SuuntoReduced Gradient

Bubble Model

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Air consists roughly of 78% Nitrogen(N2), 21% Oxygen (O2) and 1% Argon(Ar), Carbon Dioxide (CO2) and othertrace gases. When we scuba dive, wemetabolise the oxygen, but nitrogenbeing an inert gas, is stored in the bodyjust like the unseen gas in an unopenedbottle of soda water.

It all starts in the lungs, where thenitrogen is first diffused through thealveolar and capillary membranes, intosolution in the blood.

This nitrogen rich blood is thendistributed by the arteries to thevarious tissues, throughout the body.This is referred to as on-gassing. Indecompression modelling, these tissuesare commonly called compartments.The deeper and longer we dive; themore nitrogen is taken up by the bodyuntil a tissue finally reaches a point ofsaturation.

During a dive, tissues saturate withnitrogen at different rates. This isdetermined by the blood flow rate tothe tissue in question. The brain, forexample, has a very good blood supply,and is classified as a “fast” tissue, whilstjoints typically have a poor bloodsupply and are rated as “slow” tissue.There are several others in between.

The length of time that it takes fora tissue to reach a 50% saturation levelat depth is called the tissue half time

and it is usually measured in minutes. During ascent the process is

reversed, and the gas is off-loadedfrom the tissues back into the venousblood supply. This blood is thenreturned via the heart to the lungswhere the excess N2 and CO2 diffusesout through the alveoli in the lungs,

and is exhaled. This process is known as off-gassing.

The determining factor as to the extentto which a tissue off-gases is the pressuregradient (the difference between thetissue gas tension and the ambientpressure). There is a further importantfactor called the “oxygen window”.

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10 minute tissue compartment

Surface Interval (min)

5 minute delay beforeasymmetrical off-gassing starts

AsymmetricOff-gassing

ExponentialOn-gassing

Suunto Traditional

The Suunto RGBM uses an exponentialform for calculating the uptake ofnitrogen. However, because of theinfluence of microbubbles, which tendto restrict nitrogen out-gassing, anasymmetrical nitrogen elimination curve,as determined by Dr Merrill Spencer, isused. This system better protects thediver from decompression illness (DCI)and is the first in a series of microbubbleprotective measures used in SuuntoRGBM. Additionally there is an 5 minuteextra delay on the surface beforeoff-gassing starts.

THE CAUSES OF DECOMPRESSION ILLNESS

This is a naturally occurring gas tensionreduction that arises in the actualtissues and venous circulation comparedto the lungs and arterial circulation.This leaves room for the expanding gas,caused by the ascent, to go withoutexceeding the ambient pressureprovided a suitable ascent rate is notexceeded. Suunto’s designed ascentrate of 10m/minute takes full advantageof this phenomenon.

The potential for decompressionillness occurs when the ambientpressure reduction during ascent is toorapid, and some of the excess nitrogenin the body is released from solution toform bubbles. These bubbles may theninterfere with normal body functions,restricting blood flow and causingdamage to tissues and nerves.

For a diver with decompressionillness, the symptoms may commencewhile still underwater or it may takeseveral hours after surfacing. In somecases the symptoms may not show forseveral days.

When air enters the lungs, it goesthrough a maze of smaller and smaller

tubes until it reaches tiny sacs calledalveoli. Woven into the walls of these

sacks are very fine, almost transparent,capillaries. It is here in the blood/gasbarrier of the alveoli where the gas

exchange takes place and oxygen andnitrogen from the air enters the blood,and the carbon dioxide from the body

goes into the air. From here the oxygenand nitrogen rich blood is then carried

to other parts of the body.

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PRACTICAL IMPLICATIONS OF THE SUUNTO RGBM

Suunto dive computers allow the diverto personally adjust the dive computeralgorithm. Settings include personal,altitude and RGBM factor settings(Suunto Vytec).

The RGBM setting allows experienceddivers to choose between the standardRGBM model and an attenuated settingreducing the RGBM effects.

If the Suunto RGBM model predicts anexcess of microbubble build up after thedive, a flashing warning sign willprompt the diver to extend the surfaceinterval.

Ascent rate violations

A dive profile

The Suunto Reduced GradientBubble Model is a state of the artalgorithm for managing bothdissolved gas and free-gas in all itsstages in the tissues and blood of thediver.

It is a significant advance on theclassical Haldane models, which donot predict free-gas (microbubbles).Theadvantage of Suunto RGBM is a moreaccurate representation of what ishappening in the diver’s body,through its ability to adapt to a widevariety of situations.

The Suunto RGBM addresses anumber of diving circumstances thathave not been considered by previousdissolved gas models, adapting to:

• continuous multiday diving

• closely spaced repetitive dives

• dives deeper than the previous dive

• rapid ascents which produce highmicrobubble build up

The Suunto RGBM algorithmautomatically adapts its predictionsof both the effects of microbubblebuild up and adverse dive profiles inthe current dive series. It will furthermodify these calculations accordingto the personal adjustment that adiver can select.

Each tissue compartment in thedecompression model has a maximumtheoretical pressure, called M-value.Depending on the diver’s behaviourduring the dive and the personal

Depending on the conducted diveprofile and possible ascent rateviolations recommended andmandatory safety stops are added.

adjustments set, the Suunto RGBMmodel adjusts the M-values downwardsin order to protect the diver from theeffects of the generated free-gas.

Depending on circumstances, theadjustments done by the SuuntoRGBM may:

• Add Mandatory Safety stops

• Reduce no-decompression stop times

• Increase decompression stop times

• Advise an extended surface interval

Some diving patterns cumulativelyadd a higher risk of DCI, such as diveswith short surface intervals, repetitivedives deeper than earlier ones,multiple ascents and substantial multidaydiving.

SUUNTO’S PERSONAL ADJUSTMENT PROVIDES GREATER ACCURACY

The Suunto RGBM algorithmadapts its predictions of both theeffect of microbubble build up andadverse dive profiles in the currentseries of dives. All Suunto divecomputers are shipped with a defaultsetting that provides the full protectionof the Suunto RGBM algorithm.

However, some more experienceddivers may prefer not to use the fullSuunto RGBM model, and therefore,in the Suunto Vytec it is possible toadjust the algorithm to an AttenuatedRGBM Model which reduces theeffects of the Suunto RGBM model by50%.

Just as it is possible to reduce theeffect of the Suunto RGBM model it issimilarly possible to choose graduallymore conservative parameters fordecompression calculations, whenadverse personal conditions exist. Theprincipal personal factors that show acorrelation with DCI susceptibility

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Some mentioned factorsor conditions exist

Ideal Condition

include: physical and dive fitness; age,particularly for divers over the age of50; fatigue; cold water exposure,which can cause the blood vessels atthe body's extremities to close downand maintain the body's coretemperature; pre-dive exercise, whichcan create new gas nuclei; exerciseduring the dive with blood bringingadditional nitrogen to muscle tissues;post dive exercise which can causesupersaturated blood to coursethorough the blood vessels; tightfitting equipment, which can causeDCI pain around a joint where thelimb is pinched by the suit; hot showersor baths and sunbaking can all raise theskin temperature and reduce thecapacity of the skin to hold nitrogenin solution; and dehydration, whicheffects the micro circulation andtherefore the out-gassing of nitrogenafter diving.

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CONTINUOUS DECOMPRESSION OPTIMISES OFF-GASSING AND BUBBLE SUPPRESSIONTraditionally, since Haldane’s 1908

tables, decompression stops havealways been deployed in fixed stepssuch as 15m, 12m, 9m, 6m and 3m.This practical method was introducedbefore the advent of dive computers.However, when ascending, a diveractually decompresses in a series ofmore gradual mini-steps, effectivelycreating a smooth decompressioncurve.

The advent of microprocessors hasallowed Suunto to more accuratelymodel the actual decompressionbehaviour, and a continuousdecompression curve been includedin the Suunto RGBM’s workingassumption.

It is known that during the ascentthe pressure gradient across thetissues increases. That is the pressureof dissolved gas inside a specific tissuehas increased relative to the drop inthe ambient pressure. If this gradientis allowed to rise too much, thenmicrobubbling, decompression stressor DCI can occur. During the ascent

phase of the dive computer algorithmhas the information, which can beused to better control and limitpressure gradients. It is this need tolimit pressure gradients, which is theorigin of the name Reduced GradientBubble Model.

The fundamental idea behind theSuunto RGBM is to maximise theinternal pressure of any bubble withrespect to tissue tension to dissolvegas out of the bubbles and back intothe tissues. This should leave thevenous circulation and lungs lessrestricted by microbubbles, makingoff-gassing more efficient at thedissolved gas decompression stops.

During any ascent involvingdecompression-stops, Suunto divecomputers calculate the point at

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Suunto Continuous Decompression CurveTraditional Fixed Stops

Decompression Information Icons

Warning, Ceiling exceeded! Return below ceiling.

Ceiling Zone (Optimal decompression)

Decompression "Floor"

SEA FLOOR

The unique continuous decompressionused by Suunto computers provides asmooth and more natural decompressioncurve compared to traditionalpredetermined ceiling depths.

If preferred, the diver may stilldecompress at traditional fixed depths.

Suunto dive computers have anunique feature of displaying not onlythe decompression Ceiling, but also thedecompression Floor.

As long as you are below the “Floor”,i.e. still on-gassing, an upward arrow isdisplayed. Once above the floor, theleading tissues start off-gassing, and theupward arrow disappears. The optimaldecompression occurs in the CeilingZone, which is displayed by both upwardand downward arrows. If the ceilingdepth is violated a downward pointingarrow and an audible alarm will promptthe diver to descend back to the CeilingZone.

which the leading tissue compartmentcrosses the ambient pressure line(that is the point at which the tissue’spressure is greater than the ambientpressure), and off-gassing starts. Thisis referred to as the decompressionfloor. Above this floor depth andbelow the ceiling depth is the“decompression zone”. The range ofthe decompression zone, is dependenton the dive profile.

Out-gassing in the leading fasttissues will be slow at or near thefloor because the outward gradient issmall. Slower tissues may be stillon-gassing and given enough time,the decompression obligation mayincrease, in which case the ceilingmay move down and the floor maymove up. However, dwelling for aminute or two at the floor beforemoving up to the decompressionceiling will also help limit microbubblegrowth, by keeping them compressed.The decompression floor representsthe point at which the Suunto RGBMis seeking to maximise bubble

compression, while the decompression"ceiling" is maximising off-gassing.

The added advantage of having adecompression ceiling and floor isthat it recognises that in rough water,it might be difficult to maintain theexact depth to optimise decompression.However, by maintaining a depthbelow the ceiling but above the floor,the diver is still decompressing,although slower than optimal, andprovides an additional buffer tominimise the risk that waves will notlift the diver above the ceiling. Also,the continuous decompression curveused by Suunto provides a muchsmoother and a more naturaldecompression profile than thetraditional “step” decompression.

The smaller the bubble, the greaterthe force of surface tension thatminimizes the growth of the bubble.However, as the bubble grows largerthe surface tension diminishes, andthere is an increasing inclination for thebubble to expand.

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In this illustration we can see across-section of a capillary delivering

blood to muscle tissue. Friction betweenthe muscle cells create micronuclei

which attract dissolved gas from thesurrounding tissue, forming microbubbles.

The microbubbles disturb the bloodflow and slow the gas elimination

process. Microbubbles are present afteralmost any kind of dive, but are normallyfiltered out by the lungs before creating

problems. If however they are allowedto grow, they may form DCI bubbles

causing decompression illness.

The Suunto RGBM assumes that thehuman body is divided into 9 majortissue compartments, which are basedon the rate at which each tissue groupon- or off-gasses. These half-times rangefrom 2.5 to 480 minutes.

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INSTRUCTORS RISK LONG TERM DAMAGE, WITH NO SHORT TERM SYMPTOMSThe science of decompression has

been a continuously evolving processwith the fundamental causes ofdecompression illness established asearly as 1908. In the 1970s thedevelopment of Doppler technologyallowed researchers to measure theexistence of bubbles in the diver’sbody. The Doppler devices did this byreading an ultrasonic signal that wasreflected back from the bubbles inthe test subject. When the reflectedsignal was received by the Dopplerrecording device it emitted a chirpingsound. The number of chirps indicatedthe number of bubbles. It was discoveredthat small bubbles, microbubbles,were present after almost every dive,although the divers had no actualsymptoms of decompression illness.

Until recently, little has been

known about the behaviour ofmicrobubbles, other than the factthat their existence has been provenby Doppler measurement.

What has also been established isthat once formed, these bubbles areunstable. They have an ability toattract dissolved gas from surroundingtissues and the likelihood of a bubbleeither expanding or collapsing, isdetermined by a range of factors.These factors include the surfacetension that exists on the bubble’ssurface, the pressure within thebubble and the ambient pressurerelative to the bubble.

It is known that a diver, whocompletes multiple dives within agiven day or even over a number ofdays, may present with a build-up ofthese microbubbles. And a repetitive

diver, who unknowingly has alreadyaccumulated microbubbles, may havea higher predisposition to decompressionillness. It is also known that they cancause longer-term problems such asneurological damage.

This is particularly relevant toprofessional divers such as instructors,who complete a large amount ofrepetitive diving, often with manyascents in one training session.

Also, microbubbles are known toaccumulate inside the alveoli,obstructing and slowing down theoff-gassing.

Additionally, given the rightcircumstances, microbubbles cangrow to form real DCI bubbles, whichare out-of-control microbubbles thathave grown too large. It has beenrecognised that microbubbles neededto be controlled, and some divecomputer manufacturers have madetheir computers more conservative, toaccommodate microbubble formation.The Suunto solution to microbubblecontrol is based on measuring anumber of factors that correlate tomicrobubble formation, growth anddecay depending on diving behaviour.These factors are then used to adjustthe tissue limits of the basicdecompression model in real time,based on the actual diving profiles.If needed, prolonged safety stops areintroduced and/or a longer surfaceinterval prompted.

The presence of microbubbles in thetiny capillaries that form the wall of the

alveoli may obstruct the blood streamand inhibit proper off-gassing of the

nitrogen that is in the blood.

More recently, the availability ofelectron scanning microscopes hasallowed researchers to actually seemicronuclei, which are the bubbleseeds that precede microbubbleformation. These micronuclei are onlya few microns in diameter.

This has now given researchersdeeper insight into the formation and

SUUNTO RGBM CONTROLS MICROBUBBLESbehaviour of microbubbles. It is in theunderstanding of the formation anddynamics of these microbubbles, howto suppress and control them that hasled the most recent research by theSuunto technical team.

The breakthrough is that theSuunto Reduced Gradient BubbleModel controls the behaviour of

microbubbles, before they becomeDCI bubbles.

Divers who properly follow theinstructions of a dive computer thatutilises the Suunto RGBM mayexperience a reduction in divingincidents, without requiring a moreconservative dive time for most dives.

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RReeppeettiittiivvee ddiivveess -- FF11 - Microbubblesmainly occur in the venous circulationduring the surface intervals betweendives. They are washed with the bloodto the pulmonary filter (lungs), wherethey may reduce the surface area andinhibit off-gassing. This effect will continueuntil the production of microbubblesceases and the bubbles in the lungsdissipate. This continues for about threehours after surfacing. The SuuntoRGBM algorithm calculates correctionfactors to cope with this issue.

DDeepptthh sseeqquueennccee -- FF22 - In any series ofdives, the Suunto RGBM calculates thatdiving deeper than the previous divestimulates micro-nuclei into growth.During the surface interval followingsuch an event, the Suunto RGBM

recalculates future decompressionobligations based upon the depth excessof the last dive compared to the onebefore it.

MMuullttii--ddaayy ddiivviinngg -- FF33 - Pre-existingmicro-nuclei are excited into a higherenergy state by diving (compression anddecompression). They are thought toreturn to their normal energy level overtime scales of days The Suunto RGBMmulti-day factor calculates adjustmentsfor a surface interval period of 100hours.

The combination of the correctionfactors is applied to the Suunto RGBM’sM-values thus reducing the permittedsupersaturation gradient, which adjuststhe required decompression obligation.

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This picture illustrates the possibleeffect of microbubble growth within thehuman brain. The synapse is the junctionbetween a nerve terminal and the nextneurone in a long chain of neurones.Microbubbles can interpose themselvesin these junctions causing long termneurological damage by preventingelectrical transmissions. By shutting offthe bloodflow, microbubbles can alsocause tissue death, for example in thesensitive retina of the eye.

CONTROLLING BUBBLE FORMATION WHILE MAXIMISING OFF-GASSINGEarlier Haldanian dissolved gas

models assumed that in order todecompress, a diver should ascend asquickly as possible to a shallow depthto maximise off-gassing. However, byquickly reaching that shallow depth,the diver may have already formedmicrobubbles. At the very least, thesecould potentially obstruct off-gassing

and at the worst, may have alreadycaused some tissue damage.Therefore, there is a need to keep themicronuclei from forming intomicrobubbles and keep any pre-existingmicrobubbles as small as possible, byapplying an appropriate ascentprotocol.

If a diver dwells at depth, whenmicrobubbles are very small in size,their high surface tension will help tocollapse the bubble down. But thereis still the need to maximize thepressure differential as much aspossible, to allow tissues to off-gas.This is why the Suunto RGBM

considers both factors. For dissolvedgas the diver wants to maximize thepressure differential as much aspossible, but for any microbubbles,there is the need to keep the pressurehigh by staying deep. The Suunto RGBMoptimises these two contradictoryissues by both a combination of aslow ascent rate and continuousdecompression curve.

It all comes down to propercontrol of the expanding nitrogenduring an ascent. This is why Suuntohas set the maximum ascent rate at10m/minute.

This graph shows the effects of"safety stops" on microbubbling. “A” isthe bubble count over time after a diveto 36m [120ft] for 25min. “B” is thesame with a 2min stop at 3m [10ft]. “C”is still the same but with 1min at 6m[20ft] and 4min at 3m [10ft]. As can beseen bubbling is reduced by a factorof 4-6. Conclusion: free phase (bubble)reduction in pulmonary circulation isimpressive when utilizing safety stops.Graph is based on study by Pilmanis A.A1976: Intravenous Gas Emboli in Manafter compressed Air Ocean Diving.

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The effectiveness of a 10m/min ascentrate is clearly demonstrated here withthis graph showing how the surfaceradius of bubbles is significantly reducedby a slower ascent rate. Graph fromBasic Decompression Theory andApplication by Bruce Weinke, p.56.

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CREDITS

Much of the work in developing the Suunto

RGBM was pioneered by Dr Bruce Wienke.

Dr Wienke is a Program Manager in the Nuclear

Weapons Technology Simulation And Computing

Office at the Los Alamos National Laboratory

(LANL), with interests in computational

decompression and models, gas transport, and

phase mechanics. He is the developer of the

Reduced Gradient Bubble Model (RGBM), a dual

phase approach to staging diver ascents over an

extended range of diving applications (altitude,

nonstop, decompression, multiday, repetitive,

multilevel, mixed gas, and saturation).

www.suunto.com

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