In this chapter, we will review various clinical aspects of airway FB aspiration and retrieval, including diagnostic and therapeutic techniques and considerations.

Anatomy and Physiology of Swallowing

Upper Airway Embryological

Development and Anatomy

Pressure in this zone is generated primarily by the cricopharyngeal muscle, as well as the cervical esophagus and inferior pharyngeal constrictor. The UES receives innervation from the vagus nerve branches (pharyngoesophageal, superior laryngeal, and recurrent laryngeal nerves), the glossopharyngeal nerve, and sympathetic branches of the cervical cranial ganglion.

Functional Physiology of Swallowing

The development of the aerodigestive organs begins with the primitive foregut [2–5]. From onset, the glottal folds are present which represent the future vocal cords. Around the 6th–7th weeks during gestation, development of the epiglottis, aryepiglottic folds, false vocal cords, and laryngeal ventricles begins. The epiglottis arises from the hypobranchial eminence, which is also the precursor for the development of the tongue. The separation of these structures occurs around the seventh week. Eventually the larynx is formed as a result of the primitive foregut folding upon itself to create the laryngotracheal bud, which divides, and is responsible for the creation of the bronchopulmonary segments in the future. The fourth and sixth pharyngeal arches are responsible for the development of the laryngeal muscles and are innervated by branches of cranial nerve X. The intrinsic muscles of the larynx are supplied by the recurrent laryngeal nerve, while the superior laryngeal nerve supplies constrictors of the pharynx, cricothyroid, and levator palatini. Other supraglottic structures such as the pharyngeal walls, posterior one-third of the tongue, stylopharyngeus muscle are all supplied by the glossopharyngeal nerve. This nerve also provides sensory fbers to the mucosa of the oropharynx and palatine tonsils.

The esophagus is a muscular tubular structure that consists of two muscle layers: an inner circular layer and a longitudinal outer layer [6, 7]. The proximal esophagus is striated muscle, while the distal esophagus consists of smooth muscle. The upper esophageal sphincter (UES) forms the anatomic boundary where a zone of high pressure is generated between the pharynx and esophagus.

The pharyngoesophageal interface is responsible for facilitating airway protection, deglutition with safe transport of proximal esophageal contents, and clearance of volume during swallowing and emesis. The laryngeal structures are responsible for accommodating the following three functions: phonation, ventilation, and airway protection. Swallowing is a complex series of motions that requires very coordinated voluntary and involuntary movements of the oropharynx, larynx, and esophagus. Swallowing can be divided into three different phases based on the relationship between the food bolus and anatomical structure: oral, pharyngeal, and esophageal [8–10].

Oral Phase: The oral phase can further be divided into the preparatory and early transfer phases. The preparatory phase consists of various oral movements such as chewing, suckling, and masticating. The aim of this phase is to break down food and mix it with saliva with the eventual goal of making a food bolus that can easily and safely be transported. The transfer phase initiates once a decision to swallow has been made. At this point, the tongue contracts against the hard palate which leads to a squeezing motion that moves the bolus toward the oropharynx through a chute created by the posterior tongue. Subsequently, the soft palate contracts superiorly to protect the nasopharynx from nasal regurgitation. Finally, the posterior tongue contracts against the palate and there is contraction of the posterior pharyngeal wall which allows the food bolus to pass in a one-way direction toward the pharynx [11–13].

Pharyngeal Phase: At the start of this phase, the nasopharynx is sealed off by the soft palate

and the oropharynx is sealed off by the tongue pressing against the palate. Pharyngeal constrictor muscles contract in a top to down motion to propel the bolus distally. Airway protection mechanisms are of the utmost importance during this phase as this is the time where airway aspiration is most likely to occur. The opening to the trachea is protected by vocal cord closure, which is supplemented by arytenoid closure. Additionally, the epiglottis then swings down to cover and protect the laryngeal vestibule. During the pharyngeal phase, the hyoid bone moves the larynx superiorly and anteriorly. The suprahyoid and thyrohyoid muscle contractions facilitate the movement of the hyoid bone. This mechanism helps move the larynx to a position that is distant from the path of the bolus. Finally, the UES is moved upwards by facilitation of the widening and shortening of the pharynx. This motion decreases the distance the bolus has to transfer and allows the esophagus to be an open position to accept the bolus. The estimated time for transfer of a bolus through the pharynx is 1 s, with an approximate speed of 40 cm/s [14, 15].

Esophageal Phase: Once the bolus enters the esophagus through an open and relaxed UES, a series of peristaltic waves occur to transport the bolus down the esophagus. There is an initial wave of relaxation that occurs at the location of the bolus, which is followed by a wave of contraction that propels the bolus distally. When in an upright position, gravity can assist in this movement. While a liquid bolus can move distally with just gravity, a solid requires peristaltic motion to advance it toward the stomach. As the bolus is propelled, the lower esophageal sphincter (LES) relaxes and allows for passage into the stomach. The estimated travel time through the esophagus is approximately fve to 6 s.

Upper Airway Protective Refexes

Through the entire complex and well-coordinated process of swallowing, the human body has incorporated many involuntary mechanisms to protect the lungs from aspiration from birth until the end of life. The neuromuscular interface of

these re exes is so robust that its protective nature works within the space shared by laryngeal and esophageal structures [5, 16].

Laryngeal Adductor Refex: This re ex represents the best line of defense against pulmonary aspiration. Contraction of the lateral cricoarytenoid and interarytenoid muscle leads to adduction of the anterior and posterior aspects of the vocal cords, respectively. The vocalis and thyroarytenoid muscles, which make up the laryngeal tensors, assist with vocal cord closure during various physical activities. As already mentioned, additional protection is provided by descent of the epiglottis, arytenoid adduction, and laryngeal elevation.

Esophagoglottal Closure Refex (EGCR): It is important to recognize that aspiration does not have to necessarily occur with anterograde movement of contents. Retrograde movement of gastric contents is a major cause of aspiration, especially in the elderly population. There are various causes of retrograde movement such as re ux, belching, regurgitation, and vomiting. The major stimulus for the EGCR is dilation and distention of the esophagus. The vagus nerve is responsible for the afferent innervation and impulse resultant from stretch of the esophagus. Impulses from this re ex result in efferent output to the glottal structures via the recurrent laryngeal nerve. Stimulation leads to vocal cord adduction.

Pharyngoglottal Closure Refex (PGCR): It has been demonstrated that exposure of the pharynx to different quantities of instilled water leads to vocal cord adduction [17]. This adduction has a linear relationship to the amount of water instilled. There is also evidence that the elderly patients require signifcantly larger amounts ofuid volume to obtain PGCR in comparison to younger individuals. Similarly, larger volumes are required for stimulation in smokers compared to non-smokers [18].

Upper Esophageal Sphincter (UES): As mentioned above, the UES is the zone of pressure generated at the junction of the pharynx and esophagus, primarily in uenced by the cricopharyngeal muscle. The pharyngeal constrictors and proximal esophagus also act as adjunct muscles