Deep Stops

Throughout history, the key concern for divers has been how to get back to the surface without suffering decompression sickness (DCS). The British scientist J.S. Haldane combined empirical data with scientific studies to develop step-by-step decompression procedures that, along with the accumulated experience and work of many scientists, led to the development of modern decompression tables and computer algorithms.

Decompression sickness still occurs in recreational divers, but at a rate of 1 to 4 per 10,000 dives; these cases of DCS are often mild and treatable. However, the possibility of severe injury — though rare — makes divers eager to hear about any measure that might further reduce the risk. One possibility is the deep stop.

In the mid-1990s, Richard Pyle, a biomarine scientist who frequently made dives to great depths in search of fish species, noticed that sometimes he felt fatigued after dives, and at other times he felt normal. An excellent observer and trained scientist, he figured out that on dives when he had to stop during his ascent to deflate the swim bladders of his specimens, he felt much better. Soon he introduced a brief stop halfway to the surface on all his dives and formed the strong opinion that this significantly reduced his post-dive fatigue. He shared his experience with fellow divers, and the practice of deep stops became widespread among technical divers before it could be scientifically tested.

What is a deep stop? In the minds of most who practice it, the deep stop is an additional stop during ascent, introduced by divers beyond what their computer algorithm demands. However, there are now computer algorithms that claim to include deep stops, though neither these algorithms nor the practice of deep stops has been thoroughly validated.

Discussion about deep stops is not new to the scientists studying decompression safety. Since Haldane first established decompression tables, the depth of the first stop has been debated. The answers varied over time, depending on prevailing contemporary dive practices and concerns. Haldane, for example, assumed that tissues may sustain a certain level of supersaturation or critical volume of surplus gas before bubbles occur. That is why his decompression model applied a relatively quick ascent to depths he believed would drive inert gas out of the body.

Later it became clear that bubbles probably occur much earlier than Haldane assumed, and these findings led to the creation of so-called bubble models. Many dive computers on the market incorporate deeper stops than do earlier Haldanian models. Some of them are based on bubble models, while others adjust parameters of non-bubble models to achieve similar effects. However, to mimic the deep-stop practices adopted by some technical divers, some computers add stops deeper than what their models call for or give divers this option.

So the big question before divers today is: How effective are deep stops at preventing DCS, whether called for by a bubble algorithm or when used by divers independent of what their computers suggest? To help shed light on this issue, we asked several experts for their opinions.

Is it advisable for divers to insert additional deep stops regardless of what their dive computers suggest?

Peter B. Bennett: If divers have computers with a deep-stop option, they can use it. If the computers do not have a deep-stop option, they should stick to the computers’ readout.

David Doolette: If a diver is unhappy with his method of decompression planning, he should choose another method.

Christian Gutvik: No. Currently, our theoretical models and experimental results indicate that deep stops are beneficial only on long dives. We believe that decompression models will evolve to calculate optimal stops and, if and where beneficial, deep stops will be already included.

Simon Mitchell: No. Divers should not arbitrarily adjust their decompression algorithms. If the deep stops inserted are at the wrong depth and/or for the wrong period, there is a significant chance of increasing rather than decreasing the risk of the profile. If divers find the concept of deep stops appealing, they should choose an algorithm into which [deep stops] have been incorporated.

David Southerland: Not at this time. First of all, there is no formal definition of a “deep stop.” The idea that one change (the addition of deep stops) is required for every dive computer regardless of the decompression model used implies that all decompression models are broken in the same way and all require the same fix. That doesn’t seem right. The optimal stop time and stop depth for the deep stop were tested for a limited number of dive profiles in manned and animal studies, using various outcome measures. In some studies, deep stops reduced bubbles in divers, while in others its effects were profile-dependent or even increased the likelihood of DCS. More research is needed before one could advise or discourage the insertion of any deep stop.

What does it take to compare the safety of two or more decompression procedures?

Doolette: Comparing, for instance, a “deep-stops” schedule and a “traditional” shallow-stops schedule requires that each schedule be dived many times by many divers with everything else kept about the same, with a predetermined end point such as DCS or perhaps venous gas emboli (VGE) scores. For instance, the U.S. Navy Experimental Diving Unit conducted almost 400 air dives to 170 fsw for 30 minutes of bottom time with the same decompression time. Half the dives followed a deep-stops schedule, and half followed a traditional shallow-stops schedule. There was more DCS following the deep-stops schedule than the traditional schedule. An example of what is not a useful measure of safety is to compare how you feel today after decompression using nitrox and oxygen while warm in your drysuit with your memory of dives in the past with decompression conducted on your last 100 psi of air while shivering in a borrowed wetsuit.

Gutvik: Comparing the safety of two or more decompression procedures can be done observing clinical symptoms
and/or venous gas bubbles using ultrasonic detection. Clinical symptoms are rare, and it takes large numbers of trials to statistically distinguish two dive profiles. Bubble detection allows objective statistical comparison with far fewer, and less stressful, dives. To test the benefit of deep stops, procedures have to be evaluated against dives with equal total decompression length. If the deep stops are only added on top of the conventional procedure, we cannot conclude if it is the extension of decompression or the deep stop itself that is beneficial. It is possible that the extra decompression time added is more wisely spent at a shallower depth.

Bennett: Safety in this context means no DCS, especially its neurological form, which is the most common in scuba divers. By reducing the amount of bubbles in the blood vessels, a deep stop reduces decompression “stress,” and thus is safer. However, the definitive research as to whether the use of an additional deep stop reduces the incidence of neurological DCS has not been done, nor has it been done with most computers and tables in use today.

Mitchell: The difficulty of this task cannot be overstated. Ideally, the comparison would take place over very large numbers of dives that are carefully controlled for time, depth, activity and temperature using divers who complete even numbers of dives generated by both procedures. As much as possible, the divers would be blinded to the procedure used and would complete validated wellness assessments after the dives. The outcomes associated with the different procedures would be compared. Unfortunately, this sort of protocol is virtually never used because of logistical and financial difficulties. A more common approach is to compare markers of decompression stress such as venous bubble grades, measured using Doppler after dives executed according to the respective procedures. Alternatively, the risk of decompressions generated by these procedures can be modeled mathematically. Unfortunately, both of the latter two approaches are not entirely satisfactory and the resulting data can be difficult to interpret.

Southerland: Generally, I give more weight to manned studies than animal studies, and I weigh DCS outcome studies more than venous bubble load outcome studies; however, there is not universal agreement on the diagnosis of DCS. Therefore, the overall effect of adding a “deep stop” might reasonably be small enough to be lost in the noise. Perhaps “feeling better” after a dive is the proper outcome of interest rather than DCS. However, subjective feelings are quite sensitive to bias.

Should recreational divers staying within no-decompression limits be concerned about deep stops?

Mitchell: No. There is insufficient data to justify a deep stop approach in recreational, no-decompression diving. Divers should, however, pay careful attention to ascent rates, and the imposition of shallow safety stops is still considered beneficial.

Southerland: Recreational divers should be aware of the issue. If sufficient evidence becomes available, then a diver might be better off just upgrading the dive computer with a newer algorithm.

Gutvik: No. The recommended safety stop and a controlled, slow ascent rate are adequate for performing safe no-decompression dives. These dives will normally be either too shallow to consider deeper stop depths or too short for the deep stops to have an effect.

Bennett: There is no more reason for concern about deep stops than for the widely accepted shallow safety stop. Both were developed based on the reduction of bubbles in the blood vessels seen in research studies. The research on the deep stop is, in fact, more extensive and is based also on actual recreational dives. The deep stop at half the depth for 2.5 minutes significantly reduces not only bubbles, but also the critical gas supersaturation in the “fast” tissue compartments (like the spinal cord’s 13.5 minutes) without increasing the “slow” compartments usually related to limb pain. More recent research is concerned with the damaging effects of bubbles on the endothelial lining of blood vessels. Reduction of such bubbles will prevent this.

Doolette: Recreational diving within no-decompression limits conducted with a shallow safety stop has a good safety record. There is insufficient evidence to suggest a deep stop offers any advantage. 

Meet Our Experts

Peter B. Bennett, Ph.D., D.Sc., is executive director of the Undersea and Hyperbaric Medical Society. A former tenured professor at the Duke University Medical Center department of anesthesiology and director of the Duke Center for Hyperbaric Medicine and Environmental Physiology, he founded Divers Alert Network in 1980.

David Doolette, Ph.D., is a research physiologist at the U.S. Navy Experimental Diving Unit, responsible for development and testing of decompression procedures.

Christian Gutvik has a master’s degree in engineering cybernetics with specialization in mathematical modeling. He works with the Baromedical and Environmental Physiology Group at the Norwegian University of Science and Technology, where he is the main modeler of Copernicus, a decompression model based on ultrasonic detection of vascular bubbles.

Simon Mitchell, BHB, MBChB, DipDHM, DipOccMed, Ph.D., is an avid technical diver and a physician specializing in occupational medicine, hyperbaric medicine and anesthesiology. He received a doctorate for his work on neuroprotection from embolic brain injury.

David Southerland, M.D. is a captain in the U.S. Navy Medical Corps and the senior medical officer for Naval Diving and Salvage Training.

© Alert Diver — Q1 Winter 2010