Difficult Airway in the Neurosurgery Patient

Definition

Airway management is a core competency in anesthesiology, and a number of philosophies, algorithms, and tools have been established. Furthermore, airway management is influenced by the interaction between the clinical setting, patient factors, and practitioners’ skills.

A common definition is that a difficult airway is one in which a conventionally trained anesthesiologist experiences difficulty with mask ventilation, tracheal intubation, or both. Although we are aware that an increasing number of surgeries can be performed with supraglottic airways, we will focus on endotracheal tubes (ETTs) to ensure controlled ventilation of the neurosurgery patient. Difficulty in mask ventilation has been defined as an inability to maintain oxygen saturation above 90%. Some estimates state that such “cannot ventilate situations” occur approximately one in 700 times. Although no universal definition exists for difficult tracheal intubation , it has been defined as one requiring multiple attempts or requiring a change in plan. The incidence of a difficult airway in the general surgical population has been estimated to be between 5% and 15% based on the classical laryngoscopy, ^a

a Given the increase in number and availability in video assisted tools and fiber optics, implementation of the Cormack and LeHane criteria based on classical laryngoscopy may be becoming increasingly antiquated.

with a higher incidence occurring outside of the operating room. The “failed airway”—one requiring an alternative instrument or operator—has been estimated to be 3–5%.

In addition, we must also consider the largely neglected topic of extubating a difficult airway. After all, it is fair to say that in the majority of cases, a difficult airway has not become any easier through intubation. Furthermore, and often forgotten, is that nondifficult airways may, in fact, have become difficult intraoperatively as illustrated by the fact that some 80% of laryngeal injuries occur in nondifficult intubations. The Difficult Airway Society (DAS) has labeled extubation a “high-risk phase of anesthesia” as also evident in large national surveys and closed claims. While the definition of a difficult extubation refers to difficulty in removing the tube (e.g., due to a surgical stitch), at-risk of extubation failure , or simply at-risk extubation , refers to the risk of losing airway patency and/or ventilation. As a transition from a proactive and controlled situation to a somewhat uncontrolled and somewhat reactive situation, tracheal extubation is especially challenging in neurosurgery patients who often are a vulnerable and unpredictable population.

Causes of Difficult Airway

The American Society of Anesthesiologists (ASA) describes a number of difficult issues in airway management: patient cooperation and consent, difficult mask ventilation, difficult laryngoscopy, difficult intubation, difficult supraglottic airway placement, and difficult surgical airway access. We will ignore the last two points for neurosurgery but add difficult airway maintenance and causes of the at-risk extubation.

Difficulty in patient cooperation and consent may be a major factor in neurosurgery patients, who may range from cooperative elective patients to aggressive emergency patients. This may be challenging not just from a legal perspective, but also from the standpoint of adequate preoxygenation or feasibility of induction techniques (e.g., awake fiber optic intubation). Difficult mask ventilation may occur in any patient but is particularly detrimental in neurosurgery patients. Not only may this cause suboptimal induction conditions in terms of oxygenation, but also in terms of inducing hypercarbia. Causes include poor mask seal, loss of airway tonus, and increased airway reactivity (e.g., laryngospasm, bronchospasm, obstruction through aspiration, etc.). Difficult laryngoscopy is generally caused by factors impeding access to the airway or proper placement of the laryngoscope. For this reason, a fiber optic or video laryngoscopy may be a prudent first choice. Difficult intubation is largely dependent upon laryngoscopy, although problems may occur with tube placement (use a stylet!), tube size, or airway obstruction. Difficult airway maintenance can be caused by a number of factors in neurosurgery patients, including altered and changing consciousness affecting both the neurological aspect of respiration as well as airway tonus. This is particularly challenging in neurosurgery due to the unpredictability of drug effects in more sensitive patients as well as limited access to the patient’s head (see the excursion topic awake craniotomy and the stereotactic frame). Finally, the causes of at-risk extubation must be considered. Basically, all of the above mentioned causes during induction and during the operation also apply to extubation. Additional causes for the at-risk extubation include further changes in vigilance, residual drug concentrations affecting airway patency and respiratory reflexes as well as any intraoperative factors inhibiting airway function such as edema of the base of the tongue, pharynx, and/or larynx after prolonged surgery in the prone position.

Diagnosis

The diagnosis of a difficult airway is by definition one requiring actual difficulty in one of the relevant aspects, e.g., bag-mask ventilation or intubation/extubation. Because we would rather be safe than correct, we suggest diagnosing the suspected difficult airway. Unfortunately, there are no firm diagnostic criteria for the suspected difficult airway. We advise using individual judgment—preferably conducted by the anesthesiologist/intensivist directly involved in airway management—on the basis of the predisposing factors.

Predisposing Factors and Signs

One way of structuring predisposing factors is to review general preoperative and intra/postoperative factors. Although current guidelines clearly state that an anesthesiologist should perform an intubation history in addition to a general history and also perform a physical examination of the airway, they remain vague about specific examples, and evidence is limited to observational studies and case reports. Even more neglected is the topic of extubating the difficult airway. For neurosurgery patients, a number of additional patient and procedural factors can complicate the management of securing the airway. Additionally, the potential consequences of even relatively commonplace fluctuations in physiology during induction or extubation (e.g., in blood pressure, in ventilation, increases in intracranial pressure (ICP) due to coughing, stress, etc.) may be tolerated poorly in neurosurgical patients on account of their underlying pathologies.