What Makes an Airway Difficult? In short, a lot of different factors play into making an airway difficult. In general, they can be broken down into anatomic, physiologic, and logistic. We'll cover some of the logistical issues that can complicate intubations on a later post (mostly with regards to intubation in the HEMS and prehospital setting).
The anatomic factors that contribute to the difficulty of an airway are pretty well established in the literature. The "Difficult Airway Course" and our own Dr. Carleton characterize an airway as difficult based on predictors of difficulty in 4 technical operations: difficult bag valve mask (BVM), difficult laryngoscopy and intubation, difficult extraglottic device (EGD), and difficult cricothyrotomy.
Bag valve mask ventilation can be made difficult by the presence of impediments to an adequate Mask seal (trauma, beard, blood, craniofacial abnormalities, Obesity or airway obstruction, Age (>55), absence of teeth (No teeth), and Stiffness (increased lung resistance) (MOANS)
The LEMON law guides the assessment of difficult laryngoscopy. The Look of the patient (physician gestalt of how difficult the airway will be), Evaluation of the 3:3:2 rule (mouth opening, thyromental distance, and distance between hyoid bone and thyroid cartilage), Mallampati score, and presence of Obesity or airway obstruction contribute to the characterization of difficult layrngoscopy by the LEMON law.
Expected difficulty with an extraglottic device can be predicted through the use of the RODS mnemonic - Restricted mouth opening, Obstruction, Distorted airway, and Stiff lungs.
Finally difficulty in performing a cricothyrotomy can be predicted with the SHORT mnemonic - Surgery, Hematoma, Obesity, Radiation history, Tumor
The physiologic factors that can contribute to the difficulty of managing a patient's airway are perhaps somewhat less talked about than the anatomic factors. These factors however can greatly alter the way in which one chooses to manage a patient's airway.
The presence of modest hypoxia (85-90%) prior to the decision to intubate a patient may prompt you to first try non-invasive positive pressure ventilation, administer nasal as well as NRB oxygen, bag valve mask ventilate the patient after induction, or prompt you to consider delayed sequence intubation.
The presence of hypotension may alter your choice of induction agents (i.e ketamine instead of proposal) or prompt you to further resuscitate the patient with fluids, pressors, or push dose pressors before administering a sedative and paralytic. Conversely the presence of significant hypertension or the presence of a tachydysrhythmia may also alter your choice of induction/sedative agent (away from ketamine).
Likewise the presence of severe acidosis (particularly metabolic acidosis but also respiratory acidosis) may prompt you to consider a method of sedation and analgesia that maintains the patient's respiratory drive. In the setting of the metabolic acidosis, killing off the patient's respiratory compensation may precipitate a significant drop in pH, myocardial instability, and, well... you know. In the setting of a respiratory acidosis, the patient already has insufficient tidal volume (either from sedation or airway obstruction) and any further reduction in minute ventilation will only compound and worsen the accumulation of CO2.
And Now on with the Show...
Dr. Miller and Dr. Hill, in our latest podcast, review a case of a patient presenting with respiratory failure who had many predictors of difficulty (both anatomic and physiologic). The basic set up is this: Older gentleman with a big beard, shorter than you would like thyromental distance, obesity, edentulous, COPD exacerbation, CHF exacerbation with known EF of 20% and pulmonary edema on presentation, with an O2 sat of 89%, pH of 7.0 with a pCO2 of 100, atrial fib with RVR into the 160's, and modest hypertension (150/100). In the podcast, we talk through how we approached the initial patient management and the decision making process that led us to manage the airway as we did.