Decompression Sickness |
Decompression Sickness
First described in 1841, decompression sickness has gradually become better understood. Sport divers have provided a large body of material to study causing us to be able to learn more about the illness. It's safe to say that DCS is caused by the production of nitrogen bubbles in the circulation, and this is related to the depth and time of a dive and to rate at which the diver ascends from depth. DCS and AGE combined form what is known as "decompression illness".
Called "bends" by early investigators, it is now classically divided into Type I, Type II and "Type III" (a phrase coined by Bove and Neumann to describe a combination of DCS and arterial gas embolism). Type I DCS includes cutaneous manifestations and minor joint pain, or "pain only"; Type II includes severe symptoms related to the cardiopulmonary and neurological systems. Type III is a combination of AGE and DCS with neurologic symptoms.
Pain syndromes spot the pain in the limbs-not the central skeleton. It is dull, difficult to characterize and localize and is located in the shoulders, elbows and hands in divers. Compressed air workers have more pain in their lower extremities.
It is caused by bubbles, intravascular and extravascular with large gas stores in the fatty bone marrow. This is a cause of dysbaric osteonecrosis.
Neurologic Syndromes are increasing in sport divers and the spinal cord is the most commonly involved site. Symptoms include abdominal, low back, lower extremity pain, weakness and loss of feeling and function. Cerebral involvement is much more common than previously thought and may account for a portion of the "spinal cord" lesions. Peripheral nerves can also be involved causing numbness, limb pains and weakness.
Early Treatment
Recognition Symptoms usually appear 15 minutes to 12 hours after surfacing
Signs:
Blotchy rash
Paralysis or weakness
Coughing spasms
Staggering or instability
Unconsciousness
Symptoms:
Tired feeling
Itching
Pain, arms, legs or trunk
Dizziness
Numbness, tingling or paralysis
Chest compression or shortness of breath
Early Management:
Immediate oxygen breathing, continue even if person improves markedly
Stabilize patient the same way as for Air Embolism
Urgent recompression after stabilization in trauma facility
Early recompression treatment for all forms of decompression sickness
There is a lightweight, portable recompression facility that would appear to be ideal for the liveaboard or dive operation far from a fixed-base chamber. This is the 'SOS Hyperlite Stretcher'.
Exercise and Decompression Accidents
The inveterate runner or hiker on a dive trip often wants to know if there is any harm in exercising before or after diving. Of course, the problem is whether or not bubbles are induced by pre-dive exercise or by exercise immediately after a non-saturation dive.
The scientists at NASA are understandably interested in this aspect of decompression and have done studies to elucidate this problem with their extra-vehicular activity astronauts. To determine the answer to these questions, an elegant study was done by Dervay, J, MR Powell, and CE Fife, " Effective lifetimes of tissue micronuclei generated by musculoskeletal stress" in Aviat. Space and Environ. Med., 68 (Suppl), A12. (1997); Dervay, J, MR Powell BD Butler, and CE Fife. From Doppler bubble determinations in this study the following can be deduced:
All strenuous activities for about four hours prior to scuba diving will increase micronuclei, thereby increasing venous gas emboli. Musculoskeletal activity will definitely increase the number of tissue micronuclei. That is an experimental fact. These micronuclei will persist for about two to five hours – again an experimental fact. There are no studies that show clearly what happens to these bubbles when they are compressed by a dive.
It is thought that if one were to put four restful hours between exercise and diving and six between diving and exercise, a diver should be in good shape in terms of absent bubbles. That is probably sufficient for non-decompression dives.
A web site that has some diagrams and explains bubble growth can be found at:
If one were to schedule their exercise activities in the morning and diving in the afternoon, there should not be a problem with this situation. One would not need to take off a whole day as far as exercise is concerned.
In regard to the hot shower or hot tub question post-dive, there is an increase in blood flow to the skin to eliminate body heat. When this happens, blood is shunted away from muscles (a ”steal”) and flows to the skin. We have increased perfusion to the skin but that is not of much help in prevention of DCS.
This is thought to be harmful to the diver attempting to off-gas due to the shunting of blood away from the musculoskeletal areas that need to have blood flow promoted. This is done by moderate, non-straining exercise, avoiding running, climbing ladders and lifting SCUBA tanks. However, non-strenuous movement is thought to be helpful - sleeping should be avoided..
There are mixed reports of exercise causing increased DCS in altitude exposed individuals (Pilmanis). On the contrary, there is evidence that exercising while decompressing is helpful in reducing decompression accidents. Muth et al, have found that exercise increases the elimination of nitrogen post-dives that are non-DCS producing. Jankowski has shown that exercise during decompression reduces the amount of venous gas emboli.
Patent Foramen Ovale
PFO (Patent foramen ovale) is a persistent opening in the wall of the heart, which did not close completely after birth (opening required before birth for transfer of oxygenated blood via the umbilical cord). This opening can cause a shunt of blood from right to left , but more often there is a movement of blood from the left side of the heart (high pressure) to the right side of the heart (low pressure).
People with shunts are less likely to develop fainting or low blood pressure with diving than are obstructive valve lesions (such as mitral valve stenosis or aortic stenosis), but are more likely to develop fluid accumulation in the lungs from heart failure and severe shortness of breath from the effects of combined exercise and water immersion.
Ordinarily, the left to right shunt will cause no problem; the right to left shunt, if large enough, will cause low arterial O2 tension (hypoxia) and severely limited exercise capacity. In divers there is the risk of paradoxical embolism of gas bubbles (passage of bubbles into the arterial circulation) which occur in just about all divers in the venous circulation during decompression.
Blood can flow in both directions with Intra-atrial shunts at various phases of the cardiac cycle and some experts feel that a large atrial septal defect (PFO) is a contra-indication to diving. In addition, a Valsalva maneuver, used by most divers to equalize their ears during descents and ascents, can increase venous atrial pressure to the point that it forces blood containing bubbles across the PFO into the arterial circulation. Thus the usual filtering process of the lungs is by-passed.
Dr. Fred Bove, a Temple University cardiologist, did a search of the literature for patent foramen ovale in relation to diving and diving risks. Echocardiography is the tool of choice in making the diagnosis of PFO. However, it's probably not a good idea to do an echocardiogram on all divers because of the cost/benefit ratio. If you personally are concerned or are having some of the symptoms of decompression illness that are undeserved, then a bubble contrast echocardiogram should be done. Bubble contrast echocardiography appears to be the most sensitive method for detecting a shunt while color flow doppler appeared to be a poor means of detecting the shunt in a transthoracic echo.
Flying After Diving -- Cracking the DCS Code
Hyperbaric researchers shed new light on the risks of flying after diving
By Anthony K. Almon
It's been a wonderful week - probably the best dive vacation you've ever taken. The fish have been plentiful, and the reefs are teeming with activity. There's a dive this afternoon at 5 p.m., and you really wanted to have one last look at that enormous school of jacks out on the reef. There's just one minor problem. Your flight back home is at 7 a.m. tomorrow morning, and you don't want to suffer decompression sickness (DCS) because you flew too soon after diving...
Should you go on the dive? The U.S. Navy tables recommend that you wait at least two hours before you board a plane after diving; the U.S. Air Force says you should wait 24 hours; it is recommended a 12-hour minimum surface interval before flying. Which guideline should you follow? If you fly after diving, what you really want to know is:
What are my chances (or probability) of experiencing a decompression injury; and
If I do get hit, how severe might this injury be?
These two factors, the probability and the severity of injury, help determine the risk you are willing to take. So your next question might be: How much risk is right for you? You need to consider many factors before making your decision. The number of previous dives you've made on this dive trip, your general health and your age are but a few points to ponder. What if you were returning to a location where a chamber is available? Would you be willing to take more of a chance? The risk that some divers are willing to accept may be absolutely unacceptable to you.
What we want to know is how the probability of DCS changes as we increase the length of the preflight surface interval. Our first series of experiments consisted of one dive followed by a flight. A dive to 60 feet sea water (fsw)/18 meters for 55 minutes was performed, followed by a predetermined surface interval. The altitude exposure of the flight was 8,000 feet/2,440 meters for four hours. Eight thousand feet is the maximum cabin altitude allowed in a commercial aircraft.
To produce meaningful information about DCS and the preflight surface interval, some DCS must occur. This information is used to estimate the probability of decompression sickness. Note that we cannot actually measure probability - we can only estimate it statistically from experimental data. Decompression sickness that occurs in laboratory experiments is generally mild and easily treated, but we have a responsibility not to expose our volunteer research subjects - who are recreational divers - to experiments that are likely to produce DCS that does not resolve completely. The only way we could completely eliminate risk is not to conduct the experiments at all, but this leaves us where we started - with uncertain information about flying after diving. And without research, thousands of people would continue to experiment on themselves by flying home from vacation without the benefit of medical supervision provided in a laboratory study.
The protection of volunteer divers is a very important issue. We subject people to experimental protocols that we know will produce some DCS, so what do we do to protect these volunteers? First, before anyone can participate in one of our studies, a comprehensive medical history and physical are performed by a hyperbaric physician. Anyone with severe medical problems is not allowed to participate. Some of the problems which may disqualify a diver are neurological problems, chronic injuries and lung conditions that may cause gas trapping. Additionally, because diving is a hazard to an unborn child, pregnant divers are not permitted, and all women of childbearing potential must undergo a pregnancy test.
The next issue is to decide when DCS has occurred. Since there is no lab test for DCS, we must diagnose it by the presence of signs and symptoms. Unfortunately, the milder forms of DCS may have signs and symptoms that are the same as some daily-life aches and pains from which we all occasionally suffer - particularly as we get older. Because of this uncertainty, we ask divers to report any and all symptoms, no matter how mild or seemingly unrelated to the experiment. After the experiment, these are broken into three categories: "Not DCS," "Ambiguous DCS" and "Definite DCS."
"Not DCS" refers to signs or symptoms that are clearly unrelated to the experiment, such as the diver who sprained his ankle playing basketball during the preflight surface interval. (We no longer allow sports during the surface intervals.) "Ambiguous DCS" refers to signs and symptoms that may have lasted only a short time, were very mild and uncertain in the judgment of the hyperbaric physician, or may not have responded to recompression therapy. "Definite DCS" refers to clear and certain signs and symptoms that improve or resolve completely with recompression.
Within the category of Definite DCS there are two types: pain-only and neurological. We generally worry more about neurological symptoms than joint pain. Neurological signs and symptoms that have occurred in our studies have included numbness, tingling, weakness, confusion and visual disturbances, all of which resolved with recompression. Obviously, we want a lower probability of neurological DCS than either joint pain or Ambiguous DCS.
One of the most important factors in providing diver protection is the diver himself. Complete and timely reporting of any and all signs and symptoms ensures the earliest possible recompression, if needed, and the collection of good-quality data. To reduce the chances of testing a preflight surface interval that may have too high a DCS probability, we establish acceptance and rejection rules that define how many times we should test a surface interval based on the occurrence of Definite DCS in previous tests. These rules must be approved by the Duke Medical Center Institutional Review Board, which oversees all human experimentation.
The acceptance/rejection rules for a surface interval in the Flying After Diving study, based on Definite DCS, are:
Accept
0 pain-only incidents in 23 trials
1 pain-only incidents in 25 trials
2 pain-only incidents in 46 trials
Reject
Any neurological incident
2 pain-only incidents in 10 trials
3 pain-only incidents
Note that acceptance of a surface interval applies only within the Flying After Diving study and does not imply acceptance for actual use in recreational diving. The three-hour surface interval had one pain-only and two neurological incidents of Definite DCS, and seven incidents of Ambiguous DCS. A single incident of neurological Definite DCS occurred at six and at nine hours, and one Ambiguous DCS incident occurred at six hours. Because of neurological DCS, the surface intervals for three, six and nine hours were rejected. Ten-, 11- and 12-hour surface intervals, however, were considered acceptable according to the acceptance/rejection rules described above. The 10-hour surface interval produced one ambiguous symptom in 23 trials, but the 11- and 12-hour surface intervals had no symptoms in 23 and 27 exposures.
As with all research, proof lies in numbers. The more trials made at a given surface interval increases the confidence in the accuracy of the data. For example, if you performed 100 trials, you would be more confident in the accuracy of your results than if you performed only 10 trials.
There were three Definite DCS-incidents in 36 trials of the three-hour surface interval, for an 8.3 percent DCS occurrence.
When the surface interval was increased to six hours, there was one neurological incident in six trials, for a DCS occurrence of 16.7 percent.
At nine hours, there was one neurological incident in seven trials, for a 14.3 percent DCS occurrence.
The DCS occurrence was higher at six and nine hours than at three hours because the six- and nine-hour intervals had to be rejected after a single case of neurological DCS. The three-hour surface interval, on the other hand, was not rejected until 36 trials were conducted. That may be due simply to luck, but this is a classic example of having a very low level of confidence in the data for the six- and nine-hour surface intervals because there were so few trials. It is reasonable to suppose that if we could conduct more trials at six and nine hours, the DCS percentage would be lower.
The correlation between the percentage of Ambiguous DCS and the surface interval was statistically significant, suggesting that the ambiguous symptoms were a mild form of DCS. At a three-hour surface interval, for example, there was a 10 percent estimated risk of Definite DCS and nearly a 20 percent estimated risk of Ambiguous DCS. The observed DCS percentage at six and nine hours does not follow the estimated trend due to the low number of studies that were conducted.
You could decide to wait for 12 hours before your flight, and have an estimated 1 percent risk of Definite DCS and approximately 2 percent risk of Ambiguous DCS. If you want to keep your level of risk at zero, then don't dive or don't fly. Every time you dive, you are subjecting yourself to a risk of DCS. No tables guarantee absolute safety.
In making decisions about risk, you can look to existing guidelines and practical experience for clues, such as the 12-hour flying-after-diving guideline. The estimated DCS probability for this surface interval is about 1 percent. Another clue is the estimated probability for a 55-minute dive to 60 feet. The estimated risk of DCS is 0.5-1.0 percent for this dive. We have far to go before we are comfortable with our estimates of DCS probability, but the information we've gathered thus far is already making decompression safety less mysterious.
If these decisions seem arbitrary to you, keep in mind that there is no "right" or "wrong" when deciding whether a profile is "safe" or "unsafe." Whatever preflight surface interval you choose, whether by guess or by probability estimate, there will be some DCS risk - there is no way to avoid it.
Our conclusions shouldn't be tried "in the field" as gospel. All of our exposures at present are dry, resting dives under carefully monitored conditions. In no way should these results be construed as guidelines at this point. Wet, working dives may give very different results. Further testing is required before definitive guidelines can be developed.
Well, we haven't succinctly answered the question of "How long should I wait after diving before I fly?", but that wasn't the real purpose of this article. We wanted to make you think about what "safety" is and how it is determined.
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