Effect of pulmonary volume loss on the size and appearance of experimentally produced scintigraphic perfusion defects

Atelectasis is a frequent concomitant of pulmonary embolism (PE) and other conditions that mimic PE. Accordingly, we developed an animal model to study the effect of regional loss of pulmonary volume on the size and configuration of experimentally induced perfusion defects that simulated PE.

After baseline anteroposterior and lateral chest radiographs were taken, 11 pulmonary vascular occlusions were created in 10 anesthetized, intubated mongrel dogs. Three were created by balloon occlusions of segmental pulmonary arteries, and eight were created by release of autologous PE. The size and shape of the affected lung zones were determined by perfusion scintigraphy (technetium-99m-macroaggregrated albumin [99mTc-MAA]) and the results recorded. A balloon-tip Fogarty catheter then was passed through the trachea and inflated at preselected endobronchial sites to produce bronchial occlusion and volume loss, which were documented radiographically. Because one animal had two such occlusions sequentially, a total of 12 volume reduction experiments were performed. After inducing volume loss, perfusion imaging was repeated without reinjection of 99mTc-MAA to allow any changes in the perfusion defects to be recorded, measured, and compared with the degree of volume loss visualized radiographically.

Volume loss did not alter the shape of ipsilateral scintigraphic perfusion defects, but it did result in the expansion of eight of the 12 perfusion defects (range = 18-137% increase in area, Mdn = 33%) and a 28% decrease in the size of one defect located within a lobe that lost volume. Despite these quantitative changes, only three of the perfusion defects appeared substantially larger. When independently interpreted radiographs demonstrated large volume losses, quantitative changes in perfusion defects were no greater on average than when radiographic volume loss was modest.

Loss of pulmonary volume is occasionally associated with considerable ipsilateral alterations in the size of scintigraphic perfusion defects. It is probably advisable to consider the effects of anatomic distortions such as volume loss when interpreting the significance of scintigraphic perfusion defects in patients suspected of having PE.