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March 2005 Vol. 20, No. 3   RSS Feed for Undercurrent Issues
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What About Deep Stops?

— a case of the tail wagging the dog?

from the March, 2005 issue of Undercurrent   Subscribe Now

Deep stops — that is, the practice of making one or more short decompression stops at much greater depths than recommended by traditional decompression tables or a dive computer — have become standard procedure among technical divers. Many sport divers, too, have been following suit. NAUI and DAN International are even recommending that recreational divers incorporate deep “safety” stops into their diving profiles to improve their decompression safety.

“. . . The theory behind it tells us it should
work. We implemented it, we dive it, and it has
worked well so far.”

Many divers believe that profiles incorporating deep stops help thwart the bends. But much of the research supporting deep stops sounds more like marketing hyperbole than legitimate science, leaving in doubt whether there is sufficient data to support deep-stop proponents’ claims or even whether deep stops actually benefit divers on recreational exposures.

What Works, Works

Ideally, the scientific method — basically formulating a hypothesis and testing it scientifically — should be used to decide whether there are benefits to deep stops. That’s difficult when humans are the subjects, so dive procedures are often devised by getting empirical data about what works, then building a hypothesis to explain why it works. Deep stops are no exception.

The initial anecdotal reports about deep stops “working” came from Hawaiian fishermen (who made up to 12 dives per day, some more than 200 feet) and northern Australian pearl divers, who made 200-300 ft., hour-long dives twice a day, 6 days a week. Both groups had few bends cases.However, the person generally credited with bringing deep stops to the attention of the diving community is Hawaiian ichthyologist and tech diver, Dr. Richard Pyle. Some researchers still refer to deep stops as “Pyle stops.”

In the 1980s, Pyle often dived deeper than 200 ft. on air. Upon surfacing, he was fatigued frequently and had flu-like symptoms, indicating decompression stress. However, when he was collecting tropical fish, the symptoms were generally not present. To bring the fish to the surface, he made short stops along the way to puncture their swim bladders. “I didn’t know why the stops worked,” he said. “But they seemed to make a real difference.” At the time, Pyle’s observations contradicted the conventional wisdom, which essentially advised divers to get out of deep water as quickly as possible to prevent further gas loading.

Pyle published his findings in 1995. A year later, French commercial diving pioneer Andre Galerne reported that he had observed a decrease in DCS incidents when an additional deep decompression stop was added.

The Advent of Tiny Bubble Models

At the same time, physics graduate student cum tech diver Eric Maiken was investigating the research of Dr. David Yount, a professor of physics at the University of Hawaii. Yount’s work showed that under pressure tiny “microbubbles” regularly formed in liquids like those in the human body. In the mid-1980s, Yount and researcher Don Hillman developed a new decompression model which they called the “Variable Permeability Model” (VPM) that takes into account these tiny bubbles. Today there are many bubble models in use, and bubble algorithms run on the dive computers of major manufacturers.

Why We Think Deep Stops Work

Bubble models offer a plausible explanation of why deep stops work: a diver’s tissues absorb pressurized gas at varying rates, depending on the tissues. Traditional decompression tables and dive computers, which are based on Haldane’s work, seek to model the uptake of this gas and produce a decompression schedule that allows the dissolved gas to escape through the diver’s lungs (“off-gas”) without forming bubbles in the blood and tissues.

However, tiny bubble seeds or “micronuclei” also regularly form in the body as a result of natural mechanical forces, such as moving one’s joints. These micronuclei are precursors to the actual bubbles that can cause decompression illness. If the pressure is suddenly reduced (as when a diver ascends), the micronuclei can grow into larger, symptomcausing bubbles that have to be “treated” via traditional decompression or safety stops, where they shrink back down and collapse. Bubble models track them through the pressure changes. The goal is to determine ascent profiles that prevent micronuclei from expanding rapidly and turning into full-fledged bubbles.

According to the these models, making one or more deep stops provides time for these micronuclei to stabilize or collapse. That may be why divers like Pyle who practice deep stops report that they feel noticeably better. “A good feeling likely means a better exposure,” explains Dr. Bill Hamilton, who pioneered the development of mixed gas tables. “I wouldn’t sweep those reports under the rug. It’s not very quantitative but it’s valid.” Dive fatigue and flulike symptoms may be caused by bubbles that trigger a chemical cascade in the body.

Today, most decompression models that account for bubble dynamics credit divers for making deep stops and allow them to reduce time at shallower stops.

But researchers don’t know what is actually happening in a diver’s body or at what depths divers should stop. Like their Haldanian counterparts, bubble or dual-phase models are simplified abstractions of what might be occurring in a diver’s body, but they are plausible. Hamilton says that “The concept of dealing with microbubble growth — that is beating the bubbles instead of treating them — makes sense. The problem is that quantitatively, no one really knows how to do it. You need to collect data and profiles to know if it really works.”

Show Me The Data

Is there supporting data? It depends who you talk to. Dr. Bruce Wienke, program manager for Nuclear Weapons Technology at Los Alamos and decompression physicist, certainly thinks so. Wienke has developed a website he calls “Reduced Gradients Bubble Monitoring (RGBM), a decompression resource for the scuba diver (www.rgbmdiving.com), that clearly touts deep stops. He not only claims that making deep stops consistent with his model (which was licensed by NAUI) works, but also that it enables divers to reduce their overall decompression time. “Deep stop models reduce bubble counts in recreational divers — and therefore reduce risk,” he explained by phone.

When asked how he calculated the reduction, he explained that his model is fitted to his database of 2,500 dives (95% of them tech dives, 20 resulting in DCI) conducted on RGBM/bubble models. The dive profiles were reported by participating divers (as opposed to data taken from dive recorders, which log divers’ actual in-water profiles with a high degree of accuracy). Wienke sells RGBM tables ranging in price from $25-$50 from the website.

Then there is Eric Maiken’s model, the VPM-B, marketed as V-Planner (www.v-planner.com) at a price of $60-80. Maiken said in a phone interview, “We don’t claim to have validated the model. The theory behind it tells us it should work. We implemented it, we dive it, and it has worked well so far.” Maiken also said that he and his colleague Erik Baker receive no financial remuneration for their work.

Maiken and Baker estimate that thousands of dives have been made on their model. “The model works well on technical dives up to 300-350 feet for less than 50-60 minutes of bottom time,” Baker told me. “However, dives longer and deeper than that have been a mixed bag.” Like Wienke, V-Planner, a commercial enterprise run by Ross Hemingway, also maintains an online database of dives that currently contains around 1,000 selfreported VPM profiles.

Rules for the NAUI Recreational RGBM Tables
(Air, EANx32, and EANx36)

1. make a 1-minute safety stop at half the depth between
bottom and surface for depths in excess of 40 fsw (12 meters);
2. make a 2-minute safety stop in the 15 fsw (5 meter) zone
after the deeper safety stop;
3. make a 3-minute safety stop in the 15 fsw (5 meter) zone
for all bottom depths shallower than 40 fsw (12 meters).

All other NAUI RGBM Recreational Table protocols are unchanged and unaffected by the new deeper safety stop protocol.

 

Duke University’s Dr. Richard Vann, the medical director for the Divers Alert Network, is supportive of the deep stop idea. “The theory is appealing because we know that microbubbles form. If DCI is limited by preventing bubble growth, then deep stops matter,” he told me. “The difficult part is trying to find evidence. The trouble is that there is no data.”

Vann said that the selfreported dive profiles offered as evidence by Wienke and others do not constitute scientific data because dive profile logs do not accurately reflect the actual minute-to-minute depth profiles. Furthermore, trained medical personnel must evaluate divers after a dive for evidence of DCS.

DAN has accumulated a database of approximately 100,000 recreational dives recorded by in-water dive recorders. Vann and his colleagues estimate the recreational diver DCS rate at 4 per 10,000 (or 40 per 10,000 in cold water). Vann is trying to determine whether the data support the reduction of DCS via safety stops, saying the real benefit may be forcing divers to exercise buoyancy control during ascent and not rocket to the surface.

Dr. Wayne Gerth, a research physiologist at the Navy Experimental Diving Unit (Panama City, FL) agrees with Vann and says there is no quality data to support the notion that deep stops offer a benefit. “The tech community may be okay with using anecdotal data, but it’s not good enough for the Navy to use to make judgments for its divers.” Gerth is responsible for developing the next generation of Navy dive tables. Beginning in April, his team will undertake more than 700 extreme air dives. The data from those, coupled with the existing NEDU database, will help them determine which of two models — one Haldanian, one a bubble-based model — is best suited for the Navy’s new air diving tables.

DAN founder Dr. Peter Bennett, now a part of Dan International, grew interested in deep stops after reviewing DAN’s recreational diving data, which show that roughly 65% of all recreational bends incidents are Type II, or neurologically based. Bennett and his colleagues hypothesized that divers might reduce the risk of neurological bends (believed to affect fastsaturating blood and tissues) if they slowed their ascent. They monitored 1,418 recreational dives to record bubbling, a measure of possible decompression stress. They learned that a one- to three-minute deep stop, coupled with a safety stop at 15 feet, reduced bubbling when compared with a slower ascent with no deep stop. He told me that he and his team believe that a one-minute deep stop (recommended by NAUI) doesn’t appear to be long enough and that 2-3 minutes seems necessary. DAN International plans to do further study on this question.

“My goal,” explained Bennett, “is to eliminate neurological DCI from recreational diving.” Bennett thinks deep stops are a good idea but is less enamored with specific decompression models. “Models are based on our theoretical concepts of what gases are doing in the body. The trouble is that we don’t know what happens in the body — it just doesn’t add up to mathematics. That’s why our approach is to focus on the data.”

The conclusions?

Though the evidence thus far is mostly anecdotal, it’s compelling enough that it probably makes good sense for tech divers to incorporate deep stops into their protocol when making deep decompression dives. Making deep stops, though, is a separate issue from diving a bubble-based table or shortening shallow decompression stops. Tech divers like Richard Pyle often add deep stops into their existing tables as an additional conservative factor and still make the required shallow water stops based on traditional tables.

For recreational divers, where the rate of DCI is already extremely low, the case is less clear. Certainly being conservative on deep dives, cold water diving, and multi-day, multi-dive exposures is always a good idea. However, the additional time spent at depth during the deep stops should be factored in when calculating shallow stops. Failure to do so actually decreases conservatism by adding additional time at depth that isn’t balanced by shallow deco time. And of course, making a deep safety stop should never get in the way of more important operational considerations like surfacing with a sufficient supply of air (or nitrox) in your tank.

The bottom line? Vann puts it this way. “It may have a benefit. The trouble is that incident rates are so low, the benefit may be impossible to measure.”

Freelance writer Michael Menduno, founder of “aquaCORPS: The Journal for Technical Diving” (1990-1996), coined the name “technical diving.” He can be reached at michael@menduno.com

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