J Physiol Volume 537, Number 2, 330-, December 1, 2001
Journal of Physiology (2001), 537.2, pp. 330
© Copyright 2001 The Physiological Society
Exercise fizzy-ology
David J. Doolette
Anaesthesia and Intensive Care, University of Adelaide, Adelaide 5005, Australia
Email: david.doolette{at}adelaide.edu.au
MS 13178
Divers, aviators and astronauts are at risk of decompression sickness (DCS), which results from the formation of bubbles from excess dissolved gas in the blood and extravascular tissue during reduction in ambient pressure (decompression). The risk of DCS is minimised by changing breathing gas composition and gradual decompression following compressed gas undersea diving and before un-pressurised aircraft flight or space flight extravehicular activity, but DCS can still occur unpredictably. Exercise has long been considered an additional risk factor for DCS, but recent evidence in this issue of The Journal of Physiology (Wisløff & Brubakk, 2001) indicates this notion needs updating.
Unnecessary exercise has been prohibited amongst the diving and aviation communities since studies in the 1940s showed an increased risk of DCS, and unfortunately there has been limited investigation since. During diving, gas must be breathed at ambient pressure and tissues equilibrate with the elevated inspired inert gas partial pressure forming a reservoir for bubble growth; with exercise, tissues that receive increased blood flow equilibrate more rapidly. By a separate mechanism, exercise following diving or shortly before or after decompression to altitude can promote bubble formation (for references see McElroy et al. 1944; Jankowski et al. 1997; Dervay et al. 2001).
Bubbles form during decompression where dissolved gas pressure exceeds the absolute pressure in the tissues (supersaturation) plus the pressure required to maintain the bubble surface against surface tension. This tensile strength of a pure liquid is orders of magnitude higher than the supersaturation pressures that produce bubbles, which are therefore believed to grow from theoretical pre-existing gas micronuclei temporarily stabilised by adhesion to a surface (McElroy et al. 1944; Liebermann, 1957) or by a coating of surface-active molecules (Yount, 1982). A particular degree of decompression activates only those micronuclei above a corresponding critical size and the resulting micro-bubbles are unstable, growing or shrinking via diffusion of gas under the influences of dissolved gas pressure and surface tension. Exercise may activate additional micronuclei due to transient lowering of local absolute pressure in tissues by tensile forces associated with muscle contraction and tissue movement (McElroy et al. 1944).
The space exploration community has remained interested in exercise as a means of accelerating inert gas washout during oxygen breathing prior to decompression (spacesuits are only partially pressurised). Indeed the incidence of DCS in micro-gravity is lower than expected and NASA scientists believe few micronuclei are activated by non-weight bearing exercise (Dervay et al. 2001) and this underpins accelerated decompression protocols designed for International Space Station extravehicular activity. Only recently have studies shown that accelerated inert gas washout with mild underwater exercise during decompression from diving can reduce bubble formation (e.g. Jankowski et al. 1997), supporting early 20th century practice.
Exercise during the days preceding diving has received much less attention. There are three animal studies (mice, pigs and rats) and all show that several weeks of daily aerobic training dramatically reduces the incidence of severe DCS (e.g. Wisløff & Brubakk, 2001 in this issue of The Journal of Physiology, and references therein). These studies have postulated rheological changes that alter the susceptibility to DCS, modified tissue perfusion, and reduced body fat (in which nitrogen is more soluble). None of these mechanisms are convincing, particularly in light of the latest findings that a single bout of exercise is equally effective as a longer training regimen and produces a short-term (1-2 day) reduction in decompression-induced bubble formation (Wisløff & Brubakk, 2001).
Mechanisms based on gas micronuclei will remain speculative until their existence, nature and distribution are experimentally validated, but Wisløff & Brubakk (2001) suggest a nitric oxide-mediated change in surface properties of vascular endothelium, a potential site of gas micronuclei. Alternatively, exercise may have a direct effect on micronuclei. Exercise before decompression can enhance bubble formation, but the effect is only temporary, decaying with a half-life of approximately 1 h in humans (Dervay et al. 2001). The likely explanation is that micronuclei are activated but the resulting micro-bubbles dissolve if decompression does not occur during their lifetime. Since only a fraction of dissolving bubbles decay into gas micronuclei that can be reactivated into bubbles (Liebermann, 1957), it is possible that exercise without decompression depletes the population of micronuclei. Regeneration of the primordial micronuclei population may take 10-100 h (Yount, 1982), and this could explain the temporary protection against bubble formation.
There seems to be good exercise and bad exercise, and further studies must define their relation to risk of DCS. The nature of exercise that best accelerates inert gas washout without promoting bubble formation will need to be determined simulating the weightless conditions of space and underwater environments. Similarly, the nature of previous weight bearing (1 g terrestrial) exercise that best provides protection against DCS and the lifetime of this effect will need to be elucidated. Perhaps divers, aviators and astronauts will have to start exercising again.
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REFERENCES |
| DERVAY, J. P., POWELL, M. R. & FIFE, C. E. (2001). Aviation, Space, and Environmental Medicine (in the Press) |
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| JANKOWSKI, L. W., NISHI, R. Y., EATON, D. J. & GRIFFIN, A.P. (1997). Undersea and Hyperbaric Medicine 24, 59-65 |
[Medline] |
| LIEBERMANN, L. (1957). Journal of Applied Physics 28, 205-211 |
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| MCELROY, W. D., WHITELEY, A. H., WARREN, G. H. & HARVEY, E. N. (1944). Journal of Cellular and Comparative Physiology 24, 133-146 |
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| WISLØFF, U. & BRUBAKK, A. O. (2001). Journal of Physiology 537, 607-611 |
[Abstract/Full Text] |
| YOUNT, D. E. (1982). Journal of the Acoustical Society of America 71, 1473-1481 |
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