DIVING AT ALTITUDE - A REVIEW OF DECOMPRESSION STRATEGIES

Diving at altitude requires different tables from those at sea level d ue to the reduction in surface ambient pressure. Several algorithms ex trapolating sea-level diving experimental data have been proposed to c onstruct altitude diving tables. The rationale for these algorithms is reviewed together with the conservatism of the resulting tables and d ecompression computer outputs. All algorithms are based on the adaptat ion of critical tissue tensions to altitude. These are linear extrapol ation (LEM), constant ratio translation (CRT), and constant ratio extr apolation (CRE) of maximum permissible tissue tensions (M values). Eit her new tables using the altitude-adapted M values were put forward or sea-level tables are to be used through an operation called correctio n. In this review it is shown that for a given set of M values, CRT an d CRE give the same result for no-decompression-stop dives; they alway s yield more conservative results than LEM. When decompression stops a re used, CRT is more conservative than CRE. When applied to different sets of M values, the conservatism becomes a function of bottom time, depth, and altitude. The analysis shows that the tables derived using CRT of U.S. Navy (USN) schedules and CRE Boni et al. tables give more conservative results than LEM Buhlmann tables for higher altitude, lon ger bottom time, and deeper dives. Aviation altitude exposure decompre ssion sickness (DCS) data are also addressed to compare different mode l outputs. When applied to USN and Royal Navy tables, LEM yields an al titude DCS limit of 8,581 and 8,977 m, respectively. On the other hand , the altitude limit calculated using CRE applied to USN M values and LEM Buhlmann tables is found to be below 6,000 m.