As discussed in Chapter 10, DCS occurs when supersaturation of inert gas during decompression causes bubbles to form in sufficient numbers or size (and in the right location) such that some poorly defined clinical threshold is exceeded and symptoms occur. Decompression planning is the process of controlling depth, time and the ascent (‘decompression’) to reduce the probability of DCS.
Because tissue gas supersaturation is the fundamental condition required for bubbles to form, it is not surprising that all decompression planning approaches have, at their core, a means of calculating the pressure of dissolved inert gasses in a range of tissues throughout a dive. These dissolved gas pressures can then be compared with ambient pressure to establish the degree of supersaturation, and adjustments to the dive profile can be made to prevent supersaturation from exceeding safe thresholds. With the intended audience in mind, it is the express intent of this account to discuss the broad principles of adapting tissue supersaturation calculations into decompression planning tools, rather than the related mathematics. Those wishing to study the mathematical principles and methods can find relevant accounts elsewhere1,2.
As mentioned earlier, virtually any approach to decompression planning assumes that the inert gas tensions within tissues can be calculated and thereby tracked throughout a dive. As could be anticipated from the discussion of gas uptake and elimination in Chapter 10, the mathematical models that allow such calculations include tissue perfusion and the blood-tissue partition coefficient for the relevant gas(es) as inputs. It is also pertinent to reiterate that the ‘tissues’ considered in these models are not real or identifiable tissues per se. The models merely consider a range of hypothetical tissues with different inert gas kinetics and assume that the relevant real tissues behave in a manner analogous to one of the hypothetical compartments used in decompression calculations.
To illustrate the incorporation of tissue gas supersaturation data into decompression planning, the relevant events during a dive are depicted using the format introduced in Figure 12.1. It is important to appreciate that this and subsequent figures in this chapter are illustrating principles and do not purport to be scaled correctly or to –represent any particular tissue accurately.
In Figure 12.1, ambient pressure (depth) is shown on the horizontal axis, and tissue gas pressure is shown on the vertical axis. Time is not illustrated in the diagram but requires assumptions to be made about its passage, as will be described. Line A represents descent at the start of a dive. The descent occurs over a short space of time, and so there is little time for inert gas uptake and little increase in tissue gas pressure. The bottom depth is reached at the point indicated; therefore, there is no further change in ambient pressure until the ascent begins (see later). During time spent at the bottom depth, inert gas will dissolve into the tissue and the tissue inert gas tension will increase, as depicted by line B. The other notable feature in Figure 12.1 is the line labelled ‘Tissue pressure = ambient pressure’. This represents the point, for all depths, where the pressure of dissolved gas in tissue is equal to the ambient pressure and is often referred to as the ‘ambient pressure line’. It should be clear that while remaining at any particular depth, the tissue gas pressure cannot rise above this line because once tissue gas pressure equals ambient pressure, there can be no further pressure gradient to drive diffusion of gas into the tissue unless the diver descends deeper. Depending on the kinetics of individual tissues and the time spent at depth, at the end of a period at the bottom depth, tissue gas pressure may have equilibrated with ambient pressure in some ‘fast’ tissues, whereas in other ‘slower tissues’ it may not (Figure 12.2).
Figure 12.2 illustrates a hypothetical situation that could prevail at the end of a period at depth in respect of tissue gas pressures in a range of tissues (represented by the grey dots) with differing kinetic properties. Depending on the duration of the bottom time, the tissue gas pressure in one or more tissues may have reached equilibrium with ambient pressure (thus having reached the ambient pressure line), and these tissues can be described as ‘saturated’ with inert gas for that depth. Other tissues with slower kinetics will have absorbed less inert gas and will have lower tissue gas pressures.
For the purposes of illustrating the principles of decompression, this discussion temporarily ignores the fact that multiple tissues are involved and focusses on the behaviour of a single tissue. For the sake of simplicity, it is assumed that this tissue has reached the ambient pressure line (and is thus saturated with inert gas) at the end of a long bottom time. This is illustrated in Figure 12.3.
In Figure 12.3 (as in Figure 12.1), line A represents descent to the depth indicated at point 1 at the start of the dive, and line B represents the increase in tissue gas pressure as inert gas is absorbed during the time at depth. This tissue has reached the ambient pressure line (grey dot at point 2) and is thus saturated with inert gas. Line C represents the changes in tissue gas pressure and ambient pressure during the early part of the ascent. Ambient pressure decreases quickly as depth changes, whereas in this tissue the accumulated inert gas is not washed out at a rate that matches the fall in ambient pressure. In a tissue with very fast kinetics, the fall in tissue gas pressure could more closely match the falling ambient pressure, but in this tissue it can be seen that by point 3 on the ascent, the tissue gas pressure (point 4) markedly exceeds the ambient pressure (point 5) and the tissue is thus ‘supersaturated’. The supersaturation pressure is indicted by the double-ended arrow in Figure 12.3.
It should be clear at this point that the key question when modelling decompression in this way is ‘How much supersaturation is acceptable?’ The means of deriving an answer to this question introduces a controversial dichotomy in decompression science between the so-called ‘gas content models’ and ‘bubble models’. For the moment, the focus of this discussion is on the more traditional gas content models, and this approach is contrasted with bubble models later.
The original gas content model proposed by Haldane held that ascent could proceed until such time as the tissue gas pressure in any tissue reached twice the ambient pressure. This was Haldane’s often-cited 2:1 ratio. At this point a decompression stop was imposed to allow the tissue gas pressure to fall while the ambient pressure remained constant. This approach was moderately successful, but it evolved over time, and the fixed ratio concept was eventually dropped in favour of ascent rules that prescribed maximum allowable supersaturations (sometimes referred to as ‘M-values’) for different tissues across a range of depths. The most famous of these sets of rules were the Zurich Limits for 16 hypothetical tissues (the ZH-L16 limits) prescribed by A. A. Buhlmann and based on physiological predictions with subsequent empirical modifications. The principles by which these work are illustrated for a single tissue in Figure 12.4.
As in Figure 12.3, line A in Figure 12.4 represents descent to the bottom depth indicted at point 1 at the start of the dive, and line B represents the increase in tissue gas pressure as inert gas is absorbed during the time spent at that depth (bottom time). By the end of the bottom time the illustrated tissue has reached the ambient pressure line (grey dot at point 2) and is thus saturated with inert gas. As in Figure 12.3, line C in Figure 12.4 represents the changes in tissue gas pressure and ambient pressure during the early part of the ascent. In Figure 12.4, the ascent rule is shown as a series of values for maximum allowable tissue gas pressures plotted against depth and is labelled ‘supersaturation limit’ for simplicity. As is typical, especially for tissues with fast kinetics, the rule allows greater supersaturation at deeper depths. Direct ascent (line C), at a rate not exceeding a maximum prescribed by the model, proceeds until the tissue gas pressure equals the maximum allowed (point 3), at which time the first ‘decompression stop’ is imposed at a depth corresponding to point 4. After the tissue has ‘off-gassed’ sufficiently, the ascent is resumed with stops imposed each time the supersaturation limit is approached as depicted in Figure 12.4. Eventually, there is sufficient outgassing in the tissue to allow direct ascent to the surface while remaining just under the supersaturation limit.
Although diagrams such as Figure 12.4 are useful for illustrating some of the basic concepts underlying decompression planning, the process is much more complex in reality. In ‘real’ decompression modelling, we are usually not considering only one tissue but multiple tissues simultaneously. Whichever tissue is closest to its maximum supersaturation limit at any stage of the ascent becomes the ‘controlling tissue’. Typically, this will be one of the tissues with faster kinetics early in the ascent (because these tissues are likely to have accumulated higher levels of inert gas during the bottom time). This means that the early decompression stops are shorter because the faster tissues that are controlling at that point will outgas quickly. Similarly, the tissues with slower kinetics tend to be controlling later in the ascent for the shallower stops. These stops tend to be longer because the slower tissues take longer to outgas. The involvement of multiple tissues and the effect of tissue gas kinetics on decompression stop durations are not captured in diagrams such as Figure 12.4.
It is appropriate to acknowledge at this point that recreational divers undertaking entry level courses are taught ‘no decompression diving’. This means that dives are planned to be of modest depth and duration so that a direct ascent to the surface (at the correct rate) can be made at any point in the dive without the gas pressure in any tissues crossing the supersaturation limit. Because tissue inert gas pressures will reach higher levels more quickly at greater depths (where the inspired inert gas pressures are higher), the permitted duration for a no decompression dive (referred to as a ‘no decompression limit’) becomes progressively shorter as the depth increases. For example, for many years the US Navy air diving table prescribed no decompression limits for 18-, 30- and 40-metre dives as 60, 25 and 5 minutes, respectively. Dives requiring decompression stops are routinely undertaken by recreational divers who refer to themselves as ‘technical divers’ (see Chapter 62).