Monitoring bubble growth in supersaturated blood and tissue ex vivo and the relevance to marine mammal bioeffects

There have been several recent reports that active sonar systems can lead to serious bioeffects in marine mammals, particularly beaked whales, resulting in strandings, and in some cases, to their deaths. We have devised a series of experiments to determine the potential role of low-frequency acous- tic sources as a means to induce bubble nucleation and growth in supersatu- rated ex vivo bovine liver and kidney tissues, and blood. Bubble detection was achieved with a diagnostic ultrasound scanner. Under the conditions of this experiment, supersaturated tissues and blood led to extensive bubble produc- tion when exposed to short pulses of low frequency sound. right whales. 5 Although these cetaceans have not been associated with mass stranding events related to navy sonar systems, it is likely that other cetaceans will also undergo significant changes in behavior when subjected to high-intensity acoustic pulses. Rapid surfacing from a deep dive may lead to decompression sickness. In addition, it is known that exercising after diving can lead to decompression sickness in humans. 6 Analogously, abnormal extended activity resulting from sonar may induce decompression sickness in cetaceans. To address the role of direct bubble nucleation in tissue by a sound pulse, it is worthwhile to discuss the bioeffects induced by diagnostic ultrasound systems, used routinely worldwide to image the progress of healthy as well as pathological conditions in the human patient. It is no surprise, then, to recognize that ultrasound-induced bioeffects in human tissue have been studied extensively. To this date, no repeatable effects of diagnostic ultrasound exams have been reported in the general literature. This paucity of observable bioeffects was at first surprising because the acoustic pressure amplitudes used in imaging devices are in excess of the threshold for bubble nucleation and growth, i.e., cavitation—the most likely ultrasound-induced