Thermodynamic model of decompression
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The thermodynamic model was one of the first decompression models in which decompression is controlled by the volume of gas bubbles coming out of solution. In this model, pain only DCS is modelled by a single tissue which is diffusion-limited for gas uptake and bubble-formation during decompression causes "phase equilibration" of partial pressures between dissolved and free gases. The driving mechanism for gas elimination in this tissue is inherent unsaturation, also called partial pressure vacancy or the oxygen window, where oxygen metabolised is replaced by more soluble carbon dioxide. This model was used to explain the effectiveness of the Torres Straits Island pearl divers empirically developed decompression schedules, which used deeper decompression stops and less overall decompression time than the current naval decompression schedules. This trend to deeper decompression stops has become a feature of more recent decompression models.[1]
Brian A. Hills analysed the existing decompression hypotheses frequently referenced in the literature of the time, and identified three basic characteristics of comprehensive theoretical approaches to modeling decompression:[2]
- The number and composition of tissues involved
- A mechanism and controlling parameters for onset of identifiable symptoms
- A mathematical model for gas transport and distribution
Hills found no evidence of discontinuity in the incidence of decompression symptoms for exposure/depth variations, which he interpreted as suggesting that either a single critical tissue or a continuous range of tissues are involved, and that correlation was not improved by assuming an infinite range of half times in a conventional exponential model. [2] After later experimental work he concluded that the imminence of decompression sickness is more likely to be indicated by the quantity of gas separating from solution (the critical volume hypothesis) than its mere presence (as determined by a critical limit to supersaturation) and suggested that this implies that conventional (Haldanian) schedules are actually treating an asymptomatic gas phase in the tissues and not preventing the separation of gas from solution.[3]
Efficient decompression will minimize the total ascent time while limiting the total accumulation of bubbles to an acceptable non-symptomatic critical value. The physics and physiology of bubble growth and elimination indicate that it is more efficient to eliminate bubbles while they are very small. Models which include bubble phase have produced decompression profiles with slower ascents and deeper initial decompression stops as a way of curtailing bubble growth and facilitating early elimination, in comparison with the models which consider only dissolved phase gas.[4]
According to the thermodynamic model, the condition of optimum driving force for outgassing is satisfied when the ambient pressure is just sufficient to prevent phase separation (bubble formation). The fundamental difference of this approach is equating absolute ambient pressure with the total of the partial gas tensions in the tissue for each gas after decompression as the limiting point beyond which bubble formation is expected.[2]
The model assumes that the natural unsaturation in the tissues due to metabolic reduction in oxygen partial pressure provides the buffer against bubble formation, and that the tissue may be safely decompressed provided that the reduction in ambient pressure does not exceed this unsaturation value. Clearly any method which increases the unsaturation would allow faster decompression, as the concentration gradient would be greater without risk of bubble formation.[2]
The natural unsaturation increases with depth, so a larger ambient pressure differential is possible at greater depth, and reduces as the diver surfaces. This model leads to slower ascent rates and deeper first stops, but shorter shallow stops, as there is less bubble phase gas to be eliminated.[2]
Further development
The bubble models of decompression are the logical development from this model. The critical-volume criterion assumes that whenever the total volume of gas phase accumulated in the tissues exceeds a critical value, signs or symptoms of DCS will appear. This assumption is supported by doppler bubble detection surveys. The consequences of this approach depend strongly on the bubble formation and growth model used, primarily whether bubble formation is practicably avoidable during decompression.[5]
This approach is used in decompression models which assume that during practical decompression profiles, there will be growth of stable microscopic bubble nuclei which always exist in aqueous media, including living tissues.[6]
Varying Permeability Model
The Varying Permeability Model (VPM) of David Yount et al.
Reduced Gradient Bubble Model
The Reduced Gradient Bubble Model (RGBM) of Bruce Wienke
References
- ^ Doolette, DJ (2006). "A personal view of Brian Hills' contribution to decompression theory and practice". Journal of the South Pacific Underwater Medicine Society and the European Underwater and Baromedical Society.
- ^ a b c d e LeMessurier, D.H.; Hills, B.A. (1965). "Decompression Sickness. A thermodynamic approach arising from a study on Torres Strait diving techniques". Hvalradets Skrifter. Nr. 48: 54–84.
{{cite journal}}:|volume=has extra text (help) - ^ Hills, B.A. (1970). "Limited Supersaturation versus Phase Equilibration in Predicting the Occurrence of Decompression Sickness". Clinical Science, 32 (2). Portland Press. pp. 251–267. doi:10.1042/cs0380251. Retrieved 26 April 2016.
- ^ Yount, David E.; Hoffman, DC (1984). "Decompression theory: a dynamic critical-volume hypothesis" (PDF). Underwater physiology VIII: Proceedings of the eighth symposium on underwater physiology. Bethesda: Undersea Medical Society. pp. 131–146. Retrieved 9 May 2016.
{{cite web}}: Unknown parameter|editors=ignored (|editor=suggested) (help) - ^ Yount, David E. (2002). "Decompression theory - Bubble models : Applying VPM to diving" (PDF). Diving Science:. Deep Ocean Diving. p. 8. Retrieved 9 May 2016.
{{cite web}}: CS1 maint: extra punctuation (link) - ^ Wienke, BR (1989). "Tissue gas exchange models and decompression computations: a review". Undersea and Hyperbaric Medical Society, Inc. PMID 2648656. Retrieved 7 March 2016.
External links
- Resources from Rubicon Research Repository related to the work of B.A. Hills
- Wienke, BR (1987). "Thermodynamic decompression". American Academy of Underwater Sciences. Retrieved 9 May 2016.
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