Awake Tracheal Intubation in Patients with Morbid Obesity: When, Why, and How?

by Jeremy Collins, Mb. ChB., FRCA, and Jay B. Brodsky, MD

Dr. Collins is Clinical Assistant Professor, Department of Anesthesia, Stanford University, Stanford, California. Dr. Borodsky is Professor of Anesthesia, Stanford University Medical Center, Stanford, California.

Bariatric Times. 2010;7(4):8–10

Abstract
Patients with morbid obesity possess one or more anatomical features associated with difficult mask ventilation and rapidly develop hypoxemia when this is inadequate or impossible. A higher incidence of difficult intubation has also been suggested in patients with morbid obesity, and so some clinicians choose awake intubation as their primary strategy for airway management. In many cases this is unnecessary. The decision to intubate before or after anesthetic induction should consider several key factors which will be discussed and techniques of awake intubation in patients with morbid obesity are suggested.

Introduction
Every anesthesiologist fears the situation of total inability to manage a patient’s airway. When this occurs in a patient with morbid obesity (MO), the rapid development of severe hypoxemia exaggerates the critical nature of such an event. Recent data from the American Society of Anesthesiologists (ASA) Closed Claim Database justify this concern.[1] Patients with obesity were involved in 37 percent of all adverse airway events occurring during anesthetic induction, and in 58 percent of those events occurring following tracheal extubation. A large proportion of these situations resulted in either brain damage or death.

The difficult airway was defined by an ASA task force as the clinical situation in which a conventionally trained anesthesiologist experiences problems with face mask ventilation of the upper airway, tracheal intubation, or both.[2] They included a Difficult Airway Algorithm (DAA), a document that has become the foundation of airway management practice. The DAA provides a rational decision tree to follow when a difficult airway is either anticipated or arises unexpectedly. When difficulty is predicted, the algorithm may start with attempts at tracheal intubation before or after anesthetic induction.

For awake tracheal intubation, “when” is deciding from which root point of the DAA to begin—either before or following anesthetic induction. There are no preoperative tests available to provide the clinician with enough certainty to make this decision easy. The practitioner must rely on clinical judgment and consider several key factors including the following: preoperative airway assessment and predicted levels of difficulty for both tracheal intubation and mask ventilation; aspiration risk; availability of equipment and assistance; and the individual’s self-assessment of his or her own skill, knowledge, and familiarity with MO. The “why” requires knowledge of the anatomical and physiological characteristics of obesity that provide an understanding of why problems can arise. Finally, the “how” considers the different management strategies, airway adjuncts, and techniques that can facilitate successful awake intubation.

When and Why
One could approach airway management of every patient with MO with a very simple answer, “awake intubation every time.” This is an extremely cautious, unrealistic approach. The majority of patients with MO present no difficulty with airway management.[5] Even if this approach was acceptable, not all patients would cooperate. In addition, awake intubation can be technically challenging and consumes valuable time and resources. Also, awake intubation is not always uneventful. Seven percent of the ASA’s closed claims associated with death or brain damage occurred following attempts at awake intubation.[1]

A more rational approach is necessary. Rosenblatt[6] has provided a logical way of deciding when an awake intubation should be chosen with his Airway Approach Algorithm. Responses to five clinical questions determine the entry point into the DAA. The questions are as follows:

1.    Must the airway be managed?
2.    Is there potential for difficult laryngoscopy?
3.    Can supralaryngeal ventilation be used?
4.    Is the stomach empty?
5.    Will the patient tolerate an apneic period?

1. Must the airway be managed?
Avoidance of a general anesthetic is desirable for surgical patients with obesity since a regional anesthetic will minimize cardiorespiratory risks and shorten recovery time. However, regional anesthesia is technically challenging in patients with MO and failure rates are high. And, of course, regional anesthesia may not be appropriate for a particular operation. Sedation techniques in patients with MO require great care since hypoxia will occur at even light levels and airway obstruction and collapse is a risk. For patients with MO requiring general anesthesia the airway should be secured with either a tracheal tube or laryngeal mask airway (LMA).

2. Is there potential for difficult laryngoscopy? While obesity does not predict difficult tracheal intubation per se, certain features have been associated with an increased risk of difficulty.[5,7–9] These features include the following:
•    High Mallampati score (3 or 4)
•    Increased neck circumference
•    Excessive pre-tracheal adipose tissue
•    Severe obstructive sleep apnea (OSA).

Even so, most patients who have some or even all of these features frequently pose no intubation problems.  Therefore, recommendations regarding a cut-off Mallampati score or gender-specific neck circumference that can be applied to clinical practice are lacking. In addition, these risk factors are derived from studies considering direct laryngoscopy, so their application to other intubation techniques may not be valid.

Intubation problems are said to occur as much as three times more frequently in patients with MO compared to patients who do not have MO,[3,10,11] but this statement is not supported by several studies.[5,12] The reason is that “difficult intubation” is not uniformly defined. Clinical studies of laryngoscopy and intubation are biased since patients preoperatively considered to be especially difficult are frequently excluded when clinical judgment suggests awake intubation; therefore, the prediction is never tested. The laryngoscopist cannot be blinded to the patient’s body habitus. Some studies have used an intubation difficulty scale (IDS) that includes potentially subjective elements. It is worth noting that studies using the IDS have found obesity per se to be a risk factor for difficult intubation,[11] whereas those using actual view on laryngoscopy have not.[5,13]

Many of these studies are from bariatric surgical populations, groups which typically consist of a larger proportion of female patients. Higher rates of OSA, a risk factor for laryngoscopy difficulties, occur more often in men. It is therefore possible that many studies are of females in whom the anatomical impact of obesity on airway management is reduced.

Lack of a standard intubating position for patients with MO is a further cause of confusion. The standard sniffing position does not produce the same alignment of axes for tracheal intubation in patients with MO as it does in patients with BMIs less than 30kg/m[2]. Patients with MO require more elevation of the head, neck, and shoulders so that the sternum and ears are aligned in a horizontal line (head-elevated laryngoscopy position). In this position the view during direct laryngoscopy is significantly improved.[14] Therefore, in some studies suboptimal positioning may have resulted in poorer views, hence more difficulty.

3. Can supralaryngeal ventilation be used? The consequences of failure to intubate may not be serious if oxygenation can be maintained by facemask or with LMA ventilation. Difficult or impossible facemask ventilation is rare, even in patients with MO. However, increased body mass index (BMI) and OSA are independent predictors for difficult mask ventilation.[15] Adipose tissue in the face and cheeks or the presence of facial hair can interfere with the application of a tight-fitting mask.

When mask ventilation fails, “rescue” alternatives, such as supralaryngeal devices (LMA, the Combitube) must be available.[16,17] These intra-oral devices avoid many of the factors that contribute to difficult mask ventilation and can allow successful, temporary ventilation.[18] They can serve as a bridge to, or a conduit for, tracheal intubation. They should not be used as the definitive airway since they may be inadequate at the higher pressures needed to ventilate a patient with MO with resulting oropharyngeal leaks or gastric distention.

4. Is the stomach empty? When tracheal intubation has failed, rescue supraglottic ventilation can only be maintained safely when there is no gastric aspiration risk. Some consider all patients with MO at risk for gastric aspiration, but for elective surgery the fasted patient with MO is probably at no greater risk than a patient with normal weight. Exceptions are post-gastric banding surgical patients due to high preoperative residual gastric volumes.[19] Patients with MO presenting for emergency and trauma surgery, and those with severe gastric reflux disease are also aspiration risks.

5. Will the patient tolerate an apneic period? Of the questions posed, this is the only one that can be answered with certainty. Obesity is associated with a marked reduction in lung volume and significant ventilation/perfusion mismatch. Low oxygen reserves and high metabolic rate results in rapid hemoglobin desaturation if difficulty is encountered. Even with maximal pre-oxygenation, severe hypoxia will develop before spontaneous respirations return after anesthetic induction, even when the short-acting muscle relaxant succinylcholine is used.

Considering the questions posed by the Airway Approach Algorithm provides a means for deciding on whether to proceed with anesthetic induction and direct laryngoscopy, or an awake intubation. As long as the patient is properly positioned, has been adequately pre-oxygenated, and a backup plan is considered, most patients with MO can be safely intubated following anesthetic induction. However, when there is a significant risk of aspiration or when there is any doubt regarding ability to intubate or to mask ventilate, an awake intubation is better. This is particularly true for the small subset of patients with MO who have features associated with both difficult mask ventilation and difficult intubation: massive central obesity, severe OSA, high Mallampati score, and short, wide neck.

How? Intubation Strategies for Awake Intubation
Caution should be always taken when administering sedative drugs since airway obstruction may occur even with minimal sedation, especially in patients with OSA. Application of local anesthesia to the vocal cords and infraglottic structures does not increase the risk of aspiration from a full stomach, provided that the patient is not oversedated. Adequate airway anesthesia is vitally important for success. Superior laryngeal nerve blocks and trans-tracheal injection of local anesthetic via the cricothyroid membrane can improve patient comfort. Nerve blocks of the airway can be technically challenging. Noninvasive techniques, such as nebulization, may result in administration of large doses of local anesthetic with unpredictable effects as solution is lost to the atmosphere. Atomization of 2 or 4% lidocaine does provide satisfactory conditions. Careful titration of the ultra-short acting opioid remifentanil or the selective alpha-2 adrenergic agonist dexmedetomine can provide satisfactory conditions for awake intubation in patients with MO.

Flexible fiberoptic laryngoscopy is the most common approach to awake intubation, but visualization of the vocal cords may prove difficult when excess fat deposits cause airway narrowing and redundant folds of tissue. Instructing the patient to both protrude the tongue and phonate can increase the diameter of the airway and aid visualization. Sitting utilizes the effect of gravity and may also increase the pharyngeal space. Using the nasal rather than the oral route provides a straighter passage of the fiberscope, but increases the risk of epistaxis if adequate vasoconstriction of the nasal mucosa is not achieved. Supplemental oxygen can be delivered through the working channel of the flexible scope. One must avoid passing the distal tip of the flexible scope beyond the carina because barotrauma can result following oxygen insufflation. The pharyngeal airway may be splinted open during nasal fiberoptic intubation by application of nasal continuous positive airway pressure (CPAP) on the contra lateral nostril.

If technical difficulties arise with flexible fiberoptic laryngoscopy, a LMA can be used as a conduit for the fiberscope. The LMA stents surrounding soft tissues open and permit a technically easier passage of the fiberscope. Positioning and inflation of the LMA can be uncomfortable, but asking the patient to swallow the device aids insertion. Once in situ, it is usually tolerated, although swallowing and salivation are common. As well as functioning as a conduit for fiberoptic scope, the LMA provides a means of delivering oxygen. With the LMA Proseal™ (The Laryngeal Mask Company Limited, Bonn, Germany) one can even deliver positive pressure ventilation during the intubation procedure in patients with MO.

Blind passage of a tracheal tube via the intubating (LMA Fastrach™, The Laryngeal Mask Company Limited) LMA is highly successful in MO patients and can be achieved in shorter time when compared with flexible fiberoptic intubation via the Ovassapian Airway (Hudson RCI, Durham, North Carolina). The newer LMA CTrach™ (LMA PacMed Pty Ltd., Burnley Vic, Australia) permits visualization of the glottic opening on a monitor while allowing oxygenation during the intubation process.[20]

In addition to these LMA devices, videolaryngoscopy is being utililized with increasing frequency. Alignment of the axes necessary for intubation by direct laryngoscopy is not necessary and since less force is used to obtain glottic exposure, these devices may prove more suitable for awake intubation. The Pentax Airway Scope™ (Pentax Corporation, Tokyo, Japan) and the Airtraq (Prodol Meditec S.A., Vizcaya, Spain) have been used in patients wth MO.

Conclusion
The airways of most patients with MO can be managed with a conventional approach. For a subset of patients considered difficult and for those at high risk for gastric aspiration, an awake intubation may be the safest means of establishing an airway.

References
1.    Peterson GN, Domino KB, Caplan RA, et al. Management of the difficult airway: a closed claims analysis. Anesthesiology. 2005;103(1):33–39.
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Tags: Awake intubation, Morbid obesity

Category: Past Articles, Review