Decompression sickness is not just a worry for scuba divers – some “free” or “breath-hold” divers also suffer from it occasionally. This could come as a surprise to you, if you had previously taken for granted that you have to breathe pressurized gas containing nitrogen to get “bent”.
Many of you are aware, at least vaguely, of divers in the South Pacific who free-dive to harvest pearls commercially. You may also be aware of the competitive sport called breath-hold diving. Possibly, you even participate, or know people who do. Interestingly, both these types of diving, very different from each other as well as from scuba diving, can sometimes lead to decompression sickness.
First, how are they different? The most obvious difference between both types of free-diving and scuba is: no scuba, i.e., no underwater breathing gas. This leads to other differences. With less internal gas pressure in the lungs, the lung volume decreases under the pressure of increased depth. With immersion, and increasing depth, a number of other physiological changes occur, collectively known as the “diving reflex” that makes breath-hold diving feasible. These include a slowing down of the heart rate and decreased peripheral circulation, which allows near normal circulation and perfusion to the heart and brain to be maintained.
The differences between the pearl divers and competitive breath-hold divers, while in one sense quite pronounced, are essentially differences in the nature of the dive profiles. The pearl divers would dive to a range of depths, sometimes to over 100 fsw, with a bottom time of 30 to 60 seconds for a total underwater time of about one and a half minutes per dive. With approximately one minute of surface interval between dives, they would then repeat the process for perhaps 6 hours daily. By contrast, competitive breath hold divers generally do a single very deep dive, the actual depth and bottom time achieved being specific to the particular competition, but, with the aid of specialized sleds and buoyancy devices, depths of 150 – 250 meters can be – and are – achieved.
It has long been known that the pearl divers of the Tuamato archipelago (near Tahiti) do suffer some ill effects from their diving practices, a sickness known locally as Taravana which translates – very roughly – as “falling crazily”. More recently, cases of probable inner ear decompression sickness and probable cerebral decompression sickness have been diagnosed. The number of cases of Taravana or DCS decreased dramatically when the surface intervals were increased from 1 minute to 15 minutes.
In the case of competitive “breath-hold” divers, it used to be thought that decompression sickness could not occur, the idea being that the small amount of nitrogen present in a single free dive was not enough to lead to DCS. Two reported cases, one of cerebral DCS, one with both cerebral and inner-ear DCS symptoms, have shown that DCS can and does occur, even in single free dives. There also have been relatively frequent descriptions by breath-hold divers, after certain types of dives, of symptoms consistent with inner-ear DCS.
From my research group’s 2014 paper, “The lifetimes of small arterial gas emboli, and their possible connection to Inner Ear Decompression Sickness”, which was based on arterial gas emboli in scuba diving, we were able to extend the calculations to both types of breath-hold diving. In the resulting paper, “Decompression Sickness in Breath-hold diving, and its probable connection to the growth and dissolution of small arterial gas emboli”, Mathematical Biosciences, 2015, the calculated predictions, were consistent with the observed occurrence of DCS in both types of breath-hold diving.
(Copies of both the 2014 and 2015 papers can be read in the Articles section of this blog.)