Decompression Sickness: Decompression Planning ( Gas Content Versus Bubble Models )

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.

Figure 12.5 Stylized depiction of differences between decompression of a hypothetical single tissue prescribed by a gas content model (black line) and a bubble model (grey line). See text for further explanation.
Figure 12.5 Stylized depiction of differences between decompression of a hypothetical single tissue prescribed by a gas content model (black line) and a bubble model (grey line). See text for further explanation.

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’).

Figure 12.6 The stylized effect of imposing gradient factors GF 20/90 on the decompression prescribed by a gas content model for a single hypothetical tissue. See text for further explanation.
Figure 12.6 The stylized effect of imposing gradient factors GF 20/90 on the decompression prescribed by a gas content model for a single hypothetical tissue. See text for further explanation.

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).

Figure 12.7 The stylized effect of a gas content model decompression (solid black arrows) and a bubble model decompression (grey arrows) on tissue gas pressure in a hypothetical fast tissue (arrows originating at point 2) and a hypothetical slower tissue (arrows originating at point 3). See text for further explanation.
Figure 12.7 The stylized effect of a gas content model decompression (solid black arrows) and a bubble model decompression (grey arrows) on tissue gas pressure in a hypothetical fast tissue (arrows originating at point 2) and a hypothetical slower tissue (arrows originating at point 3). See text for further explanation.

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.