Can Freediving Cause DCS?
By Fred Bove, M.D., Ph.D.
A reader recently asked me if freedivers ever developed decompression sickness. This has been studied in several countries but the best information comes from the natives of the Tuamotu Archipelago in Polynesia, where freedivers in the past made 40 to 60 dives a day to depths of 100 to 140 feet to gather pearls. Their descents were assisted by a lead weight and they ascended by pulling themselves up a rope tethered to a collection basket. An assistant then raised the basket to retrieve the shells gathered on the bottom. Descent times were 30 to 50 seconds, total dive time, about 100 seconds and the surface interval between dives was four to six minutes. Many of the divers developed a severe illness called Taravana, the symptoms of which are similar to those of decompression sickness-paralysis, visual changes, hearing loss and dizziness. Some divers died. Many who survived had permanent brain and spinal cord injuries. Although Taravana is most likely decompression sickness, some symptoms do not fit the picture of decompression sickness and other causes, such as hypoxia, have been proposed.
The mechanism whereby freedivers can develop decompression sickness (DCS) was studied by Dr. P. Paulev in Denmark. He studied the freedivers who accompanied trainees as they performed the free ascents required to qualify for naval submarine duty. Dr. Paulev described his findings in a Danish naval medical officer. He states, 'The author has intimate knowledge of the event, because the medical officer happens to be himself.' He performed about 60 dives to 100 feet with a two minute bottom time and surface intervals of one to two minutes. After about five hours of freediving, he noticed pain, paralysis of the legs, nausea, visual changes and weakness of the right arm. He was treated in an hyperbaric chamber and, following a full treatment table, all abnormalities disappeared.
Dr. Paulev calculated the nitrogen in his tissues after the repetitive breath-hold dives. He determined the short surface intervals did not allow tissue nitrogen to be eliminated and thus it was equivalent to that resulting from a continuous dive. Further studies by Dr. E. Lanphier indicated that the ratio of dive time to surface time and the rate of ascent were important factors in the development of decompression sickness from freediving. He calculated that a ratio of surface interval to dive time of one gave a depth exposure equivalent to about 50 percent of the actual depth of the dive. Thus a dive to 100 feet with a 90 foot second dive and a 90 second surface interval would be equivalent to a continuous dive to about 50 feet. If ascent rate was rapid, the equivalent depth would be about 65 percent of actual depth (65 feet). These calculations explain why many repetitive freedives in the 100 to 140 foot range will eventually cause decompression sickness. Those who freedive for three to five hours will greatly exceed the no decompression times for their equivalent depths and are very prone to developing severe neurologic DCS.
The Taravana syndrome and the experience of freedivers in submarine escape training provide adequate data that this is a real phenomenon. For the average freediver Taravana is not a problem, but if you want to dive for pearls making 130 foot dives every two minutes for five hours, you will get DCS. By increasing the ratio of surface time to dive time to two (e.g. 90 second dive, 180 second surface interval), equivalent depth would be about 30 feet when freediving to 100 feet and no risk of decompression sickness would occur.
The information on Taravana was published in 1965 in a book entitled Breathhold Diving and the Ama of Japan, edited by H. Rahn and T. Yokoyama. It is publication number 1341 of the National Research Council, Washington, D.C.Another situation that has not been studied involves freediving after scuba diving. There are several stories of divers getting DCS this way. A surface interval spent freediving is not really a surface interval and the calculation of decompression will not be correct. The best advice is to avoid freediving during surface intervals. If you want to snorkel, stay on the surface.
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