Lung Lesions Induced by Continuous- and Pulsed-Wave (Diagnostic) Ultrasound in Mice, Rabbits, and Pigs

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Abstract
These studies documented the presence or absence of macroscopic and microscopic intraparenchymal hemorrhage in individual lung lobes of mice, rabbits, and pigs exposed to continuous- and pulsed-wave (diagnostic) ultrasound; we described the character of and lesions associated with the hemorrhage and compared differences in the lesions among species and exposure conditions to investigate the pathogenetic mechanisms and species differences associated with ultrasound-induced lung hemorrhage. In a series of three sequential interdependent studies, 312 mice, 91 rabbits, and 74 pigs were divided at random into experimental groups and exposed to continuous-wave ultrasound (3 kHz modulated at 120 Hz) of acoustic pressure levels ranging from 0 to 490 kPa for 5, 10, or 20 minutes. In a fourth study, three mice, 43 rabbits, and six pigs were divided at random into experimental groups and exposed to pulsed-wave ultrasound (3- and 6-MHz center frequency) of peak rarefactional acoustic pressure levels ranging from 0 to 5.6 MPa for 5 minutes. Macroscopic lesions induced by continuous- and pulsed-wave ultrasound consisted of dark red to black areas of hemorrhage that extended from visceral pleural surfaces into lung parenchyma. Hemorrhage appeared spatially related to the edges of lung lobes where pleura of dorsal and ventral surfaces met, occurred in specific lung lobes in all three species, and appeared anatomically related to lung that was closest to and in contiguous alignment with the ultrasound transducer and thus the path of the sound beam. Macroscopic lesions were similar in all species under all exposure conditions for both continuous- and pulsed-wave ultrasound; however, hemorrhage was not induced in pig lung exposed to pulsed-wave ultrasound at any peak rarefactional acoustic pressure level. Eighteen mice (145 kPa exposure pressure), 60 rabbits (145-460 kPa exposure pressure), and 58 pigs (145-490 kPa exposure pressure) from study 3 were used for microscopic evaluation of lung exposed to continuous-wave ultrasound; three mice (6 MHz; 2.9 and 5.4 MPa), 39 rabbits (3 and 6 MHz; 2.3-5.4 MPa), and six pigs (3 and 6 MHz; 3.3, 5.4, and 5.6 MPa) from study 4 were used for microscopic evaluation of lung exposed to pulsed-wave ultrasound. Microscopic lesions and the character of hemorrhage induced by continuous-wave ultrasound were different from those induced by pulsed-wave ultrasound. Lesions induced by continuous-wave ultrasound under all exposure conditions were similar in all three species. Lesions induced by pulsed-wave ultrasound under all exposure conditions were similar in all three species. Microscopic lesions induced by continuous-wave ultrasound affected alveoli and alveolar septa and consisted of severe hemorrhage, accumulation of substantial volumes of protein-rich plasma admixed with fewer numbers of cells, coagulated protein, and acute coagulative necrosis. Microscopic lesions induced by pulsed-wave ultrasound consisted of severe alveolar hemorrhage without accumulation of substantial volumes of plasma, infrequent foci of coagulated plasma proteins, and absence of acute coagulative necrosis involving alveolar septa. Results of these studies suggest that 1) the principal target for the biologic effects of ultrasound in lung is the microvasculature of alveolar septa, 2) differences in the character of the hemorrhage induced by the two wave forms may reflect fundamental distinctions in physical/biological interactions that the wave forms have with cellular junctions or membranes that ultimately lead to vasolytic events and hemorrhage, and 3) innate interspecies differences in anatomy and physiology of lung may determine differences in species susceptibility to ultrasound-induced lung hemorrhage.

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