Conspicuity of the azygos vein on thoracic radiographs and association with right heart disease in dogs

Introduction <br/>The azygos vein (AV) has not been described as a visible structure on normal canine survey thoracic radiographs. However, the authors noticed a small vascular structure occasionally superimposed on the aortic arch on lateral radiographs(double vessel sign, DVS). The purpose of this study was to investigate whether this structure is the AV and whether AV conspicuity is associated with right heart disease in dogs. <br/>Methods<br/>A single-centre, retrospective, descriptive, case-control study with institutional ethical approval was performed using canine lateral thoracic radiographs. . The control group included dogs without thoracic disease but with thoracic radiography and CT. Radiographs were assessed for DVS. The AV at level of the aortic arch was identified on CT. A sagittal maximum-intensity-projection CT image including the AV and aorta, mimicking a radiograph, was assessed for DVS. The case group included dogs with echocardiographically confirmed <br/>right heart disease and thoracic radiographs. CT/echocardiography were performed within 15 days of radiography. A Chi-square test was performed to compare the radiographic conspicuity of the DVS between the two groups. <br/>Results<br/>Thirty-two case and forty-six control dogs met the inclusion criteria. Nineteen control dogs had radiographic DVS, all of which had a DVS and corresponding AV on CT. No statistical difference was found in radiographic AV conspicuity between the case and control groups P=0.46). <br/>Conclusions<br/>A small vascular structure superimposed on the aortic arch seen on canine lateral radiographs represents the AV. In dogs, the DVS is a normal radiographic anatomic feature and does not indicate right- sided heart disease.