Exercise and Fitness in Decompression Sickness Risk

In Train Exercise is Exercise Training, I mentioned the ongoing question in diving physiology research of how exercise can affect the risk of decompression sickness (the bends).

It seems that exercise done during a scuba dive at the bottom increases the amount of nitrogen gas you absorb from the air you breathe from your tanks. This makes more gas which could contribute to decompression sickness on the way up. Mild exercise on a "decompression hang" (waiting at specific shallower depths for a few minutes on the way up) seems to help let more gas dissolve out while you breathe, and may lower risk. Exercise soon after surfacing may increase gas coming out and increase risk according to other work. Some interesting studies look into whether exercise done days before a dive can reduce risk by "using up" specific components that decompression sickness bubbles need to be able to form. The kind of exercise and timing seems important. I will post more on this another time.

Some work looks at physical fitness, and whether that affects risk of decompression sickness (DCS). Would someone in better physical shape have lowered risk? What constitutes being in better shape? Is it body fat? Is it the amount of oxygen you can use to exercise? How might any one of those components affect DCS risk?

I am writing this from the UHMS scientific meeting, explained in Exercise and Medicine Underwater and at High Pressure. 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 (pronounced vee-oh-too-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.

• What about physical fitness and risk of decompression sickness in space? Several studies here at the meeting address that. Astronauts who go outside the space vehicle go to lower pressures, similar to divers coming up from a dive. Many considerations, including exercise, go into their preparation for that. The interesting story is posted in Space Walks.


• Here is the next exciting article from this meeting: Does Hyperbaric Treatment Heal Sprains?
  

Labels: aerobic, aerospace, hyperbaric, scuba

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