More About Exercise Before Diving

In the March issue of Undercurrent, we examined the DCS risk posed by exercise before, during, and after scuba diving. We concluded that strenuous exercise four hours before and six hours after diving should be avoided.

We did not, however, address the possible effects of exercising well before the first dive, a topic that has received a flurry of interest in the wake of recent studies from the Norwegian University of Science and Technology.

While research conducted in the early 1990s suggested that exercising the day before a dive suppresses inert gas bubble production and reduces the incidence of DCS, this line of inquiry was not given much attention until the present Norwegian investigations. These studies have shown that a single episode of high-intensity aerobic exercise 24 hours before a chamber dive decreases the number of bubbles more than fourfold and cuts the maximum bubble size in half.

Interestingly, other studies by members of this research group have shown that heavy exercise suppressed bubble formation and reduced DCI-related deaths in rats when performed 20 hours prior to a chamber dive but had no meaningful effect at 48, 10, or five hours prior. High-intensity exercise half an hour before a dive actually increased bubble formation.

Also investigated were the effects of a drug known to release nitric oxide within the body -- not to be confused nitrous oxide, the "laughing gas." When given to rats for five days and then 20 hours before a chamber dive, or when nitric oxide itself was administered 30 minutes prior, both bubble formation and death were significantly diminished.

Why particular regimens of exercise interfere with inert gas bubble formation is unclear. However, this later study has generated the hypothesis that high-intensity exercise may enhance the production of nitric oxide, a compound that relaxes capillary walls, allowing them to expand. This change might allow micronuclei generated by exercise to be more readily eliminated, thereby inhibiting the growth of bubbles of sufficient size to cause DCS.

Many experts have suggested that physically fit divers are at less risk for DCS because of healthier and more numerous capillary systems to efficiently eliminate inert gases. If so, nitric oxide's capacity to enlarge capillaries transiently, and perhaps render their surfaces less sticky and less likely to trap particles like microbubbles, may mimic the effects of fitness -- improved capillary blood flow and filtration.

These findings don't contradict our earlier advice regarding minimizing exertion before, during, or after a dive. However, if vigorous activity is undertaken, it might best be done around 24 hours before diving. And, once you start getting wet on a dive vacation, the simplest and safest approach is it to delay the weight lifting and marathon training until after returning home.

As for a magic pill, the Norwegian investigators speculate that within a decade it may be possible to develop a nitric oxidereleasing drug for divers that would imitate the effects of general physical fitness. However, there are many questions to be addressed, including whether indeed nitric oxide is a major player. So, given the uncertainties and the research to be done, speculation that a pill will allow divers to stay down longer without increased risk of DCS reminds us of the always sound scuba advice -- when in a difficult spot, remain hopeful and don't hold your breath.

-- Doc Vikingo