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The function of decompression models has changed with the availability of Doppler ultrasonic bubble detectors, and is no longer merely to limit symptomatic occurrence of decompression sickness, but also to limit asymptomatic post-dive venous gas bubbles.<ref name="Papadopoulou 2013" /> A number of empirical modifications to dissolved phase models have been made since the identification of venous bubbles by Doppler measurement in asymptomatic divers soon after surfacing.<ref name="Huggins 1981"/>
===Efficiency and safety===
Two criteria that have been used in comparing decompression schedules are efficiency and safety, where decompression efficiency is defined as the ability of a schedule to provide acceptable safety from decompression sickness in the shortest time spent decompressing, and decompression safety, or converely, risk, is measured by the probability of decompression sickness incurred by following a given schedule for a given dive profile. Since it is impracticable to eliminate all risk using current knowledge of the effects of several variables, risk is estimated by statistical analysis of the recorded outcomes of exposure and decompression profiles, and an acceptable risk is stipulated, which may vary depending on the circumstances of the application.<ref name="Edel 1980" />
=== Tissue compartments ===
One attempt at a solution was the development of multi-tissue models, which assumed that different parts of the body absorbed and eliminated gas at different rates. These are hypothetical tissues which are designated as fast and slow to describe the rate of saturation. Each tissue, or compartment, has a different half-life. Real tissues will also take more or less time to saturate, but the models do not need to use actual tissue values to produce a useful result. Models with from one to 16 tissue compartments<ref name="Buhlmann 1984" /> have been used to generate decompression tables, and [[dive computer]]s have used up to 20 compartments.<ref name="Validation workshop" />
For example: Tissues with a high [[lipid]] content can take up a larger amount of nitrogen, but often have a poor blood supply. These will take longer to reach equilibrium, and are described as slow, compared to tissues with a good blood supply and less capacity for dissolved gas, which are described as fast.<ref name="DAN intro to DCS" />
Fast tissues absorb gas relatively quickly, but will generally release it quickly during ascent. A fast tissue may become saturated in the course of a normal recreational dive, while a slow tissue may have absorbed only a small part of its potential gas capacity. By calculating the levels in each compartment separately, researchers are able to construct more effective algorithms. In addition, each compartment may be able to tolerate more or less supersaturation than others. The final form is a complicated model, but one that allows for the construction of algorithms and tables suited to a wide variety of diving.<ref name="DAN intro to DCS" /> A typical dive computer has an 8–12 tissue model, with half times varying from 5 minutes to 400 minutes.<ref name="Validation workshop" /> The [[Bühlmann tables]] use an algorithm with 16 tissues, with half times varying from 4 minutes to 640 minutes.<ref name="Buhlmann 1984" />
Tissues may be assumed to be in series, where dissolved gas must diffuse through one tissue to reach the next, which has different solubility properties, in parallel, where diffusion into and out of each tissue is considered to be independent of the others, and as combinations of series and parallel tissues, which becomes computationally complex.<ref name="Goldman 2007" />
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===Ingassing model===
<!-- target for redirect [[Ingassing]] -->
The half time of a tissue is the time it takes for the tissue to take up or release 50% of the difference in dissolved gas capacity at a changed partial pressure. For each consecutive half time the tissue will take up or release half again of the cumulative difference in the sequence ½, ¾, 7/8, 15/16, 31/32, 63/64 etc.<ref name="Bookspan"/> Tissue compartment half times range from 1 minute to at least 720 minutes.{{sfn|Yount|1991|p=137}} A specific tissue compartment will have different half times for gases with different solubilities and diffusion rates. Ingassing is generally modeled as following a simple inverse exponential equation where saturation is assumed after approximately four (93.75%) to six (98.44%) half-times depending on the decompression model.<ref name="Huggins 1992 Chapter 2"/><ref name=logodiving /><ref name="Maiken" /> There is normally no phase change during ingassing after the gases are dissolved in the blood of the pulmonary circulation in the lungs. They remain in solution in whichever tissues they reach by perfusion and diffusion, so the model is fairly robust. The exception is for [[isobaric counterdiffusion]] which can induce bubble growth and posssibly bubble formation when a gas of different solubility is introduced to the breathing mixture.{{sfn|Hamilton|Thalmann|2003|pp=477–478}}<ref name="Lambertson 1989" />
This model may not adequately describe the dynamics of outgassing if gas phase bubbles are present.<ref name="Wienke 1990" /><ref name="Yount 1990" />
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=== Decompression obligation ===
A decompression obligation is the presence in the tissues of sufficient dissolved gas that the risk of symptomatic decompression sickness is unacceptable if a direct ascent to surface pressure is made at the prescribed ascent rate for the decompression model in use. A diver with a decompression ceiling can be said to have a decompression obligation, meaning that time must be spent outgassing during the ascent additional to the time spent ascending at the appropriate ascent rate. This time is nominally and most efficiently spent at decompression stops, though outgassing will occur at any depth where the arterial blood and lung gas have a lower partial pressure of the inert gas than the limiting tissue. When a decompression obligation exists, there will be a theoretical safe minimum depth known as the [[decompression ceiling]]. {{visible anchor|Obligatory decompression stops}} will be indicated at a depth at or below the current ceiling.<ref name="Doolette et al 2015" />
=== Time to surface ===
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Gas switching during decompression on open circuit is done primarily to increase the partial pressure of oxygen to increase the [[oxygen window]] effect, while keeping below [[Oxygen toxicity|acute toxicity]] levels. It is well established both in theory and practice, that a higher oxygen partial pressure facilitates a more rapid and effective elimination of inert gas, both in the dissolved state and as bubbles.
In closed circuit rebreather diving the oxygen partial pressure throughout the dive is maintained at a relatively high but tolerable level to reduce the ongassing as well as to accelerate offgassing of the diluent gas. Changes from helium-based diluents to nitrogen during ascent are desirable for reducing the use of expensive helium, but have other implications. It is unlikely that changes to nitrogen based decompression gas will accelerate decompression in typical technical bounce dive profiles, but there is some evidence that decompressing on helium-oxygen mixtures is more likely to result in neurological DCS, while nitrogen based decompression is more likely to produce other
==== Altitude exposure, altitude diving and flying after diving ====
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* Early decompression stress biomarkers
* The effects of normobaric oxygen on blood and in DCI first aid
{{expand section|<ref name="
===Practical effectiveness of models===
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The practical efficacy of gas switches from helium-based diluent to nitrox for accelerating decompression has not been demonstrated convincingly. These switches increase risk of inner ear decompression sickness due to counterdiffusion effects.<ref name="Mitchell 2016" />
Besides the basic dive profile and gas mixes, and the residual gas load from previous dives, three groups of factors are considered likely to have significant influence on decompression stress, the evolution of bubbles in the diver, and development of symptoms. These are exercise, before, during and after the dive, Thermal status, during and after the dive, including the effects on perfusion distribution and changes during the dive, and the set of factors grouped under the label "predisposition", such as the state of hydration, physical fitness, age, biological health, and other characteristics which could affect the uptake and release of gases in the diver. Currently these factors cannot be used to make reproducible predictions about decompression risk, and some cannot be numerically evaluated in real time.<ref name="Fogarty 2025" />
{{expand section|from<ref name="Blömeke 2024" >{{cite magazine |url=https://indepthmag.com/thalmann-algorithm/ |title=Dial In Your DCS Risk with the Thalmann Algorithm |magazine=InDepth |first=Tim |last=Blömeke |date=3 April 2024 |access-date=15 April 2024 |archive-date=16 April 2024 |archive-url=https://web.archive.org/web/20240416190438/https://indepthmag.com/thalmann-algorithm/ |url-status=live }}</ref> |date=April 2024}}
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<ref name="DAN data uploads">{{cite web |url=https://www.daneurope.org/send-your-dive-profile |title=Send your Dive Profile |last=<!-- not specified --> |website=daneurope.org |publisher=DAN Europe |access-date=13 February 2016 |archive-date=8 April 2016 |archive-url=https://web.archive.org/web/20160408170051/http://www.daneurope.org/send-your-dive-profile |url-status=live }}</ref>
<ref name="DAN intro to DCS" >{{cite web |url=https://dan.org/health-medicine/health-resource/dive-medical-reference-books/decompression-sickness/introduction/ |title=Chapter 1: Introduction to Decompression Sickness |website=dan.org |access-date=31 August 2025 }}</ref>
<ref name="DAN projects">{{cite web |url=https://www.daneurope.org/our-projects |title=Our Projects |last=<!-- not specified --> |work=DAN Europe website |access-date=13 February 2016 |archive-date=11 April 2016 |archive-url=https://web.archive.org/web/20160411164440/https://www.daneurope.org/our-projects |url-status=dead }}</ref>
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<ref name="Eckenhoff 1986" >{{cite journal |title=Direct ascent from shallow air saturation exposures |journal=Undersea Biomedical Research |volume=13 |issue=3 |pages=305–16 |last1=Eckenhoff |first1=R.G. |last2=Osborne |first2=S.F. |last3=Parker |first3=J.W. |last4=Bondi |first4=K.R. |year=1986 |publisher=Undersea and Hyperbaric Medical Society, Inc. |pmid= 3535200 }}</ref>
<ref name="Edel 1980" >{{cite report |url=https://diving-rov-specialists.com/index_htm_files/scient-b_73-analysis-deco-tables-calculated-by-non-u_s.pdf |title=Analysis of Decompression Tables Calculated by non-U.S. Navy Methods |via=diving-rov-specialists.com |first=Peter O. |last=Edel |date= 31 March 1980 |publisher=Sea-Space Research Company Inc. |___location=Harvey, Louisiana }}</ref>
<ref name="EOW" >{{cite journal |title=The Extended Oxygen Window Concept for Programming Saturation Decompressions Using Air and Nitrox |last1=Kot |first1=Jacek |first2=Zdzislaw |last2=Sicko |first3=Tadeusz |last3=Doboszynski |year=2015 |journal=PLOS ONE |doi=10.1371/journal.pone.0130835 |pages=1–20 |pmid=26111113 |pmc=4482426 |volume=10 |issue = 6 |bibcode=2015PLoSO..1030835K |doi-access=free }}</ref>
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<ref name="FAA" >{{cite web |url=http://www.faa.gov/pilots/safety/pilotsafetybrochures/media/dcs.pdf |publisher=[[Federal Aviation Administration]] |title=Altitude-induced Decompression Sickness |access-date=21 February 2012 |archive-date=3 February 2012 |archive-url=https://web.archive.org/web/20120203141302/http://www.faa.gov/pilots/safety/pilotsafetybrochures/media/dcs.pdf |url-status=live }}</ref>
<ref name="Flook 2004" >{{cite book |last=Flook |first=Valerie |title=Excursion tables in saturation diving - decompression implications of current UK practice
<ref name="
<ref name=gernhardt >{{cite journal |last=Gernhardt |first=M.L. |title=Biomedical and Operational Considerations for Surface-Supplied Mixed-Gas Diving to 300 FSW. |editor1-last=Lang |editor1-first=M.A. |editor2-last=Smith |editor2-first=N.E. |journal=Proceedings of Advanced Scientific Diving Workshop |publisher=Smithsonian Institution |place=Washington, DC |year=2006 }}</ref>
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<ref name="LeMessurier and Hills" >{{cite journal |last1=LeMessurier |first1=H. |last2=Hills |first2=B.A. |year=1965 |title=Decompression Sickness. A thermodynamic approach arising from a study on Torres Strait diving techniques |journal=Hvalradets Skrifter |volume=48 |pages=54–84 }}</ref>
<ref name=logodiving >{{cite web |url=
<ref name="Maiken" >{{cite web |url=http://www.decompression.org/maiken/Bubble_Decompression_Strategies.htm |title=Part I: background and theory. Bubble physics |last=Maiken |first=Eric |year=1995 |work=Bubble Decompression Strategies |access-date=11 March 2016 |archive-date=12 April 2016 |archive-url=https://web.archive.org/web/20160412210256/http://www.decompression.org/maiken/Bubble_Decompression_Strategies.htm |url-status=live }}</ref>
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