Reverse Profiles and Decompression

Taking the deepest dive first is the mantra that is repeated to us by practically all diving agencies. Breaking this “rule” seems to be one of the biggest taboos in diving. But is that the case? Maybe not, or at least not always, as can be seen from the results of a conference organized by the American Academy of Underwater Sciences and the Smithsonian Institution from which the information for this article is taken and where you can find the references.  

Let’s start with a brief overview of the decompression theory. During respiration, the only gas metabolized is oxygen. The remainder (nitrogen when breathing air) does not participate in any physiological reaction.  At sea level, our body is in a state of saturation for nitrogen; with each breath, this gas is ventilated in and out of the lungs. As the ambient pressure increases during diving, the partial pressure of nitrogen (and any other inert gas present in the respiratory mixture) increases, causing its dissolution in the tissues following Henry’s law. During ascent, this excess gas returns to the gaseous phase and is eliminated from the lungs.  Problems begin when the process generates bubbles large enough to interfere with blood circulation, causing local blockages. In this case, the diver is a victim of decompression sickness. To avoid these problems, limits on time, depth, and speed rate of ascent have been developed as dive tables or algorithms for dive computers.

Over the years, one “rule” seems to have become dominant: take the deepest dive first and, throughout a single dive, reach the maximum depth at the beginning and then start the ascent. Violating this procedure is considered to cause possible interference with the decompression process, increasing the risk of decompression sickness.

However, there are situations where an inverse profile, shallower dive first, or return to maximum depth after ascending to a shallower depth during a single dive, is unavoidable. For example, cave diving forces divers to follow the elevation profile of the cave, which may require several “ascents and descents.”  

A study of decompression theories, beginning with Haldane’s early work in 1908, has shown no theoretical or experimental support for the need to avoid inverse profiles. The limitation would seem rather practical because the dive tables, as they have been created, penalize the inverse profiles by significantly reducing the bottom time allowed in subsequent dives (Lewis, 1999). 

For example, using the NAUI tables, making a direct profile with a first dive at 27 m for 20 minutes and a second dive at 21 m for 19 minutes (maximum bottom time allowed) with a surface interval of one hour between the two, we can spend a total dive time (cumulated between the two) of 39 minutes. Reversing the profile (21 m for 20 minutes followed by 27 m for the maximum allowed of 9 minutes), our dive time will be 29 minutes. The inverse profile, therefore, causes a reduction in the total bottom time of the two cumulative dives by almost 25%.

However, this reduction in allowed no decompression dive time does not necessarily indicate an increase in decompression risk based on the physiological effects of inverse profiles. In practice, the rule would be more a consequence of “habits” than the result of scientific studies. In fact, in the medical literature, there does not seem to be any reference to the fact that inverse profiles cause an increase in the risk of compression (Egstrom, 1999). Inverse profiles, however, cause a substantial increase in bubbles and gas release, and these effects must be adequately considered in the decompression algorithm (Yount, Maiken, Baker, 1999). From the analysis of multiple inverse profiles, the mechanism of bubble formation and the risk of decompression sickness would seem to be the same as for direct profiles (Weathersby, Gerth, 1999). Both bubble dynamics models and different algorithms used in dive computers show no difference in decompression stress between direct and inverse profiles (Gerth and Thalamann, 1999; Gernhardt, 1999).

What has been noted is that inverse profiles are often associated with periods of intense activity with multiple dives for many consecutive days. This would cause increased decompression risk rather than the reverse profile (Vann et al. 1999). In addition, the more complex calculations required to process the parameters of inverse profile dives push the limit of dive computer algorithms with the potential for increased decompression risk (Mutzbauer, 1999).

In conclusion, making the deepest dive first or reaching the maximum depth in a single dive right away is more for practical reasons than an obligation for physiological reasons, mainly if you use the tables to program dive times on multiple dives. A direct profile will allow a cumulative dive time between the two dives greater than permitted in a reverse profile. As a last but important note, it should be noted that these considerations apply only to dives without decompression within 40 meters and with a maximum depth differential not exceeding 12 meters. In addition, it is always necessary to ensure that the tables or the computer used allow an inverse profile, even if penalizing.