Can high-frequency sound affect gas-bubble dynamics? A study in the intact prawn Palaemon elegans

Evidence for the initiation of decompression sickness by exposure to intense underwater sound

Mass stranding of cetaceans (whales and dolphins), in close association with the activity of naval sonar systems, has been reported on numerous occasions. Necropsy showed bubble-associated lesions similar to those described in human decompression sickness (DCS). We examined the hypothesis that exposure to underwater sound may potentiate DCS. Rats were subjected to immersion and simulated dives with and without simultaneous acoustic transmissions at pressure levels and frequencies of 204 dB/8 kHz and 183.3 dB/15 kHz. DCS severity was assessed using the rotating wheel method. Recording of somatosensory evoked potentials (SSEPs) was employed under general anesthesia as an electrophysiological measure of neurologic insult. A significantly higher rate of decompression sickness was found among animals exposed to the 204-dB/8-kHz sound field. Significantly higher pathological SSEPs scores were noted for both underwater sound protocols. Pathological SSEPs scores in animals immersed during the acoustic transmissions, but without changes in ambient pressure, were comparable to those observed in animals exposed to the dive profile. The results demonstrate induction of neurological damage by intense underwater sound during immersion, with a further deleterious effect when this was combined with decompression stress. The study outcome has potential implications for human diving safety and may provide an explanation for the mass stranding of cetaceans purportedly associated with sonar activity.

High-frequency sound transmissions under water and risk of decompression sickness

We tested the possible occurrence of a neurological insult secondary to high-frequency sound exposure. Immersed, anesthetized rats were subjected to a simulated diving profile designed to induce decompression sickness, while exposed to the transmission of an acoustic beacon. Intermittent sound at a pressure level of 184.5 dB re 1 microPa at 1 m (1.7 kPa), a frequency of 37 kHz, and with a duration of 4 ms, was transmitted in a duty cycle of 0.26%. Four groups, each containing nine animals, were included in the study as follows: group 1, immersion only, no sound exposure; group 2, immersion with sound exposure; group 3, diving simulation when immersed, no sound exposure; group 4, diving simulation when immersed, with sound exposure. Somatosensory evoked potentials (SSEPs) were recorded the day before the study, and a second recording was made 30 min after immersion. Some of the SSEP components disappeared after the dive in 3 rats from group 3 and 2 rats from group 4. SSEP components could not be identified in a significantly larger number of animals from groups 3 and 4, compared with groups 1 and 2. No differences were found in wave latency, amplitude or conduction time. Our data show that the high-frequency sound exposure employed did not contribute to the development of the neurological insult.

Hyperbaric oxygen may reduce gas bubbles in decompressed prawns by eliminating gas nuclei

It is accepted that gas bubbles grow from preexisting gas nuclei in tissue. The possibility of eliminating gas nuclei may be of benefit in preventing decompression sickness. In the present study, we examined the hypothesis that hyperbaric oxygen may replace the resident gas in the nuclei with oxygen and, because of its metabolic role, eliminate the nuclei themselves. After pretreatment with oxygen, prawns were 98% saturated with nitrogen before explosive decompression at 30 m/min. Ten transparent prawns were exposed to four experimental profiles in a crossover design: 1) 10-min compression to 203 kPa with air; 2) 10-min compression with oxygen; 3) 10-min compression with oxygen to 203 kPa followed by 12 min air at 203 kPa; and 4) 10 min in normobaric oxygen followed by compression to 203 kPa with air. Bubbles were measured after explosive decompression. We found that pretreatment with hyperbaric oxygen (profile C) significantly reduces the number of bubbles and bubble volume. We suggest that hyperbaric oxygen eliminates bubble nuclei in the prawn.

High-frequency sound field and bubble formation in a rat decompression model

High-frequency sound might cause bubble enlargement by rectified diffusion. The purpose of the present study was to investigate gas bubble formation in the immersed diving animal during exposure to high-frequency sound. Anaesthetised rats were subjected to a simulated diving profile while immersed inside a hyperbaric chamber. An acoustic beacon (pinger) was placed ventral to the animal's abdomen, transmitting at an intensity of 208.9 dB re 1 micro Pa and a frequency of 37 kHz. Six groups of eight animals were included in the study as in Table 1, breathing air (n = 4) or Nitrox 72/28 (n = 2), at a depth of 0 m, 30 m or 40 m. Immediately after decompression, the intestinal mesenterium was imaged, and frames were acquired digitally. The number of bubbles and their radii were analysed and compared among the groups. The mean bubble density for group 1 was 1.35 +/- 0.18 bubbles/mm(2), significantly higher when compared with the other groups (p < 0.0001). The average bubble radius for groups 1 and 2 was similar (12.57 +/- 4.1 and 10.63 +/- 1.8 microm, respectively), but significantly larger than in the other groups (p < 0.0002). The percentage of bubbles with a radius greater than 50 microm was significantly higher in group 1 (p < 0.0001). The results suggest that commercially available underwater pingers might enhance bubble growth during deep air diving.