A question from a club member this month about reverse dive profiles. On a recent dive holiday to Vanuatu the dive leaders took them on a dive to 20m in the morning, followed by a dive to 30-35m after an appropriate surface interval i.e. a reverse dive profile. This goes against everything we were taught as student divers but it seems to be occurring more and more often. Have the divemasters gone mad or was the traditional teaching to do the deepest dive first just an old wife’s tale?
As soon as we leave the surface we breathe gases at increased pressure. The increased partial pressure of nitrogen exceeds the pressure of nitrogen in the body’s tissues and so the tissues take up more nitrogen. When we return to the surface this gradient is reversed and nitrogen leaves the tissues, enters the blood stream and is exhaled via the lungs. If the pressure gradient that develops from the tissues to the lungs is too great (a critical pressure gradient can be defined mathematically), then bubbles form which can then cause the syndrome of decompression sickness (DCS). Bubbles are more likely to form with the rapid development of a large gradient (rapid ascent from a prolonged dive at a great depth), if bubbles already exist from a previous dive or if a problem exists with the elimination of nitrogen (poor blood flow due to hypothermia developing during the dive). In addition many poorly understood factors may contribute to bubble formation or the development of symptoms e.g. alcohol/dehydration, obesity, viral infections or heart abnormalities of certain types.
We now recognise that most no-decompression dives cause measurable bubble formation that cause no symptoms or illness. The presence of these bubbles and residual nitrogen in the tissues prior to a repetitive dive must be taken into consideration when planning the dive profile (US Navy defines a repetitive dive as one more than 10minutes but less than 12 hours from your last dive).
But to return to the original question, does it matter whether the deep dive (a large "decompression stress") is performed first followed by a more benign dive, or whether a small nitrogen/bubble load is followed by a greater one?
This rule of deeper dive first can be traced to recreational dive manuals written in the early 1970s but earlier origins cannot be found. Certainly there is no reference to the issue in the US dive manual or other occupational diving texts. In 1999 at the Smithsonian Institution a workshop examined all the theoretical, experimental and practical data for reverse profile diving. The experts concluded that for no-decompression diving using air or nitrox to depths less than 40m, no reason exists to prohibit reverse profile diving. They add that any depth differentials should be 12m or less, and that this conclusion should not be extrapolated to any other types of diving.
On a practical note, one can calculate that longer bottom times are gained by doing the deeper dive last, with no increase in risk as long as the tables are accurately followed.
