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Doppler Bubble Detection and Scuba Divers

Discussion in 'Ask Dr. Decompression' started by King_Neptune, Sep 29, 2000.

  1. King_Neptune

    King_Neptune Founder

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    I have heard about Doppler bubble detectors being used for dive table testing. Are any of these Doppler devices suitable for the recreational diver?

    =-)

     
  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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    Doppler ultrasound bubble detectors were introduced into diving physiology by Dr. Merrill Spencer in the late 1960s. I worked with Dr. Spencer at the Institute of Applied Physiology and Medicine for almost 15 years, and, over that time, learned a great deal about what they can and cannot do.

    By way of background, Merrill told me that it was at a Man In The Sea conference in Seattle that he first became interested in this area. He had been working with Doppler blood flowmeters and, while calibrating them, occasional air bubbles in the fluid stream gave big and annoying signal artifacts. While discussing this one night at the conference dinner with Dr. Dennis Walder, he remarked about the bubble artifacts. Dennis remarked, “You can actually detect small air bubbles like that? Have you even tried this device with divers?” This started the whole business in operation.

    Originally it was believed that bubbles in the blood stream shortly preceded the onset of the “bends.” If detectable, decompressions might be controlled and proceed at a faster rate and also be safer. When tests were made with US Navy dive tables, it was found, however, that bubbles were present in many cases, even when no DCS appeared. We know today that DCS is primarily the result of gas bubbles in tissues, and these bubbles lie outside of the circulatory system. The Doppler bubbles detected are not directly causative of joint pain. There is a general trend, though, of bubble number and DCS risk; good decompression schedules produce fewer gas bubbles in divers than more severe ones.

    In practice, a table can tested and even though no one gets DCS, comparisons can still be made with existing schedules in that the best procedures will produce the fewest bubbles in the greatest percentage of test divers. This was the principle used when the Recreational Diver Planner was tested, first in the laboratory and then in open water dives [Hamilton, RW, RE Rodgers, MR Powell, and RD Vann. Development And Validation Of No-stop Decompression Procedures For Recreational Diving. Diving Science and Technology. February 28, 1994, (pp. 78 + appendix)].

    At NASA, Doppler bubble detection devices have been employed for more than a decade to test decompression procedures for EVA. [Space suits are at a much lower pressure than the Shuttle Orbiter cabin and astronauts could get DCS if some type of countermeasure was not used.] Again, in a group of test subjects, the better procedures will not produce many bubbles in the aggregate test subject population.

    Doppler bubbles come from many tissues of the body, especially muscle and fat, and the physiological effects of these bubbles is quite complex [MR Powell, MP Spencer, and O von Ramm. Ultrasonic Surveillance of Decompression. In: Physiology and Medicine of Diving, 3rd Edition, (P. Bennett, D. Elliott, eds.) pp. 404-434, Baillière Tindall, London.] While Doppler methods are useful for table testing, what most divers conjure up are thoughts that this device could be applied directly to themselves to optimize decompression. Unfortunately, time has indicated that nothing is that simple, and an individual “decompression meter” is more science fiction than fact. The temporal relationship between bubbles and DCS is not always a good one. Furthermore, tables are developed to result in a very low level of bubbles being generated in divers. What will happen is that the diver will note that he/she is a “non bubbler” and possibly attempt to increase their bottom time. The result could be disastrous and is one reason why I have never been comfortable with the concept of self-Dopplered, do-it-yourself decompression schedules.

     
  3. rcohn

    rcohn Manta Ray

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    Dr. Deco,

    Things seem to be quiet, so I thought I’d ask for your comments on the current practice of some advanced technical divers customizing their own tables based on how they feel after decompression. With frequent macho discussions on the Techdiver list of excessive deco schedules and “weenie” divers, this practice may become more common. Note this is nothing I have any intention of trying, just intellectual curiosity. This practice was commented on by Yount, Maiken, and Baker (“Implications of the Varying Permeability Model for Reverse Dive Profiles”, Proceedings of the Reverse Dive Profile Workshop, pg. 30, Oct. 1999.) implying that there may be some “scientific” justification to this practice.

    Are divers better off basing table modifications on Doppler bubbles or how they feel? Actually, I’m guessing you’ll say they should use the existing tables.

    A related question from the sport diving perspective involves that standard recommendation that divers apply tables and computers very conservatively. Clearly, these tools were developed to cover all divers, no matter how unfit or susceptible to DCS. They may be overly conservative for divers with high levels of fitness or inherent low susceptibility to DCS. Are there any reasonable approaches to establishing what is an appropriate level of conservatism when using standard tools for an individual sport diver?

    Thanks for all the information you’re providing.

    Ralph
     
  4. 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:

    This is not a question with a short answer. I will try by breaking it into several parts.

    [sp][1.] The dive schedules normally followed by recreational divers are no-decompression dives. These are the well-known no-decompression limits (NDL) . In general, this means that the primary controlling factor is the partial pressure of the inert gas that has dissolved in the tissues of the body. This is largely controlled by the applied pressure (depth) and the duration of time at depth(s). This determines the dose of nitrogen. [We have also discussed to some degree that the uptake, and elimination, are regulated by the blood supply. In the development of NASA decompression schedules, the blood perfusion is controlled by certain types of mild exercise and monitored by oxygen uptake – a measure of physical activity].
    [sp] These NDLs are relatively standard for all dive tables although they are modified to give more safety by allowing shorter NDLs for the more conservative tables. [These are discussed in the Hamilton et al. book referenced in other questions where the DSAT Recreational Dive Planner was described.] Whether one uses SCUBA tables, dive computers, or “technical diver” decompression tables, the NDLs are roughly equivalent.

    [sp][2.] The second controlling factor in decompression is gas phase (bubble) formation. In recreational NDL diving, even if gas bubbles form, it matters little since one is always at the surface and finished with decompression. {This will change if one makes repetitive dives, but this is a different matter – see the questions and answers in the “free diving” section of this FORUM.} When one is in a decompression scenario with different “stops,” the question of this gas bubble formation is of paramount importance. The classical Haldanian scheme is a so-called single-phase model, and all of the gas is assumed to be in the dissolved state unless frank DCS develops. We know from work of several decades with Doppler bubble detectors that this assumption is not true, although the importance of this for the quantitative development of tables is debated. To remedy this matter, two decades ago it was proposed that nitrogen is not only dissolved but also in the gaseous state (in bubbles), and that it is eliminated faster when one remains deeper for the decompression stops (doctoral thesis of Brian Hills, 1968). These are the so-called two-phase models.

    [sp][3.] The schedules utilized by many technical divers have been derived from research of David Yount , PhD, using observations of bubble growth in gels. These models control the bubbles by modeling the fact that they shrink during the bottom phase of the dive and can be removed from the normal concentration of bubbles present in tissues. This is the Variable Permeability Model (VPM). Deeper initial decompression stops are employed since gas-phase nitrogen is eliminated faster at deeper depths. Divers making use of these schedules claim that they have less fatigue and a general feeling of well being when the deeper stops are used. (Fatigue has been reported in decompression for decades.) Since a reduction in lethargy is a hallmark of these decompression procedures, many divers take careful note of this and use it as a “biomarker” for their decompression.
    [sp] What is the genesis of this fatigue? It is not really known, but many ascribe it to subclinical, “asymptomatic” bubble formation in the brain. I personally believe that the explanation is correct, at least to a large degree.

    [sp][4.]Only if one were using multi-stop decompression tables would this concept be of importance. However, many technical divers perform this type of diving on a regular basis. The decompression schedules are commercially available, and the one dive computer ( VYPER developed by Bruce Weinke, PhD) has the “Reduced Gradient Bubble Model” in it. The major problem with these models is that they have not been rigorously tested in a crossover test of one- and two-phase models under similar conditions. This is more a lack of funds than a lack of desire.

    [sp][5.] With regard to the estimation and understanding of the degree of conservatism for each divers particular circumstance, I am not aware of any methodology. I personally believe that certain types of chamber dives could by used in a standardize test, but nothing exists at the current time that is agreed upon by everyone. As far as the NDLs go, nothing really need be changed.
     

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