Bronchoscopy is a diagnostic and therapeutic procedure to visualize the airway and its pathological alterations. Knowledge of the upper and lower respiratory tract structures is the cornerstone of bronchoscopic anatomy. Jakson and Huber1 in the year of 1943 were the first ones to recognize the importance of a systematic classification of the tracheobronchial tree. In fact, their classification is the basis of the international nomenclature system approved by the British Thoracic Society in 1949.2 The aim of this article is to describe the gross structural anatomy visible while doing a flexible bronchoscopy, during its passage, when the patient is in supine position. Bronchoscopy involves examination of both upper airway and lower respiratory tract. However, in intubated and tracheostomized patients, the upper airway is bypassed and bronchoscopic examination of only lower respiratory tract is possible beginning from the lower part of trachea.
The bronchoscopic anatomy of upper airway includes mouth, nose, pharynx, and larynx. The mouth and nasal cavity are separated from each other by soft and hard palate. The bronchoscope can be introduced through nose as well as mouth leading to oropharynx, and then onwards further into respiratory passages (Fig. 1).
- The bronchoscopic anatomical boundary of mouth is made by lips anteriorly, hard and soft palate superiorly, floor of the tongue inferiorly, and the oropharynx posteriorly (Fig. 2)
- Via the oral approach, bronchoscope is introduced in-between the lips, passed through the oral cavity, over the tongue and below the hard and soft palates, to the oropharynx and then to larynx.
- The nose begins anteriorly with two nostrils and communicates posteriorly with the nasopharynx (Fig. 3)
- The nasal septum divides nasal cavity into two nasal fossae (Fig. 3)
FIG. 4: Sagittal view showing the palate dividing the nasal and oral cavities, while pharynx is seen as the common conduit to both respiratory and alimentary tract.
- The lateral wall of nasal cavity has three bony projections called turbinates or conchae. These are described as superior, middle, and inferior turbinates. The superior turbinate is not visible during bronchoscopy (Fig. 5)
- Via the nasal approach, the bronchoscope is inserted through nostrils, passed usually below the inferior turbinate or between the middle and inferior turbinates to the nasopharynx, over the uvula to the oropharynx, and then to larynx.
- The pharynx is a muscular tube that connects the nasal and oral cavities to the larynx and esophagus. Pharynx is unique in a way that it is common to both the alimentary and the respiratory tracts (Fig. 4)
- The pharynx begins at the base of the skull and ends inferior to the cricoid cartilage
- The nasopharynx is found between the base of the skull above and the soft palate below, and is a continuation of the nasal cavity (Fig. 7).
- The middle part of pharynx called oropharynx extends anteriorly between soft palate above and base of tongue below; and from tip of uvula to epiglottis behind (Fig. 8).
- The most distal part of the pharynx also called laryngopharynx or hypopharynx, lies posterior to larynx between the superior border of the epiglottis and inferior border of the cricoid cartilage (C6), at which point it becomes continuous with the esophagus (Fig. 8)
- During routine bronchoscopy, hypopharynx is not negotiated, as from oropharynx the bronchoscope is directly passed into larynx.
- The larynx is a 5–7 cm long communicating structure between the upper and lower respiratory tract7
- The larynx starts at the tip of epiglottis and ends at the lower border of cricoid cartilage, consisting of three areas (Fig. 9) as follows:
- Supraglottic area
- Glottic area
- Subglottic area.
- From oropharynx during bronchoscopy, the supraglottic area is seen consisting of epiglottis, vallecula, and base of the tongue (Fig. 10)
- The epiglottis is a leaf-shaped structure arising from the base of tongue. It is the first laryngeal structure seen when the bronchoscope is in the oropharynx
- The base of the tongue joins the anterior surface of the epiglottis and forms the median and lateral glossoepiglottic folds
- The space between the median and lateral glossoepiglottic folds called vallecula, which is a pouch like fold between the epiglottis and the base of the tongue
- Lateral to the aryepiglottic folds are the piriform fossae of the pharynx. The piriform sinuses are pyramid shaped with the bases superior at the level of the laryngeal inlet and the apices at the inlet of the esophagus (Fig. 12)
- From the aryepiglottic folds, there are thick folds of mucous membrane going up to the true vocal cords. These are called vestibular folds, ventricular folds, or false vocal cords. The vestibular fold is formed by the vestibular ligament extending from lateral wall of epiglottis to arytenoid cartilage
- The true vocal cords lie below the false vocal folds, extending from the anterior surface of the arytenoid cartilage to the thyroid cartilage. The true vocal cords are glistening shining, ivory or pearl white colored, which meet anteriorly to form anterior commissure just below the laryngeal surface of epiglottis. Posteriorly, the true vocal cords are separated from each other articulating with vocal process of arytenoid cartilage. This intercartilaginous area is called posterior commissure. The bronchoscope is introduced between the true vocal cords as near to posterior commissure as possible
- Above the true vocal cords is the ventricle, superiorly bound by the false vocal cords
- The opening between the two true vocal cords is called the rima glottidis, or glottis through which the bronchoscope is passed further down the airways.
- The subglottic space begins from the inferior border of the true vocal folds and terminates 2 cm below at the level of the cricoid cartilage (Fig. 14)
- The cricoid is the lower margin of the larynx and forms the only complete cartilage ring in the airway (Fig. 15)
FIG. 16: Trachea dividing into major bronchi and then further subsequent lobar and segmental bronchial divisions.
- From below the cricoid cartilage, the lower respiratory tract also called the tracheobronchial tree begins.
The lower respiratory tract forms the tracheobronchial tree, which is like a branching of a tree. It begins with trachea and then divides into right and left bronchial tree going into right and left lungs, respectively (Fig. 16).
The right lung has three lobes called right upper, middle, and lower lobes. The right upper lobe has three segments—apical, anterior, and posterior segments. The right middle lobe has two segments called medial and lateral segments. The right lower lobe consists of five segments, which include one superior segment and four basal segments termed as medial, anterior, lateral, and posterior basal segments. To summarize, the right lung has three lobes supplied by respective lobar bronchi and 10 segments supplied by their respective segmental bronchi (Fig. 17).
The left lung has two lobes called left upper and left lower lobes. The left upper lobe has two divisions called upper division and lingua. The upper division of left upper lobe has two segments termed as apicoposterior and anterior segments. The lingular division has two segments called superior and inferior lingular segments. The left lower lobe consists of four segments, which include one superior segment and three basal segments called anterior, lateral, and posterior basal segments. To summarize, the left lung has two lobes supplied by their respective lobar bronchi and eight segments supplied by their respective segmental bronchi (Fig. 17).
- As it passes downwards, it follows the curvature of the spine and courses slightly backward. Near the tracheal bifurcation, it deviates slightly to the right
- One-third of the trachea is extrathoracic and two-thirds is intrathoracic. The extrathoracic or cervical segment ends at the sternal manubrium and encompasses about the first six tracheal rings
- The length of the trachea varies according to age. In neonates it is 3 cm, in the pediatric population the range is 7–10 cm,10 and in adults it is approximately 15 cm
- The diameter of trachea varies in accordance with the gender of the patient. In men, the average diameter of trachea is 22 mm, the coronal diameter is 13–25 mm, and sagittal diameter is 13–17 mm. The tracheal diameter in men is somewhat larger as compared to women who have average diameter 19 mm, coronal 10–21 mm, and sagittal diameter 10–23 mm.11 The average diameter in neonates and pediatrics age group is 6 mm and 10 mm, respectively
- In adults, the trachea is supported anterolaterally by 16–20 incomplete C-shaped cartilages
The normal carina is sharp vertical ridge like projection in the center, which causes bifurcation of trachea into right main bronchus at 25–30° and left main bronchus at 45° angle from the midline (Fig. 20).
In children, the carina is usually more displaced to the right and with time it becomes more medial and the angulation tends to be more obtuse.
Right Bronchial Tree
The right bronchial tree begins with right main bronchus after bifurcation of carina.
Right Main Bronchus
- The right main bronchus (Fig. 21) is short and more vertical compared to left main bronchus. In men, the average length of right main bronchus is 2.0 cm, whereas it is approximately 1.5 cm in women. The average diameter is 17.5 mm in men and 14 mm in women
FIG. 22: Right main bronchus seen dividing into right upper lobe bronchus and bronchus intermedius with right carina 1 (RC1) in the middle.
- Right main bronchus divides into two divisions (Fig. 22):
- Right upper lobe bronchus
- Bronchus intermedius
- The secondary carina dividing the right upper lobe from the bronchus intermedius is called the Right Carina 1 or RC-1 (Fig. 22).
Right upper lobe bronchus
- The right upper lobe bronchus is seen originating from the right lateral wall of right main bronchus
- Right upper lobe bronchus usually branches into three segmental bronchi (Fig. 23) termed as:
- Apical segment bronchus
- Anterior segment bronchus
- Posterior segmental bronchus
- The openings of the three segmental bronchi of right upper lobe form a triangle. The anterior and posterior segment bronchi are together seen opposite to each other, while the apical segment bronchus forms the apex of the triangle.
- After generating the right upper lobe bronchus, right main bronchus becomes the bronchus intermedius (Fig. 24), which extends for approximately 2–2.5 cm and then splits into two divisions (Fig. 25):FIG. 25: Bronchus intermedius is seen dividing into right middle lobe and right lower lobe bronchi with right carina 2 (RC2).
- Right middle lobe bronchus
- Right lower lobe bronchus
- The length of right lower lobe bronchus is very small and it immediately gives off its superior segment. Therefore, the bronchoscopic view from the bronchus intermedius shows three openings appearing almost at the same level (Fig. 26). The medial opening is of right middle lobe bronchus, the lateral opening just opposite to right middle lobe bronchus is superior segment of right lower lobe and in-between the two is the opening of remaining right lower lobe bronchus
- The secondary carina between the right middle lobe bronchus and the bronchus to the right lower lobe is named the Right Carina 2 or RC-2 (Fig. 25).
Right Middle Lobe Bronchus
- It divides into two divisions (Fig. 27):
- Lateral segment bronchus
- Medial segment bronchus
- The lateral segment bronchus is visualized more distally as compared to medial segment bronchus, which originates more obliquely.
FIG. 28: Right lower lobe bronchus seen dividing into its superior segment and the common division of four basal segments.
Right Lower Lobe Bronchus
- Right lower lobe bronchus (Fig. 28) divides into five segmental bronchi:
- Superior segment bronchus
- Medial basal segment bronchus
- Anterior basal segment bronchus
- Lateral basal segment bronchus
- Posterior basal segment bronchus
- The superior segment bronchus of right lower lobe arises posteriorly opposite and across to the origin of middle lobe bronchus. The remaining four basal segments of right lower lobe arise from a common division, which is seen in-between the openings of right middle lobe bronchus and superior segment bronchus of right lower lobe (Fig. 26)
- The medial basal segment bronchus arises a bit more distally on the medial wall of right lower lobe bronchus separately from subsequent division of right lower lobe, which finally divides into anterior basal, lateral basal, and posterior basal segment bronchi (Fig. 29).
Left Bronchial Tree
- The left main bronchus is usually 4–5 cm long. Its lumen is narrow and relatively horizontal as compared to right main bronchusFIG. 31: Left main bronchus seen dividing into left upper lobe and left lower lobe bronchi with left carina 1 (LC1) in the middle.FIG. 32: Left main bronchus seen dividing into left upper lobe and left lower lobe bronchi with left carina 1 (LC1) in the middle. Further divisions of left upper lobe bronchus also visible.FIG. 33: Left upper lobe bronchus seen dividing into its upper division and lingular bronchus by Left carina 2 (LC2).
- The left main bronchus divides into two lobar bronchi for their respective lobes (Fig. 31) which are:
- Left upper lobe bronchus
- Left lower lobe bronchus.
Left Upper Lobe Bronchus
- Upper division bronchus
- Lingular bronchus, which is also called lower division bronchus.
- Upper division of left upper lobe bronchus:
- The upper division of left upper lobe is lateral to lingular division
- This divides into two segmental bronchi (Fig. 34) called:
- Apicoposterior bronchus
- Anterior segment bronchus
- The apicoposterior bronchus as the name suggests is most apicoposterior and lateral. It subsequently divides into the:
- Apical subsegment bronchus
- Posterior subsegment bronchus
- The subcarina separating the upper division of left upper lobe bronchus from the lingular segment is termed Left Carina-1 or LC-1(Fig. 32).
- Lingular division:
FIG. 36: Left lower lobe bronchus seen dividing into its superior segment and the common opening of three basal segments.
- It is 2–3 cm long and divides into two divisions (Fig. 35) termed as:
- Superior lingular segment bronchus
- Inferior lingular segment bronchus
- The secondary carina separating the lingular division from the left lower lobe bronchus is called Left Carina-2 or LC-2 (Fig. 32).
Left Lower Lobe Bronchus
- The left lower lobe bronchus (Fig. 32) is slightly longer than right lower lobe bronchus by about 1 cm
- The left lower lobe bronchus divides into its four segments:
- Superior segment bronchus
- Anterior basal segment bronchus
- Lateral basal segment bronchus
- Posterior basal segment bronchus
- The left lower lobe initially gives rise to the superior segment bronchus, which is posteriorly located separately and at a greater distance from the its basal divisions (Fig. 36)
- The three basal pyramid bronchi called left anterior basal, left lateral basal, and left posterior basal segment bronchi are in mirror shape compared to those on the right (Fig. 37)
- The left medial basal segmental bronchus is usually but not always absent.
NOMENCLATURE OF AIRWAYS
Jackson-Huber classification is most commonly used that describes and names the divisions in accordance with the anatomic space orientation.1 Yamashita Japanese classification classifies bronchial tree with numerical nomenclature,14 which assigns numbers to the segmental airways (Table 1). The other less popular classification is Boyden's,15 which is more for surgical purposes. This classification also divides bronchial tree numerically. The Boyden surgical anatomy refers to the anterior and posterior segments of the upper lobe as B2 and B3. This nomenclature is usually not used by bronchoscopists, who prefer the Japanese System using anterior as B3 and posterior as B2. However, the Jackson-Huber classification is most widely used and less complex.
The anatomy of respiratory tract is complex and includes both upper and lower airways. It is not something just to get past in order to do the bronchoscopy. The spatial orientation of the airways is key to recognition of various anatomical structures in the airways. A proper understanding of airway anatomy is necessary for one to become an expert bronchoscopist. This will aid in performing specialized diagnostic and therapeutic bronchoscopic procedures.
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