Radiographic determination of the tracheal indices at caudal cervical, thoracic-inlet, and intra-thoracic trachea in non-bulldog brachycephalic breeds without evidence of cardiorespiratory disease

Background Congenital tracheal hypoplasia is a component of airway syndrome affecting a wide variety of brachycephalic dog breeds. Several radiographic procedures were utilized to assess vertical tracheal diameter (VTD) in dogs. The objective of this study was to calculate the tracheal indices at the caudal cervical, thoracic-inlet, and intra-thoracic tracheal regions on the right lateral thoracic radiograph to further establish a screening protocol for diagnosis of tracheal hypoplasia in non-bulldog brachycephalic breeds. Dogs without clinical or radiographic evidence of tracheal, respiratory, or cardiovascular abnormalities were investigated. The absolute and average VTDs were normalized by manubrium length (ML), thoracic-inlet distance (Ti-D), and proximal 3rd rib width (PR3-W). Manubrium-tracheal index (M-TI), thoracic inlet-tracheal index (Ti-TI), and proximal R3-tracheal score (PR3-TS) were calculated. Correlations between averaged VTD and each of the normalizing parameters (ML, Ti-D, and PR3-W), and between M-TI and each of the previously established procedures (Ti-TI and PR3-TS) were determined.Results Eighty healthy subjects met the inclusion criteria for the study. There were significant differences (P <= 0.0001) among the means of absolute and normalized VTDs at the 3 tracheal levels. The smallest VTD was identified at the thoracic inlet. The average tracheal diameter showed a better correlation with ML (rs=0.81, P < 0.0001) compared to Ti-D and PR3-W. There was a strong correlation (rs=0.83, P < 0.0001) between the averaged M-TI and Ti-TI.Conclusion Radiographic M-TI could be an alternative to traditional procedures to assess the tracheal lumen in non-bulldog brachycephalic dogs. M-TI < 0.39, < 0.30, or < 0.34 at caudal cervical, thoracic inlet, or intrathoracic trachea, respectively, may indicate tracheal hypoplasia in non-bulldog brachycephalic breeds. Screening of tracheal diameter using M-TI should be recommended. However, further investigation of non-bulldog brachycephalic breeds with cardiac and/or respiratory disease is indicated.