Stationary Stops: Yes or No?

[Texass]

Does swimming around at 30/20/10' provide the same benefit assuming you maintain a depth say within a 2' leeway? Which may lead to another question on how much bobbing leeway do you have at a stop?

I remember reading that Dr. Hill's(?) goats died doing just a slow ascent so I understand that a definite stop is necessary. Is the stop simply a function of pressure, or does activity also effect the off gassing?

Thanks,

 

[Uncle Pug]

Activity affects off gassing in a positive way if it is gentle swimming since inactivity would slow perfusion. What you don't want to do is create bubble nuclei by excessive exertion.

As for making discrete stops and being able to hold them... this is a wonderful skill to have... but I doubt it really makes much difference unless your excursions are really great.

For recreational dives the key is a very slow ascent from the last 30 feet... and spending plenty of time look at stuff shallow. It would be better IMO (recreational dives using air/nitrox) to move up or down somewhat to keep yourself interested and occupied thereby spending plenty of time than to try to hold an exact stop and suface as soon as possible.

 

[Charlie99]

I remember reading that Dr. Hill's(?) goats died doing just a slow ascent so I understand that a definite stop is necessary.

Is the stop simply a function of pressure, or does activity also effect the off gassing?

My understanding is that a definite stop is NOT necessary. The Hill vs Haldane comparison is more a difference in the shape of the ascent rather than stops vs. continuous ascent. Hill did linear ascent at the same rate all the way up.

If one did Haldane stops with sufficiently small increments of depths, it would become a continuous ascent, but it would be more rapid ascent down deep, with the rate slowing dramatically at shallow depths.

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A search on Scubaboard should return some comments by Dr. Deco to the effect that mild exercise at a deco stop increases circulation and promotes offgassing. IIRC, he has done tests with astronauts simulating the effect of exercising on the offgassing needed before going to the very low pressure envirionment needed in EVA suits.

 

[Uncle Pug]

The shape of the curve is what counts... and as Charlie says... ascend slower as you get shallower... paraphrased.

 

[DepartureDiver]

Hill did linear ascent at the same rate all the way up.

I'm not sure what you mean or which study you are refering to, but Dr. Hill's thermodynamic model definately has stops.

As to the mild exercise issue, this is an interesting question I am curious about. While exercise promotes circulation, the question must be asked where is the circulation occuring. If blood flow is greatly increased to certain areas of the body, it may be reduced at other sites. So which site is the beneficial site? I'm guessing if the swimming is very mild, the circulation rate may be increased without diverting blood to the swim muscles and away from other muscle/tissues/organs. However, if blood flow is diverted, then off-gassing may be slowed in other areas.

 

[Uncle Pug]

If you weren't swimming when you on-gassed then don't swim while you off-gas.

 

[jbd]

Does swimming around at 30/20/10' provide the same benefit assuming you maintain a depth say within a 2' leeway? Which may lead to another question on how much bobbing leeway do you have at a stop?

I remember reading that Dr. Hill's(?) goats died doing just a slow ascent so I understand that a definite stop is necessary. Is the stop simply a function of pressure, or does activity also effect the off gassing?

Thanks,

On gassing and off gassing are both diffusion and perfusion related. As noted by Uncle Pug the perfusion part is affected by activity. The key is the right kind of activity. Gentle activity keeps blood flowing but something strenous tends to set off a shower of bubbles so to speak.

Time is also a factor and Uncle Pug has posted about dive profiles in regards to time at depth and time in the shallows. Try a search for some of these gems of info.

Also look around for info on dual phase gas modeling--RGBM. BRW has some books out about this.

 

[Charlie99]

I'm not sure what you mean or which study you are refering to, but Dr. Hill's thermodynamic model definately has stops.

I'm referring (as did I believe the orginal post) to Sir Leonard Hill, a British goat-bending contemporary of Haldane, not a 1976 Australian thermodynamic modeler Brian Hill(s).

Dan Europe article discussing history of linear ascent vs more rapid ascent with stops.

 

[ScubaDadMiami]

While I am at deco stops, I will open and close my hands (like the way that they tell you to pump up before a blood test). I will also gently make a crawling action with my legs. I don't do either of these things the whole time but just occasionally. I mostly try just to relax especially if I have not been working hard on the earlier portion of the dive.

One thing to consider is that, if you are breathing higher oxygen concentrations during your ascent, oxygen is a vasocontstrictor. Additionally, if you become chilled to any degree during the dive, the body will attempt to keep the core warmest by further shutting down flow in the extremities. This is why I place emphasis for this gentle stretching on the arms and legs.

 

[Dr Deco]

Hello Readers:

As indicated by Uncle Pug, gentle swimming activity is a good idea. This must include movement of both the arms and legs. It is the “muscle pump” that is a major player and that means that muscles must be moving to cause gas washout from any specific volume of tissue.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :1book:
http://wrigley.usc.edu/hyperbaric/advdeco.htm


Reference

Jankowski LW, Tikuisis P, Nishi RY. Exercise effects during diving and decompression on postdive venous gas emboli. Aviat Space Environ Med. 2004 Jun; 75(6): 489 - 495.

BACKGROUND: Exercise and diving have generally been associated with an increased risk of decompression sickness (DCS), thus accounting for the lack of studies involving exercise during decompression. However, theoretical and observational evidence contrary to this association motivated the present investigation on the effects of moderate, intermittent exercise during diving and/or during decompression on Doppler venous gas bubbles activity following a dive.

HYPOTHESIS: Doppler bubbles observed at both the heart and subclavian vein sites after diving should be reduced if moderate exercise is performed during decompression vs. remaining inactive.

METHODS: In a water-filled hyperbaric chamber, 39 healthy male subjects were compressed to a pressure of 450 kPa (45 msw) for 30 min followed by 55 min of staged decompression. Subjects were either active or inactive at the bottom phase (450 kPa) and/or during the decompression. Activity comprised three 5-min intervals of moderate arm or leg exercise at the bottom and five such intervals during decompression. After decompression, Doppler bubbles were monitored at the heart (precordium) and subclavian vein sites using Doppler detection.

RESULTS: Number of bubbles was unchanged as to whether subjects were active or sedentary during the bottom phase of the dive. However, it was significantly lower for all indices examined (p < 0.05) after dives in which exercise was performed during decompression vs. inactive decompression.

CONCLUSION: Moderate, intermittent physical activity during decompression decreases VGE activity after diving.