Temporal and spatial evaluation of lesion reparative responses following superthreshold exposure of rat lung to pulsed ultrasound.

This study characterized the reparative responses in rat lung. Forty-five adult female rats were exposed at two sites over the left lung to 3.1-MHz superthreshold pulsed ultrasound. The repair of lung lesions was evaluated from 0 through 44 days postexposure. Macroscopic lesions at 0 days postexposure were large bright red ellipses of hemorrhage. By 1 and 3 days postexposure, lesions were the same size and dark red to red-black, but, by 3 days postexposure, lesions had a raised surface appearance. From 5 to 10 days postexposure, lesions grew smaller in size, progressed from red-gray to yellow-brown, and retained a raised surface appearance. From 13 through 44 days postexposure, lesions gradually decreased in size, had a faint yellow-brown discoloration, and gradually lost the raised surface appearance. By 37 and 44 days postexposure, lung returned to near normal morphology, but had small areas of light yellow-brown discoloration in the areas where lung was exposed. Microscopic lesions at 0 and 1 days postexposure were areas of acute alveolar hemorrhage. By 3 days postexposure, lesions had loss of alveolar erythrocytes and the formation of hemoglobin crystals. From 5 through 44 days postexposure, iron in degraded erythrocytes was processed to hemosiderin and was negligible in quantity at 44 days postexposure. The proliferation of resident cells (likely alveolar epithelial cells, fibroblasts and endothelial cells) and the infiltration of inflammatory cells in lesions declined in intensity as the lesions aged and was minimal by 44 days postexposure. Under the superthreshold exposure conditions described, lesions induced by ultrasound do not seem to have long-term residual effects in lung.