I recently completed a Decompression Procedures course with Blue Label Diving and, as a result, feel comfortable weighing the factors that go into creating a deco schedule for my dives. As a former physiology major, decompression theory was right up my alley. Luckily, instructor Por Parasu Komaradat had lots to offer on the topic – going far beyond the minimum teaching standards of the course.
Here are some of the basics of decompression theory, from the understanding of a diver who has recently been introduced to the topic:
In the early 1900s, John Scott Haldane begun some of the earliest experiments on the effects of decompression on the body. He found that greater depths increase pressure on the body and force inert gas to move from the lungs into the blood and then into tissues. When divers discuss these gasses, we know that nitrogen is a primary gas considered, as it makes up about 79% of air. Helium and other gasses, however, follow the same trends and are important if present in our gas mix.
Haldane found that external pressure goes down when the body comes up from depth. Suddenly, gas pressure in tissues is greater than pressures of the same gasses in the blood and lungs. The difference causes gas to move out of tissues, into the blood, and eventually into the lungs where it is exhaled. As long as the difference in pressure between the tissues and external environment is not too great, this process is unlikely to cause harm. However, when a diver ascends from depth too quickly, the difference between the gas pressure in the tissues and the external environment becomes too great and the risk of gas bubble formation is increased. Every open water diver knows that these bubbles can contribute to decompression illness.
Haldane realized that the difference between external pressure and tissue pressure is what causes the movement of gas molecules and that, if this difference is too great, bubbles can form. He also found that gas moves into and out of tissues at different rates and grouped the tissues into 5 compartments, ranging from “fast” tissues that quickly on-gas and off-gas to “slow” tissues that take much longer.
After Haldane, Robert Workman defined “M-Values.” The M-Value of a tissue compartment describes the ratio of tissue pressure to external pressure at which gas bubbles are likely to form. These concepts form the basis for modern Neo-Haldanian decompression theory, and the number of tissue compartments as well as accepted M-Values have been revised several times.
As divers, we are concerned with avoiding decompression illness (DCI). We do this by following a schedule of deco stops. These stops equalize the pressure difference between tissues and the external environment to reduce the risk of bubble formation and DCI.
Modern dive computers compute these deco stops based on algorithms – complicated mathematical models – to minimize bubble formation. Many algorithms exist and are constantly being revised. Two of common schools of algorithms are Gradient Factor Models, such as the ZHL16b algorithm, and Variable Permeability Models, such as the VPM-b algorithm. Both schools aim to reduce DCI risk. However, they consider different factors as primary to achieving this goal.
Modern Buhlmann Gradient Factor models consider 16 tissue compartments with different rates of gas movement. The difference between external and tissue pressure is the main consideration for choosing deco stops. A deco diver as able to set an upper and lower “gradient value,” which defines how close to the M-Value it is acceptable for the tissue pressure to reach. The values chosen by a diver depend on how conservative they would like to be, and factors to consider include gas requirements and composition, the dive environment, repetitive diving, and a diver’s individual physiology and status on a particular day. The lower value describes the percent of the M-Value allowed at the beginning of decompression and the upper value describes the percent allowed at shallower stops. A lower GF-Low value tends to give a deeper initial stop.
Variable Permeability Models consider bubble growth as a primary DCI risk factor and aim to minimize it. Three important forces are considered when describing inert gas bubbles. For simplicity, the pressures can be thought of as internal bubble pressure, external tissue pressure, and bubble surface tension. Internal bubble pressure describes the force of the gas molecules within preexisting bubbles on outside tissues. This force would cause the bubble to expand if it was not offset by the external tissue pressure, which describes the force of the same gasses pushing inward on the bubble. Just like the difference in external and tissue pressure in gradient factor models, the difference between these forces describes whether bubbles expand or contract. The third pressure, bubble surface tension, complicates the model. The molecules at the interface between the gas bubble and the tissue are attracted to each other and, therefore, exert a force that causes the bubble to contract. A bubble that neither grows nor contracts, then, can be described by this equation:
External tissue pressure + Bubble surface tension = Internal bubble pressure
As a diver ascends, external tissue pressure decreases and can cause bubble growth. Furthermore, surface tension becomes less strong as the bubble grows and can cause an uncontrolled “positive feedback mechanism” leading to rapid bubble growth. This means that controlling the difference between bubble and tissue pressure is critical and variable permeability models consider these pressures to reduce bubble growth.
There is no clear-cut answer for which decompression algorithm is best for reducing DCI risk. Ultimately, a diver must carefully consider dive conditions and experiment with algorithms and their settings to make an informed choice.
Lots of decompression research exists and is available to a determined individual. For those of us who want this information presented in an organized, easily understood and applicable manner, I’d encourage you to enroll in a course on decompression procedures. It’s the best way to receive this information, which is truly complicated and often difficult to understand, in a relatable way to diving – and to learn how to plan dives around these models and utilize your dive computer to its full capacity.
An understanding of dive theory at a fundamental level, I find, makes diving more interesting and enjoyable. I hope this article lays a foundation to decompression theory and encourage every diver to pursue their own research.