In this chapter, we scope the importance of functional anatomy and physiology of the upper airway. The upper airway has an important role in transporting air to the lungs. Both the anatomical structure of the airways and the functional properties of the mucosa, cartilages, and neural and lymphatic tissues influence the characteristics of the air that is inhaled. The airway changes in size, shape, and position throughout its development from the neonate to the adults. Knowledge of the functional anatomy of the airway in these forms the basis of understanding the pathological conditions that may occur. The upper airway extends from the mouth to the trachea. It includes the mouth, the nose, the palate, the uvula, the pharynx, and the larynx. This section also describes the functional physiology of this airway. Managing the airway of a patient with craniofacial disorders poses many challenges to the anesthesiologist. Anatomical abnormalities may affect only intubation, only airway management, or both. This section also focuses on the abnormal airways in obesity, pregnancy, children and neonate, and patients with abnormal facial defects.
- upper airway
The upper airway has an important role in conducting air to the lungs. Both the anatomical structure of the airways and the functional properties of the mucosa, cartilages, and neural and lymphatic tissues influence the characteristics of the air that is inhaled . The upper airways begin with the nasal cavity and continue over nasopharynx and oropharynx to the larynx and the extrathoracic part of the trachea. The structure and function of this system have a major influence upon the conduction of the air to the lower airways . Functions of the airway include phonation, olfaction, digestion, humidification, and warming of inspired air . Clinical application of anatomical and physiological knowledge of respiratory system improves patient’s safety during anesthesia . This chapter focuses on airway anatomy and physiology, which form the basis for airway management and endotracheal intubation, and also for anesthesiologists.
2. Functional anatomy and physiology of airway
The knowledge of normal anatomy and anatomic variation is important in guiding anesthesiologists in airway management planning. The airway can be divided into upper airway, which includes the nasal cavity, the oral cavity, the pharynx, and the larynx, and the lower airway, which consists of tracheobronchial tree (Figure 1) .
2.1. The upper airway
2.1.1. Nasal cavity
The nose originates in the cranial ectoderm and is composed of the external nose and the nasal cavity . The nose is divided into the external nose and the nasal cavity . The external nose is a pyramidal structure, situated in the midface, with its base on the facial skeleton and its apex projecting anteriorly . The external nose is formed by an upper framework of bone, a series of cartilages in the lower part, and a small zone of fibro fatty tissue that forms the lateral margin of the nostril (the ala). The upper framework of bone is made up of the nasal bones, the nasal part of the frontal bones, and the frontal processes of the maxillae . The paired nasal bones form the external nose superiorly and two sets of paired cartilage inferiorly. The upper lateral cartilages provide the shape of the middle third of the nose and support for the underlying nasal valve. The paired nasal bone form consists of two parts, the upper nose and the lower cartilage. The upper lateral cartilage provides protection which is of the shape of the middle third part of the nose and supports the nasal valve. The lower lateral cartilage segments are butterfly shaped and consist of medial and lateral crures. The medial crus forms the columellar, while the lateral crus forms the nasal area. These crures together form the nasal vestibule deficit. Cartilage is supported by nasal septum . The nasal cavity is divided into two compartments by the nasal septum. One of them opens out into the nostrils. The other compartment is the nasopharynx, which opens to the concha or the posterior nasal opening. The vestibule, which includes the nostrils between the small flat nose hairs, is a small aperture .
Deviations of the septum are very common; in fact, they are present to some degree in about 75% of the adult population. When the rapid growth in this region, septal cartilage, occurs from an unspecified minor dislocation, the deformity as often as the appearance of the second tooth structure often does not manifest itself. A distribution that supports this traumatic theory is that men are more often affected than women . Due to the possibility of septum deviation, before passing instrumentation, through the nasal passages, the more open side should be determined . The lateral wall of the nasal passages includes
The olfactory nerve (I) innervates the region designated as the nose-specific olfactory area, which covers an area of 2 cm2 in the uppermost part of the nose and the lateral wall of the nasal cavity. The nerves of common sensation are derived from the nasociliary branch of the first division of the trigeminal nerve (V1) and also from the second, or maxillary division (V2) .
The nose is the main portal of air exchange between the inner and the outer environment. The nose creates favorable conditions for approximately 37.8° and 100% relative humidity of respiratory air required for vital functions, and it plays a role in conjunction with local defense and filtering of particulate matter and gases introduced. There is also a role for the individual in defending and delighting smells. In a healthy adult, the total nasal airway resistance is relatively stable, but the airflow of each nasal cavity changes in a reciprocal manner (as the flow increases in one space, the flow decreases on the other). This change in airflow, known as the nasal cycle, reflects changes in the vascular involvement of the canals and septal tuberculosis. The normal individual is unaware of this return, because the total airway resistance remains constant. During the cycle, the water vapor saturation level of the breathing air is not affected. The warning center for the nasal cycle is located in the hypothalamus .
2.1.2. Oral cavity
Oral cavity consists of mouth, palate, teeth, and tongue. The mouth cavity is bounded by the alveolar arch of the maxilla and the mandibula, and teeth in front, the hard and soft palate above, the anterior two-thirds of the tongue and the reflection of its mucosa forwards onto the mandible below, and the oropharyngeal isthmus behind . For a secure intubation, it is important that the anesthesiologist should evaluate the condition of
Mouth opening is an important parameter for intubation, and its definition is the distance between the mandibular and the maxillary central incisor teeth. Temporomandibular joint dysfunction, congenital fusion of the joints, trauma, tissue contracture around the mouth, and trismus may limit mouth opening . The Mallampati score is a scoring scale for estimating the size of the tongue according to the oral cavity, and it can be useful in predicting whether or not the laryngoscope will be easy to move with the laryngoscope blade. In addition, it also assists in whether or not the opening of the mouth to allow intubation . Protrusions of the anterior teeth are among the factors affecting intubation. During laryngoscopy and placement of the intubation tube, the anterior teeth and tongue will affect the imaging on the oral cavity . A small mandibular space may fail to adequately accommodate tongue displacement, thus interfering with visualization of the larynx .
Nasopharynx → between the nares and the hard palate;
Velopharynx or retropalatal oropharynx → between the hard palate and the soft palate;
Oropharynx → from the soft palate to the epiglottis;
Hypopharynx → from the base of the tongue to the larynx (Figure 7).
Pharynx is a tube-like passage that connects the posterior nasal and oral cavities to the larynx and the esophagus. It is separated into nasopharynx, oropharynx, and laryngopharynx . The pharynx is a muscle tube extending from the base of the skull to the level of the cricoid cartilage and connecting the nasal and oral cavities to the larynx and the esophagus . To facilitate understanding of its functions, the pharynx can be divided into three or four parts. These four structures form the appropriate route for air passage from the nose to the lung. It also has other physiological functions such as phonation and swallowing. There are 20 or more airway upper muscles surrounding the airway and actively constricting and expanding the upper respiratory tract lumen. These muscles can be divided into four groups: muscles that regulate the soft palate position (ala nasi, tensor palatini, levator palatini), tongue (genioglossus, geniohyoid, hyoglossus, styloglossus), hyoid device (hyoglossus, genioglossus, digastric, geniohyoid, sternohyoid), and posterolateral pharyngeal walls (palatoglossus) pharyngeal constructors). These muscle groups interact in a complex way to keep the airway open and close. Soft tissue structures form the walls of the upper airway and tonsils including soft palate, uvula, tongue, and lateral pharyngeal walls (Figure 4) . The pharyngeal muscle structure seen in the patient who is awake helps to maintain airway patency. However, during anesthesia, the loss of pharyngeal muscle tone is one of the major causes of upper airway obstruction .
The oral cavity enters the oropharynx via oropharyngeal isthmus, which is limited by palatoglossal arches, soft palate, and lingual dorsum . The oropharynx begins with a soft palate and extends to the epiglottic level. The lateral walls contain, respectively, palatoglossal folds and palatopharyngeal folds, referred to as front- and back-faceted (tonsillar) columns. These layers include palatine tonsils and cause hypertrophy of the tonsils, leading to airway obstruction . The anterior wall of the oropharynx is mainly limited with the soft palate, the tongue, and the lingual tonsils, and the posterior wall is delimited by a muscular wall of the upper, middle, and inferior contraction muscles lying in front of the cervical vertebrae. The minimum diameter of the upper airway during waking, retropalatal oropharynx as a primer, is of interest as a potential localization of collapse during sleep .
The anatomical position, composition, associated musculature, and innervation of the larynx all contribute to this structure’s capabilities . The cartilaginous frame of the larynx is made up of different nine cartilages . The arytenoid, corniculate, and cuneiform cartilages are paired, whereas the thyroid, cricoid, and epiglottis are unpaired (Figure 8) .
They are associated by ligaments, membranes, and synovial joints that are lined by the hyoid bone via the thyrohyoid ligaments and the membrane . The epiglottic, thyroid, and cricoid cartilages make up the three unpaired cartilages and are arranged superior to inferior, respectively. The thyroid cartilage, with the epiglottic cartilage superior, predominates anteriorly and forms the laryngeal prominence (i.e., Adam’s apple), while the predominate cartilage dorsally is the cricoid cartilage which sits inferior to the thyroid cartilage . This laryngeal prominence is appreciable from the anterior neck and serves as important landmarks for percutaneous airway techniques and laryngeal nerve blocks . The thyroid cartilage is the largest one and forms a protective shield-like shape in front of the vocal cords . The cricoid cartilage, which lies below the thyroid cartilage and above the entrance to the trachea, is the only complete ring of the laryngeal skeleton. The cricoid cartilage encloses the subglottic region of the larynx. Stenosis may form if the mucosa in this region is injured, as can occur with a prolonged endotracheal tube intubation . The paired arytenoid cartilages are found on the dorsal aspect of the larynx, attached superiorly to the cricoid cartilage. Both arytenoid cartilages give off a lateral extension (muscular process) and anterior extension (vocal process) which aid in supporting the vocal ligaments . The arytenoids are pyramidal-shaped (Figure 9) cartilages positioned on the upper border of the posterior cricoid cartilage; these attach at the synovial cricoarytenoid joints. The arytenoids serve as attachment sites for some of the intrinsic muscles of the larynx and allow complex movement and fine adjustment of the vocal cords . In addition, each arytenoid cartilage has an associated corniculate and cuneiform. These two small, paired cartilages border the opening into the laryngeal vestibule both dorsally and laterally cartilage.
The corniculate cartilage can be found at the apex of both arytenoid cartilages. The cuneiform cartilage can be found seated anterior and lateral to both arytenoids. These cartilages form connections via numerous membranes, ligaments, and synovial joints .
There are two essential
The larynx is subdivided into three regions: the supraglottis, glottis, and subglottis. The space between the vocal cords is termed the glottis; the portion of the laryngeal cavity above the glottis is known as the supraglottis, and the portion inferior to the vocal cords is known as the subglottis .
2.1.3. Lower airway
2.1.4. The main bronchi
In full inspiration, the bifurcation level is at T6. The right main bronchus is shorter, wider, and more perpendicular than the left bronchus. This situation can be explained by the transformation into a shorter and wider structure because the embryologically will feed larger lungs. In addition, the aortic arc is the reason for the placement to be more perpendicular due to the position (at 25° perpendicular to the 45° left) . This is the result of a greater possibility of foreign bodies and endotracheal tubes entering the right bronchial lumen . The bronchi are supplied by the bronchial arteries from the aorta and drained by the azygos vein on the right and the hemiazygos vein on the left, and also, some drainage by pulmonary veins and the bronchial veins .
3. Evaluation of the complex airway
3.1. Pediatric airway differences
The pediatric airway changes significantly from birth to adulthood. These changes affect the development of the skull, oral cavity, throat, and trachea. The head is larger than the body in infants and young children. Due to the absence of paranasal sinuses, the facial skeleton is smaller in neonates compared with neurocranium. Oral cavity is small at birth. It grows in the first year of life due to the significant growth of the mandibles and teeth in the following period. In neonates, the tongue has a flat surface and limited lateral mobility and appears relatively large in the small mouth space. Neonatal laryngeal and tracheal structures are especially important for anesthesiologist. The larynx appears more prominently during direct laryngoscopy, but when compared with adults, the surrounding structure is loosely embedded. External manipulation allows direct laryngoscopic intubation to be easily carried to a position where it is possible. If the epiglottis is not removed by the bladder of the laryngoscope, the glottic appearance on the laryngoscopy is prevented long, narrow, and often U- or V-shaped (“flopping”) . Glottis is higher in the newborn (C2/C3) than in the vertebrae, and after 2 years, it falls to the normal position in C5 . In newborn, vocal cords are shorter, and anterior glottis, which normally corresponds to two-thirds in a larger child, constitutes about 50% of the newborn. The newborn larynx is conical, but in a larger child, it is approximately cylindrical. Though the larynx is thought to be widest in the supraglottic region and narrowest in the subglottic region, this suggests that the narrowest portion of the magnetic resonance imaging (MRI) studies may be in glottic. Also, the cricoid ring is the narrowest part of the neonatal airway and is an ellipsoid-shaped mucosa layer which is highly sensitive to trauma. Bypassing the air leak at this level from the untrained tracheal tube does not guarantee avoidance from the pressure points and the next payment . Intubation tubes with small tracheal internal diameter cause a significant increase in airway resistance and this can lead to an exaggerated mucosal injury. The tracheal length depends on the child’s age and height but is not dependent on body weight. During the operation, changes in the head position may lead to a displacement of the tracheal tube and reevaluate the position of the tube with the head’s new position. Verification of the position of the tracheal tube clinically (chest movement, auscultation) or by other means (chest radiography, fluoroscopy, ultrasonography, or bronchoscopy) is recommended.
3.2. Congenital disease
It may produce abnormalities of the head, neck, or upper airway . Cardiovascular, nervous, musculocutaneous, or excretory system disease is more often tabulated with these abnormalities. Crouzon, Goldenhar, Pierre Robin, and Treacher Collins syndromes are known for their abnormal head and neck. The patients with micrognatia, retrognatia, and macroglossia must be remembered for the congenital diseases in childhood . The most significant vascular malformations are vascular rings, usually of aortic arch origin, encircling the trachea. Tracheomalacia, congenital tracheal stenosis, shortened trachea, and bronchogenic cysts can contribute to difficult airway management . Infants with congenital malformation syndromes associated with cardiovascular anomalies and skeletal dysplasia have a shortened trachea significant percentage . Soft tissue changes that cause airway management difficulties are usually divided into two categories as those that disturb the motion of the airway and limit the movements that disturb the airway by mass effects. Soft tissue changes that limit airway motion usually affect mouth opening. Microstomy, a feature of Freeman-Sheldon syndrome, is a condition in which the movement of oral tissues that do not respond to stomach relaxation is limited. Other rare diseases that limit the movement of airway tissue include fibrofacial myositis ossificans and dermatomyositis. The mass effects on the airway due to soft tissue abnormalities may be the result of congenital, end-of-life, or subsequent disease outcomes of surgical interventions . Macroglossia is one of the most common problems appearing with birth, and the tongue expands and fills the oral cavity, making it difficult to see the larynx. Macroglossia occurs in Beckwith-Wiedemann syndrome, Down syndrome, Sturge-Weber syndrome, and in a variety of dystrophically related syndromes .
3.3. Obese patients
Perioperative management in obese patient, including airway management, is an increasing and a worldwide concern for the anesthesiologist. Since obese patients have an increased fatty tissue distributed in a truncal fashion, obesity may have an important and negative impact on the airway patency and respiratory function. Respiratory function and airway patency can be significantly altered by this change in position . Airway assessment of the obese patient should be performed with the patient in both the sitting and supine positions. Respiratory function and airway patency can be significantly altered by this change in position . Body weight may not be as critical as the location of excess weight. Massive weight in the lower abdomen and hip area may be less important than when the weight is in the upper body area. A short, thick, immobile neck caused by cervical spine fat pads will interfere with rigid laryngoscopy. Furthermore, the redundancy of soft tissue structures inside the oropharyngeal and supralaryngeal area may also make visualization of the laryngeal structures difficult. Mask ventilation should be difficult in the obese patient. When a high positive pressure is required to ventilate the patient, the chance of inflating the stomach is increased. Rapidly oxygen desaturation during apnea, secondary to a reduced functional residual capacity, limits intubation time. In the case of a cannot-intubate-cannot-ventilate situation, access to the neck for transtracheal jet ventilation or establishing a surgical airway (emergency tracheostomy or cricothyroidotomy) will also be more complex .
Maternal, fetal, surgical, and personal factors in pregnancy cause an increase in the incidence of unsuccessful intubation. The mucosa of the upper respiratory tract becomes more vascular and edematous, which increases the risk of bleeding and swelling in the airway . These changes cause the Mallampati score to increase as the pregnancy progresses and during labor. Airway edema may be exacerbated by preeclampsia, oxytocin infusion, intravenous fluids, and Valsalva maneuvers during labor and delivery. A decreased functional residual capacity and increased oxygen requirements accelerate the onset of desaturation during apnea and are further exacerbated in obese patients. Progesterone reduces the lower esophageal sphincter tonus, which results in gastric reflux. Risk of reflux is further increased because of delayed gastric emptying after prolonged painful delivery and opioid administration. Enlarged breasts can make laryngoscopy difficult . Airway anatomy may become distorted during prolonged labor or toxemia, leading to an edematous soft tissue encroachment of the upper airway . At last, in cases of fetal distress or maternal hemorrhage, the emergency nature of the circumstances compounds airway management problems .
All illustrations were designed and colored by architect Ceren Yoldaş. www.yoldasmimarlik.com 00 90 5425346896, Doğan Demircioğlu Cad. Özlem Sitesi C Blok Denizli Turkey.