Exercise and decompression sickness: a matter of intensity and timing

The paper in this issue of The Journal of Physiology entitled ‘Aerobic exercise before diving reduces venous gas bubble formation in humans’, by Dujic et al. (2004) extends their findings in animals (Wisloff & Brubakk, 2001) to human subjects, and confirms that exercise performed prior to exposure to modest pressures, equivalent to 18 m of sea water depth, can ameliorate venous bubble formation upon decompression. Since bubble formation is associated with decompression sickness (DCS), this article impacts broadly on the field of diving physiology. If confirmed, it potentially affects a large population. First, in addition to pilots and diving professionals who are impacted by profession, there are an estimated 854 000 new certifications for SCUBA divers issued worldwide each year (Professional Association of Diving Instructors, web page). Second, the article provides clarification and changes the commonly accepted view of the effects of exercise on decompression-induced bubble formation. Third, the theoretical explanation for the response whereby a reduction in bubble nuclei may result from the heavy exercise, thereby accounting for the reduced bubble formation upon decompression, represents a plausible and potentially important avenue for more research. The previously held general concepts of exercise and bubble formation resulting from decompression involved the understanding that exercise at depth would increase blood flow and consequently nitrogen uptake. With more nitrogen in the body the time required to unload the additional nitrogen would then be greater. Indeed, studies have shown that if exercise occurred during one dive, and a similar dive was conducted with no exercise, more nitrogen was eliminated while resting at sea level following the dive when exercise was performed (Dick et al. 1984). This is in line with the finding that more bubbles were detected when subjects exercised during or immediately following completion of decompression, and the intravascular bubbles impede the elimination of inert gases. Summarization of prior reports generally led to the conclusion that exercise performed either prior to or during exposure to pressure would increase the risk of DCS (Vann & Thalmann, 1993). Such impressions suggested that otherwise unexplainable cases of DCS might have been due to strenuous exercise performed prior to an incident. This was the case even in a highly controlled US Navy dive (Hughes & Eckenhoff, 1986). On the other hand, more recent reports indicate that if exercise is performed at a modest intensity during decompression, Doppler-detected gas emboli are reduced (Jankowski et al. 1997). The beneficial effects of exercise during decompression are similarly thought to be a consequence of increased blood flow but in this case resulting in increased gas elimination. So the timing of exercise performed at depth and the intensity of the exercise become critical factors in whether or not the effects are harmful or beneficial. The controversy over the impact of exercise on the outcome of decompression is not new. In Haldane's time, both Royal and US Navies routinely imposed exercise on their divers during decompression, believing that the elevated circulatory state would accelerate the elimination of nitrogen. The practice ended when many experiments demonstrated that various exercise regimens before, during and following decompression either from depth or to altitude increased the incidence of DCS. A long period followed where exercise was prohibited during and following decompression. Later, sporadic reports appeared showing beneficial effects of exercise during decompression. So far no definitive conclusion can be drawn regarding the effect of exercise on DCS before, during or after decompression. However, it is apparent that a beneficial effect of exercise performed during decompression is dependent on type and intensity of exercise. The present study stresses the importance of the timing of the pre-dive exercise. In other studies by this group that were conducted in rats, a beneficial effect of pre-dive exercise did not occur if the exercise was done too close to, i.e. within 10 h, or too much in advance of, e.g. 48 h, the dive time (Wisloff et al. 2004). If the mechanism of exercise-induced suppression of bubble formation is related to nitric oxide (NO) production, then the timing of NO administration in relation to its prophylactic effects on bubble formation should be similar. In the same study, Wisloff et al. (2004) reported that administration of an NO-releasing agent reduced bubble formation even when given 30 min prior to hyperbaric exposure. Thus, the timing of the responses to exercise and NO administration were quite different. It is not clear why exercise performed only within a window of time near 24 h prior to the dive can produce the effect if the mechanism is linked to an NO-dependent reduction in bubble nuclei. Whether or not the effect of exercise on bubble formation operates through the formation of NO as the authors hypothesize, or through yet another mechanism, the present study significantly advances our understanding of the effects of pre-dive exercise, and provides another means of ameliorating the formation of bubbles upon decompression in humans. The contribution of this effect will need further standardization and study before pre-dive exercise can be widely adopted as a predictable safeguard against DCS. For instance, better knowledge is needed of how much exercise is necessary to provide a certain degree of protection, and how this can measured. Clarification of the mechanism may be helpful in this regard.