Thermodynamic model of decompression
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]
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 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.
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.