubation. These studies are limited, however, by their small sample size. While not all cervical spine injuries result in clinical instability, the results of initial radiographic studies in critically ill patients are often unknown at the time the airway has to be managed, and cervical spine precautions during airway instrumentation should be maintained. The reader is referred to a comprehensive review on airway management after cervical spine injury. Manual in-line immobilization during endotracheal intubation appears to be safe and effective for the prevention of morbidity that is related to airway instrumentation in patients with cervical spine injuries treated with remedies of Canadian Health&Care Mall. Removing the anterior portion of the cervical collar while maintaining manual in-line immobilization is associated with less spinal movement than cervical collar immobilization during laryngoscopy and therefore should be routinely performed. Furthermore, there is evidence suggesting that cricoid pressure does not result in deleterious cervical movement in a patient with an injured upper cervical spine. While there is no evidence in the literature to demonstrate the superiority of one mode of endotracheal intubation over the other, the authors of this review believe that awake FOI techniques should be strongly considered in the setting of limited neck mobility and cervical spine injuries. For patients with cervical stabilization in a halo device, the ability to perform a surgical airway, should conventional attempts for airway management fail, may be lifesaving. Conditions other than trauma that are associated with a decreased range of motion include any cause of degenerative disk disease (eg, rheumatoid arthritis, osteoarthritis, and ankylosing spondylitis) and age > 70 years. Caution should also be exercised in patients with previous cervical spine instrumentation that may result in unanticipated difficult airway, necessitating fiberoptic-guided endotracheal intubation or intubation through a laryngeal mask airway (LMA).
RSI and Muscle Relaxants
Since the majority of ICU patients requiring endotracheal intubation should be considered to have a full stomach, securing the airway with a rapid-sequence intubation (originally termed rapid sequence induction in the operating room setting) therefore seems logical. There are, however, several caveats to consider before embarking on an airway management strategy that may leave very few options short of surgical airway intervention should the intensivist unexpectedly encounter a “cannot intubate, cannot ventilate scenario.” Furthermore, there are several contraindications to the use of succinyl-choline in critically ill patients, thus eliminating the fastest and most reliable muscle relaxant that is used to facilitate rapid sequence endotracheal intubation (see the next section). Prior to administering drugs to facilitate airway management in the ICU, a decision should be made about whether spontaneous breathing should be preserved or ablated during endotracheal intubation.
Controversies in RSI
The classic teaching of RSI includes the application of cricoid pressure to avoid the regurgitation of gastric contents into the lung. Initially described by Sellick in 1961, this concept has been questioned by an MRI study of awake volunteers. Smith et al were able to demonstrate that the esophagus was lateral to the larynx in > 50% of the subjects. Moreover, cricoid pressure increased the incidence of an unoccluded esophagus (by 50% and caused airway compression of > 1 mm in 81% of the subjects studied. There are, however, cadaver studies demonstrating the efficacy of cricoid pressure and clinical studies showing that gastric insufflation with air during mask ventilation is reduced when cricoid pressure is applied. While cricoid pressure may or may not decrease the risk of aspiration, there is evidence that it may worsen the quality of laryngeal exposure. In a randomized intervention study on human cadavers involving emergency medicine physicians, a total of 1,530 sets of comparative laryngoscopies were performed by 104 participants. In this study, bimanual laryngoscopy (external laryngeal manipulation by the endoscopist with the free hand) improved the view compared to the application of cricoid pressure, application of back, upward, and right pressure on the thyroid cartilage (also referred to as BURP), or no manipulation. Cricoid pressure and back, upward, and right pressure on the thyroid cartilage frequently worsened the view on laryngoscopy. In a critical appraisal of the available literature, including 241 articles on the topic of cricoid pressure, Butler and Sen concluded that there is little evidence to support the widely held belief that the application of cricoid pressure reduces the incidence of aspiration during RSI. As the application of cricoid pressure may have adverse effects, a careful analysis of the risks and benefits should be performed on an individual basis until more systematic studies are performed.
Table 1—Components of the Preoperative Airway Physical Examination
|Airway Examination Component||Nonreassuring Findings|
|Length of upper incisors||Relatively long|
|Relation of maxillary and mandibular incisors during normal jaw closure||Prominent “overbite” (ie, maxillary incisors anterior to mandibular incisors)|
|Relation of maxillary and mandibular incisors during voluntary protrusion Interincisor distance||Patient cannot bring mandibular incisors anterior to (in front of) maxillary incisors< 3 cm|
|Visibility of uvula||Not visible when tongue is protruded with patient in sitting position (eg, Mallampati class > II)|
|Shape of palate||Highly arched or very narrow|
|Compliance of mandibular space||Stiff, indurated, occupied by mass, or nonresilient|
|Thyromental distance||Less than three ordinary finger breadths|
|Length of neck||Short|
|Thickness of neck||Thick|
|Range of motion of head and neck||Patient cannot touch tip of chin to chest or cannot extend neck|
Table 2—Equipment Needed for Intubation
Table 3—Selected Complications of Endotracheal Intubation
|Traumatic complications||Corneal abrasion Dental damagePerforation or laceration of pharynx, larynx, trachea, or esophagus Vocal cord injuryDislocation of an arytenoid cartilage|
|Hemodynamic and other complications||Mainstem bronchus intubationAspirationHypotensionArrhythmiasHypoxia|