Every time you see a patient in preop, you do your routine airway assessments to predict whether patient has an easy or difficult airway. Have you ever asked yourself what the assessments really tell you and why some patients would have difficult airways based on your Mallampati score, neck circumference mouth opening or the protruding teeth?
It is important to look at the airway as a whole in a more systematic approach rather than based on the individual bedside tests such as mallampati, or thyromental distance. We all have experienced that patient has a difficult airway despite the fact that MP score is I. So It is crucial to understand why we are measuring those particular parameters which can help us have a deeper understanding of airways. For example, it may help us understand why patient with MP I would be a difficult intubation. The images below are taken from Greenland’s work based on his two-curved theory. You can see the previous post Perpetuation of Three-Axis Alignment Theory and the Sniffing Position
With all the airway assessments we do, we are essentially assessing the volume of oropharyngeal cavity and the compliance of the submandibular tissue. If we recall in physics, the volume = length x width x height. When we are assessing airway in 3D, we measure the incisors to hyoid which is very difficult to measure clinically . However, it correlates well with the thyromental distance (see the red arrow). The TMJ to incisor correlates very well with the length of the jaw. Finally, TMJ to TMJ is the width of the palate. There you have your volume of the airway. As you can imagine, if you have a tiny small tunnel, it would be difficult to visualize anything down the tunnel.
Now lets assess the function of DL. As you put your blade in and perform direct laryngoscopy, , you are increasing the volume of the oropharyngeal cavity by pulling forward with the laryngoscope blade on the mandible. As shown below, as you pull forward with with your blade, your line of vision is increased and hence you can see the vocal cord with the line of sight. However, if one of the variables decreases, such as length (short thyromental distance), high palate, or short chin, the volume of the oropharyngeal cavity is reduced. Hence, the line of vision is shortened and more angled, it is more difficult to see the vocal cord or pass the tube to the vocal cord (there is an upslope that the tube needs to overcome to pass the tube). See the example below with short thyromental distance.
The list below explains how each airway assessment affects the volume, therefore, it causes a more steep line of vision and it is difficult to be able to visualize the vocal cord.
- Mallampati score: Tongue size in relation to the size of oropharygeal cavity. If you have a big tongue, the relative volume of oral cavity is reduced.
- Thyromental distance: low incisor to hyoid; Larynx is relatively posterior to other upper airway structures
- TMJ- TMJ: high/narrow palate; A narrow palate decreases the oropharyngeal volume and room for laryngoscope blade
- Thickness/length of neck: difficulty in aligning the upper airway axes.
- Protruding maxillary teeth: laryngoscope blade to enter mouth in a more cephalad direction; reduce the volume of the anterior column and difficulty in line of sight.
- Short chin/small jaw: difficulty aligning the primary and secondary curves to bring epiglottis in line of sight
It is easier to predict that the bigger and flatter the tunnel, the easier to pass the tube through the vocal cord. Sometimes we find out after induction, it is difficult to intubate due to poor month opening as patient had radiotherapy to the submandibular tissues from treatment of laryngeal cancer. Patient had perfect MP score and mouth opening when they were awake. Again, the reduced compliance essentially affects volume of oropharyngeal cavity as you can’t displace the jaw out of the way to visualize the cord easily. If you feel this is a little confusing, please refer back to my previous post Perpetuation of Three-Axis Alignment Theory and the Sniffing Position
Ok, enough for the physics. I feel that the inter-relationship between the airway anatomy and airway tests is not well addressed in many textbooks. I hope my quest for the answers would help you as well to have a better and deeper understanding of the difficult airways. Airway assessment is an important component of our pre-op assessment, but it is more important to look at the whole airway instead of based on individual tests. It is helpful to reassess when we run into an unexpected difficult airway to figure out where the problem lies and what is the best solution to deal with that particular problem. Difficult airway is a symptom of an issue, not an entity in itself!
- Adnet F, Borron SW, Lapostolle F, Lapandry C. The three axis alignment theory and the “sniffing position”: perpetuation of an anatomic myth? Anesthesiology. 1999 Dec;91(6):1964-5. PubMed PMID: 10598648.
- Greenland KB. A proposed model for direct laryngoscopy and tracheal intubation. Anaesthesia. 63(2):156-61. 2008.
- Greenland KB, Eley V, Edwards MJ, Allen P, Irwin MG. The origins of the sniffing position and the Three Axes Alignment Theory for direct laryngoscopy. Anaesth Intensive Care. 2008 Jul;36 Suppl 1:23-7. Review. PubMed PMID: