The airway is incredibly dynamic because it is precious real estate in a very unfortunate location. The aerodigestive tract requires multiple reflexes to protect the lungs from aspirating the contents of its shared, glutinous roommate. Aspiration is either anterograde and retrograde. It took me a while to realize that almost all pneumonias are technically aspiration pneumonias. Viruses, strep, and mycoplasma are all aspirated, anterograde. However, clinicians reserve the term “aspiration pneumonia” specifically for retrograde aspiration events.
Reflexes that Protect the Airway
The entryway to the lungs is the glottis. Its porticullis includes the epiglottis descent, vocal cord closure, and aryepiglottic adduction. Together they are termed Laryngeal Adductor Reflex. Anterograde aspiration triggers LAR through the pharyngoglottal closure reflex where pharyngeal sensations lead to closure of the glottus. Experiments show that rapid and large volumes in the pharynx induce a complete closure whereas slow and low volumes induce only partial closure. Smokers and elderly demonstrate blunting of this reflex. Pharyngeal sensations alone cause closure of the glottis for only 0.5s; too quick to be noticed. However, if the quantity was sufficient to trigger the irrepressive pharyngeal swallow, the glottis remained closed for 3s. Due to this reflex, humans are one of the few animals that cannot breath and swallow at the same time.
Retrograde aspiration must bypass the autonomous contractile apparatus of the LES. This is more of a static barrier. The UES is controlled by the cricopharyngeus muscle and has been thought to be important in preventing GER. But it is very unreliable. Sometimes, the UES clamps down to prevent refluxate. Other times, it relaxes to promote belching and regurgitation. What is clear is that decreased levels of consciousness dramatically reduce UES tone. Distention of the esophagus triggers LAR through the esophagoglottal closure reflex and can be reproduce with inflating a foley tip in the esophagus. This phenomena is likely why mechanical food obstruction often produces respiratory sx despite not involving the airway at all. Interestingly, this reflex is absent in esophagitis and in 50% of patients > 79yo.
It was traditionally thought that all of these reflexes were mediated through the brain stem (nucleus solitarius in dorsomedial oblongata to nucleus ambiguous in dorsal motor nucleus). But fMRI show involvement of cortical areas for swallowing, and urge to cough. Specifically the insular and somatosensory cortex. Other studies show that decreased sensation of foods (body temp and bland foods) decrease the swallowing reflex.
Reflexes that DON’T Protect the Airway
The teaching in medschool is to assess for a gag reflex but this is flawed. Physiologically, people do not prevent aspiration by gagging. And gagging is not part of a normal swallowing reflex. Rather, it is triggered by thoughts and emotions. Foreign bodies in the oropharynx do not, per se, trigger gagging. Otherwise, eating or drinking would be impossible.
Pharyngeal sensation and gag reflex in healthy subjects (Lancet 1995) addresses the futility of assessing gag reflexes. n=128. The interobserver reliability of gag reflex testing is low (10/15) due to the variable amount of pressure used to stimulate the reflex . Up to 30% of healthy young adults and 44% of healthy older adults do not have a gag reflex.
In addition to aspiration, the airway is at constant risk for intrinsic obstruction via the tongue and soft palate. Genioglossus tone is necessary to prevent the tongue from obstructing the airway. Tensor palatini tone is also necessary to stiffen the soft palate during respiratory cycles. Decreased levels of consciousness, including the sleep state, reduces muscle tone in all airway muscles and can lead to OSA.