Tracheal dimensions are determined by the difference in pressures between the tracheal lumen and an external region. For example, in the cervical portion of the trachea, size is determined by the gradient between atmospheric and intratracheal pressure, whereas in the thoracic portion of the trachea, size is determined by the difference between intrapleural pressure and intratracheal pressure. In a study1 of healthy humans, CT was used to identify a 35% change in tracheal cross-sectional area at the level of the aortic arch between maximal inspiration and forced expiration. The change in area was primarily related to a decrease in tracheal height, and an associated change in tracheal shape was detected. Human and canine tracheas are anatomically similar in that both have semicircular cartilaginous rings that are connected dorsally by a dorsal tracheal membrane.2 These anatomic similarities suggest there may also be considerable fluctuation in the tracheal dimensions in healthy dogs between inspiration and expiration.
In dogs, the severity of tracheal collapse is evaluated on the basis of a grading scheme developed in 1982 by use of tracheoscopy, by which the percentage of tracheal obstruction is compared with an estimation of typical tracheal diameter in the dog if it were healthy.3 Grades range from I, which corresponds to a decrease in tracheal diameter of < 25%, to IV, which signifies that < 10% of the trachea remains patent.3 However, if tracheal diameter in healthy dogs fluctuates as it does in healthy humans, then a 35% change in dimension could actually be clinically normal.1
Dynamic changes in tracheal dimension are most important in identifying tracheal collapse, which is a problem that affects humans as well as small- and toy-breed dogs.4 In humans, CT is the imaging method of choice for defining tracheal collapse and a change in tracheal diameter of 70% to 100% can be evident in affected individuals.
The purpose of the study reported here was to measure induced changes in tracheal height, width, and cross-sectional area during forced inspiration and end expiration in healthy dogs to determine the degree of change possible at 3 regions of the trachea: cervical, thoracic inlet, and thoracic. Computed tomography was used rather than radiography or fluoroscopy because of the usefulness of CT images for measuring height, width, and cross-sectional area changes. To the authors' knowledge, there is no information available regarding fluctuations in tracheal dimensions between inspiration and expiration in healthy anesthetized dogs. This lack of reference data could result in misinterpretation of regular changes in the trachea throughout respiration and inappropriate diagnosis of tracheal collapse, particularly in dogs undergoing cervical and thoracic CT. We hypothesized that tracheal diameter in healthy dogs would vary by < 25% at any tracheal location between phases of respiration in a manner similar to the changes that occur in humans.
x/i Helical Scanner, GE Medical Systems, Milwaukee, Wis.
CT Perfusion 2, GE Medical Systems, Milwaukee, Wis.
GE Advantage Workstation 4.2, GE Medical Systems, Milwaukee, Wis.
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