Tracheal Structure

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The adult male trachea averages 11.8 cm in length (range 10 to 13 cm) from the lower border of the cricoid cartilage to the top of the carinal spur, varying with the patient's height. There are usually from 18 to 22 cartilages within this length, approximating almost two rings per cm.1 Cartilaginous rings may be incomplete or bifid. The lateral tracheobronchial angles are located slightly higher than the carinal spur so that the length of the trachea proper along its lateral wall is slightly shorter than that measured anteriorly in the midline to the carinal spur (Figure 1-1). The carina in the adult projects quite consistently on the body surface at the level of the sternal angle since it is held in place by the aortic arch. The right main bronchus continues more vertically, whereas the left is always more horizontal with respect to the trachea. The angles between the bronchi and trachea vary quite widely. In infants, the subcarinal angle between the bronchi is much wider and the bronchi lie more transversely.

The level of the trachea and of its lesions is often described by reference to specific vertebrae. Since levels vary in an individual with cervical flexion and extension as well as with respiration and deglutition, and among individuals by age, spinal curvature, anteroposterior diameter of the thorax, and body build, denomination by vertebral level is of little use. More to the point is the length of airway from the vocal cords to carina, or better, of the trachea from cricoid to carina, using the carinal spur as the lower point of mea surement. Also important are measures of length of uninvolved trachea above and below a lesion, plus the longitudinal extent of the lesion. These distances are easily determined, although without absolute precision, by imaging (see Chapter 4, "Imaging the Larynx and Trachea") and bronchoscopy (see Chapter 5, "Diagnostic Endoscopy"). As a practical rule, I consider about three-fifths of the juvenile trachea to reside above the sternal notch, about one-half in the young adult, and one-third or less in older adults. In the first two groups especially, the proportion of cervical trachea varies with cervical extension and flexion.

In the adult, the tracheal lumen is often roughly ovoid, flattened anteroposteriorly (Figure 1-2). The rings are normally C-shaped, with the posterior membranous wall connecting the arms of the "C" in an essentially straight line measuring generally less than one-third of the circumference of the trachea (see Figure 1-2A-C). The proportion of cross-sectional cartilaginous length to length of membranous wall does not change from infancy during growth.2 An adult tracheal ring is about 4 mm high. Therefore, there are about two rings per cm of trachea. The length and diameter of the trachea is roughly proportional to the size of the individual. Men generally have a trachea of larger diameter than women (see Figures 1-2B,C). In the adult male, the external diameters of the trachea measure about 2.3 cm coronally and 1.8 cm sagitally. Corresponding figures in the female are 2.0 and 1.4 cm. The tracheal wall is about 3 mm thick. The trachea narrows somewhat as it progresses distally to the carina, more notably in children. These are important points in selecting an endotracheal tube, especially for ventilation, where the tube may be in place for a long time. A small man or woman, even if obese, will nonetheless have a trachea of shorter length and narrower diameter. An excessively wide tube can produce subglottic erosion and consequent stenosis.

Griscom and Wohl together with Fenton measured the length, diameters, cross-sectional area, and volume of the trachea in children under the age of 6 years, asleep or resting quietly, and of older children and adolescents at total lung capacity using computed tomography (CT) scans (Table 1-1).3-5 All parameters correlated with body height with little variance for gender in young children. Considerable variation in measurements of length and diameter are recorded in the literature depending upon the artifacts of handling specimens, the methods of observation, and whether the observations were made in the living or postmortem. The techniques involved measurements of fixed and unfixed autopsy specimens, the use of x-ray or CT images, and bronchoscopy. In small children, body weight may actually correlate better with tracheal growth than height or age. Increase in length outstrips growth of cross-sectional area in the first year of life.6 Thereafter, the rate of lengthening falls below the rate of area growth until puberty.2 Initially, the anteroposterior diameter is slightly greater than the transverse, producing a nearly circular lumen (see Figure 1-2A). Gradually, as the child grows, the adult configuration emerges. At first, the trachea is somewhat funnel-shaped, but the discrepancy between the area at the subcricoid level and the carina gradually diminishes, first to a cylindrical form and later to the more ovoid adult shape (see Figure 1-2B).2,6 After age 14 years, female tracheae generally stop growing whereas male tracheae continue to enlarge in cross section but not in length.4 Great care must be taken not to use excessively large endotracheal tubes for ventilation in infants and children. Formulas for selecting tube size with relation to age are not of great value because of individual variation.

The shape of the adult trachea varies even without disease. Some remain nearly circular rather than becoming ovoid (see Figure 1-2 C). In others, the sagittal diameter may be greater than the coronal. A slightly triangular configuration occurs less often. Unique and unexplained distortions also occur. The cross-sectional area as well as shape changes dynamically with intraluminal pressure alterations due to cough (see Figure 1-2B), respiration, and ventilation; tracheal length and volume vary similarly. The aorta may displace and deform the lower trachea (see Figure 1-2D).

The cross-sectional configuration of the trachea may be markedly altered with increasing age, particularly in the presence of chronic obstructive lung disease. The lower two-thirds of the trachea may gradually become flattened from side to side with a consequent decrease in lateral diameter and increase in anteroposterior diameter (see Figure 1-2E). This deformation is called, "saber sheath trachea."7 In this

Thyroid cartilage

Superior cornu

Inferior cornu

Cricothyroid membrane

Anterior thyroid lamina

Cricoid cartilage

Lesser cornu —i

Greater cornu

Thyrohyoid membrane

Superior thyroid notch

Anterior thyroid lamina

Cricoid cartilage

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