Dive tables are pre-calculated and pre-printed implementations of a decompression algorithm that divers can use to plan their dives. These were most popular before the 1990s among recreational scuba air divers wanting to plan simple dives, and their use was taught as part of all recreational scuba diving courses. The most frequently used items of information were the no decompression limits (see earlier) provided by these tables. Thus, divers could look up the allowable bottom time they could spend at any particular depth and still make a direct ascent to the surface without decompression stops. Even though no decompression was prescribed for dives ‘inside’ the no decompression limits, most training agencies advocated the use of a 3- to 5-minute ‘safety stop’ at 3 to 5 metres during the final part of the ascent, as an added precaution. Such stops are probably useful in reducing the incidence of DCS on routine no decompression dives.
The dive tables also provided a series of steps (with minimal calculation) for divers to account for the effect of inert gas accumulated but not yet eliminated after previous dives when planning further ‘repetitive’ dives. In air diving, this is referred to as ‘residual nitrogen’, and its presence, not surprisingly, has the effect of reducing the no decompression limit for subsequent dives.
The use of dive tables has declined dramatically with the rise in use of dive computers (see later), and some entry level diving courses no longer teach their use. Whether this is a good or bad thing is impossible to say. Tables were inexpensive and readily available, whereas in the early days this was not true of computers. More recently, however, entry level dive computers have become much more reasonably priced and have the advantages of avoiding calculation errors, thus ensuring that the diver who carries one is receiving accurate time and depth information, and most of these computers provide ascent rate alarms.
Planning software that runs a decompression algorithm (and often multiple decompression algorithms) can be purchased for use on desktop, laptop and tablet computers, as well as telephones. Such software is effectively an electronic dive table and is often used to generate tables that are transcribed onto underwater slates for specific missions. The advantage of such software is that the decompression plan can be tailored specifically to the diver’s equipment, gas mixes and decompression preferences (e.g. gas content model, bubble model, gradient factors). The multitude of combinations and permutations of circumstances that can be ‘run’ by such software would be very difficult, if not impossible, to replicate on pre-printed tables.
Dive computers that the diver carries underwater have become increasingly popular since the early 1990s and are now almost ubiquitous. These computers run software with one or more decompression algorithms programmed into them and with various levels of adaptability for decompression planning. They track time and depth exposures in real time and provide a constant display of parameters such as depth, dive duration, decompression ceiling and expected time to surfacing. These parameters are continuously updated as the depth varies and the dive duration lengthens, all with little or no effort on the part of the diver. Advanced computers allow the diver to choose the equipment being used (e.g. open-circuit or rebreather systems) and the gas mixes being breathed and to adjust decompression preferences such as gradient factors. Some computers can connect to the oxygen cells in a rebreather (see Chapter 62) so that they ‘know’ the inspired partial pressure of oxygen used by the diver and can incorporate this information in calculating decompression. Advanced computers that perform these functions and provide all relevant information in a head-up display constantly visible to the diver are now available.
As described earlier, gas content models regulate ascent and impose decompression stops to maintain tissue supersaturation below empirically derived thresholds across the range of tissues with different kinetic behaviour. Such models have been very successful, but are not invariably so; that is, DCS can certainly still occur even when divers decompress according to the model. The occurrence of such events always results in interest in alternative approaches that may (potentially) be more successful. Moreover, as discussed in Chapter 10, it has long been known that even decompressions performed in accordance with established guidelines frequently result in the formation of venous gas emboli (VGE), whose numbers can be correlated (albeit imprecisely) with the risk of DCS. Much of the early research that revealed this VGE phenomenon took place when the use of gas content models to control decompression was almost ubiquitous. Thus, the emerging recreational technical diving world of the late 1990s and early 2000s was fertile ground for well-meaning advocates of alternative approaches to decompression.
A school of thought that had been around for some time, but came to prominence during this period, was the so-called ‘bubble-model’ approach. Bubble model advocates had taken note of the frequently high VGE counts after decompression conducted according to gas content models and advanced the notion that, at least in part, the failure of these models to control bubble formation effectively could increase the risk of DCS even when the diver did everything right. Moreover, they proposed that initiation of bubble formation probably occurred during exposure to the relatively large supersaturations allowed by gas content models during the long ascent to the first decompression stop. Using advanced physics, bubble modellers purported to be able to quantify bubble formation from micronuclei (see Chapter 10) of a given size for a given level of supersaturation, and their calculations suggested that shorter initial ascents (and therefore smaller initial supersaturations) than allowed by gas content models would result in ‘excitation’ of smaller populations of micronuclei and therefore help prevent initiation of bubble formation. It was even suggested that by imposing deeper initial decompression stops a diver could reduce the requirement for the shallow decompression stops later in the ascent because initiation of bubble formation would have been controlled earlier. A stylized comparison between these two approaches to decompression using the same format as previous figures is shown in Figure 12.5.
As in Figures 12.3 and 12.4, line A in Figure 12.5 represents descent to the bottom depth indicted at point 1, 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 12.4, the supersaturation limit or M-value line as prescribed by a gas content model is depicted, and if the diver was following such a model, then direct ascent (line C) would proceed until the tissue gas pressure equalled the maximum allowed (point 3), at which time the first decompression stop would be imposed at a depth corresponding to point 4. After the tissue has off-gassed sufficiently, the ascent would be resumed with stops imposed each time the maximum supersaturation is approached.
In contrast, the ascent prescribed by a typical bubble model (depicted by the grey arrows) involves shorter initial ascents, deeper initial decompression stops and smaller initial supersaturations. A bubble model could also (as depicted) allow surfacing with a tissue gas supersaturation greater than the maximum allowed by the gas content model, based on the belief that the process of bubble initiation had been controlled earlier and that this allowed exposure to greater supersaturation later in the ascent.
There was a compelling theoretical attraction to the concept of using ‘deep stops’ to ‘control bubble formation early in the ascent’. There were also some widely discussed anecdotal observations from several prominent divers that insertion of deep stops into their ascents seemed to result in feeling less fatigued after dives. In the early 2000s these factors, combined with the burgeoning influence of Internet communication, became an article of faith among deep recreational technical divers that bubble model approaches to decompression were superior even though no formal testing of the algorithms had been undertaken. There was widespread adoption of the two most readily available bubble model algorithms (the varying permeability model [VPM] and the reduced gradient bubble model [RGBM]). It largely went unnoticed when VPM was revised into VPM-B to increase shallow stop time after reports of DCS began to emerge. Gas content models with their relatively rapid early ascents and longer shallow stops were derided as being a recipe for ‘bending and mending’ (an allusion to causing bubble formation with supersaturation of fast tissues early in the ascent and then fixing the problem with long shallow stops late in the ascent).
The use of gas content models did persist, perhaps because they were easier to understand or to program for use in computers, but even users of these algorithms began to manipulate them to make them behave more like bubble models. One technique for such a manipulation that became and remains popular is the use of so-called ‘gradient factors’. This involves limiting supersaturation to less than permitted by the conventional supersaturation limit by redefining maximal permissible supersaturation as a fraction of the difference between ambient pressure and the limit. These fractions have come to be known as gradient factors. Thus, if a diver elects to limit supersaturation to 80 per cent of the usual difference between ambient pressure and the supersaturation limit, this is referred to as ‘gradient factor 80’ or ‘GF 80’. Typical implementations of the gradient factor method require the diver to select two gradient factors: the first (often referred to as GF-Low) notionally controls supersaturation in the fast tissues early in the ascent, and the second (often referred to as GF-High) controls supersaturation in the slower tissues at the point of surfacing. The algorithm then interpolates a series of modified M-values in between these two user-specified points. Not surprisingly, lowering the first gradient factor limits supersaturation in the fast tissues early in the ascent by imposing deeper decompression stops, and lowering the second will produce longer shallower stops to reduce supersaturation in the slower tissues at the point of surfacing. Choosing a low GF-Low and a higher GF-High produces a decompression profile that resembles a bubble model decompression. This is illustrated in Figure 12.6 for a GF-Low of 20 per cent and a GF-High of 90 per cent (in common use this terminology would be abbreviated to ‘GF 20/90’).
As in previous figures, line A in Figure 12.6 represents descent to the bottom depth indicted at point 1, 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. At the start of decompression, the initial ascent (line C) is allowed to proceed until the tissue reaches 20 per cent of the supersaturation limit, at which point a stop is imposed at the depth corresponding to point 3. The ascent then continues with further stops imposed when the tissue supersaturation approaches the modified supersaturation limit defined by a line joining the chosen GF-Low (black dot labelled 20 per cent) and the chosen GF-High (black dot labelled 90 per cent). If this approach is compared with the two profiles shown in Figure 12.5, it is clear that it is now very different from the unmodified gas content model decompression and substantially similar to the bubble model decompression. For obvious reasons, the use of gradient factors with a low GF-Low and bubble model decompressions are collectively referred to as ‘deep stop’ approaches to decompression.
The ZH-L Buhlmann gas content model that forms the basis for most tables and computers designed to guide decompression diving by recreational divers was subjected to some human testing; albeit minimal for the trimix diving and depth range for which it is now implemented. The bubble models and gradient factor manipulations of gas content models have had essentially no testing. It is acknowledged that the preceding discussion of bubble model theory represents a gross oversimplification of a complicated matter, but the fact remains, no matter how attractive the theory, it has never been tested in a practical sense. Advocates frequently cite the ubiquitous nature of deep stop approaches as some sort of proof that they are optimal, but this is an invalid argument in the absence of comparative outcome data. These approaches clearly work in the majority of dives, but whether they are optimal is an unresolved question.
Debate over this issue has been rekindled with the publication of several studies that have suggested that the emphasis on deep stop approaches to decompression may need to be reconsidered. Several of these studies have focussed on measuring VGE after diving and suggest that deep stops may not reduce the appearance of VGE as previously widely assumed. However, by far the most significant development has been the 2011 publication of a study performed by the US Navy Experimental Diving Unit (NEDU) at Panama City in Florida3. The investigators compared outcomes after air dives to 170 feet for 30 minutes with same-duration decompression on air prescribed by either a gas content model or a bubble model. Both decompression protocols are US Navy models that are not used by recreational divers, but they nevertheless have characteristics that reflect the respective approaches; the gas content model allows greater supersaturation in faster tissues early in the ascent and distributes decompression time shallower, and the bubble model imposes deeper stops early in the ascent and thereby distributes decompression time deeper. The remarkable feature of this study was that the primary outcome measure was DCS in human subjects. The divers performed a standardized workload during the bottom time, and temperature effects were standardized across the groups by having all divers wear no thermal protection in water at a temperature of 30°C. There were 11 cases of DCS in 198 dives in the deep stops group and 3 cases in 192 dives in the shallow stops group. The trial was ceased at this point because the difference became significant on sequential analysis.
This result was not the outcome expected or hypothesized by the investigators. Attempts to explain it have focussed on the likelihood that protection of fast tissues from supersaturation early in the ascent does not seem to be as effective as thought, and it comes at the expense of increased supersaturation in the slow tissues later in the ascent because they are continuing to absorb gas during deep stops. This principle is illustrated in Figure 12.7. As in previous figures, line A in Figure 12.7 represents descent to the bottom depth (indicted at point 1), 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 tissue represented by the grey dot at point 2 has reached the ambient pressure line and is thus saturated with inert gas, whereas the slower tissue represented by the grey dot at point 3 is still absorbing inert gas. At the start of decompression, gas content model decompression allows an the initial ascent (line C) to proceed until the faster controlling tissue reaches the maximum supersaturation limit, where the first decompression stop is imposed at a depth indicated by point 4. In contrast, the bubble model allows a shorter ascent (line D) to the first stop at a depth indicated by point 5. In the slower tissue, whose tissue gas pressure at the beginning of ascent is indicated by point 3, the gas content model decompression gives the tissue little more time to absorb inert gas as illustrated by line E because absorption ceases and outgassing begins once the tissue reaches then crosses the ambient pressure line. In contrast, the deep stops prescribed by the bubble model will result in further inert gas absorption by this tissue (line F).
The NEDU study forces us to question whether the proposed benefit of using a bubble model (protection of fast tissues early in the ascent) is worth the disadvantage of the increased gas loading that occurs in slower tissues as a result. Bubble model advocates have tried to portray the study as irrelevant because the experiments involved air diving and used a deep stop profile that is not exactly the same as that prescribed by VPM-B. Nevertheless, analyses of ‘real-world’ VPM-B profiles prescribed for dives using accelerated decompression on oxygen (such as are typically undertaken by technical divers) suggest that the same disadvantageous pattern of protecting fast tissue from supersaturation early in the ascent at the expense of slower tissue supersaturation later still occurs.
It is clear that the optimal approach to decompression from the deep bounce dives undertaken by recreational technical divers is not established; however, it seems plausible to suggest that we have evolved an approach that risks overemphasizing deep stops. One trend that is emerging as this book goes to press is the use of gradient factors to reduce the emphasis on deep stops (by increasing the GF-Low) and re-emphasizing shallow stops (by decreasing the GF-High). Thus, whereas gradient factor combinations such as 10/90 were popular during the height of belief in deep stops, it is increasingly common to see combinations such as 40/70 or 50/70 now.
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).