Measurement of the left and right caudal pulmonary artery diameter. The maximum diameter (black arrows) of the right and left caudal lobe pulmonary arteries at the level of the first bifurcation of the trachea were measured on ventrodorsal thoracic radiographs. A, caudal pulmonary artery; B, caudal lobe main bronchus; V, caudal pulmonary vein.