Simulation of gas bubbles in hypobaric decompressions: roles of O2, CO2, and H2O

To gain insight into the spatial features of bubbles that may form in aviators and astronauts, we simulated the growth and decay of bubbles in two hypobaric decompressions and a hyperbaric one, all with the same tissue ratio (TR), where TR is defined as tissue PN 2 before decompression divided by barametric pressure after. We used an equation system which is solved by numerical methods and accounts for simultaneous diffusion of any number of gases as well as other major determinonts of bubble growth and absorption. We also considered two extremes of the number of bubbles which form per unit of tissue. RESULTS: A) Because physiological mechanisms keep the partial pressures of the «»metabolic« gases (O 2 , CO 2 , and H 2 O) nearly constant over a range of hypobaric pressures, their fractions in bubbles are inversely proportional to pressure and their large volumes at low pressure add to bubble size. B) In addition, the large fractions facilitate the entry of N 2 into bubbles. and when bubble density is low, enhance an autocatalytic feedback on bubble growth due to increasing surface area. C) The TR is not closely related to bubble size; that is when two different decompressions have the same TR, metabolic gases cause bubbles to grow larger at lower hypobaric pressures. We conclude that the constancy of partial pressures of metabolic gases, unimportant in hyperbaric decompressions, affects bubble size in hypoboric decompressions in inverse relation to the exposure pressure