Tracheal collapse is a common cause of cough in mature, small-breed dogs. This disease is defined either by narrowing of the lumen of the trachea, prolapse of the dorsal tracheal membrane, or a combination of those abnormalities.1 Tracheal narrowing is a multifactorial process that results from weakening of the tracheal cartilage.2,3 It has been proposed that tracheal collapse is often associated with concurrent weakening of the bronchial cartilage.1 In dogs, collapse is frequently exacerbated by coexisting conditions such as laryngeal paralysis, heart disease, or bronchitis and identifying these concurrent conditions is critical in the medical management of tracheal collapse.1,4 Grading the degree of collapse is important for recognition of the location and severity of airway collapse, monitoring progression of disease, and identification of potential surgical candidates among affected animals.1
For evaluation of coughing dogs in which tracheal collapse is a potential cause, currently available methods include radiography (preferably including inspiratory and expiratory views),5 fluoroscopy,6 ultrasonography,7 CT,8 and bronchoscopy.1 Disadvantages of ultrasonography include the inability to simultaneously evaluate the thorax for coexisting diseases and a poor-quality image because of air in the trachea. A disadvantage of CT and bronchoscopy is the requirement for anesthesia; dogs with airway collapse can have a difficult recovery from anesthesia as a result of complications, such as increased respiratory effort, that lead to further airway irritation or collapse. Also, endotracheal intubation can exacerbate tracheal inflammation, thereby initiating a vicious cycle of tracheal irritation and collapse.1,9 Nevertheless, bronchoscopy allows for assessment of the extent and degree of airway collapse, analysis of airway injury associated with collapse, and collection of airway samples for detection of additional diseases.
Fluoroscopy can be used for detection and grading the severity of tracheal collapse in veterinary patients6; however, this technique is generally available only at university and large referral hospitals, whereas conventional radiography can be performed at most small animal hospitals and clinics. Furthermore, fluoroscopic procedures result in exposure of the patient and the technical staff who perform the examination to doses of radiation that are greater than those sustained during standard radiographic procedures. Radiography has been reported to detect tracheal collapse in 59%10 to 84%11 of dogs, but to our knowledge, the accuracy of radiography for the detection and severity assessment of tracheal collapse, compared with a real-time diagnostic technique such as fluoroscopy, has not been assessed in dogs. The purpose of the study reported here was to compare the use of radiography and fluoroscopy for detection and grading of tracheal collapse in dogs with fluoroscopically confirmed tracheal collapse. The intent was to determine the ability of radiography to provide diagnostic evidence of tracheal collapse when it is present in dogs. An additional goal of the study was to determine whether the location and degree of collapse (as detected fluoroscopically) were accurately represented by radiographic findings.
Criteria for Selection of Cases
Medical records at the Veterinary Medical Teaching Hospital of the University of California, Davis, were searched for dogs with tracheal collapse for which the diagnosis had been confirmed via fluoroscopy during the period from February 6, 2001, to February 6, 2006. Only those dogs for which at least 1 lateral thoracic radiographic view (including the cervical portion of the trachea) was available were included in the study. Fluoroscopic images were digitized to provide consistency and optimize determination of the change in tracheal diameter.
Procedures
For each dog, the following information was abstracted from the medical record: age, breed, sex, reproductive status, BCS (assessed by use of a scale of 1 to 912), the presence or absence of a heart murmur, and the presence and severity of dental disease. All fluoroscopic examinations were obtained with the dog restrained in right lateral recumbency. Regular respiration was observed and recorded on video. A wooden spoon was used to manipulate the trachea and induce a cough to allow fluoroscopic evaluation of the trachea during a coughing episode.
A board-certified radiologist (REP) evaluated all fluoroscopic videos and radiographic views. The cervical, thoracic inlet, thoracic, and carinal regions of the trachea and the main stem bronchi were individually evaluated for collapse both on fluoroscopic and radiographic images. If an area of the trachea could not be clearly seen on an image, it was marked as not assessed and was not included in calculations. Findings were considered positive if collapse was detected in a region at any phase of the respiratory cycle. The fluoroscopic images were evaluated separately in the 4 tracheal regions and the main stem bronchi during inspiration, expiration, and an episode of cough.
In the cervical, thoracic inlet, thoracic, and carinal regions of the trachea, the degree of collapse was graded as 0%, 25%, 50%, 75%, or 100% by measuring the decrease in luminal diameter associated with collapse, compared with the estimated tracheal width in each individual dog (Figure 1). Invagination of the dorsal tracheal membrane into the tracheal lumen was considered evidence of a redundant tracheal membrane. The contribution of the redundant dorsal tracheal membrane to luminal narrowing was included in the overall estimated degree of airway collapse (Figure 2). This semiquantitative grading scheme was developed to approximate the widely used grading scheme first proposed by Tangner and Hobson.10 The main stem bronchi were evaluated only for the presence or absence of collapse, and no severity grade was assigned.
Lateral radiographic views of the thorax of a dog with fluoroscopically confirmed tracheal collapse. A—View obtained during the inspiratory phase of respiration. Notice that there is a diffuse bronchial pattern but no evidence of collapse in the visible portion of the trachea. B—View obtained during the expiratory phase of respiration. Notice that the main stem bronchi (black arrows) and the carinal portion of the trachea (white arrow) have collapsed. The extent of the collapse (decrease in luminal diameter) in the carina was graded as 100% in this dog.
Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1870
Lateral radiographic views of the thorax of a dog with fluoroscopically confirmed tracheal collapse. A—View obtained during the inspiratory phase of respiration. Notice that there is a diffuse bronchial pattern but no evidence of collapse in the visible portion of the trachea. B—View obtained during the expiratory phase of respiration. Notice that the main stem bronchi (black arrows) and the carinal portion of the trachea (white arrow) have collapsed. The extent of the collapse (decrease in luminal diameter) in the carina was graded as 100% in this dog.
Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1870
Lateral radiographic views of the thorax of a dog with fluoroscopically confirmed tracheal collapse. A—View obtained during the inspiratory phase of respiration. Notice that there is a diffuse bronchial pattern but no evidence of collapse in the visible portion of the trachea. B—View obtained during the expiratory phase of respiration. Notice that the main stem bronchi (black arrows) and the carinal portion of the trachea (white arrow) have collapsed. The extent of the collapse (decrease in luminal diameter) in the carina was graded as 100% in this dog.
Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1870
Figure 2—Lateral radiographic view of the cervical area of a dog with fluoroscopically confirmed tracheal collapse. Notice the invagination of the dorsal tracheal membrane into the tracheal lumen within the cervical and thoracic inlet regions; the redundant membrane reduces the luminal diameter of the trachea by approximately 50% in the cervical region (white arrows). Because the dorsal aspect of the trachea can still be seen (black arrows) in this view, this confirms that membrane redundancy rather than collapse of the tracheal rings has occurred.
Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1870
Figure 2—Lateral radiographic view of the cervical area of a dog with fluoroscopically confirmed tracheal collapse. Notice the invagination of the dorsal tracheal membrane into the tracheal lumen within the cervical and thoracic inlet regions; the redundant membrane reduces the luminal diameter of the trachea by approximately 50% in the cervical region (white arrows). Because the dorsal aspect of the trachea can still be seen (black arrows) in this view, this confirms that membrane redundancy rather than collapse of the tracheal rings has occurred.
Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1870
Figure 2—Lateral radiographic view of the cervical area of a dog with fluoroscopically confirmed tracheal collapse. Notice the invagination of the dorsal tracheal membrane into the tracheal lumen within the cervical and thoracic inlet regions; the redundant membrane reduces the luminal diameter of the trachea by approximately 50% in the cervical region (white arrows). Because the dorsal aspect of the trachea can still be seen (black arrows) in this view, this confirms that membrane redundancy rather than collapse of the tracheal rings has occurred.
Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1870
Dogs were divided into 2 groups for data analysis. Group 1 dogs had lateral cervical and thoracic radiographic views available for assessment. At the study institute, the protocol is to obtain lateral thoracic radiographic views when full inspiration is attained by the patient. Therefore, these study images were presumed to be taken during the stage of full inspiration. Group 2 dogs had both inspiratory and expiratory lateral cervical and thoracic radiographic views available for assessment.
Statistical analysis—The percentage of radiographic views that revealed any form of collapse was calculated. In addition, the percentage of radiographic views in which collapse was evident in a region that had been fluoroscopically confirmed as positive for collapse was calculated. With fluoroscopy as the gold standard, sensitivity, specificity, positive predictive value, and negative predictive value were determined for radiography with regard to detection of tracheal collapse in each of the 5 regions (cervical, thoracic inlet, thoracic, and carinal regions of the trachea and the main stem bronchi). Mean ± SD grade of collapse for each region as determined via fluoroscopy and radiography was calculated. A Student t test was used to assess for significant differences between grades of collapse determined via radiography and fluoroscopy. In addition, the contribution of the redundant dorsal tracheal membrane was removed from the grade of collapse determined from fluoroscopic and radiographic images, and a Student t test was used to determine whether grades were significantly changed from the grades that included the redundant dorsal membrane. For all statistical comparisons, a value of P < 0.05 was considered significant.
Results
Sixty-two dogs met the inclusion criteria for the study. Thirty-two (52%) dogs were female, and 30 (48%) were male; the age range was 1 to 15 years (mean ± SD age, 9.3 ± 3.2 years). Among the dogs, breeds included 10 (16%) Yorkshire Terriers, 10 (16%) mixed-breed dogs, 9 (15%) Pomeranians, 4 (6%) Maltese, 3 (5%) Miniature Poodles, 3 (5%) Toy Poodles, 3 (5%) Pugs, 2 (3%) Cocker Spaniels, 2 (3%) Soft Coated Wheaten Terriers, and 2 (3%) Shih Tzus. There was also 1 (1.6%) each of other breeds as follows: Australian Shepherd, Japanese Chin, Border Collie, Schipperke, Dachshund, Miniature Pinscher, Pekingese, Papillon, Poodle, Jack Russell Terrier, American Eskimo, and Bull Mastiff. The BCSs for all the dogs ranged from 3 to 9 (mean BCS, 6.2). Among the 62 dogs, only 1 (1.6%) was underweight (BCS within the range of 1 to 3), 15 (24.2%) were considered of normal body condition (BCS, 4 or 5), and 46 (74.2%) were over-weight (BCS within the range of 6 to 9). A heart murmur was detected in 12 (19.4%) dogs. Mild dental disease was present in 10 (16.1%) dogs, whereas moderate to severe dental disease was present in 38 (61.3%) dogs.
Fluoroscopically, tracheal collapse was evident in at least 1 region of the trachea or main stem bronchi in all dogs. Fluoroscopic views obtained during the inspiratory phase of respiration revealed tracheal collapse more commonly in the thoracic inlet (36/48 [75.0%] dogs) and cervical (34/43 [79.1%] dogs) regions than in the carinal (27/57 [47.4%] dogs) and thoracic (24/57 [42.1%] dogs) regions. During the expiratory phase of respiration, fluoroscopic views revealed tracheal collapse less commonly in the cervical (9/43 [20.9%] dogs) and thoracic inlet (9/46 [19.6%] dogs) regions and more commonly in the thoracic (14/49 [28.6%] dogs) and carinal (18/49 [36.7%] dogs) regions. Dogs with cervical collapse during expiration had static collapse in this region that did not change with respiratory phase. During cough, collapse was occasionally detected in the cervical (4/25 [16.0%] dogs) region but was more common in the thoracic inlet (26/43 [60.5%] dogs), thoracic (40/46 [86.9%] dogs), and carinal (43/46 [93.5%] dogs) regions of the trachea.
Among all dogs, 58 (93.5%) had radiographic evidence of luminal attenuation. Single lateral cervical or thoracic radiographic views were available for 54 dogs (group 1), whereas both inspiratory and expiratory lateral thoracic radiographic views were available for the remaining 8 dogs (group 2). In group 1 dogs, radiography revealed some degree of tracheal collapse in 50 of 54 (92.6%) dogs. However, the region of collapse identified radiographically agreed with that identified fluoroscopically in only 26 of the 54 (48.1%) cases. Among the 5 anatomic regions of collapse, 270 points of possible agreement between fluoroscopic and radiographic findings were identified. Twelve regions could not be evaluated via 1 or the other modality; thus, there were 258 points of possible agreement. Sensitivity, specificity, positive predictive value, and negative predictive value for detection of collapse in each of the 5 regions by use of radiography in group 1 dogs were calculated (Table 1).
Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of radiography for detection of tracheal collapse in 4 tracheal regions and main stem bronchi in dogs with fluoroscopically confirmed tracheal collapse determined on the basis of data for 54 dogs (group 1) for which single lateral cervical or thoracic radiographic views were available and 8 dogs (group 2) for which lateral radiographic views obtained during the inspiratory and expiratory phases of respiration were available for assessment.
| Group | Region | Sensitivity | Specificity | PPV | NPV |
|---|---|---|---|---|---|
| 1 | Cervical portion of the trachea | 0.76 | 0.48 | 0.46 | 0.78 |
| Thoracic inlet | 0.76 | 0.59 | 0.79 | 0.56 | |
| Thoracic portion of the trachea | 0.45 | 0.57 | 0.88 | 0.13 | |
| Carina | 0.51 | 0.57 | 0.89 | 0.15 | |
| Main stem bronchi | 0.13 | 0.95 | 0.80 | 0.42 | |
| Overall | 0.50 | 0.64 | 0.75 | 0.37 | |
| 2 | Cervical portion of the trachea | 1 | 0 | 0.43 | NA |
| Thoracic inlet | 0.86 | 0 | 0.86 | 0 | |
| Thoracic portion of the trachea | 0.75 | NA | 1 | 0 | |
| Carina | 0.75 | NA | 1 | 0 | |
| Main stem bronchi | 0.14 | 1 | 1 | 0.14 | |
| Overall | 0.67 | 0.67 | 0.81 | 0.08 |
NA = Not applicable.
From the radiographic and fluoroscopic images, the grade (degree) of collapse in group 1 dogs was assessed (Table 2). In general, radiography revealed more dogs with mild collapse (25% decrease in tracheal diameter) than fluoroscopy. Via fluoroscopy, the degree of collapse was graded as 75% to 100%, particularly in the thoracic and carinal regions, much more frequently than via radiography. Via radiography, the grades of collapse in the thoracic inlet, thoracic, and carinal regions were underestimated; on comparison of the fluoroscopic and radiographic assessments, the same grade of collapse was assigned in only 9 of the 62 (15%) dogs (Figure 3).
Results of severity grading (number of dogs/total number of dogs in category) of tracheal collapse in 4 regions assessed in fluoroscopic and radiographic images obtained from dogs with fluoroscopically confirmed tracheal collapse. Group 1 included 54 dogs for which single lateral cervical or thoracic radiographic views were available, and group 2 included 8 dogs for which lateral radiographic views obtained during the inspiratory and expiratory phases of respiration were available for assessment. Fluoroscopic images were evaluated separately in the 4 tracheal regions during inspiration and expiration, and the worst grade is reported.
| Group | Region | Fluoroscopic grade (% collapse) | Radiographic grade (% collapse) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 25 | 50 | 75 | 100 | Mean ± SD grade (median) | 0 | 25 | 50 | 75 | 100 | Mean ± SD grade (median) | ||
| 1 | Cervical | 28/44 | 3/44 | 11/44 | 2/44 | 0/44 | 17.9 ± 25.1 (0) | 17/44 | 18/44 | 6/44 | 2/44 | 1/44 | 22.3 ± 23.7 (25) |
| Thoracic inlet | 16/50 | 9/50 | 8/50 | 6/50 | 11/50 | 42.6 ± 39.1 (25) | 17/48 | 19/48 | 7/48 | 5/48 | 0/48 | 25.0 ± 23.9* (25) | |
| Thoracic | 6/51 | 11/51 | 13/51 | 8/51 | 13/51 | 53.7 ± 34.1 (50) | 29/52 | 17/52 | 6/52 | 0/52 | 0/52 | 13.9 ± 17.3* (0) | |
| Carina | 7/54 | 10/54 | 11/54 | 8/54 | 18/54 | 59.3 ± 36.1 (50) | 26/52 | 16/52 | 6/52 | 2/52 | 2/52 | 19.9 ± 25.9* (12.5) | |
| 2 | Cervical | 4/7 | 1/7 | 1/7 | 1/7 | 0/7 | 21.4 ± 30.4 (0) | 0/7 | 4/7 | 2/7 | 1/7 | 0/7 | 39.3 ± 19.7 (25) |
| Thoracic | 1/8 | 0/8 | 4/8 | 1/8 | 2/8 | 59.4 ± 32.6 (50) | 1/8 | 3/8 | 2/8 | 2/8 | 0/8 | 40.6 ± 26.5 (37.5) | |
| Thoracic | 0/8 | 0/8 | 5/8 | 2/8 | 1/8 | 62.5 ± 18.9 (50) | 2/8 | 5/8 | 1/8 | 0/8 | 0/8 | 21.9 ± 16.0* (25) | |
| Carina | 0/8 | 0/8 | 5/8 | 0/8 | 3/8 | 68.8 ± 25.9 (50) | 2/8 | 1/8 | 2/8 | 2/8 | 1/8 | 46.9 ± 36.4 (50) | |
Radiographic grade is significantly (P < 0.05) different from fluoroscopic grade in this region.
Lateral radiographic (obtained during inspiration; A) and fluoroscopic views (obtained at maximal collapse during expiration; B) of the thoracic portion of the trachea of a dog with fluoroscopically confirmed tracheal collapse. Radiographically, the extent of collapse of the thoracic inlet, thoracic, and carinal regions (black arrows) of the trachea appears to be approximately 25% (A). Fluoroscopically, the extent of collapse of the thoracic inlet, thoracic, and carinal regions (black arrows) of the trachea appears to be 100% (B), indicating that the degree of tracheal collapse was underestimated via radiography.
Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1870
Lateral radiographic (obtained during inspiration; A) and fluoroscopic views (obtained at maximal collapse during expiration; B) of the thoracic portion of the trachea of a dog with fluoroscopically confirmed tracheal collapse. Radiographically, the extent of collapse of the thoracic inlet, thoracic, and carinal regions (black arrows) of the trachea appears to be approximately 25% (A). Fluoroscopically, the extent of collapse of the thoracic inlet, thoracic, and carinal regions (black arrows) of the trachea appears to be 100% (B), indicating that the degree of tracheal collapse was underestimated via radiography.
Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1870
Lateral radiographic (obtained during inspiration; A) and fluoroscopic views (obtained at maximal collapse during expiration; B) of the thoracic portion of the trachea of a dog with fluoroscopically confirmed tracheal collapse. Radiographically, the extent of collapse of the thoracic inlet, thoracic, and carinal regions (black arrows) of the trachea appears to be approximately 25% (A). Fluoroscopically, the extent of collapse of the thoracic inlet, thoracic, and carinal regions (black arrows) of the trachea appears to be 100% (B), indicating that the degree of tracheal collapse was underestimated via radiography.
Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1870
In all 8 of the group 2 dogs, examination of both inspiratory and expiratory lateral thoracic radiographic views revealed some degree of tracheal collapse. The region of collapse identified radiographically agreed with that identified fluoroscopically in 5 of the 8 cases. Among the 5 anatomic regions of collapse, 40 points of possible agreement between fluoroscopic and radiographic findings were identified. One region could not be evaluated via fluoroscopy; thus, there were 39 points of possible agreement. Sensitivity, specificity, positive predictive value, and negative predictive value for detection of collapse in each of the 5 regions by use of radiography in group 2 dogs were calculated (Table 3).
Sensitivity, specificity, PPV, and NPV of radiography for detection of tracheal collapse after removal of the contribution of the dorsal tracheal membrane in 4 tracheal regions and main stem bronchi in dogs with fluoroscopically confirmed tracheal collapse determined on the basis of data for 54 dogs (group 1) for which single lateral cervical or thoracic radiographic views were available and 8 dogs (group 2) for which lateral radiographic views obtained during the inspiratory and expiratory phases of respiration were available for assessment. Values for the main stem bronchi with or without the inclusion of dorsal tracheal membrane redundancy are the same.
| Group | Region | Sensitivity | Specificity | PPV | NPV |
|---|---|---|---|---|---|
| 1 | Cervical portion of the trachea | 0.29 | 0.89 | 0.33 | 0.87 |
| Thoracic inlet | 0.50 | 0.70 | 0.67 | 0.53 | |
| Thoracic portion of the trachea | 0.44 | 0.82 | 0.90 | 0.27 | |
| Carina | 0.51 | 0.57 | 0.89 | 0.15 | |
| Main stem bronchi | 0.13 | 0.95 | 0.80 | 0.42 | |
| Overall | 0.40 | 0.83 | 0.79 | 0.47 | |
| 2 | Cervical portion of the trachea | 1 | 1 | 1 | 1 |
| Thoracic inlet | 0.83 | 1 | 1 | 0.5 | |
| Thoracic portion of the trachea | 0.5 | NA | 1 | 0 | |
| Carina | 0.75 | NA | 1 | 0 | |
| Main stem bronchi | 0.14 | 1 | 1 | 0.14 | |
| Overall | 0.59 | 1 | 1 | 0.32 |
See Table 1 for key.
From the inspiratory and expiratory radiographic and fluoroscopic images, the grade (degree) of collapse in group 2 dogs was assessed (Table 2). Radiography revealed more dogs with mild collapse (25% decrease in tracheal diameter) than fluoroscopy. Via fluoroscopy, the degree of collapse was graded as 75% to 100%, particularly in the thoracic and carinal regions, more frequently than via radiography. Assessments of inspiratory and expiratory lateral thoracic radiographic views underestimated the degree of collapse in most instances, but this was only significant for findings in the thoracic region. On comparison of the fluoroscopic and radiographic assessments, the same grade of collapse was assigned in only 1 of the 8 dogs in group 2.
Partial invagination (redundancy) of the dorsal tracheal membrane into the tracheal lumen has been reported13,14 in clinically normal humans, but the importance of dorsal membrane redundancy in dogs with tracheal collapse is unknown. Therefore, we evaluated the sensitivity, specificity, positive predictive value, and negative predictive value of radiographic assessments for detection of tracheal collapse in both groups after eliminating the contribution of the dorsal membrane to the overall presence and grade of collapse. For group 1 dogs, removal of the contribution of the redundant membrane from the fluoroscopic grading scheme for tracheal collapse resulted in a significant (P = 0.025) decrease in the grade of collapse in the cervical region, compared with the initial value. There were no significant differences between the initial and recalculated fluoroscopic severity grades for the thoracic inlet (P = 0.498), thoracic (P = 0.636), and carinal (P = 1.000) regions. For group 2 dogs, removal of the contribution of the redundant membrane from the fluoroscopic grading scheme for tracheal collapse did not result in significant differences between the initial and recalculated fluoroscopic severity grades for the cervical (P = 0.810), thoracic inlet (P = 0.498), thoracic (P = 0.636), and carinal (P = 1.000) regions. For group 1 dogs, removal of the contribution of the redundant membrane from the radiographic grading scheme for tracheal collapse resulted in significant decreases in the grade of collapse in the cervical (P < 0.001) and thoracic inlet (P = 0.004) regions, compared with the respective initial value (Table 3). There were no significant differences between the initial and recalculated radiographic severity grades for the thoracic (P = 0.470) and carinal (P = 1.000) regions. For group 2 dogs, removal of the contribution of the redundant membrane from the radiographic grading scheme for tracheal collapse resulted in a significant decrease in the grade of collapse in the cervical region (P = 0.008), compared with the initial value. There were no significant differences between the initial and recalculated radiographic severity grades for the thoracic inlet (P = 0.175), thoracic (P = 0.483), and carinal (P = 1.000) regions.
Discussion
Results of the present study have indicated that abnormalities in the trachea of most dogs (50/54 [92%] dogs) that have fluoroscopically confirmed tracheal collapse could be detected via examination of lateral radiographic views of the cervical region and thorax. Inclusion of a lateral thoracic radiographic view obtained during the expiratory phase of respiration increases the ability of radiography to detect attenuation of airway diameter (as determined by findings for all 8 dogs in group 2 in the present study). Although these results appeared quite encouraging, further assessment revealed that the data are somewhat misleading. In group 1 dogs, radiographic and fluoroscopic identification of collapse in the same region was apparent in only 26 of 54 cases. Examination of additional expiratory lateral thoracic images improved the frequency of such corroborative findings only slightly (as determined in 5/8 dogs in group 2). Assuming that the findings of fluoroscopy were correct for the 62 study dogs, collapse was identified in an incorrect location in 27 (44%) dogs and was not detected at all in 5 (8%) dogs via radiography.
One possible explanation for this high number of false-positive results for tracheal collapse determined via radiography is that the degree of dynamic tracheal luminal change during tidal respiration in clinically normal dogs has yet to be defined. In clinically normal humans, CT has revealed that the tracheal diameter can change by 12%13 to 32%14 between phases of maximum inspiration and expiration; the data from those studies13,14 have suggested that a change in tracheal diameter of 28% to 70% between the inspiratory and expiratory phases must be identified before the diagnosis of tracheomalacia is made in humans. This range of values is likely higher than that expected in dogs because forced inspiration and expiration cannot easily be achieved during radiography or fluoroscopy in canids and the degree of change in tracheal diameter between inspiration and expiration is therefore likely to be less. However, the configuration of the trachea is similar in dogs and humans, and some degree of diameter change is likely to occur in clinically normal dogs during respiration. If dogs in the present study that had ≤ 25% luminal attenuation of airway diameter during any phase of respiration (evident radiographically or fluoroscopically) were considered clinically normal, the number of positive results for tracheal collapse identified via radiography would substantially decrease. Therefore, it is possible that some dogs in our study that were considered to have tracheal collapse on the basis of either fluoroscopic or radiographic findings were actually undergoing normal respiration-associated changes in tracheal luminal diameter. This mild attenuation of luminal diameter would be easier to detect via radiography than fluoroscopy because of higher image resolution attained radiographically. Moreover, the dynamic nature of the luminal change may be detected by use of 1 imaging technique but not another. Further assessment of the change in tracheal luminal diameter that occurs in dogs during normal respiration is necessary to determine criteria that define clinically important tracheal collapse, thereby ensuring that this disorder is not overdiagnosed in this species.
Similarly, the contribution of the redundant dorsal tracheal membrane to tracheal collapse is unknown in dogs. The presence of a redundant tracheal membrane was detected more frequently via radiography (evident in 67 views obtained from 62 dogs) than it was via fluoroscopy (evident in 17 views obtained from 62 dogs). A redundant dorsal tracheal membrane was identified most often in the cervical portion of the trachea via radiography and in the thoracic portion of the trachea via fluoroscopy. Perhaps the lesser image resolution of fluoroscopy combined with difficulty in obtaining images of the cervical region prevented detection of a redundant dorsal membrane in the cervical region. Regardless, the number of dogs with a redundant membrane (particularly in the cervical or thoracic inlet regions) appears high in the study population. In the grading scheme for tracheal collapse reported by Tangner and Hobson,10 dorsal tracheal membrane redundancy is considered a part of tracheal collapse. However, it is widely accepted that some clinically normal dogs with no signs of cough can have radiographic evidence of dorsal tracheal membrane redundancy, as may some clinically normal humans.13 After removal of the contribution of the redundant dorsal tracheal membrane to the tracheal luminal attenuation detected fluoroscopically in the present study, we found that only the results for the cervical portion of the trachea were substantially changed from the initial findings. The grade of collapse assessed radiographically decreased significantly in the cervical and thoracic inlet regions after the contribution of the redundant membrane was removed from the grading process. This implies that dorsal tracheal membrane redundancy occurs more commonly in the cervical and thoracic inlet regions and less commonly in the thoracic and carinal regions of the trachea in dogs. Evidently, the degree of dorsal tracheal membrane redundancy detected in clinically normal, small-breed dogs must be further defined before its role in tracheal luminal attenuation can be completely understood.
In our study, a high percentage of dogs with collapse in either the thoracic or carinal regions of the trachea was detected fluoroscopically. In fact, the degree of collapse was most severe in the region of the carina among the study dogs. In contrast, collapse was most commonly detected in the cervical or thoracic inlet regions radiographically. In a previous study,15 the thoracic inlet was the area most commonly and severely affected with tracheal collapse in dogs. Assuming that the fluoroscopic findings were correct, our results contradict current opinion regarding the most common location of tracheal collapse in dogs. Perhaps this is related to the fact that much of the veterinary medical literature has used radiography to identify tracheal collapse in dogs.1 Tracheal collapse in the cervical and thoracic inlet regions may be more easily detected via radiography because those areas are more likely to collapse during inspiration, and in clinical practice, radiographic views of the thorax of dogs are most commonly acquired during the inspiratory phase of respiration. Fluoroscopy can be used to obtain images during the entire respiratory cycle as well as during an induced cough. Expiration and cough are more likely to result in collapse of the intrathoracic portion of the trachea. Thus, one might conclude that fluoroscopy would be more sensitive for detection of collapse of the intrathoracic portion of the trachea that has probably remained undetected previously.
It has long been accepted that collapse of the extrathoracic portion of the trachea occurs during inspiration and that collapse of the intrathoracic portion occurs during expiration. It is interesting to note that, in the present study, most but not all tracheal collapse in the cervical and thoracic inlet regions was detected fluoroscopically during the inspiratory phase of respiration and that collapse in the thoracic and carinal regions was detected fluoroscopically during the expiratory phase and the induced cough. It is not entirely clear why fluoroscopy sometimes revealed inspiratory collapse of intrathoracic airways and expiratory collapse of extrathoracic airways in the study dogs. One report15 indicates that the tracheal luminal diameter in dogs with tracheal collapse is abnormally small in the dorsal-to-ventral direction during both inspiration and expiration. However, that group of investigators did not speculate on why this might occur.
The predominance of mature, small-breed dogs of both sexes included in the present study is similar to previously reported1,9 demographics of dogs with tracheal collapse. Concurrent obesity has been reported in dogs with tracheal collapse11,16 and this was supported in the present study, in which 46 of the 62 (74.2%) dogs had BCSs greater than the ideal values of 4 or 5. The presence of heart murmurs in 12 (19.4%) dogs and severe dental disease in 38 (61.3%) dogs is consistent with the common occurrence of heart murmurs, dental disease, and tracheal collapse in mature, small-breed dogs.1,9
A retrospective study has inherent limitations. In our investigation, radiographic and fluoroscopic examinations were not standardized, as could be assured in a prospective study. Prospective assessment of dogs would ensure that fluoroscopic images of the cervical portion of the trachea were of adequate quality and that inspiratory and expiratory thoracic radiographic views were obtained for each study dog. A larger sample size of dogs with thoracic radiographic views obtained during inspiration and expiration would enable more accurate comparison of those images with single radiographic views obtained during maximum inspiration with regard to assessing the sensitivity of radiography for detection of tracheal collapse and accuracy of collapse localization. Further studies involving larger sample sizes of dogs with thoracic radiographic views obtained during inspiration and expiration for comparison with findings in clinically normal dogs are also needed. Additionally, in the present study, bias may have been created because only 1 radiologist evaluated the radiographic and fluoroscopic images; a review of the images by multiple radiologists could perhaps decrease potential bias created by an individual investigator.
Radiographic evaluation appears to be a worthwhile procedure for screening dogs with potential tracheal collapse. Examination of both inspiratory and expiratory radiographic views obtained from a given dog is likely to improve a clinician's ability to detect tracheal collapse, compared with examination of single lateral radiographic views. However, care must be taken not to overinterpret radiographic findings of a mild degree (≤ 25%) of tracheal luminal attenuation because, as the results of the present study suggest, mild collapse detected via radiography does not always correlate with collapse detected via fluoroscopy. Furthermore, compared with fluoroscopy, radiography appeared to underestimate the grade of tracheal collapse in the intrathoracic regions. More information is necessary to understand the degree of tracheal diameter fluctuation during respiration in clinically normal dogs and the contribution of dorsal tracheal membrane redundancy to true tracheal collapse. Overall, radiography and fluoroscopy appear to be complementary imaging techniques for the detection and severity assessment of tracheal collapse in dogs.
ABBREVIATIONS
| CT | Computed tomography |
| BCS | Body condition score |
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