Chapter 5- Decompression and Recompression Decompression and Recompression 153 5 10 20 40 75 and fatty tissues are all representative of various body tissue types. Each of the tissues behaves differently under pressure, taking up and giving off gases at different rates, but all tend towards reaching equilibrium with ambient pressure. Rather than model the body directly, by measuring the gas uptake of human muscle and other specific tissues, a broad range of theoretical tissues or compartments was established to span the types of human body tissues. These terms will be used interchangeably. The speed with which one of these compartments absorbs or eliminates gases is defined by its half-time (figure 5-2). The compartment’s half-time is the time required for the compartment to absorb or eliminate half of the pressure difference between what it started with and the ambient pressure it is exposed to. After the first half-time, 50% of the gas would be gone and 50% would remain. After the second half-time, 50% of the remaining gas would be gone, for a total of 75% gone. After the third half-time, 87.5% of the excess gas has left the tissue. After six half-times, more than 98% of the excess gas has gone, which is sufficiently close to 100% for normal usages (figure 5-3). Haldane and his co-workers used tissue half-times, called compartments, of 5, 10, 20, 40, and 75 minutes to model the body. During the course of a dive, the pressures in the various tissues were calculated. If the absolute pressure in any tissue exceeded that at 20 mswa (66 fswa), the diver would not be allowed to surface directly. For each diving depth, the maximum length of time before stops were required was calculated. Haldane also proposed a method of “stage” decompression, where the diver would ascend to the shallowest stop possible without exceeding the 2:1 ratio. A diver would remain at the stop until enough gas left the tissues, so that he could safely ascend to the next 3 msw (10 fsw) stop, or safely surface. Tables based on these principles were produced for the Royal Navy and formed the basis for further work by other researchers. U.S. Theory The U.S. Navy used the Haldane Royal Navy tables until their own were developed in 1937, by the U.S. Navy Experimental Diving Unit in Washington, D.C. The major difference between the two sets of tables was the use of different surfacing ratios for the different tissues. It was also decided that only the inert portion of the gas was important from a decompression viewpoint, because the oxygen portion was metabolized during pressure reduc- Circulation and Pressure Gradient Fast Medium Body Tissues Slow FIGURE 5-2. HALF-TIME DIAGRAM. Dive 1 Number of Half-Times Passed Original Pressure Percent of 50 75 100 0 1 2 3 4 5 6 0 1 2 3 4 5 6 FIGURE 5-3. INGASSING AND OFFGASSING IN AN EXPONENTIAL TISSUE.
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