This chapter is most relevant to Section F10(iv) from the 2017 CICM Primary Syllabus, which expects the exam candidates to "Describe the function and structure of the upper, lower airway and alveolus". The alveolus and lower airways are sufficiently interesting that they should get their own chapters. Here, we will take a trip down the air passages from the vibrissae to the larynx.
Unfortunately, like most anatomy, this is an exercise in listing facts, which is susceptible to being mindlessly memorised and regurgitated. In terms of exam value, this topic is rarely explored in the written exam. Question 24 from the second paper of 2016 asked about the structure of the airways, but only the adult human trachea and bronchi were asked about. The larynx came up in Question 14 from the second paper of 2020. Surgical anatomy of the upper airway has come indirectly in various ways, eg. in the context of discussing anatomy for intubation or tracheostomy, but no direct "describe the anatomical relations of the larynx" questions have ever been asked. Additionally, though the college has never asked about the role of upper airway musculature in respiration (focusing on the diaphragm and chest wall instead), but the possibility remains that they one day will. In order to prepare for this horrifying possibility, this chapter has been compiled mainly using the 8th edition of Last's. For an excessively detailed review of the upper aiway muscles, one may also turn to the 2009 article by Jordan & White, which mainly deals with sleep apnoea.
When one refers to the upper airways, one generally means "everything above the larynx". This is a nod to the gatekeeper function of the larynx as the separator of the "shared" aerodigestive functions of the pharynx and mouth from the purely respiratory functions of the lower airway. Gaga et al (2000) and most other authors group the larynx together with the upper structures.
Question 14 from the second paper of 2020 asked specifically for the anatomy of the larynx. A surprising amount of detail was expected. "Candidates were expected to address the location of the larynx, its relations, the cartilages (single and paired), ligaments, muscles (intrinsic and extrinsic), innervation (sensory and muscular) and blood supply (including venous drainage)". As will soon become apparent, that is quite a lot:
The upper airway is full of muscles, the anatomy of which is the nightmarish province of ENT and maxillofacial surgeons. For the intensivist, it will likely suffice to know that their function in respiration is basically to not collapse. And to collapse, apparently, is their natural tendency. As during inspiration the intrathoracic pressure drops by a few fractions of a cm H2O, so the upper airway pressure also drops, and the squishy mass of tissue which is your upper airway would naturally implode upon itself if it did not make a special effort to stay open. This special effort manifests as a muscular contraction which must by all accounts be viewed as an important part of the respiratory muscle function at least as much as the diaphragm, insofar as its absence would result in a failure of airflow.
This upper airway muscle contraction is a coordinated reflex arc, known as the pharyngeal dilator reflex. As for many reflexes, the arc is activated by mucosal stretch receptors, which respond to negative intrapharyngeal pressure. This was investigated beautifully by Horner et al (1991). The authors demonstrated the role of pharyngeal receptors by recording an EMG pharyngeal dilator muscles (mainly genioglossus) while smearing cocaine paste on the mucosae of healthy volunteers to anaesthetise those receptors ("minor side effects that were noted after the study included ... increased alertness", the authors cackled). The responses to negative pressure were extremely brrisk, averaging 34 millisconds - well faster than any possible voluntary response, confirming that a reflex arc was responsible. Further cocaine help to confirm that the trigeminal, superior laryngeal and the glossopharyngeal nerves were involved in this reflex arc.
Thus, genioglossus is thought to be the muscle most responsible for upper airway patency. It is the largest of the group of airway dilator muscles, which also include several others:
The palatal muscles:
The pharyngeal constrictor muscles:
The hyoid muscles:
The genioglossus is innervated by the medial branch of the hypoglossal nerve, the unilateral lesion of which will result in the tongue deviating towards the side of the lesion when it is protruded. As one can plainly see from where it is placed anatomically, the contraction of the genioglossus pulls the base of the tongue and the hyoid bone anteriorly, opening the airway (it is in essence the same as the thing you do with the jaw thrust). It does this automatically when you assume a supine position, because otherwise gravity would pull the base of the tongue down, occluding the airway.
The activation of the pharyngeal dilator reflex is moderated by descending input into the involved cranial nerve nuclei. During states of decreased consciousness (be they sleep or anaesthesia) this airway reflex is depressed, and the airway may collapse (Fregosi & Ludlow, 2014).
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