The Difficult Airway: Definition and Prevalence
Airway difficulty can be considered under two distinct headings: a) difficult mask ventilation (DMV); and b) difficult tracheal intubation. These may be encountered together or in isolation.
DMV can be defined as the inability of an unassisted anesthesiologist a) to maintain oxygen saturation, measured by pulse oximetry, >92%; or b) to prevent or reverse signs of inadequate ventilation during positive-pressure mask ventilation under general anesthesia. In a study of 1,502 patients, DMV was considered present when the anesthetist found that the difficulty was clinically relevant and could have led to potential problems if mask ventilation had to be maintained for a longer time. There were 75 patients (5%) with DMV but in only 13/75 (17%) had this been predicted. Two subsequent studies reported a DMV rate of approximately 8% and 2%.
Difficult tracheal intubation (DTI) is tracheal intubation requiring multiple intubation attempts in the presence or absence of tracheal pathology. However, there is no universal definition and because the expertise of the intubator, the equipment used, and the number of attempts made may vary, the reported rates of DTI differ. Using direct laryngoscopy only, DTI has been reported in 1.5% to 8.5% of patients-with tracheal intubation impossible in up to 0.5% of the population.[7,15] Failure to intubate the trachea occurs in one in 2,000 in the nonobstetric population and one in 300 in the obstetric population. DTI may be the result of difficulty in visualization of the larynx-termed difficult direct laryngoscopy (DDL)-or anatomic abnormality (distortion or narrowing of the larynx or trachea).
Visualization of the larynx is usually described using the Cormack and Lehane grades with grades 3 and 4 indicating DDL. The incidence of DDL is 1.5% to 8% in general surgical patients but higher in patients undergoing cervical spine surgery (20%) or laryngeal surgery (30%). Other grading systems for visualization of the larynx exist, including a modified Cormack and Lehane and the Percentage of Glottic Opening scale.
The need for equipment other than a direct laryngoscope may also help define DTI, although devices such as the gum elastic bougie (introducer) may or may not be viewed as part of standard technique. Many of these issues are addressed in the intubation difficulty scale (IDS), which uses seven variables to calculate a score. An IDS score of 5 has been used to define DTI and, in a large study, occurred in 8% of patients.
Crit Care Med. 2008;36(7):2163-2173. © 2008 Lippincott Williams & Wilkins
Cite this: The Difficult Airway in Adult Critical Care - Medscape - Jul 01, 2008.