The normal roentgen appearance of the pulmonary vasculature of an upright human being is typified by a caudal flow pattern because of gravity. The pressure differential between the apex and the base of the lung is approximately 22 mmHg in adults in the upright position.223 Therefore, more flow under higher distending pressure is expected in the lower-lobe vessels than in the upper. Normally, one sees very little vascularity above the hilum, whereas more and larger vessels are found below the hilum. Since the pulmonary resistance is normal, all vessels taper gradually in a treelike manner from the hilum toward the periphery of the lung. The right descending pulmonary artery measures 10 to 15 mm in diameter in males and 9 to 14 mm in females^24 (see 0* -Bi Fig. 12-2).
Abnormal pulmonary vascularity can be classified into two categories, either in terms of volume or in terms of distribution210 25 (0+;0; Table 12-4).
In the evaluation of pulmonary vasculature, the caliber of the vessels is more important than the length or the number. As long as the PBF pattern remains normal, with a greater amount of flow to the bases than to the apices, the volume of the flow is proportional to the caliber of the pulmonary arteries (see Fig. 12-2). In addition to measuring the right descending pulmonary artery, one also may assess the pulmonary blood volume by comparing the size of the pulmonary artery with that of the accompanying bronchus where they are viewed on end. Normally, the two structures have approximately equal diameters.2,26 When the artery-bronchus ratio is greater than unity, increased blood flow is suggested. Conversely, when the ratio is smaller than unity (see Fig. 12-2), decreased flow is likely.
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