Bühlmann decompression algorithm: Difference between revisions

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{{Short description|Mathematical model of tissue inert gas uptake and release with pressure change}}

The '''Bühlmann decompression model''' is a [[neo-HaldanianHaldane's decompression model|neo-Haldanian model]] which modelsuses Haldane's or Schreiner's formula for inert gas uptake, a linear expression for tolerated inert gas pressure coupled with a simple parameterised expression for alveolar inert gas pressure and expressions for combining Nitrogen and Helium parameters to model the way [[inert gases]] enter and leave the human body as the [[ambient pressure]] and inspired gas changes.<ref name="AAB1984" /> VersionsDifferent parameter sets are used to create [[decompression tables]] and in personal dive computers to compute no-decompression limits and decompression schedules for dives in real-time, allowing [[Underwater diving|divers]] to plan the depth and duration for dives and the required [[decompression stops]].

The model (Haldane, 1908)<ref name="haldane" /> assumes [[Decompression theory#Perfusion limitedlimitedar tissues and parallel tissue models|perfusion limited gas exchange and multiple parallel tissue compartments]] and uses an [[Exponential decay|inverse exponential modelformula]] for in-gassing and out-gassing, both of which are assumed to occur in the dissolved phase.

Bühlmann, however, assumes that safe dissolved inert gas levels are defined by a critical difference instead of a critical ratio.

Multiple sets of parameters were developed by Swiss physician Dr. [[Albert A. Bühlmann]], who did research into decompression theory at the Laboratory of Hyperbaric Physiology at the University Hospital in [[Zürich]], [[Switzerland]].<ref name="AAB1982" /><ref name="spums1999" />

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Building on the previous work of [[John Scott Haldane]]<ref name="haldane" /> (The Haldane model, Royal Navy, 1908) and Robert Workman<ref name="Workman65" /> (M-Values, US-Navy, 1965) and working off funding from [[Shell Oil Company]],<ref name=Pressure94/> Bühlmann designed studies to establish the longest [[Half time (physics)|half-times]] of nitrogen and helium in human tissues.<ref name="AAB1984" /> These studies were confirmed by the ''Capshell'' experiments in the [[Mediterranean Sea]] in 1966.<ref name="Pressure94" /><ref name="pmid6053671" />

===Alveolar inert gas pressure===

The basic idea (Haldane, 1908)<ref name="haldane" /> is to represent the human body by multiple tissues (compartments) of different saturation half-times and to calculate the partial pressure <math>P</math> of the inert gases in each of the <math>n</math> compartments (Haldane's equation):

The Bühlmann model uses a simplified version of the [[alveolar gas equation]] to calculate alveolar inert gas pressure

<big><math>PP_{alv} = P_0[P_{amb} +- (P_{gasH_{2}0} - P_0)+ \cdot (frac{1 - 2^RQ}{-\frac{t_{exp}RQ} P_{t_CO_{1/2}}})]\cdot Q</math></big>

with the initial partial pressureWhere <math>P_0P_{H_{2}0}</math>, is the partialwater vapour pressure inat the37°C breathing(conventionally gasdefined as 0.0627 bar), <math>P_{gasCO_{2}}</math> (minus the vapourcarbon dioxide pressure of(conventionally waterdefined inas the lung of about0.0534 60&nbsp;mbarbar), <math>Q</math> the timeinspired ofinert exposuregas fraction, and <math>t_{exp}RQ</math> andthe respiratory coefficient: the compartment-specificratio of carbon dioxide production to oxygen consumption. The Buhlmann saturationmodel half-timesets <math>t_{1/2}RQ</math>. to 1, simplifying the equation to

<math>P_{alv} = [P_{amb} - P_{H_{2}0}]\cdot Q</math>

When the gas pressure drops, the compartments start to off-gas.

===Tissue inert gas exchange===

===Nitrogen (air, nitrox) set of parameters===

Inert gas exchange in haldanian models is assumed to be perfusion limited and is governed by the ordinary differential equation

To calculate the maximum tolerable pressure <math>P_{t.tol}i.g.</math>, the constants <math>a</math> and <math>b</math>, which are derived from the saturation half-time as follows (ZH-L 16 A):

<math>a = \fracdfrac{2\,\textmathrm{atmd}P_t}{\sqrt[3]mathrm{t_{1/2d}t} = k(P_{alv} - P_t)</math>

This equation can be solved for constant <math>P_{alv}</math> to give the Haldane equation:

<math>b = 1.005 - \frac{1}{\sqrt[2]{t_{1/2}}}</math>

<math>P_t(t) = P_{alv} + (P_{t}(0) - P_{alv}) \cdot e^{-kt}</math>

are used to calculate M-Value (<math>P_{t.tol}i.g.</math>):

and for constant rate of change of alveolar gas pressure <math>R</math> to give the Schreiner equation:

<math>P_{t.tol}i.g.= \frac{P_{amb.}}{b}+a</math>

<math>

P_t(t) = P_{alv}(0) + R(t - \dfrac{1}{k}) - (P_{alv}(0) - P_{t}(0) - \dfrac{R}{k}) e^{-kt}

</math>

===Tissue inert gas limits===

Similarly to Workman, the Bühlmann model specifies an affine relationship between ambient pressure and inert gas saturation limits. However, the Buhlmann model expresses this relationship in terms of absolute pressure

<math>P_{igtol} = a + \frac{P_{amb}}{b}</math>

Where <math>P_{igtol}</math> is the inert gas saturation limit for a given tissue and <math>a</math> and <math>b</math> constants for that tissue and inert gas.

The constants <math>a</math> and <math>b</math>, were originally derived from the saturation half-time using the following expressions:

<math>a = \frac{2\,\text{bar}}{\sqrt[3]{t_{1/2}}}</math>

<math>b = 1.005 - \frac{1}{\sqrt[2]{t_{1/2}}}</math>

The <math>b</math> values calculated do not precisely correspond to those used by Bühlmann for tissue compartments 4 (0.7825 instead of 0.7725) and 5 (0.8126 instead of 0.8125).<ref name="Buhlmann-a-b-2002" />

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Versions B and C have manually modified<ref name="Buhlmann-a-b-2002" /> the coefficient <math>a</math>.

In addition to this formulation, the Bühlmann model also specifies how the constants for multiple inert gas saturation combine when both Nitrogen and Helium are present in a given tissue.

===Versions===

Several versions of the Bühlmann set of parameters have been developed, both by Bühlmann and by later workers. The naming convention used to identify the set of parameters is a code starting ZH-L, from Zürich (ZH), Linear (L) followed by the number of different (a,b) couples (ZH-L 12 and ZH-L 16)<ref name="Buhlmann-a-b" />) or the number of tissue compartments (ZH-L 6, ZH-L 8), and other unique identifiers.

<math>a = a_{N_2} (1 - R) + a_{He} R</math>

For example:

<math>b = b_{N_2} (1 - R) + b_{He} R</math>

where <math>a_{N_2}</math> and <math>a_{He}</math> are the tissue's <math>a</math> Nitrogen and Helium coefficients and <math>R</math> the ratio of dissolved Helium to total dissolved inert gas.

===Ascent rates===

Ascent rate is intrinsically a variable, and may be selected by the programmer or user for table generation or simulations, and measured as real-time input in dive computer applications.

The rate of ascent to the first stop is limited to 3 bar per minute for compartments 1 to 5, 2 bar per minute for compartments 6 and 7, and 1 bar per minute for compartments 8 to 16. Chamber decompression may be continuous, or if stops are preferred they may be done at intervals of 1 or 3 m.<ref name="Buhlmann 1984" />

==Applications==

The Buhlmann model has been used within dive computers and to create tables.

===Tables===

Since precomputed tables cannot take into account the actual diving conditions, Buhlmann specifies a number of initial values and recommendations.

* Atmospheric pressure

* Water density

* Initial tissue loadings

* Descent rate

* Breathing gas

* Ascent rate

In addition, Buhlmann recommended that the calculations be based on a slightly deeper bottom depth.

===Dive computers===

Buhlmann assumes no initial values and makes no other recommendations for the application of the model within dive computers, hence all pressures and depths and gas fractions are either read from the computer sensors or specified by the diver and grouped dives do not require any special treatment.

==Versions==

Several versions and extensions of the Bühlmann model have been developed, both by Bühlmann and by later workers. The naming convention used to identify the set of parameters is a code starting ZH-L, from Zürich (ZH), Linear (L) followed by the number of different (a,b) couples (ZH-L 12 and ZH-L 16)<ref name="Buhlmann-a-b" />) or the number of tissue compartments (ZH-L 6, ZH-L 8), and other unique identifiers.

{| class="wikitable"

|+ZH-L16C Parameters (bar minute units)

|-

! !! 1 !! 2 !! 3 !! 4 !! 5 !! 6 !! 7 !! 8 !! 9 !! 10 !! 11 !! 12 !! 13 !! 14 !! 15 !! 16

|-

| <math>h_{N2}</math> || 5.0|| 8.0|| 12.5|| 18.5|| 27.0|| 38.3|| 54.3|| 77.0|| 109.0|| 146.0|| 187.0|| 239.0|| 305.0|| 390.0|| 498.0|| 635.0

|-

| <math>a_{N2}</math> || 1.1696|| 1.0|| 0.8618|| 0.7562|| 0.62|| 0.5043|| 0.441|| 0.4|| 0.375|| 0.35|| 0.3295|| 0.3065|| 0.2835|| 0.261|| 0.248|| 0.2327

|-

| <math>b_{N2}</math> || 0.5578|| 0.6514|| 0.7222|| 0.7825|| 0.8126|| 0.8434|| 0.8693|| 0.8910|| 0.9092|| 0.9222|| 0.9319|| 0.9403|| 0.9477|| 0.9544|| 0.9602|| 0.9653

|-

| <math>h_{He}</math> || 1.88|| 3.02|| 4.72|| 6.99|| 10.21|| 14.48|| 20.53|| 29.11|| 41.20|| 55.19|| 70.69|| 90.34|| 115.29|| 147.42|| 188.24|| 240.03

|-

| <math>a_{He}</math> || 1.6189|| 1.383|| 1.1919|| 1.0458|| 0.922|| 0.8205|| 0.7305|| 0.6502|| 0.595|| 0.5545|| 0.5333|| 0.5189|| 0.5181|| 0.5176|| 0.5172|| 0.5119

|-

| <math>b_{He}</math> || 0.4770|| 0.5747|| 0.6527|| 0.7223|| 0.7582|| 0.7957|| 0.8279|| 0.8553|| 0.8757|| 0.8903|| 0.8997|| 0.9073|| 0.9122|| 0.9171|| 0.9217|| 0.9267

|}

'''ZH-L 12 (1983)'''

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* ZH-L 16 C (air, nitrox): The set of parameters with more conservative “a” values for tissue compartments #5 to 15. For use in dive computers.

* ZH-L 16 (helium): The set of parameters for use with helium.

* ZH-L 16 ADT MB: set of parameters and specific algorithm used by Uwatec for their trimix-enabled computers. Modified in the middle compartments from the original ZHL-C, is adaptive to diver workload and includes [[Decompression practice#Profile determined intermediate stops|Profile-Determined Intermediate Stops]]. Profile modification is by means of "MB Levels", personal option conservatism settings, which are not defined in the manual.<ref name="Galileo" >{{cite book|title=Technical diving software for Galilio: User manual |url=https://www.scubapro.com/sites/scubapro_site/files/trimix_user_manual-eng.pdf |author=<!--not specified--> |publisher=Scubapro |access-date = 18 September 2019 }}</ref>

'''ZH-L 6 (1988)'''

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* ZH-L 8 ADT MB: A version of the ZHL-8 ADT claimed to suppress MicroBubble formation.<ref name="Smart" />

* ZH-L 8 ADT MB PDIS: [[Decompression practice#Profile determined intermediate stops|Profile-Determined Intermediate Stops]].<ref name="dykcen">{{cite web|url=http://www.dykcen.dk/PDF/Instruktor%20info/PDIS_Algorithm.pdf|title=Diving with PDIS (Profile-Dependent Intermediate Stop)|last=Staff|work=Dykkercentret website|publisher=Dykkercentret ApS|access-date=5 March 2016|___location=Frederiksberg|archive-url=https://web.archive.org/web/20161017170523/http://www.dykcen.dk/PDF/Instruktor%20info/PDIS_Algorithm.pdf|archive-date=17 October 2016|url-status=dead|df=dmy-all}}</ref>

* ZH-L 8 ADT MB PMG: Predictive Multi-Gas.<ref name="Scubapro-Luna-PMG" />

==Ascent rates==

Ascent rate is intrinsically a variable, and may be selected by the programmer or user for table generation or simulations, and measured as real-time input in dive computer applications.

The rate of ascent to the first stop is limited to 3 bar per minute for compartments 1 to 5, 2 bar per minute for compartments 6 and 7, and 1 bar per minute for compartments 8 to 16. Chamber decompression may be continuous, or if stops are preferred they may be done at intervals of 1 or 3 m.<ref name="Buhlmann 1984" />

==References==

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|isbn=978-3-642-55939-6 |doi=10.1007/978-3-642-55939-6 |publisher=Springer-Verlag, p. 158 |date=2002 |lang=de }}</ref>

<ref name="Völlm-1">{{cite web |url=https://www.divetable.info/BS_ZH/Buehlmann_Algorithms.pdf |title=Bühlmann algorithm for dive computers |last=Völlm |first=Ernst |access-date=29 July 2023 |archive-date=10 September 2024 |archive-url=https://web.archive.org/web/20240910013821/https://www.divetable.info/BS_ZH/Buehlmann_Algorithms.pdf |url-status=live }}</ref>

<ref name="Buhlmann-2019">{{cite web|url=https://www.divetable.info/BS_ZH/altitude_diving.pdf |title=Bühlmann Memorial Symposium 29.03.2019|last=Mueller|first=Beat|access-date=29 July 2023|archive-date=19 April 2022|archive-url=https://web.archive.org/web/20220419155841/https://www.divetable.info/BS_ZH/altitude_diving.pdf|url-status=live}}</ref>

<ref name="dykcen">{{cite web|url=http://www.dykcen.dk/PDF/Instruktor%20info/PDIS_Algorithm.pdf|title=Diving with PDIS (Profile-Dependent Intermediate Stop)|last=Staff|work=Dykkercentret website|publisher=Dykkercentret ApS|access-date=5 March 2016|___location=Frederiksberg|archive-url=https://web.archive.org/web/20161017170523/http://www.dykcen.dk/PDF/Instruktor%20info/PDIS_Algorithm.pdf|archive-date=17 October 2016|url-status=dead|df=dmy-all}}</ref>

<ref name="Scubapro-Luna-PMG">{{cite web|url=https://scubapro.johnsonoutdoors.com/sites/default/files/2022-08/galileo_luna_pmg_multilanguage_3.pdf |title=Predictive Multi Gas for Galileo Luna|last=Scubapro|access-date=29 July 2023 }}</ref>

<ref name="Galileo" >{{cite book |title=Technical diving software for Galilio: User manual |url=https://www.scubapro.com/sites/scubapro_site/files/trimix_user_manual-eng.pdf |author=<!--not specified--> |publisher=Scubapro |access-date=18 September 2019 |archive-date=13 April 2019 |archive-url=https://web.archive.org/web/20190413220658/https://www.scubapro.com/sites/scubapro_site/files/trimix_user_manual-eng.pdf |url-status=live }}</ref>

<ref name="haldane">{{cite journal |last1=Boycott |first1=A.E. |last2=Damant |first2=G.C.C. |last3=Haldane |first3=John Scott |author-link3=John Scott Haldane |title=Prevention of compressed air illness |journal=Journal of Hygiene |publisher=Cambridge University Press |volume=8 |issue=3 |pages=342–443 |year=1908 |url=http://archive.rubicon-foundation.org/7489 |pmid=20474365 |access-date=2009-06-12 |pmc=2167126 |doi=10.1017/S0022172400003399 |archive-url=https://web.archive.org/web/20110324171946/http://archive.rubicon-foundation.org/7489 |archive-date=2011-03-24 |url-status=usurped }}</ref>

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<ref name="Pressure94">{{cite journal |last1=Völlm |first1=T.G. |title=Leading diving researcher dies unexpectedly: Albert A Bühlmann, 1923 - 1994 |journal=Pressure, Newsletter of the Undersea and Hyperbaric Medical Society |volume=23 |issue=3 |year=1994 |pages=1–3 |issn=0889-0242 }}</ref>

<ref name="Scubapro-Luna-PMG">{{cite web |url=https://scubapro.johnsonoutdoors.com/sites/default/files/2022-08/galileo_luna_pmg_multilanguage_3.pdf |title=Predictive Multi Gas for Galileo Luna |last=Scubapro |access-date=29 July 2023 |archive-date=29 July 2023 |archive-url=https://web.archive.org/web/20230729105615/https://scubapro.johnsonoutdoors.com/sites/default/files/2022-08/galileo_luna_pmg_multilanguage_3.pdf |url-status=live }}</ref>

<ref name="spums1999">{{cite journal |last1=Wendling |first1=J |last2=Nussberger |first2=P |last3=Schenk |first3=B |title=Milestones of the deep diving research laboratory Zurich |journal=South Pacific Underwater Medicine Society Journal |volume=29 |issue=2 |year=1999 |issn=0813-1988 |oclc=16986801 |url=http://archive.rubicon-foundation.org/6003 |access-date=2009-04-02 |archive-url=https://web.archive.org/web/20120203155311/http://archive.rubicon-foundation.org/xmlui/handle/123456789/6003 |archive-date=2012-02-03 |url-status=usurped }}</ref>

<ref name="Smart">{{cite web|url=http://users.skynet.be/sky68333/Theorie/Tabellen/Ver_bijl.pdf|title=Smart microbubble management|last=Staff|work=In Depth|publisher=Uwatec|access-date=12 March 2016|archive-date=21 September 2005|archive-url=https://web.archive.org/web/20050921115411/http://users.skynet.be/sky68333/Theorie/Tabellen/Ver_bijl.pdf|url-status=dead}}</ref>

<ref name="Workman65">{{cite journal |last=Workman |first=Robert D. |title=Calculation of decompression schedules for nitrogen-oxygen and helium-oxygen dives.|year=1965 |journal=Navy Experimental Diving Unit Panama City Fl |volume=Research rept. |url=https://apps.dtic.mil/sti/citations/AD0620879 |access-date=2023-07-29 }}</ref>

<ref name="Workman65">{{cite journal |last=Workman |first=Robert D. |title=Calculation of decompression schedules for nitrogen-oxygen and helium-oxygen dives.|year=1965 |journal=Navy Experimental Diving Unit Panama City Fl |volume=Research rept. |url=https://apps.dtic.mil/sti/citations/AD0620879 |access-date=2023-07-29 }}</ref>

.<ref name="Smart">{{cite web|url=http://users.skynet.be/sky68333/Theorie/Tabellen/Ver_bijl.pdf|title=Smart microbubble management|last=Staff|work=In Depth|publisher=Uwatec|access-date=12 March 2016|archive-date=21 September 2005|archive-url=https://web.archive.org/web/20050921115411/http://users.skynet.be/sky68333/Theorie/Tabellen/Ver_bijl.pdf|url-status=dead}}</ref>

}}