Scuba diving may seem a long way from the class room and the point of theory may not be obvious, but it can be vital in making important decisions, especially about decompression models (aka tables). This article will outline some important theory and hopefully remind you that no dive profile is completely safe. Bear in mind that a model is only as good as it has been verified to be and there are too many factors involved to guarantee prevention of DCI.
A fundamental problem in the design of decompression models is that the rules that govern a single dive and ascent do not apply when bubbles already exist, as these will delay nitrogen elimination and equivalent decompression may result in decompression illness (DCI). Therefore repetitive diving, multiple ascents within a single dive and surface decompression procedures are significant risk factors for DCI.
The ideal dive profile creates the greatest possible gradient for nitrogen elimination from the tissues without causing bubbles to form. However it is far from clear whether this is possible. Some decompression models assume that stable bubble micro nuclei 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 ultrasound in divers soon after surfacing.
Building Bert’s observation that dissolved nitrogen causes DCI, the first model that was verified experimentally, was developed by Haldane and is based on the following principles and concepts:
Nitrogen dissolves into tissues and becomes completely saturated after a certain amount of time. (Henry’s law)
The degree of saturation is determined by the ambient pressure, so that a given tissue above atmospheric pressure contains more nitrogen than the same tissue at 1 atmosphere.
The difference between the ambient pressure and a tissue’s partial pressure is called the pressure gradient.
On ascent, the partial pressure may be higher than the ambient pressure. But the body can tolerate some amount of pressure gradient without DCI.
If the pressure gradient becomes too high, the dissolved nitrogen cannot be eliminated quickly enough (by exhalation) and nitrogen bubbles form.
Partial pressure is the fraction of the total pressure that a single gas exerts in a mixture.
Tissue compartments are areas of the body that absorb gas at different rates and are categorized by how fast they uptake gas. Haldane introduced the concept of halftimes and suggested 5 tissue compartments with half times of 5, 10, 20, 40 and 75 minutes.
Tissue compartments do not correspond to anatomic tissues but one compartment will on gas and off gas at the same rate. Fast tissues on-gas and off-gas in shorter half-times than slow tissues. Areas that are well supplied by blood, such as the lungs and abdominal organs, absorb nitrogen faster than other tissues. Slower tissues include fat, fatty marrow and cartilage and saturation is reached when the pressure gradient is 0. This means after one half-time a compartment is 50% saturated, but it is not 100% saturated after two half-times, since each time the pressure gradient is halved. After two half-times a compartment is 75% saturated. After three, 87.5%. Four, 93.8 and for simplicity, we say a compartment is 100% saturated after 6 half-times.
How large can a gradient become before it’s a problem? Haldane assumed from his observations that the gradient could be a maximum of twice ambient pressure for all tissues. He assumed that an ascent from 30m (4 bar) to 10m (2 bar), or from 10m (2 bar) to the surface when saturated would not cause DCI. This was very conservative for shallow dives and not conservative for deep dives. In the 1960s Robert D. Workman of the U.S. Navy Experimental Diving Unit revised Haldane’s model with further experimental work to allow each tissue compartment to tolerate a different amount of supersaturation which varies with depth. This required a standardised ascent rate, which he took to be 18m per minute. His work allowed greater flexibility and safety to be built into tables for a range of depths and he introduced the term “M-value” to indicate the maximum amount of supersaturation each compartment could tolerate at a given depth. There are M-values for each compartment for each decompression stop. In no-decompression diving, however, we only have to be concerned with the values for the pressure at the surface. The PADI recreational dive planner grew out of Workman’s work and this is the reason why 18m per minute is the ascent rate required. The precise algorithms used in dive computers and tables are proprietary and therefore not available, but the PADI recreational dive planner now has a long safety history and is a useful benchmark.
We now have a complete model for predicting how nitrogen moves in and out of the tissues and at what point this process becomes a problem. However it remains a model and is therefore not true for every dive for every diver. So keep it conservative and stay within the limits of your computer!