3.4 Omitted Decompression
Among the challenges with very lengthy decompression diving is that everything can go to plan, until it doesn’t. It’s a very, very fine line. Light was shed on this fine line for me during my early visits to Lee Stocking Island and NOAA’s Caribbean Research Center. The Dive Officer at the time would frequently take the more experienced divers out on deeper forays to provide ‘support’ for his own deep excursions, which were typically made solo. The direction to us was always, “If we get chased out of the water [by sharks], we’re going to pull out of here, head in X-direction, anchor, and get me back in the water”. Sounds simple enough, but in practice this would have been a near Herculean effort to achieve and at the same time does not get anyone bent in the process. Fortunately, we never had to enact this plan.
Years later, at the very same location, I had the misfortune of botching a few ascents due to a leaking overpressure relief valve on my rebreather, which resulted in catastrophic floods on multiple dives. Being forced to bailout from an ideal pO2 to an open-circuit profile for decompression can be a massive time delta – gaining more than an hour of decompression required for the dives I was running. Fortunately, we were smart about open-circuit bailout contingencies. Additionally, the miscalculation of minutes on the bottom can come with the expense of literally hours during decompression, and these contingencies need to be accounted for, particularly when rebreathers are used.
When $#!^ really hits the fan, it is also important to understand that the surface is likely not an option, or at least not without accepting that decompression sickness will be a probability, not an uncertainty. In many of the remote locations dived for science and exploration which are only made possible with small, cost-effective footprints, developing contingencies for omitted decompression, or just generally botched decompression, need to be addressed as risk mitigating steps within the dive operations plan.
Decompression can go bad for any number of reasons including, but not limited to:
- failed life support equipment
- premature exhaustion of primary or bailout life support equipment
- overextended stays resulting in severe hypothermia, mental fatigue, or panic
- need to abort given an in-water injury
- chased out of the water by sharks or other aggressive marine life
In its simplest form, a useful tool for addressing omitted decompression is following the US Navy’s recommended omitted decompression procedure as follows:
- descend to first stop depth
- follow the original schedule up to 30fsw
- multiply the 30, 20, and 10 fsw stops by 1.5x’s
Note that this Navy procedure is for air diving and should not be misconstrued as a magical solution for deep mixed-gas dives with lengthy decompression. The general strategy is simple enough to be kept in the back of one’s mind as a last-ditch effort and, most importantly, encourage the thought process and expectations in returning to the water under these dire circumstances.
Most deep and decompression dives today are made using multiple computers, so in practice it would make sense to re-enter the water and clear out the computer (plus some reasonable buffer) in a highly emergent omitted decompression scenario. There are bodies of work evolving to more carefully consider in-water recompression as a practical tool, though continued research is needed.
As a bit of a disclaimer, plan dives to do everything possible to avoid an omitted decompression emergency. In the very unfortunate event that this become a reality, be confident in field response to DCS incidents and emergent evacuation. Best practice is always providing well-practiced first response for diving emergencies, with forward movement towards getting the victim to appropriate medical facilities and, ideally, a hyperbaric treatment center.
Increasingly more aggressive diving is taking place in very remote areas where evacuations, let alone chamber access, are not always attainable within a reasonable time period. In these environments, additional risk mitigating steps may involve various means to establish oxygen treatments under pressure and, ideally, with medical support telepresence. These capabilities exist and are becoming more practicable with low-cost portable habitats. Coupling expert know how of rebreather atmospheric management with the space of a simple habitat, extremely lengthy planned or omitted decompression schedules can be carried out.