Exercise before after or during the bottom phase of a dive

Exercise during exposure to increased ambient pressure (during the bottom phase of the dive) appears to increase the incidence of DI. The probable explanation of this is that the increased perfusion during exercise leads to an increased uptake of inert gas that must be subsequently eliminated during decompression. Conversely Vann et al have shown that exercise during decompression stops may reduce the incidence of DI by an opposite mechanism 25. Exercise prior to or following diving are risk factors however and there are at least three reasons for this:

• Rapidly flowing blood, especially in the area of bifurcation of vessels, may create foci of relative negative pressure through a Venturi effect. Small numbers of molecules of gas from the surrounding supersaturated blood may then diffuse into these foci down a partial pressure gradient. The resulting localized collections of small numbers of gas molecules are known as gas micronuclei and are thought to act as a 'nidus' for bubble formation.

• Distraction between articulating joint surfaces causes extreme reductions in ambient pressure and gradients of up to 270 atm. This process - called tribonucleation - is thought to offer some explanation for the variable incidence between different forms of diving and compressed air activities vs. the ultra-low incidence of DI during extravehicular activity in space. In the latter case it is thought that the absence of gravity reduces articular shearing forces and in the absence of ongoing production these micronuclei seem to disappear.

• Increased local CO2 production in exercising muscle may play a role since CO2 is a highly diffusible gas and might contribute to the formation of gas micronuclei.

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