Definition and Incidence of the Difficult Airway

Key Points

The inability to manage a difficult airway (DA) is responsible for a large proportion of deaths and morbidity directly attributable to anesthesia.
While it is important to have common definitions for common problems, the literature continues to report variable nomenclature related to DA management. It is important to have an understanding of the incidence of significant airway problems for the specialty to make advances in this area.
Difficult mask ventilation (DMV) has been reported at a rate of 1 to 2 per 100 anesthetics, and impossible mask ventilation (IMV) can be expected at a rate of 1 to 2 times per 1000 anesthetics.
Most studies report the failure rate for supraglottic airway (SGA) devices at approximately 2% for the classical laryngeal mask airway (CLMA) and flexible LMA (FLMA) and 1% for the intubating LMA (ILMA), unique LMA (ULMA), and ProSeal LMA (PLMA). A 2015 study showed difficulty in 0.5% of SGA uses and failure of the device 0.2% of the time.
Fewer data exist regarding failure for video-assisted laryngoscopy (VAL) than for direct laryngoscopy (DL), but the incidence of failed intubation with these devices ranges from 0.4% to 2.6% in patients with a normal airway and 2.4% to 3.6% for patients with a DA.
When difficult laryngoscopy is defined as a Cormack-Lehane (CL) grade 2 or 3 view requiring multiple attempts or blades, the reported incidence varies from 1% to 18% of surgical cases, but the majority of these patients are successfully intubated.
Unsuccessful intubation with DL occurs at a rate of 5 to 35 per 10,000 anesthetics, and the cannot intubate/cannot oxygenate (CICO) scenario occurs at a rate of 0.01 to 2 patients per 10,000 anesthetics.
Large multicenter studies will be required to refine our understanding of the incidence of serious airway problems.

Introduction

The fundamental responsibility of an anesthesiologist is to ensure adequate gas exchange for the patient. Failure to maintain oxygenation for more than a few minutes could result in catastrophic anoxic injury. Data from closed claims of respiratory-related malpractice in 1990 reported brain damage or death in over 85% of patients. In the closed claims data from 2006, subsequent improvements in airway management techniques and monitoring standards reduced the number of intubation-related claims, but difficulties with airway management during emergence remain among the leading causes of serious perioperative problems. It has been estimated that the inability to successfully manage a difficult airway (DA) is responsible for as many as 30% of deaths and the majority of cardiac arrests directly attributable to anesthesia.

In general, greater degrees of difficulty in maintaining airway patency are more likely to engender greater risk of brain damage or death. Before discussing the specific management of a DA, we must (1) define the DA, (2) classify the degrees of difficulty in maintaining a patent airway, and (3) determine the incidence of each class or type of DA. In this discussion, it is assumed that a reasonably well-trained anesthesia provider always attempts to maintain airway patency.

Definition and Classification of the Difficult Airway

There are three common ways of maintaining airway patency and gas exchange. First, mask ventilation (MV) delivers inspired gas via a mask that is sealed to the patient’s face, while the natural airway from the face to the vocal cords is kept patent with or without external jaw thrust maneuvers or internal upper airway devices. Second, inspired gas can be delivered via a supraglottic airway (SGA), such as a laryngeal mask airway (LMA). Third, with tracheal intubation, inspired gases are delivered via a tube that traverses the vocal cords, providing continuity from the respiratory circuit to the trachea. Maintenance of airway patency via a surgically inserted device is not discussed in this chapter.

The term difficult airway spans a spectrum of clinical situations ( Fig. 8.1 ) from difficulty or inability to ventilate the patient with a face mask or SGA to difficulty or inability to intubate the trachea. The combined “cannot intubate/cannot oxygenate” (CICO) scenario carries the highest risk of brain damage or death. To better describe the layers of difficulty, we have chosen several categories: difficult mask ventilation (DMV) or impossible mask ventilation (IMV); difficult placement of or ventilation via an SGA; difficult laryngoscopy; and difficult intubation (DI) using a direct laryngoscope, videolaryngoscope (VL), or flexible intubation scope (FIS), such as a flexible bronchoscope.

Fig. 8.1, Conceptualization of the difficult airway. Difficult intubation (DI) is one or more of the following: difficult laryngoscopy, difficult video-assisted laryngoscopy, or difficult flexible scope visualization. The widespread use of supraglottic airways (SGAs) has elevated them to immediate rescue devices in cannot intubate/cannot oxygenate situations. The triad of difficult mask ventilation, difficult SGA placement, and DI increases the risk of hypoxic brain injury and death.

Difficult or Impossible Face-Mask Ventilation

Causes of Difficult Mask Ventilation

There are two main causes of inadequate MV. One cause is an inability to establish an adequate seal between the face and the mask, causing a leak of respiratory gas. The second cause is inadequate patency of the airway at the level of the nasopharynx, oropharynx, hypopharynx, larynx, or trachea. These manifest as either an inability to generate adequate airway pressure to drive gas into the lungs or an inability to move gas into the lungs despite an adequate driving pressure.

Definition of Difficult Mask Ventilation

The American Society of Anesthesiologists (ASA) Practice Guidelines for Management of the Difficult Airway define DMV as the inability “to provide adequate ventilation (as confirmed by end-tidal carbon dioxide detection, for example) because of one or more of the following problems: inadequate mask seal, excessive gas leak, or excessive resistance to the ingress or egress of gas.”

The most commonly cited MV scale in contemporary literature was described by Han and colleagues in 2004 ( Table 8.1 ). In this scale, the progressive grades of difficulty are (1) ventilated by mask; (2) ventilated by mask with oral airway or other adjuvant, with or without muscle relaxant; (3) DMV defined as “inadequate, unstable, or requiring two providers,” with or without muscle relaxant; and (4) IMV with an inability to mask ventilate, with or without muscle relaxant. Langeron and colleagues defined DMV as “the inability of an unassisted anesthesiologist to maintain oxygen saturation >92%, as measured by pulse oximetry, or to prevent or reverse signs of inadequate ventilation during positive-pressure MV under general anesthesia.” In that study, MV was considered difficult if one or more of six criteria were present:

inability of the unassisted anesthesiologist to maintain oxygen saturation >92% using 100% oxygen and positive-pressure MV;
significant gas flow leak by the face mask;
need to increase gas flow to >15 L/min and to use the oxygen flush valve more than twice;
no perceptible chest movement;
need to perform a two-handed MV technique; or
change of operator required.

Table 8.1

Han Mask Ventilation Scale and Incidence of Difficult Mask Ventilation

Grade	Description	Incidence, n (%)
1	Ventilated by mask	37,857 (71.3)
2	Ventilated by mask with oral airway/adjuvant, with or without muscle relaxant	13,966 (26.3)
3	Difficult ventilation (inadequate, unstable, or requiring two providers) with or without muscle relaxant	1,141 (2.2)
4	Unable to mask ventilate with or without muscle relaxant	77 (0.15)

El-Ganzouri and colleagues defined DMV as the “inability to obtain chest excursion sufficient to maintain a clinically acceptable capnogram waveform despite optimal head and neck positioning and use of muscle paralysis, use of an oral airway, and optimal application of a face mask by anesthesia personnel.”

Incidence of Difficult Mask Ventilation

Two studies by Kheterpal and colleagues on DMV and IMV represent the largest investigations to date on the topic. The incidence of DMV was 1.4% in 22,660 patients and 2.2% in a subsequent study of 50,000 patients. ^, The incidence of IMV ranged from 0.15% to 0.16% in these two large studies. ^, Langeron and colleagues reported a 5% incidence of DMV with 1 of 1502 patients impossible to ventilate with a face mask (0.07%). Thomsen and colleagues reported a DMV or IMV (Han grade 3 or 4) incidence of 0.07% in 658,104 anesthetic records. Other large prospective studies show an incidence ranging from 0.07% to 1.4%, although this was not always the primary outcome being assessed. ^, ^, In summary, DMV can be expected between 1 and 2 times per 100 anesthetics, and IMV can be expected between 1 and 2 times per 1000 anesthetics.

Techniques for improving airway patency include the head-tilt, jaw-thrust, and chin-lift maneuvers, as well as insertion of oral or nasal airways. If mask seal is poor, the practitioner may choose a different face mask, use a two-handed or two-person technique, insert bolsters between the alveolar ridge and the cheeks, or employ other methods to improve the interface between the face and the mask. When two providers are needed, ideally the primary practitioner stands at the patient’s head and initiates jaw thrust with the left hand at the angle of the left mandible and left-sided mask seal, while the right hand compresses the reservoir bag. The standard position for the primary practitioner is shown in Fig. 8.2 . The secondary (assisting) person stands at the patient’s side, at the level of the patient’s shoulder, facing the primary practitioner. The right hand of the secondary practitioner should cover the left hand of the primary practitioner and contribute to left-sided jaw thrust and mask seal, and the left hand of the secondary person initiates right-sided jaw thrust and mask seal. In this way, all four hands are doing something important without interfering with one another, and there is almost no redundant effort. With this positioning, the secondary person can watch the monitors continuously, manipulate the larynx externally, and hand equipment to the primary practitioner.

Fig. 8.2, Optimal two-person mask ventilation effort. The primary practitioner stands at the head of the patient and uses left and right hands in the standard classical fashion. The secondary (assisting) person stands facing the primary practitioner at the level of the patient’s shoulder and uses the right hand to help achieve left-sided jaw thrust and mask seal while the left hand achieves right-sided jaw thrust and mask seal.

Difficulty With Supraglottic Airways

SGAs have become a mainstay of airway management. They have a role in routine airway management and are an essential part of all difficult airway algorithms (DAAs). Most studies of SGAs describe first-attempt and overall success rates. Difficulties with the devices include failure of insertion, failure to form a clear passage to the trachea from obstruction or laryngospasm, and failure to form an effective seal in the airway. ^,

Most of the studies examining difficulties with SGAs focus on LMAs (Teleflex, Inc., Morrisville, NC). Among these, the failure rate is 1% for the LMA Unique (ULMA), intubating LMA (LMA Fastrach; ILMA), and the LMA ProSeal (PLMA), and it is 2% for the LMA Classic (CLMA) and LMA Flexible (FLMA). ^,

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