8 - Compartmental decompression models and DCS risk estimation

Abstract

Decompression models that describe the distribution of excess dissolved inert gas in the body and provide a means of estimating the risk of incurring decompression sickness (DCS) are introduced. Both older deterministic models, the first of which was developed by J.S. Haldane over a century ago, and more recent probabilistic models are described. The models are further distinguished by whether they consist of independent parallel compartments, or interconnected compartments. A compartment, in the context of the independent compartment models, is viewed as a collection of tissues with similar values of their inert gas exchange rate constants. Alternately, and particularly in the context of the interconnected compartment models, a compartment can be viewed as a physical region in the body containing dissolved inert gas, which plays a role in the development of DCS. Interconnected compartments exchange dissolved inert gas both with the circulatory system and with each other, while independent compartments are isolated from one another and exchange dissolved inert gas only with the circulatory system. All the exchanges of dissolved inert gas—whether with the circulatory system or with contiguous compartments—occur by diffusion, which is driven by a dissolved inert gas tension gradient. For the independent compartment models, the dissolved gas uptake and elimination follows monoexponential kinetics, while for the interconnected models uptake and elimination rates are multiexponential, i.e., they are sums of exponentials. Comparisons of the risk predictions by some independent parallel compartment models and an interconnected compartment model are provided. All these models contain adjustable parameters whose numerical values must be determined before the model(s) can be used. The parameter values are determined by calibration against empirical datasets. The latter are compilations of dive profiles (e.g., depth vs. bottom time), together with their outcomes (whether or not a particular dive profile resulted in DCS). Calibration methods used for decompression models, including the Levenberg-Marquart method and simulated annealing, are described in detail.