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The trachea

image of The trachea
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Abstract

The trachea is a semi-rigid air-filled tube that connects the larynx with the bronchial system. The trachea ends caudally with a crest, called the carina, which divides and channels the airflow into the two mainstem bronchi. In a normal dog and cat the carina should be located at the level of the fourth or fifth intercostal space. The chapter considers Radiographic anatomy; Interpretive principles; Diseases.

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Figures

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10.1 Lateral radiograph of a normal canine thorax. Note the divergence of the trachea from the thoracic vertebra, and narrowing of the most caudal part of the trachea. The trachea ends with the carina at the level of the fifth intercostal space (arrowhead).
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10.3 Lateral radiograph of a normal canine thorax with annotations for the calculation of the tracheal diameter:thoracic inlet ratio. The thoracic inlet distance (black arrow) is measured from the ventral aspect of the vertebral column at the midpoint of the most cranial rib to the dorsal surface of the manubrium at its point of minimal thickness. The tracheal diameter (white arrow) is measured between the internal surfaces of the tracheal wall oriented perpendicularly to the tracheal long axis at the point where the thoracic inlet line crosses the midpoint of the tracheal lumen.
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10.4 Lateral tracheal radiograph of a 12-year-old Labrador Retriever (with dilated cardiomyopathy) with flexed neck position, resulting in dorsal deviation of the distal trachea. Repeated lateral radiograph with the neck in a neutral position. Note lack of kinking of the caudal trachea.
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10.5 Lateral cervical radiograph of a 7-year-old Labrador Retriever with no signs of respiratory disease. Notice the narrowed tracheal lumen caused by the ventrally deviated dorsal tracheal wall. CT image obtained immediately after radiography with similar recumbence and without chemical constraint. The cross-sectional area of the trachea at the level of the sixth cervical vertebra is reduced due to the invagination of the dorsal trachealis muscle into the lumen. Notice also the slight rotation of the tracheal height axis in relation to the dorsoventral axis of the body. This obliquity accentuates the luminal narrowing on the lateral radiograph.
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10.6 CT image of the cranial thorax of a 4-year-old Yorkshire Terrier with an acute episode of respiratory distress 2 years after intraluminal stent placement for tracheal collapse. Radiographic evaluation revealed increased opacity of the stented tracheal lumen and in the right caudal thorax. The CT image reveals the presence of gravity-dependent tracheal fluid (meniscus-shaped fluid–gas interface), but no evidence of obstructive granulation or other tissue, and partially consolidated cranial lung lobes. Final diagnosis was a diaphragmatic hernia and pneumonia for which the dog was successfully treated. Notice the mild streak artefacts emanating from the metallic stent, which do no prevent diagnosis.
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10.8 Lateral view of the thorax of a 1-year-old terrier with tracheal hypoplasia. Note the uniform narrowing along the length of the trachea. The tracheal diameter:thoracic inlet ratio was 0.11. (Courtesy of the Animal Health Trust)
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10.9 CT image of the trachea at the level of the sixth cervical vertebra of a Beagle cross dog with mucopolysaccharidosis VII. Notice the thickened and overlapping tracheal cartilages, resulting in a reduced lumen.
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10.10 Lateral thoracic radiograph of a 4-year-old Domestic Shorthair cat with tracheal stenosis (arrowed). Note the marked dilatation of the segments of trachea cranial and caudal to the lesion as well as the hyperinflated lung fields.
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10.11 Dorsally reconstructed tracheal CT image of a 13-year-old Domestic Shorthair cat with a 3-week history of increased respiratory effort. Notice the stenotic lumen and thickened wall of the trachea at the level of the first intercostal space (arrowheads). A granuloma was surgically resected.
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10.13 Positioning of a dog for the tangential craniocaudal view of the trachea at the thoracic inlet. The central ray (yellow line) is directed at the ventral aspect of the thoracic inlet. Due to the obliquity of the beam only a parallel grid with grid lines oriented parallel to the long axis of the table, or a table top technique without a grid, can be used to obtain this view. This view requires manual restraint and should therefore only be taken if absolutely essential. (Courtesy of I. Schwarz) Tangential rostrocaudal radiograph of a 1-year-old Akita with a normal trachea. Notice the circular radiolucent tracheal lumen. (Courtesy of D. Rodriguez) Tangential rostrocaudal radiograph of a 4-year-old Pomeranian with tracheal collapse. The residual cross-sectional lumen of the collapsed trachea can be seen as a crescent-shaped gas opacity in the thoracic inlet (arrowheads). (Courtesy of D. Rodriguez)
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10.14 Diagram illustrating the dependence of tracheal collapse on phase of respiration. During inspiration a slightly positive pressure surrounds the cervical trachea as long as air moves towards the thorax. This positive pressure collapses the cervical trachea if it lacks sufficient stability. In the thoracic area, the pressure surrounding the trachea is lower than the pressure within the trachea, which results in tracheal distension. During expiration, the thoracic pressure exceeds the intratracheal pressure. This reduces the diameter of the thoracic trachea as long as air flows towards the larynx. However, the cervical trachea is distended due to the luminal pressure exceeding the outside pressure. (Adapted from with permission). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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10.15 Lateral radiograph centred on the thoracic inlet of a 15-year-old Miniature Poodle showing partial collapse of the extrathoracic portion and a distended intrathoracic portion of the trachea during inspiration. Notice also the radiolucent lung fields and mainstem bronchi. Lateral radiograph demonstrating partial collapse of the intrathoracic portion and normal diameter of the cervical portion of the trachea during expiration. Notice the more opaque caudodorsal lung field and collapsed mainstem bronchi.
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10.16 Lateral cervicothoracic radiograph of a 16-year-old Chihuahua with complete collapse of the cervical and thoracic portion of the trachea. There is no distinguishable radiolucent lumen visible. CT image of the caudal cervical trachea of a 7-year-old Papillon with chronic inspiratory dyspnoea. The image was acquired during natural inspiration. The tracheal lumen is completely collapsed, the tracheal cartilage is flattened and the dorsal trachealis muscle is stretched.
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10.17 Lateral cervicothoracic radiograph of a 16-year-old Chihuahua with extensive cervical and thoracic tracheal collapse. Notice the straight ventral and undulating dorsal aspect of the collapsing tracheal wall.
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10.18 Lateral cervical radiograph of a 6-year-old Domestic Shorthair cat with chronic inspiratory stridor. Notice the marked collapse of the caudal cervical trachea. A congenital deformation in the cranial end of the trachea with significant luminal obstruction was found on autopsy. (Reproduced from with permission from the )
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10.19 Lateral cervicothoracic radiograph of a 7-month-old Domestic Shorthair cat suffering from a tracheal intubation injury caused during a routine spay procedure. The laceration site is not directly visible but the secondary subcutaneous emphysema and pneumomediastinum are. The tracheal wall is highlighted by gas on both sides.
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10.20 Lateral thoracic radiograph of a 4-year-old Domestic Shorthair cat with complete tracheal rupture due to a road traffic accident. Notice the absence of tracheal rings within the ballooned radiolucent area extending from the second to fourth intercostal space. This gas bubble contained in local mediastinal structures is sometimes referred to as a pseudotrachea. (Courtesy of E. Friend)
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10.22 Lateral cervical radiograph of a 7-year-old Rottweiler with chronic upper respiratory distress. There is a soft tissue and mineral opaque mass arising from the ventral aspect of the tracheal wall, inhibiting further passage of the endotracheal tube. An oesophageal tube can be seen dorsally. Final diagnosis was a tracheal chondrosarcoma.
Image of 10.23
10.23 Contrast-enhanced CT image at the level of the sixth cervical vertebra of a 10-year-old Domestic Shorthair cat with chronic inspiratory dyspnoea. There is a contrast-enhancing (bright) soft tissue mass (arrowed) arising from the dorsal wall of the trachea (T) adjacent to the gas-distended oesophagus (O). Final diagnosis was a tracheal lymphoma. Tracheal resection and chemotherapy maintained this cat in remission for almost 2 years.
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10.24 Lateral thoracic radiograph of a 5-year-old Domestic Shorthair cat with a radiopaque foreign body in the caudal trachea. (Courtesy of E. Friend) Lateral radiograph of a cat with a severed endotracheal tube in the thoracic trachea.
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10.25 Lateral radiograph of a 5-year-old Lurcher with several nodules (arrowheads) in the caudal thoracic trachea representing granulomas. (Courtesy of A. Holloway)
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10.26 Lateral thoracic radiograph of a 3-year-old Labrador Retriever with rodenticide toxicity (warfarin) causing submucosal tracheal and pulmonary haemorrhage. Notice the marked tracheal narrowing and increased opacity in the midcaudal lung field. DV thoracic radiograph. There is marked cranial mediastinal widening and increased opacity in the left caudal lung lobe as a result of haemorrhage.
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10.27 Lateral thoracic radiograph of a dog with mitral valve endocardiosis and resulting cardiomegaly. Notice the straight course of the thoracic trachea, parallel to the thoracic spine, due to the enlarged cardiac silhouette.
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10.28 Lateral thoracic radiograph of a 10-year-old Cavalier King Charles Spaniel with a heart base tumour. Notice the focal marked dorsal tracheal deviation pivoting over the cranial cardiac silhouette. Reconstructed dorsal plane thoracic CT image demonstrating the lateral deviation and partial compression of the caudal thoracic trachea as well as the left cranial lung lobe.
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10.29 Lateral thoracic radiograph of an 8-month-old West Highland White Terrier with pulmonic stenosis. The protruding dilated main pulmonary artery segment is superimposed on the ventral aspect of the caudal thoracic tracheal lumen, creating the impression of a hat (i.e. the ‘hat sign’).
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10.30 VD thoracic radiograph of a dog with a PRAA. Notice the leftward deviation of the caudal thoracic trachea (arrowhead) caused by entrapment between the right aortic arch and the left-sided ductus arteriosus. This radiographic sign is relatively specific for PRAA and has been described as a hallmark for this condition. (Courtesy of J. Buchanan)
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10.31 Lateral thoracic radiograph of a 15-year-old Domestic Shorthair cat with a mediastinal carcinoma occurring 2 years after successful radiation treatment of a thymoma. Notice the dorsal tracheal deviation pivoting at the level of the fourth rib. There is also visible cranial lung lobe atelectasis (air bronchograms) and pleural effusion. Lateral thoracic radiograph of a dog with perihilar lymphadenopathy causing ventral deviation and compression of the caudal thoracic trachea. Lateral thoracic radiograph of an 11-year-old Domestic Shorthair cat with a large thymoma causing dorsal tracheal deviation and caudal displacement of the carina (eighth intercostal space). The heart is located at the caudodorsal aspect of the mass. The caudal displacement of the carina is pathognomonic for a cranial mediastinal mass. Lateral thoracic radiograph of a 2-year-old Irish Setter with a neuroendocrine tumour in the cranial mediastinum causing ventral deviation of the cranial thoracic trachea.
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10.32 Cranial close-up of a lateral thoracic radiograph of a dog with pleural effusion. The trachea is straight but parallel to the thoracic spine, a common feature in pleural effusion.
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10.33 Lateral cervical radiograph of a 2-month-old Weimaraner with canine strangles. There is severe submandibular and retropharyngeal lymphadenopathy, the latter causing marked ventral deviation of the larynx and cranial trachea. Lateral cervical radiograph of a 10-year-old Dobermann with a mineralized thyroid tumour causing marked dorsal deviation of the cranial trachea.

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