The next speaker was Dr. Peter Tikuisis who spoke on "The Evolution of Bubble Evolution in Diving." He was the first speaker to present slides using a Power Point format projecting images from a laptop computer. This appears to be the way of the future. Overview: The talk was an overview on the role of bubbles, their formation, stabilization, and subsequent growth during decompression.
He started with a historical background of bubbles. In 1659, Robert Boyle surmised that bubble formation contributed to decompression sickness (DCS). In 1878, Paul Bert demonstrated that decompression bubbles are largely composed of nitrogen thereby establishing a direct link between the gas breathed and the bubbles formed during decompression. And in 1951, E.N. Harvey stated unequivocally that bubbles cause DCS.
Dr. Tikuisis differentiated between bubbles caused by boiling which are vapour bubbles limited by temperature and gas bubbles which are diffusion limited. It is the latter that concerns us in decompression sickness. What is a bubble? It is essentially a space filled with gas that exerts a pressure in opposition to the outside hydrostatic pressure and surface tension of the bubble (depending on the medium, an additional elastic force might also be present). These factors were expressed in equation form in the slide presentation. Bubbles that form spontaneously require gas supersaturation levels that far exceed that attained in conventional diving. Decompression bubbles form either from a mechanical disturbance (eg., tribonucleation) which causes the liquid to separate or from pre-existing gas nuclei. The latter can be stabilized by contamination on the bubble surface, in crevices having the correct surface chemistry and geometry, or by finite volume constraints. To grow, bubbles must exceed the 'critical' radius of the medium which depends on the amount of dissolved gas, hydrostatic pressure, and surface tension.
The DCIEM bubble model was developed with many of the above considerations and calibrated against Doppler-detected bubble scores in divers. The model is used to calculate the growth and decay of bubbles generated by a dive/decompression profile. The maximum bubble radius attained can then be used to predict the severity of the decompression. In conclusion, the phenomenon of bubble evolution is reasonably well understood and can be applied for the prediction of bubble scores and/or DCS. It appears that bubble formation is an inevitable consequence of diving, and thus the key to the prevention of DCS is bubble management.
Summary:
1. Bubbles are the precursors to decompression sickness.
2. Bubbles likely form from turbulence, mechanical disturbances, and/or pre-existing gas nuclei.
3. Bubbles can be stabilized by contamination on the bubble surface, in crevices having the correct surface chemistry and geometry, or by finite volume constraints.
4. Bubbles grow if their initial size (after formation) exceeds the critical radius (defined by the amount of gas dissolved in solution) which decreases with increased gas supersaturation.
5. Bubble-based models can be used to predict the severity of decompression.
At this point there was a refreshment break and the posters could be viewed. There was a pictorial presentation of the history and activities of the Chapter by Dr. Mary Anne Rokitka. There was the paper "History of exercise during decompression" by Louis Jankowski and Ron Nishi. (This paper is published in Undersea and Hyperbaric Medicine. Ed)
Hal Koch and Peter Tikuisis discussing a poster with an audience member at the break.
