Defensive Dive Profile Planning




Dive computers generally work as designed, but the mathematical algorithms do not evaluate many of the factors that can alter the decompression risk of a given exposure.


Limited gas supply and less-than-stellar thermal protection once worked to cap decompression stress for the typical diver. Increased choices for gas supply and improved thermal protection have enabled divers to go further and longer. Dive computers have likewise expanded the freedom to explore. The square profiles of the past can be replaced by complex dive profiles that are easily tracked by these little boxes.

Decompression safety may be achieved by staying within dive computer or dive table limits, but decompression sickness (DCS) can develop even after dives that remain within prescribed limits. Dive computers generally work as designed, but the mathematical algorithms do not evaluate many of the factors that can alter the decompression risk of a given exposure. Building in modest buffers at every step of the diving process can help ensure good outcomes. This article will discuss concepts important for conservative practices, some of the pitfalls that must be overcome, and practical strategies for defensive dive-profile planning and implementation.
Conceptual Control
Know the risks. Diving is used for both work and pleasure, and in the vast majority of cases it concludes without problems. The risks, though, should not be ignored. Understanding them is a critical step in preparedness. Early recognition of issues can resolve many before they become troublesome.

Take responsibility for your safety. Do not give any other person or any device complete authority over your activity. Some divers will follow a divemaster they just met without question; others will follow a computer without thinking about what it does not know or will expect it to get them out of trouble they may create. Any person or device can make mistakes. Make sure that you are actively and intentionally involved in every step of every dive, able to lead yourself when necessary.


The popularization of the safety stop was probably the most significant evolution in decompression safety for recreational diving in the past 30 years.


Understand the available tools. Reliance on dive computers is now the norm for many divers. While you do not have to be a decompression modeler to dive safely, it is important to have a clear conceptual understanding of how the decompression algorithms that you will rely on work. It is equally important to know what they do not consider and that they can be wrong. Ask questions, learn, and develop plans for the "just in case" events.

Evaluate information critically. One of our human quirks is the abundance of faith we put in what appears on a screen or gauge, such as a gas gauge or dive computer screen. This can even spill over to what we read on the Internet, regardless of provenance. The tendency for blind faith must be kept in check. Maintain an open, critical mind to fully assess information and use it appropriately.

Know your risk tolerance. Risk is inherent to life; it cannot be fully avoided if one is to live, but it can be managed. Tolerance varies among individuals and situations. Generally, tolerance increases as the perceived benefit increases and decreases as the severity of the potential injury increases. Knowing your own comfort zones will help you plan and act to stay within them.

Maintain a safety-oriented mental state. When rules are broken or limits are violated with no obvious repercussions, there can be a gradual shift away from thinking of them as important. This can lead to "normalization of deviance," in which something once thought of as unacceptable becomes acceptable. The problem is that decompression stress is a relatively invisible hazard. We do not change color as we fill with inert gas, and decompression stress may not be perceived until a critical stage is reached. We can feel good right up to the point that we feel very bad. Vigilance is required to maintain good practice.


Choose dive partners whose risk tolerance, goals and attitudes align with your own.


Reinforce safety messaging. Thinking or teaching "do this or get hurt" can be counterproductive to safety-oriented practice. As described above, the first time the line is crossed without injury the rule will become less important. After it has been crossed a few times the rule may seem irrelevant, or the individual may perceive himself or herself as being endowed with special protection. Both of these viewpoints can lead to poor choices. Flipping the focus to "do this and be safer" can provide much healthier reinforcement. When nothing bad happens, the positive benefits of the practice are reinforced. Both peace of mind and good practice are promoted.

Avoid mission creep. Even the best intentions can be pushed aside by trouble-free diving and personal comfort. This can be exemplified on multiday dive trips. The intensity of diving frequently increases as the trip continues. It is not uncommon for a person developing DCS during a trip to describe their most conservative practice as their norm. Electronic dive logs, however, frequently show an erosion of safety buffers over successive days.

Pick your partners well. The mindset and practice of others in your group can radically affect your risk. Choosing those with complementary goals, objectives and attitudes can help ensure that the activity remains within your comfort zone. If someone you are diving with pushes you beyond your comfort zone, remember the first two rules: Know the risk, and take responsibility for your own safety.

Use tools to defend your practice. Selecting appropriate conservatism settings on your dive computer can reduce the need to argue over no-decompression limits or decompression profiles. Going back to the faith we often have in computers, differences in the selected settings may prompt discussions that help everyone gain insight. A critical mind is essential at this point to weigh the merits of the often heartfelt beliefs of those participating in the debate. Understanding the available tools is important for understanding the options and levels of conservativism. An article about gradient factors in the Fall 2015 issue of Alert Diver might be helpful to this end.1
Practical Strategies
Solid knowledge, awareness, critical thinking and smart partner selection provide the foundation for good diving practice. Implementation requires further thought. Employing a number of small buffers can produce a web of protections that can mean giving up little in the way of opportunity while maintaining a high degree of conservatism.

The

Bubbles can be seen in a diver’s heart on the screen of a portable
ultrasound device. The right heart is shown on the left side, with the
right ventricle above the right atrium.
dive profile is the single most important determinant of the ultimate decompression risk of a dive. The shift from square profiles to multilevel profiles can produce powerful advantages.

Going deep increases the rate of inert gas uptake and the ultimate amount to be eliminated, but going to the extremes of one's training can be enticing. Multilevel diving offers a good way to scratch that itch while maintaining good decompression safety. Choosing sites appropriate for multilevel dives is a great way to start. In the simplest case, swimming outbound at one depth and back at a shallower depth can limit inert gas uptake and extend the controlled inert gas elimination period. Decompression stress is minimized, and the diver can experience different zones during a single dive.

Out and back is fine for many recreational exposures, but as maximum depths increase, it becomes increasingly important to spend progressively more time at progressively shallower depths. Dive sites that make this easy facilitate optimized dive profiles.

The high relative rate of pressure change in the shallowest zone makes it critical in determining the overall decompression stress. Next to backing off the intensity of a dive, the most important decompression safety buffer is time spent in the shallow regions during ascent. For much recreational diving this can be considered the depth range shallower than 25 feet. The popularization of the safety stop was probably the most significant evolution in decompression safety for recreational diving in the past 30 years. The three-minute stop is good, but it is even better if it follows a progressive multilevel profile and is extended as gas supply and conditions allow.

Figure 1 shows the dive profile of a decompression dive in which the diver completed decompression 10 to 20 feet deeper than the dive computer algorithm required and then extended the time spent in the relatively shallow zone after the obligatory stop period before surfacing. This may be a more conservative ascent profile than was required, but the worry-free endpoint is reflected in the absence of bubbles seen in the heart during postdive monitoring.


Figure 1.


There are times when over-applying well-intended rules can get in the way of safety. For example, divers are frequently taught to surface with a reserve of 500 psi in their tanks. If the concern for surfacing with this reserve becomes so compelling that safety stops are abbreviated, the rule becomes counterproductive. Dives should be planned to be finished with a reserve of air, but using some of that supply to extend a safety stop is probably a high-benefit compromise. Having said this, any deviations from established rules should be discussed postdive and actions should be taken to avoid unnecessary future violations.

Another area in which safety can be put at risk is reverse dive profiles. If all other things are equal, planning the deepest dive first makes sense in that it is consistent with good practice for multilevel diving. But all things are frequently not equal, and, as far as we know, the body does not actually register whether inert gas accumulates at pressure A or pressure B; the important thing is the total accumulation and the subsequent pressures achieved to eliminate it from the body. Practically speaking, the order of the maximum depth between two dives can be unimportant. Concerns arise when the "deepest dive first" rule is applied with such rigor that an unnecessarily deep dive is conducted for no other reason than to allow a second deep dive when it must be scheduled later (for example, to meet a suitable tide state). Mindless fixation on rules can create problems. Dive planning should be thoughtful.

Surface intervals also need to be considered. There is a trend toward progressive shortening, probably as a function of mission creep and perceived efficiency. Surface intervals are important for inert gas elimination. The minimum reasonable surface interval will vary with the exposure, but focusing on the minimum is not conservative practice. If short surface intervals are necessary, the severity of the dive profiles should be moderated.

The ends of dive trips often require consideration of the final surface interval before flying. Flying-after-diving plans are often based on guidelines produced at a DAN workshop.2 The recommended minimum preflight surface intervals were developed from the available data: 12 hours after single dives within no-decompression limits; 18 hours after multiple dives per day or multiple diving days; and "substantially longer than 18 hours" after decompression dives. An added challenge is that these guidelines apply only to aircraft cabin pressures equivalent to altitudes in the 2,000- to 8,000-foot range. Additional buffers are recommended since it cannot be known with certainty whether cabin altitudes might exceed this range. Planning a surface interval of at least 24 hours following diving is a good rule of thumb, and an extra safety buffer can be gained through more conservative exposures on the final day of diving. Driving to altitude postdive can similarly induce additional decompression stress; it also requires appropriate pre-travel surface intervals.

Ultimately, the best way to protect yourself and your partners is to build conservatism into all aspects of dive planning and execution. The net effect can be a high level of safety, often with relatively little compromise in your diving experience. When good habits are established and peace of mind maintained, the best diving in the world is possible. The thoughtful and well-informed diver remains the most important factor in producing safe outcomes.
References
1. Pollock NW. Gradient Factors: A pathway for controlling decompression risk. Alert Diver 2015; 31(4): 46-9.

2. Sheffield PJ, Vann RD, eds. DAN Flying After Recreational Diving Workshop Proceedings; May 2, 2002. Durham, NC: Divers Alert Network, 2004.

© Alert Diver — Q1 Winter 2016