Thermodynamic model of decompression

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Decompression profiles based on the Thermodynamic model compared with the US Navy table for the same depth and bottom time Thermodynamic vs US Navy decomnpression profiles 1.svg
Decompression profiles based on the Thermodynamic model compared with the US Navy table for the same depth and bottom time

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]

Contents

Concept

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]

  1. The number and composition of tissues involved;
  2. A mechanism and controlling parameters for onset of identifiable symptoms;
  3. 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, an effect variously known as the oxygen window, partial pressure vacancy and inherent 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]

Natural unsaturation also increases with increase in partial pressure of oxygen in the breathing gas. [5]

The thermodynamic model is based on the following assumptions: [6]

The requirement to maintain an ambient pressure high enough to prevent bubble growth leads to a significantly deeper first stop than the dissolved phase models which assume that bubbles do not form during asymptomatic decompression. [6]

This model was a radical change from the traditional dissolved phase models. Hills was met with considerable skepticism and after several years of advocating two-phase models, eventually turned to other fields of research. Eventually, the work of other researchers provided enough impact to gain widespread acceptance for bubble models, and the value of Hills' research was recognised. [6]

Further development

The bubble models of decompression are a 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. [7]

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. [8]

Varying Permeability Model

The Varying Permeability Model (VPM) is a decompression algorithm developed by D.E. Yount and others for use in professional and recreational diving. It was developed to model laboratory observations of bubble formation and growth in both inanimate and in vivo systems exposed to pressure. [9] The VPM presumes that microscopic bubble nuclei always exist in water and tissues that contain water. Any nuclei larger than a specific "critical" size, which is related to the maximum dive depth will grow during decompression. The VPM aims to minimize the total volume of these growing bubbles by keeping the external pressure relatively large, and the inspired inert gas partial pressures low during decompression.

Reduced Gradient Bubble Model

The reduced gradient bubble model (RGBM) is a decompression algorithm developed by Dr Bruce Wienke. It is related to the Varying Permeability Model. [10] but is conceptually different in that it rejects the gel-bubble model of the varying permeability model. [11]

It is used in several dive computers, particularly those made by Suunto, Aqwary, Mares, HydroSpace Engineering, [10] and Underwater Technologies Center. It is characterised by the following assumptions: blood flow (perfusion) provides a limit for tissue gas penetration by diffusion; an exponential distribution of sizes of bubble seeds is always present, with many more small seeds than large ones; bubbles are permeable to gas transfer across surface boundaries under all pressures; the haldanean tissue compartments range in half time from 1 to 720  minutes, depending on gas mixture. [10]

Related Research Articles

<span class="mw-page-title-main">Decompression sickness</span> Disorder caused by dissolved gases forming bubbled in tissues

Decompression sickness is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompression. DCS most commonly occurs during or soon after a decompression ascent from underwater diving, but can also result from other causes of depressurisation, such as emerging from a caisson, decompression from saturation, flying in an unpressurised aircraft at high altitude, and extravehicular activity from spacecraft. DCS and arterial gas embolism are collectively referred to as decompression illness.

<span class="mw-page-title-main">Dive computer</span> Instrument to calculate decompression status in real time

A dive computer, personal decompression computer or decompression meter is a device used by an underwater diver to measure the elapsed time and depth during a dive and use this data to calculate and display an ascent profile which, according to the programmed decompression algorithm, will give a low risk of decompression sickness.

Diving physics, or the physics of underwater diving is the basic aspects of physics which describe the effects of the underwater environment on the underwater diver and their equipment, and the effects of blending, compressing, and storing breathing gas mixtures, and supplying them for use at ambient pressure. These effects are mostly consequences of immersion in water, the hydrostatic pressure of depth and the effects of pressure and temperature on breathing gases. An understanding of the physics is useful when considering the physiological effects of diving, breathing gas planning and management, diver buoyancy control and trim, and the hazards and risks of diving.

The reduced gradient bubble model(RGBM) is an algorithm developed by Bruce Wienke for calculating decompression stops needed for a particular dive profile. It is related to the Varying Permeability Model. but is conceptually different in that it rejects the gel-bubble model of the varying permeability model.

The Varying Permeability Model, Variable Permeability Model or VPM is an algorithm that is used to calculate the decompression stops needed for ambient pressure dive profiles using specified breathing gases. It was developed by D.E. Yount and others for use in professional diving and recreational diving. It was developed to model laboratory observations of bubble formation and growth in both inanimate and in vivo systems exposed to pressure. In 1986, this model was applied by researchers at the University of Hawaii to calculate diving decompression tables.

In diving and decompression, the oxygen window is the difference between the partial pressure of oxygen (PO2) in arterial blood and the PO2 in body tissues. It is caused by metabolic consumption of oxygen.

The Thalmann Algorithm is a deterministic decompression model originally designed in 1980 to produce a decompression schedule for divers using the US Navy Mk15 rebreather. It was developed by Capt. Edward D. Thalmann, MD, USN, who did research into decompression theory at the Naval Medical Research Institute, Navy Experimental Diving Unit, State University of New York at Buffalo, and Duke University. The algorithm forms the basis for the current US Navy mixed gas and standard air dive tables. The decompression model is also referred to as the Linear–Exponential model or the Exponential–Linear model.

In physiology, isobaric counterdiffusion (ICD) is the diffusion of different gases into and out of tissues while under a constant ambient pressure, after a change of gas composition, and the physiological effects of this phenomenon. The term inert gas counterdiffusion is sometimes used as a synonym, but can also be applied to situations where the ambient pressure changes. It has relevance in mixed gas diving and anesthesiology.

<span class="mw-page-title-main">Decompression (diving)</span> Pressure reduction and its effects during ascent from depth

The decompression of a diver is the reduction in ambient pressure experienced during ascent from depth. It is also the process of elimination of dissolved inert gases from the diver's body which accumulate during ascent, largely during pauses in the ascent known as decompression stops, and after surfacing, until the gas concentrations reach equilibrium. Divers breathing gas at ambient pressure need to ascend at a rate determined by their exposure to pressure and the breathing gas in use. A diver who only breathes gas at atmospheric pressure when free-diving or snorkelling will not usually need to decompress, Divers using an atmospheric diving suit do not need to decompress as they are never exposed to high ambient pressure.

Altitude decompression or hypobaric decompression is the reduction in ambient pressure below the normal range of sea level atmospheric pressure. Altitude decompression is the natural consequence of unprotected elevation to altitude, while hypobaric decompression is due to intentional or unintentional release of pressurisation of a pressure suit or pressurised compartment, vehicle or habitat, and may be controlled or uncontrolled, or the reduction of pressure in a hypobaric chamber.

<span class="mw-page-title-main">Decompression practice</span> Techniques and procedures for safe decompression of divers

The practice of decompression by divers comprises the planning and monitoring of the profile indicated by the algorithms or tables of the chosen decompression model, to allow asymptomatic and harmless release of excess inert gases dissolved in the tissues as a result of breathing at ambient pressures greater than surface atmospheric pressure, the equipment available and appropriate to the circumstances of the dive, and the procedures authorized for the equipment and profile to be used. There is a large range of options in all of these aspects.

<span class="mw-page-title-main">History of decompression research and development</span> Chronological list of notable events in the history of diving decompression.

Decompression in the context of diving derives from the reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the tissues during this reduction in pressure.

<span class="mw-page-title-main">Decompression theory</span> Theoretical modelling of decompression physiology

Decompression theory is the study and modelling of the transfer of the inert gas component of breathing gases from the gas in the lungs to the tissues and back during exposure to variations in ambient pressure. In the case of underwater diving and compressed air work, this mostly involves ambient pressures greater than the local surface pressure, but astronauts, high altitude mountaineers, and travellers in aircraft which are not pressurised to sea level pressure, are generally exposed to ambient pressures less than standard sea level atmospheric pressure. In all cases, the symptoms caused by decompression occur during or within a relatively short period of hours, or occasionally days, after a significant pressure reduction.

<span class="mw-page-title-main">Pyle stop</span> Type of short deep decompression stops in addition to the standard profile

A Pyle stop is a type of short, optional deep decompression stop performed by scuba divers at depths well below the first decompression stop mandated by a conventional dissolved phase decompression algorithm, such as the US Navy or Bühlmann decompression algorithms. They were named after Richard Pyle, an American ichthyologist from Hawaii, who found that they prevented his post-dive fatigue symptoms after deep dives to collect fish specimens.

<span class="mw-page-title-main">Haldane's decompression model</span> Decompression model developed by John Scott Haldane

Haldane's decompression model is a mathematical model for decompression to sea level atmospheric pressure of divers breathing compressed air at ambient pressure that was proposed in 1908 by the Scottish physiologist, John Scott Haldane, who was also famous for intrepid self-experimentation.

<span class="mw-page-title-main">Decompression equipment</span> Equipment used by divers to facilitate decompression

There are several categories of decompression equipment used to help divers decompress, which is the process required to allow divers to return to the surface safely after spending time underwater at higher ambient pressures.

Brian Andrew Hills, born 19 March 1934 in Cardiff, Wales, died 13 January 2006 in Brisbane, Queensland, was a physiologist who worked on decompression theory.

<span class="mw-page-title-main">Physiology of decompression</span> The physiological basis for decompression theory and practice

The physiology of decompression is the aspect of physiology which is affected by exposure to large changes in ambient pressure, and involves a complex interaction of gas solubility, partial pressures and concentration gradients, diffusion, bulk transport and bubble mechanics in living tissues. Gas is breathed at ambient pressure, and some of this gas dissolves into the blood and other fluids. Inert gas continues to be taken up until the gas dissolved in the tissues is in a state of equilibrium with the gas in the lungs,, or the ambient pressure is reduced until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again.

Inner ear decompression sickness, (IEDCS) or audiovestibular decompression sickness is a medical condition of the inner ear caused by the formation of gas bubbles in the tissues or blood vessels of the inner ear. Generally referred to as a form of decompression sickness, it can also occur at constant pressure due to inert gas counterdiffusion effects.

References

  1. ↑ 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.
  2. 1 2 3 4 5 LeMessurier, D.H.; Hills, B.A. (1965). "Decompression Sickness. A thermodynamic approach arising from a study on Torres Strait diving techniques". Hvalradets Skrifter (48): 54–84.
  3. ↑ Hills, B.A. (1970). "Limited Supersaturation versus Phase Equilibration in Predicting the Occurrence of Decompression Sickness". Clinical Science. 38 (2): 251–267. doi:10.1042/cs0380251. PMID   5416153.
  4. ↑ Yount, David E.; Hoffman, DC (1984). Bachrach A.J.; Matzen, M.M. (eds.). "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.
  5. ↑ Van Liew, Hugh D; Conkin, J; Burkard, ME (1993). "The oxygen window and decompression bubbles: estimates and significance". Aviation, Space, and Environmental Medicine. 64 (9): 859–65. ISSN   0095-6562. PMID   8216150.
  6. 1 2 3 Powell, Mark (2008). "Specific bubble models". Deco for Divers. Southend-on-Sea: Aquapress. ISBN   978-1-905492-07-7.
  7. ↑ Yount, David E. (2002). "Decompression theory - Bubble models : Applying VPM to diving" (PDF). Diving Science. Deep Ocean Diving. p. 8. Retrieved 9 May 2016.
  8. ↑ Wienke, BR (1989). "Tissue gas exchange models and decompression computations: a review". Undersea Biomedical Research. 16 (1): 53–89. PMID   2648656. Archived from the original on June 2, 2016. Retrieved 7 March 2016.{{cite journal}}: CS1 maint: unfit URL (link)
  9. ↑ Yount, DE (1991). "Gelatin, bubbles, and the bends". In: Hans-Jurgen, K; Harper Jr, DE (Eds.) International Pacifica Scientific Diving... 1991. Proceedings of the American Academy of Underwater Sciences Eleventh Annual Scientific Diving Symposium held 25–30 September 1991. University of Hawaii, Honolulu, Hawaii. Archived from the original on January 13, 2013.{{cite journal}}: CS1 maint: unfit URL (link)
  10. 1 2 3 Wienke, Bruce R; O’Leary, Timothy R (13 February 2002). "Reduced gradient bubble model: Diving algorithm, basis and comparisons" (PDF). Tampa, Florida: NAUI Technical Diving Operations. pp. 7–12. Retrieved 12 January 2010.
  11. ↑ Campbell, Ernest S (30 April 2009). "Reduced gradient bubble model". Scubadoc's Diving Medicine. Retrieved 12 January 2010. – Bruce Wienke describes the differences between RGBM and VPM
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