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What factors increase/decrease the risk of DCI?

Discussion in 'Ask Dr. Decompression' started by rcohn, Sep 26, 2000.

  1. rcohn

    rcohn Manta Ray

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    I have a question about factors that contribute to the likelihood of DCI. I have heard various opinions lately from different authorities that are often in conflict with what is traditionally taught. What does the latest scientific research show? How much of the old guidelines are based on disproved theories?

    For example:

    http://www.divernet.com/medical/ddoc896.htm
    Diving after a bend
    by Dr Peter Wilmshurst

    “Traditionally, it was taught that by being female, older, obese, dehydrated, drinking alcohol, being hot, being cold, or working hard during the dive, one could predispose oneself to decompression illness. There is no good evidence that any of these is implicated in decompression illness when it occurs inside the tables in amateur divers. If they start with an unproven premise, it is no wonder that some doctors reach unsound and inconsistent conclusions about the cause of a diver's bend.…

    The causes of decompression illness are:
    • Failure to follow a safe decompression profile dictated by an accepted algorithm.
    • The presence of an intracardiac right to left shunt (usually a patent foramen ovale, but sometimes an atrial septal defect or a pulmonary arteriovenous fistula). All of these allow bubbles to pass from the right side of the heart through to the arterial circulation, where they can be distributed around the body causing some, but not all, types of decompression illness.
    • Lung disease predisposing to pulmonary barotrauma, which allows bubbles to enter the arterial circulation.
    • Unknown causes.”

    DAN has also said that dehydration is not a factor in the bends in a recent article, ”Although dehydration doesn’t cause decompression illness, increased fluid losses may contribute if a problem occurs…” (Joel Dovenbarger, “Exercise Training and Scuba Diving”, Alert Diver, May/June 2000, pg. 21)

    There is a belief in the tech diving community that increased physical fitness will improve a divers ability to off-gas allowing more aggressive decompression. This was supported by Dr. Bill Hamilton in Alert Diver, May/June 2000, pg. 42:

    ”There is no doubt that fitness needs to be emphasized more and for divers of all levels. In addition to the obvious benefits of enhanced aerobic ability and athletic performance, a good level of fitness helps in decompression. A diver who is extremely fit can decompress more aggressively than someone who isn’t fit. An overweight diver may have more problems offgassing because of less efficient circulation and because of the way fat takes up gas.”

    So Dr. Deco, can you separate the science from the fiction?

    Thanks,

    Ralph Cohn
     
  2. Dr Deco

    Dr Deco Medical Moderator Staff Member

    # of Dives: I just don't log dives
    Location: Issaquah [20 miles east of Seattle], Washington.
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    [sp]Thank you for your question, which is by no means a simple one and cannot get a short answer. I must add at the onset that the ideas concerning DCI are as extensive and varied as the researchers in the field. Since this disorder of DCS was first found around 1850, ideas have come in and out of vogue. These are some observations and ideas that have been developed at JSC/NASA, and some of these are applicable to recreational SCUBA divers. The observations that we make, however, apply to altitude DCI, although much of this information is applicable to divers.
    [sp]In the past, the causes of DCI were difficult to elucidate, since the gas loads could be very different in each diver at the close of the bottom phase. In the case of altitude decompressions, all individuals initially have the same gas load (that is, they are all saturated at sea level pressure) and thus variations are caused either by the person’s physiology, or what they did before or during the depressurization phase.

    [sp]We can separate factors into [1] individual characteristics, [2] activities before depress and [3] activities during depress. Let us look at each of these in turn:

    1. INDIVIUAL CHARACTERISTICS

    [sp][a.] It is fairly clear from data that, in a controlled laboratory setting, all individuals do NOT demonstrate the same response to a given depressurization. The tendency to acquire JOINT-PAIN DCS definitely varies from person to person, and some are definitely more susceptible. (This has been known for decades in animal subjects also.) This applies likewise to bubble formation as measured with Doppler devices. {At NASA, we do not test individuals for sensitivity and segregate them out of a particular job (such as EVA, “space walks”). The current philosophy is to generate depressurization schedules that are applicable to all - but a selection could probably be made were the decision made by management to do so.}
    [sp][b.] If we accept that some people are more susceptible to JOINT-PAIN DCS, then we must also accept the possibility that, if you are once “bent,” you could be bent again (unless something changes your particular susceptibility).
    [sp][c.] An important question for us at NASA is the nature of that DCS-provoking characteristic(s). One determinant in DCS is the ability for gas bubbles to grow from preformed tissue micronuclei that are generated in the body (apparently) from musculoskeletal movement. Surface tension is a determinant in this nuclei “formation/growth/lifetime” process. {We are all aware that blowing bubbles with pure water is possible but difficult. The addition of a small amount of soap will greatly facilitate bubble formation; this is an everyday example of surface tension. The same bubble-formation process no doubt occurs in your body.} The QUANTITATIVE significance of this is not currently known, however. We are looking into this, but the jury is still out.
    [sp][d.] For NEUROLOGICAL DCS, the presence of a patent foramen ovale (PFO) is of importance, but this must be coupled with the generation of bubbles during decompression. Bubble formation is very common in altitude depressurizations but less so in diving situations. Thus the PFO question is very important to NASA but might be less so to SCUBA divers, unless they have a neurological hit. [Dr. Wilmshurst presented his views to a NASA workshop recently, and I share his viewpoints on the significance of this entity. This viewpoint is not equally shared by some of my NASA colleagues, however.]

    2. ACTIVITIES BEFORE DEPRESSURIZATION

    [sp][a.] Because we are concerned with DCS in space, the activity level is very important. In a large series of laboratory tests, we found that walking (exercise of the lower extremities) BEFORE depressurization was capable of provoking bubble formation during to depress up to three hours later. Since astronauts do not actually walk during “spacewalks,” all of our decompression schedules are now developed with seated subjects. For a diver, strenuous activities, such as carrying one’s gear from the car to the boat and then lugging it around prior to a dive are probably nucleating events and likely to contribute to increased DCS risk. Some of this research actually dates back to the second World War when DCS in high altitude bomber crews was studied.

    [sp][b.] Very strenuous activities at depth are bad, e.g., tugging to free an anchor.

    3. ACTIVITIES DURING DECOMPRESSION

    [sp][a.] Again, we do not have the test subjects walk during the decompressions in the altitude chamber. The resultant change in DCS risk between seated and walking individuals is between three and ten-fold (not 10% but 10 TIMES). We attribute this to tissue micronuclei formation, although we do not have direct evidence that this is indeed the mechanism. The message to SCUBA divers is not to climb ladders with full gear, do not strain the arms while boarding the dive boat, don’t lug heavy gear around on deck, etc.

    [sp][b.] Blood flow is important for the washout of dissolved gas following the dive. We modify the gas loads of the individuals with specific,graded (but mild) exercise (the oxygen consumption is measured). This greatly accelerates inert gas washout (in the altitude case, they are breathing oxygen by mask). The take-home lesson is maintain moderate physical activity post-dive, and don’t go to sleep following the dive. This exercise-augmented washout was known even during WW II, but they combined it with the wrong type of exercise (e.g., jumping jacks) and RENUCLEATED themselves. Naturally one should maintain a good fluid load so as not to compromise circulation and not to reduce surface tension by concentrating surfactants (e.g., proteins in the body’s fluids)

    [sp]As you can see from this response, none of these risk-modifying factors are listed as increasing the possibility of DCS in classical, canonical teaching.
     
  3. rcohn

    rcohn Manta Ray

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    Thank you for the excellent response.

    One more question relating to the folowing comment.


    " [b.] Blood flow is important for the washout of dissolved gas following the dive. We modify the gas loads of the individuals with specific,graded (but mild) exercise (the oxygen consumption is measured). This greatly accelerates inert gas washout (in the altitude case, they are breathing oxygen by mask). The take-home lesson is maintain moderate physical activity post-dive, and don’t go to sleep following the dive. "

    Conventional wisdom has been to avoid hot tubs and hot showers post dive because this would increase blood flow to the skin, speeding off-gassing and invalidating the decompression model. Anecdotal stories have suggested that this increases the risk of DCS. It has always seemed to me that faster off-gassing should tend to decrease the DCS risk. It would appear that this might be comparable to the moderate exercise you discuss, the blood flow increases without causing tribonucleation in the joints which would create additional microbubbles. Is this extrapolation reasonable?

    By the way, any suggestions on how to get gear on and off a dive boat, or to a rocky shore dive without straining?

    Thanks,

    Ralph
     
  4. Dr Deco

    Dr Deco Medical Moderator Staff Member

    # of Dives: I just don't log dives
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    I believe that the major difference between exercise and the hot tub is the regional circulations that are promoted by the former. What we wish to do is to promote blood blow in the muscles and their adjacent tendons and ligaments. This is done by exercise that increases the heart rate, local concentrations of vasodilators (to open the capillaries), and squeezes the capillaries (the muscle pump). In addition, this exercise carries over in what is termed Post Exercise Hyperemia or “Excess Post-Exercise Oxygen Consumption.”

    In the case of the hot tub, we have an increase of blood flow, but it is primarily 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.

    As far as handling of equipment, what I have in mind is that someone (a non diving helper) would lift your tanks on to the boat from the water and help you up the ladder so that the strain is minimized. From the boat to the shore, the equipment would be carried by a “roadie.” That is the beauty of science – when applied correctly it reduces the work!


    [Edited by Dr Deco on 10-24-2000 at 02:41 PM]
     
  5. rcohn

    rcohn Manta Ray

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    Dr. Deco,

    Thank you again for your answers. I'm sorry if my questions don't have simple answers. I have many resources where I can find answers to the simple questions but this is the only option for gaining insight to the harder questions. Hopefully, this question will have a short answer.

    I was rereading this thread and had a follow-up question. You said:

    and

    So walking seems to be a bad idea, although I think not walking three hours prior to dive time may be difficult. You have also recommended moderate activity post dive,

    I would assume (perhaps incorrectly) that walking is still a poor moderate physical activity option in the post dive period. If so what activity would you recommend?

    By the way I thought your comments on post dive exercise seem to correlate rather well with Richard Pyle’s experience as related in "Confessions of a Mortal Diver II"
    http://divingthenorthwest.com/scuba/tech/bendstory2.htm

    Thanks again.

    Ralph Cohn


    [Edited by rcohn on 10-10-2000 at 11:03 AM]
     
  6. Dr Deco

    Dr Deco Medical Moderator Staff Member

    # of Dives: I just don't log dives
    Location: Issaquah [20 miles east of Seattle], Washington.
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    Ralph:

    While we do not have our test subjects walk, I do not see a good alternative for recreational divers under the usual circumstances. The reason for the proscription of walking for test subjects in altitude decompression studies at NASA is so we can mimic the decompression conditions in space as closely as possible. While we refer to “space walks,” the astronauts do not actually do weight-bearing activity in space.

    Walking by the divers on the boat is fine since the tables were designed with that activity in mind. What they were not tested for was the vigorous activity that is somewhat common in the recreational scenario. In table testing protocols, here are the activities that are NOT performed. These are strong points of difference between the laboratory and the field:

    [1.] Laboratory subjects never climb ladders – and never walk around, or climb, with diving gear. These activities produces nuclei because the equipment is heavy. This is DCS provoking and has been well documented for several decades:

    [2.] Laboratory subjects never lift diving gear such as scuba air tanks. They do not bend over at the waist thus exercising and stress the tissues of their spinal cord.

    [3.] Laboratory subjects never engage in any strenuous activity that causes them to perform Valsalva-like maneuvers. These activities can result in arterialization of bubbles from the venous side to the arterial side.

    The article by Richard Pyle was very interesting. He appeared to have had a very large gas bubble load in the venous system following his dives (he comments on having the “chokes.”) Being very active in the gas uptake phase of a dive and relatively quiet and restive in the decompression phase will cause an underestimation of the required decompression time. I have communicated with Richard by e-mail, but I have not yet heard this story(!)

     
  7. rcohn

    rcohn Manta Ray

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    Thanks very much for the answer. I've liked Richard Pyle's stories and articles, although he seems to be a bit less conservative diver than most of us, to say the least. I did find the surprising effectiveness of his O2 treatment without recompression to be quite interesting as well.

    Ralph
     

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