Decompression Algorithms - Decompression Models

Decompression Models

A fundamental problem in the design of decompression tables is that the rules that govern a single dive and ascent do not apply when some tissue bubbles already exist, as these will delay inert gas elimination and equivalent decompression may result in decompression sickness.

One attempt at a solution was the development of multi-tissue models, which assumed that different parts of the body absorbed gas at different rates. Each tissue, or compartment, has a different half-life. Fast tissues absorb gas relatively quickly, but will release it quickly during ascent. A fast tissue may become saturated in the course of a normal sports dive, while a slow tissue may hardly have absorbed any gas. By calculating the levels in each compartment separately, researchers are able to construct better tables. In addition, each compartment may be able to tolerate more or less supersaturation than others. The final form is a complicated model, but one that allows for the construction of tables suited to a wide variety of diving. A typical dive computer has a 8–12 tissue model, with half times varying from 5 minutes to 400 minutes. The Bühlmann tables have 16 tissues, with half times varying from 4 minutes to 640 minutes.

The ideal decompression profile creates the greatest possible gradient for inert gas elimination from a tissue without causing bubbles to form, but it is not certain whether this is practically possible: some of the decompression models assume that stable bubble micronuclei always exist. However, the dissolved phase decompression models are based on the assumption that bubble formation can be avoided. The bubble models make the assumption that there will be bubbles, but there is a tolerable total gas phase volume or a tolerable gas bubble size, and limit the maximum gradient to take these tolerances into account. A number of empirical modifications to dissolved phase models have been made since the identification of venous bubbles by doppler measurement in asymptomatic divers soon after surfacing.

Repetitive diving, multiple ascents within a single dive, and surface decompression procedures are significant risk factors for DCS.