Pulmonary vascularity: radiographic considerations.

Distribution of pulmonary vascularity, as imaged on the routine chest radiograph, may be used as an accurate and sensitive indicator of underlying pathophysiologic change. The lungs, like most other human organ systems, have extensive reserve capability. Such reserves exist in terms of excess alveolar capacity in the pulmonary vascular bed to compensate for certain vascular alterations. This available vascular reserve is potentially available to compensate for situations in which there is an imbalance created between the size (capacity) of the vascular bed and the amount of blood (content) that it must accommodate. Such imbalances can result from decrease in the size of the vascular bed secondary to destruction or to normal physiologic response mechanisms or from an increase in the amount of blood coursing through the vascular bed. Either occurrence or combination of occurrences necessitates recruitment of available pulmonary vascular reserves. Recognition of this recruitment phenomenon and understanding of its underlying pathophysiologic significance enable relatively sophisticated diagnostic interpretation of the chest radiograph.