Diving and Aerobic Capacity

[bobhog]

There are many obvious benefits accompanying an increase in aerobic capacity. What effect if any does an increase in aerobic capacity have on diving and DCS or DCI risk?

In another post, many claims have been made concerning deep diving and a supposed connection between high aerobic capacity, also referred by a VO2 maximum number, and DCS risk. In the absence of hard science (control, grouping, measurements, and theories) it would be dangerous to imply that being fit creates a margin of safety.

Any thoughts here?

 

[CameronMartz]

Bobhog,

I don't know what your standard for hard science is, but there is a *ton* of research examining the overlap between exercise physiology and hyperbaric physiology. You can search the archives here for several posts where I list studies in different areas- BMI, body fat %, VO2max, pre-dive and post-dive exercise...

You can also do your own secondary research, if that is appealing to you, by going to Gene Hobbs' incredibly useful website http://rubicon-foundation.org . Search under terms like "dcs VO2max" (without the quotes), "dcs fat," and whatever other terms you're interested in. You will be amazed.

Cameron

 

[bobhog]

Hi Cameron

Thanks. "hard science" is a reference to the loose posts that suggest some connections to things that have not been established in a controlled situation.

Again, I have posted this question in an attempt to collect information directly related to the subject at hand. The Rubicon website is very nice. I have found nothing to this writing relating to the topic at hand. I've searched utilizing your suggested terms plus many others.

That's why I'm looking here, in this Marine Science and Physiology discussion group. I figured this should be as good a place as any to start.

Again, anyone with knowledge of research related to aerobic capacity (cardio-respiratory fitness level) and risk of decompression sickness/illness, please let me know. Additionally, I would be interested in knowing about research indicating the relationship between aerobic capacity and bottom time, ditto... please let me know.

Cameron, I noticed you have a book titled "Fitness for Divers". In that regard, what research did you cite for your book? What gains are made through exercise that also relate to what advanced capabilities underwater?

Thanks!

 

[CameronMartz]

There is little abilty to control all of the variables you'd like to control when studying human physiology. Every study done on divers leaves significant questions unanswered because of this, and this is what I mean when I question your threshold for hard science. We're not studying microbes in a petri dish, and we can't just go around bending people on purpose to see who survives and who doesn't.

So, were left with studies that look at accident data, studies that bend rats and other animals, studies that look at military divers, studies that decompress people to altitude from surface saturation, and studies that sit people in dry chambers instead of having them submerged swimming around. None of these are ideal, but we have chosen to apply a lot from these types of studies in all areas of decompression research.

The list of research regarding fitness is long. I include a summary of 40 different studies in my book, and many more have been produced since it was first published two years ago. 16 of those summarized are on fitness and resistance to DCS alone, going all the way back to 1907 where researchers noted that "stouter" caisson workers got bent more frequently than "less stout" workers. The answers to your final question take up half of the book, each chapter with references to peer-reviewed research.

Again, I have provided more detailed lists on this board before, but here are just a random few to get you started. Click the "See all related articles..." link at each of these pages to see more abstracts on the subject.

http://www.ncbi.nlm.nih.gov/sites/e...ez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
"This work showed that bubble production after hyperbaric exposures depends on several individual factors. The effects of age, weight and VO2max are more significant than the effect of %BF."


http://www.ncbi.nlm.nih.gov/sites/e...med.Pubmed_ResultsPanel.Pubmed_RVAbstractPlus
"Younger, slimmer, or aerobically fitter divers produced fewer bubbles compared with older, fatter, or poorly physically fit divers."


http://www.ncbi.nlm.nih.gov/sites/e...ez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
"Age and height showed no significant influence on DCS incidence, but persons of either sex with higher body mass index and lower physical fitness developed DCS more frequently."

Cameron

 

[bobhog]

Cameron, don't get bent out of shape. forgive the pun...

Thanks, the links you provided are a good starting point. As a physiologist, I have read and seen ample spurious claims especially since recently joining this board that are downright misleading and in the context of diving, are dangerous. "Hard science" means not to use terms like "prove" because as you indicate, there is no certainty in anything we think we know. Hard science means taking observed actions and reactions and measurable components and applying them to a hypothesis. Many times, I read artifact, subjective analysis, and "intuition" that is masked as "science".

The quoted articles support my suspected outcomes that fitness level may be associated with reduced DCS but I wonder what levels of aerobic capacity are thresholds for positive outcomes? Numerous studies indicate that regardless of aerobic capacity, exercise prior to diving may reduce the susceptibility of DCS

Dujić Z, Duplančić D, Marinović-Terzić I, Baković D, Ivanćev V, Valić Z, Eterović D, Petri N, Wisløff U, Brubakk AO. Aerobic exercise before diving reduces venous gas bubble formation in humans. J Physiol. 2004;555:637–642.

And this paper on incidence of DCS with exercise indicates contradictory benefits of during exercise (Exercise and decompression sickness: a matter of intensity and timing).

True, researchers won't deliberately induce DCS in their human research subjects but you seem to imply that this is what science requires. Actually, microbubble formation can be measured in vivo (inside the human body) while exposed to increasing or decreasing external pressure while controlling for increased risk of DCS. So, yes, we must expose research subject to increased risk of DCS in order to understand the risk of DCS for differing populations. Would it not be irresponsible to not do so?

In a follow-up on this, what benefits other than reduced DCS are there for divers who regularly engage in aerobic exercise (who summarily have higher VO2max)?


Thanks!

 

[Thalassamania]

Cameron,

You're cherry picking your references. What about:

Here at the UHMS scientific meeting, one of the studies presented by French naval researchers is, "Does the VO2 max value predict the formation of intravascular circulating bubbles during decompression of healthy divers?" VO2 max is the most oxygen you can use when doing the most exercise you can do. It is usually higher in people who can do more aerobic exercise (other factors also contribute). The maximum amount of exercise an average person can do is about ten times their resting level of oxygen use. Marathoners usually max at around 20 times better than resting levels. A top aerobic athlete can use about 30 times resting level (a horse - more than twice the top human max). Someone badly out of shape, or with heart disease or other problems that limit ability to get oxygen to cells, generally has a low VO2 max. You can raise your level with regular exercise at any age. It is not set.

In the French study, divers were tested for VO2 max a week before their experimental dive. They avoided any physical exercise 48 hours before the dive. Then half completed a dive in a dry hyperbaric chamber and the other half in the open sea with the same dive profile and decompression stop according to French military decompression table MN90. After the dive they were all tested for presence of small decompression bubbles in the bloodstream.

Bubbles can form in the body painlessly after a dive without creating decompression sickness. It is not the case that bubbles always form after every dive, as often thought. Certain bubbles can be detected audibly (they sound like pops and squeeks) using Doppler ultrasound, and other kinds of instruments being developed. I will post more another time about these bubbles and what ultrasound can and can't determine about bubbles and decompression sickness.

The French researchers found that bubble formation in both types of dive was related to the age and body mass index of the divers, but not to VO2max.

Being in good shape makes many aspects of diving safer, even if it doesn't affect risk of decompression sickness. Being in better aerobic shape helps you swim more easily against currents that may take you away from your dive site or boat. Strengthening your body through weightlifting with good body mechanics helps you lift and haul gear with less chance of injury, and practicing all your physical skills helps you be more able to rescue someone or yourself.

 

[bobhog]

Good quote Thalassamania.

I've cited that French study in other areas, too. I think it brings up a very important point for new divers that being in "good shape" doesn't mean being capable of doing more dangerous tasks. Based on what I have found in my search for studies, the scientific community really has not done a lot of work trying to understand diving and aerobic capacity but that's not to say that work is not in progress.

I find it interesting that many tout or imply fitness as a form of protection. Fitness is important but in relation to diving, still not clearly understood.

It's just that diver's need to understand that they are still constrained by RDPs or conservative multicompartmental algorithms despite their fitness level.

 

[bobhog]

It is also worth noting that I have yet to find dive studies on high VO2 max groups that discuss differences with N2 uptake and off-gassing when compared to a less aerobically fit control group. I am interested to know, since aerobically conditioned people have a higher degree of muscular vascularization, if this group has a higher loading of N2, even if they respire at the same rate.

I am also interested in citations from studies relating aerobic fitness level to dive respiration rate.

 

[Gene_Hobbs]

No time this week but thought this paper is well worth a read.

Exercise conditioning reduces the risk of neurologic decompression illness in swine.
Broome, Dutka, and McNamee. Undersea Hyperb Med. 1995 Mar;22(1):73-85.
RRR ID 2188

I agree that it is sad that there is not much available but would it honestly broaden our knowledge base that much? Since funding for diving research has not really changed much since the early 80's, I have to wonder about 'WHAT' we really should be looking into (Hobbs et. al. 2007 UHMS abstract. What can the medical community do for technical divers? [data]).

The abstract by Dr. Castagna et. al. (the French work mentioned above) is excellent but it is worth noting for the non-scientific folks reading this thread that bubble formation is an interesting measure but it does not speak to actual inert gas wash-out. That measure was not taken. I was more partial to the way they presented this work:

Haemodynamic changes induced by submaximal exercise before a dive and its consequences on bubble formation.
Blatteau et. al. Br J Sports Med. 2007 Jun;41(6):375-9. Epub 2006 Nov 30.
PubMed ID: 17138641

Since the last thread involving "Heart Rate" measures broke down, and it was not worth bring that back, it is worth noting that:

The median error in estimating VO2 from heart rate was 12 percent on the surface, 23 percent submerged, and 31 percent in the dry chamber. Heart rate was not a good predictor of VO2 during dry or submerged exercise in the hyperbaric chamber.

Exercise Heart Rate as a Predictor of Oxygen Consumption During Decompression from Saturation Diving.
Shykoff and Knafelc. 2002 NEDU report
RRR ID: 5344

 

[CameronMartz]

Cameron, don't get bent out of shape. forgive the pun...

I think you might be reading more into my posts than I intended. You set a high bar with the language you used in your first two posts, and I was merely attempting to prepare you for what you were going to find.

The quoted articles support my suspected outcomes that fitness level may be associated with reduced DCS but I wonder what levels of aerobic capacity are thresholds for positive outcomes?

Unfortunately, with the nature of the research involved, we cannot determine thresholds for many effects in diving. We suspect that dehydration, for example, increases risk of DCS. How hydrated must you be to not see an associated risk? We can't determine that. We can only see a vector, but that alone is enough to suggest ways to change our behavior on dive day.

That is the way it is with the research on fitness. We suspect that better fitness means reduced risk of DCS, but there is no reasonable way to caculate a threshold, say, of VO2max, above which you consistently see benefit and below which you don't.

True, researchers won't deliberately induce DCS in their human research subjects but you seem to imply that this is what science requires. Actually, microbubble formation can be measured in vivo (inside the human body) while exposed to increasing or decreasing external pressure while controlling for increased risk of DCS. So, yes, we must expose research subject to increased risk of DCS in order to understand the risk of DCS for differing populations. Would it not be irresponsible to not do so?

How are you going to get subjects to agree to incur a significant risk of DCS in the name of research? You might get some highly motivated people, as some military studies do, but it just doesn't happen to a level of statistical significance, especially considering all of the decompression questions we'd like to research this way. Heck, I've spoken with researchers about how difficult it is to get subjects for studies that require study of internal body temperature (guess where the thermometer goes <g>).

Otherwise, we do have researchers doing their best to study various physiological variables while measuring associated risk of DCS. You can see the different methods used by reading through the many decompression studies you'll find at Rubicon and PubMed, but they all fall under the limitations listed in my previous post.

In a follow-up on this, what benefits other than reduced DCS are there for divers who regularly engage in aerobic exercise (who summarily have higher VO2max)?

Increased physical reserves, delay of panic response, reduced gas consumption, shorter time to equilibrium pressure, reduced cost of free phase gas formation, bigger oxygen window... Some of these are certainly open for debate, but all have a basis in research.

Cameron