Decompression Theory: a Dynamic Critical-volume Hypothesis

As emphasized by Hills (1,2), there are two basically different approaches to decompression optimization. The first is to devise a convenient calculational method and then modify it empirically until it is in reasonable agreement with the available data. The second is to develop a theoretical model from fundamental physical and physiological principles and then attempt to quantify its response to changes in exposure pressure. A key issue in either case is the identification of the proper decompression criterion. The empirical approach is illustrated by the method of Haldane (3). The Haldane decompression criterion is expressed as a pressure ratio, which has been interpreted a posteriori as a supersaturation limit for the formation of bubbles whose mere presence is assumed to cause symptoms (4). Alternatively, the ratio could represent a critical volume of separated gas or a critical degree of embolism that the body can tolerate (5). This second possibility was mentioned already in Ref. 3; however, since it is not rigorously compatible with the assumptions of exponential gas exchange and of symmetric gas uptake and elimination, it cannot properly be regarded as a bona fide part of the Haldane scheme (1,2). · The theoretical approach is illustrated by the method of Hills (6), which is based on the principle of phase-equilibration. In Hills' regime, bubble formation is assumed to be so profuse in the relevant tissues that all gas in excess of equilibrium is ''dumped'' into the gas phase within a few minutes after a pressure reduction. If one further assumes that the volume of separated gas is critical, the result is not a pressure ratio but a zero-supersaturation criterion for decompression (1,2). The physiological circumstances implicit in the Haldane method ( 4) represent the "best case" in the sense that little or no gas has come out of solution.