Does the Airway Examination Predict Difficult Airway Management?

INTRODUCTION

Over the past few decades, advancements such as the laryngeal mask airway (LMA), the flexible intubation scope (FIS), and video-assisted laryngoscopy (VAL) have revolutionized the field of airway management. Nevertheless, difficulty with airway management still poses a challenge to anesthesia practitioners. A recent study analyzing claims in the Anesthesia Closed Claims Project database demonstrated that in the period from 2000 to 2012, adverse respiratory events accounted for 25% of claims against anesthesiologists.

VAL has been widely adopted over the last 20 years and has been shown to lead to improved glottic visualization, improved intubation success, and a high rate of success after failed direct laryngoscopy (DL). ^{, ,} Despite these advances, however, difficult intubation still remains the most common cause of respiratory events; of the respiratory events responsible for death or brain damage, difficult intubation accounted for 27%, followed by inadequate oxygenation (20%) and aspiration (15%). Overall, however, there has been improvement in the number of claims related to difficult intubation. The 7-year period from 1993 to 1999 contained 93 claims where difficult tracheal intubation was identified as the primary damaging event, whereas the 13-year period from 2000 to 2012 contained 102. Nevertheless, despite the decrease in the yearly number of claims related to difficult tracheal intubation, anesthetic-related deaths and brain damage are still most commonly caused by airway management problems. In fact, there has been a higher proportion of death as an outcome related to difficult intubation in the most recent time period of 2000 to 2012 claims (73%; n = 74 of 102) compared with 1993 to 1999 (42%; n = 39 of 93). In 76% of the more recent claims, preoperative predictors of a potentially difficult airway were present, and in 73% of claims with adequate data for assessment, judgment errors in management were present.

The 4th National Audit Project (NAP4) was a 1-year audit performed by the Royal College of Anaesthetists and the Difficult Airway Society of the United Kingdom from 2008 to 2009 that aimed to determine the incidence of major complications of airway management in anesthesia. It gave a point estimate of one airway-related death per 180,000 general anesthetic procedures and a 1 in 22,000 incidence of adverse airway events. In their editorial on the NAP4 report, O’Sullivan and colleagues suggest that the real incidence of an adverse airway event is likely to be more common than 1 in 5500 and may thus be experienced on a “regular” basis. Notably, the data from NAP4 demonstrated that a formal airway assessment was conducted in only 35 of 133 cases of airway-related events occurring during anesthesia (26%). When an airway assessment was performed, however, difficulty was anticipated correctly in the majority (i.e., in 25 of 35 cases). This is suggestive that an airway examination is worthwhile.

O’Sullivan and colleagues also point out that, in a review by Yentis in 2002, the aggregate positive predictive value of several trials on difficult airway predictors was roughly 0.25. ^, If the group identified as potentially difficult to intubate is regarded as having a “disease” in need of some form of specialized “treatment” for airway management (e.g., awake intubation), then this number needed to treat for preventing harm from failed intubation would be 4, which is much lower than other standard-of-care treatments.

In summary, studies such as the Closed Claims Project and NAP4 suggest that better prediction of and preparation for difficult airway management might lead to a reduction in adverse airway events. Preoperative evaluation of the airway can be accomplished by a thorough history and physical examination as related to the airway; in addition, various measurements of anatomic features and noninvasive clinical tests can be performed to enhance this assessment. In this chapter, we will review the multitude of airway assessment tests and discuss their utility in identifying and preparing for potential difficult airway management.

DESCRIPTIONS OF TERMS

Five terms are important to review and analyze in this area: failed intubation, difficult laryngoscopy, difficult tracheal intubation, difficult mask or supraglottic airway (SGA) ventilation, and difficult SGA placement. The American Society of Anesthesiologists (ASA) Task Force on Management of Difficult Airway suggests the following descriptions:

Failed intubation, or the inability to place an endotracheal tube after multiple intubation attempts, is a clear-cut endpoint. There is a fairly uniform reported incidence of failed intubation of approximately 0.05% of surgical patients or 1:2230 and approximately 0.13% to 0.35% of obstetric patients or 1:750 to 1:280. ^,

Difficult laryngoscopy is described as an inability to visualize any portion of the vocal cords after multiple attempts at conventional laryngoscopy, and many investigators use Cormack-Lehane (CL) grades III and IV or grade IV alone to define this ( Fig. 16.1 ). According to these definitions, the incidence of difficult DL ranges from 1.5% to 13% in patients undergoing general surgery. ^{, , , , , , ,}

Difficult tracheal intubation (DTI) is described as tracheal intubation requiring multiple attempts, in the presence or absence of tracheal pathology. The incidence of DTI is higher than failed intubation and has been reported to be 1.2% to 3.8%. ^{, , ,} DTI is often, but not always, the end result of difficult laryngoscopy, which depends on the operator’s level of expertise, patient characteristics, the type of laryngoscopy (direct or video-assisted), and specific circumstances. Thus it has been suggested that the definition of DTI be based on a uniform understanding of the best attempt at performing laryngoscopy/intubation and should use the number of attempts and time as boundaries only. A best attempt should incorporate the effect of changing the patient’s position; the effect of changing the length or type of laryngoscope blade; and the effect of simple maneuvers, such as backward, upward, rightward pressure (BURP) and optimal external laryngeal manipulation (OELM).

Difficult mask or SGA ventilation refers to the inability of an experienced airway practitioner to provide adequate face mask ventilation because of one or more of the following problems: an inadequate mask seal, excessive gas leakage, or excessive resistance to the ingress or egress of gas. It is clear from clinical experience that there are grades of difficulty, similar to DTI. The incidence of difficult mask ventilation (DMV) also varies in the literature from 0.08% to 15%. ^{, , , ,}

Difficult SGA placement is a situation in which inserting an SGA such as the LMA is difficult. It is defined as requiring multiple attempts to place an SGA to achieve adequate ventilation. Indices of clinically adequate ventilation are generally expired tidal volume greater than 7 mL/kg and leak pressure greater than 15 to 20 cm H ₂ O. Ramachandran and colleagues noted a 1.1% failure rate of the LMA Unique in a study of 15,795 patients in ambulatory and nonambulatory settings.

PREDICTION OF THE DIFFICULT AIRWAY: THE PROBLEM

As previously mentioned, despite the decrease in yearly claims related to difficult intubation, difficult airway management is still the most common cause of anesthetic-related deaths and brain damage. As such, it is meaningful to define the actual problem: Is control of the airway going to be difficult after induction of anesthesia? Although intubation of the trachea is usually the end goal of induction of general anesthesia in the operating room, what is more important is the ability to maintain oxygenation and ventilation. If the problem is judged to be that the patient will be difficult to intubate but easy to mask ventilate, it is appropriate to induce anesthesia and attempt to intubate according to a previously thought-out plan because failed attempts at intubation can be managed by ventilating the patient and then reattempting intubation. Analogously, if it is judged that the patient will be difficult to mask ventilate but easy to intubate, it would be acceptable to perform a rapid sequence intubation to control the airway without wasting efforts attempting to mask ventilate. Successfully navigating these scenarios relies on the judgment made after examining the patient and maintenance of oxygenation throughout attempts at securing the airway.

The incidence of DMV has been examined in several studies. Williamson and colleagues analyzed 2000 incident reports and indicated a 15% incidence of DMV in patients who had difficult or failed intubation. El-Ganzouri and colleagues found an incidence of 0.08% in their study of 10,507 patients and determined that approximately 100,000 patients would be required to apply a multivariate analysis. They defined DMV as the inability to obtain chest excursion sufficient to maintain a clinically acceptable capnogram waveform despite optimal head and neck positioning, use of muscle paralysis, use of an oral airway, and optimal application of a face mask. Langeron and colleagues observed a 5% incidence of DMV, defined as the inability of an unassisted anesthesiologist to maintain oxygen saturation at greater than 92% or to prevent or reverse signs of inadequate ventilation during positive-pressure mask ventilation (MV) under general anesthesia. Lower rates of DMV have been reported in prospective studies by Asai and colleagues (1%–4%), Rose and Cohen (0.9%), and El-Ganzouri and colleagues, as mentioned earlier. Obviously, a standardized definition is lacking for DMV, which could explain the variation in the incidence.

The incidence of intubation difficulty also varies according to the population studied and the definition of DTI used. There is no universally accepted definition of DTI. Most of the larger studies concentrate on DTI broadly defined as a difficult rigid laryngoscopic view (CL grades III and IV or grade IV only), without the best attempt used. To be useful, a classification of laryngeal view should predict difficulty (or ease) of tracheal intubation, which requires the views to be associated with increasing degrees of intubation difficulty. Nonetheless, in a study of 1200 patients, Arne and colleagues found a significant difference between the incidence of CL grades III and IV laryngoscopic views and the occurrence of DTI in the general population, because many of the grades III and IV views were actually easy intubations. Thus one of the problems in the prediction of the difficult airway is that a DTI is often not identified until laryngoscopy is performed; and, as previously mentioned, there are discrepancies in the literature as to what defines difficulty.

A meaningful evaluation of the available literature requires an assumption about a reasonable level of expectancy in terms of sensitivity and specificity of the tests used for prediction of DTI. Thus if at least 9 of 10 DTIs are to be predicted, a sensitivity of 90% would be required. Likewise, if one assumes that one false alarm a week is acceptable, in a hypothetical practice of 10,000 cases a year, it would correspond to a specificity of 99.5%. A number of investigators have attempted to achieve the goal of predicting difficult laryngoscopy or DTI, or both, by combining different predictors and deriving multivariate indices so that the occurrence of false-negative results is decreased and the PPVs are increased. ^{, ,} To date, however, no single multifactorial index can be applied to all of the various surgical populations.

EVIDENCE