Barotrauma of the Ear

Barotrauma of the Ear

Barotrauma in divers is physical damage to body tissues caused by a difference in pressure between a gas space inside the body, and the surrounding fluid.

Barotrauma typically occurs when the diver is exposed to a significant change in ambient pressure, such as during ascents or descents, or during uncontrolled decompression of a pressure vessel, (i.e. Chamber).

There are several types of barotraumas that affect divers, today we will look at Middle Ear Barotrauma or M.E.B.T. Understanding M.E.B.T. will give your good insight to the other types of barotraumas.

Divers may first incur the symptom of an “ear squeeze” prior to the actual barotraumas. Pain is never a good thing during diving… or really any other time.

  

Ear Function & Anatomy

It will be useful in understanding the subject to have an understanding of the basic function and anatomy of your ear.

On the side of your head you have a substantial bit of cartilage and skin commonly identified as “an ear”. This is actually the Auricle and can be fun to nibble on. The passage way going from the Auricle into your head is the External Auditory Meatus or ear canal. Next you run into the Tympanic Membrane or ear drum. On the inside of the Tympanic Membrane is the middle ear. The middle ear is an air space with three bones (Malleus, Incus & Stapes) connecting the Tympanic Membrane to the inner ear, and the opening to the Eustachian Tube. Next is the inner ear made up of the Cochlea and Semicircular Canals. (I left the Semicircular Canals off the diagram for simplicity.) These structures are filled with a liquid made up of nearly all water, and therefore not compressible. There are two very important features on the Cochlea: The Round Window and the Oval Window. These windows are a flexible connective tissue. 

Sound waves enter via the Auricle, pass through the Canal and strike the Tympanic Membrane. The Membrane turns the waves into vibrations which are transmitted via the three bones into the liquid of the Cochlea via the Oval Window. The Auditor Nerve has sensors in the Semicircular Canals which transmit signals to your brain and you interpret the meaning of it all! But what happens to the vibrations in the Cochlea fluid? That energy must be dissipated somehow; the structural material making the Cochlea is very hard, un-giving skull bone. That is where the Round Window comes in. It absorbs the vibrations like a trampoline. The Round Window actually vibrates out into the middle ear space, transferring the energy into the air and displacing air and energy down the Eustachian Tube. There you have it. So simple I am sure it is an accident of nature.
If we simplify the whole thing, and straighten it out it would look something like this:

The “U” shape is filled with water, the center island and area surrounding the top, bottom and right side are bone. The missing Semicircular Canals would be represented attached to the top right section.

Barotrauma can affect the external, middle or inner ear. It is most common in the middle ear and is experienced by 10% - 30% of divers.

Causes

There is really only one mechanism of injury for the diver to consider: breathing gas in the middle ear or gases in the external ear becomes trapped and expands causing damage. This can occur on descent or ascent (reverse squeeze). The gas may become trapped a few different ways:

·      The Eustachian Tube is blocked. We see this most commonly when the diver has a cold or allergies. It can happen if the diver has used a sinus medicine or nasal spray to clear up his sinuses and the affects of the medicine wear off during the dive. This is one reason not to use such medicines. (Another reason is; some of these medicines are in the class of drugs known as Oxygen Exciters. Oxygen Exciters when used with ambient pressures somewhere more than 1 ATM can impede normal cognitive functions.): Yes, that is a sad face experience.

·     Middle ear infections can cause the middle ear space to fill with fluid and the exit to the Eustachian Tube to be blocked.

·     The Eustachian Tube can be compromised by infection or trauma.

·      A wetsuit hood or similar piece of SCUBA gear worn over the ear can cause an external Barotrauma rupturing the Tympanic Membrane. (Ouch!)  Wax can also cause a similar situation. There are those divers who use “ear beer” to clean out their ears before and after diving. I have never used it and have no recommendation. Except, don’t drink the concoction: 1/3 rubbing alcohol, 1/3 hydrogen peroxide, 1/3 white vinegar. It’s up to you!  

The Pathology

The pathology (a cool word for injury in this case) is normally tearing or rupturing of the Tympanic Membrane. Less often it is a similar trauma to the Round Window. It is almost never, or maybe even never, damage to the Oval Window. Even as pressure builds on the outside of the Tympanic Membrane pushing the three bones into the Oval Window, hydraulically transferring pressure via the Cochlea fluids onto the Round Window, causing it to tear or rupture before the Oval Window will. Which is a good thing; because it is a lot easier for the Round Window to heal vs. the Oval Window which would likely have the Stapes bone in the way now. Yes, another accident of nature.

Bottom Line

Be sure to dive only when your ears and sinuses are clear. If you have a chronic problem or are experiencing seasonally allergies, see a M.D. who is a diver him/herself and is well acquainted with dive medicine.

While performing descents or ascents if you feel some pressure or discomfort in your ears: Stop, reverse direction until the pressure or discomfort goes away, begin your equalization process and then continue on.

Be smart and live to dive another day my friends. 

Regulators...what is the difference?

Regulators...what is the difference?

First Stages

(I would like to give a special thanks to my friends at SCUBAPRO for providing the illustrations...it is good to have friends!)

FIRST STAGES
The task of a first stage regulator is quite simple: reduce high pressure air coming from the tank to a consistent intermediate pressure. They are generally divided into PISTON and DIAPHRAGM categories, depending on the mechanism used to control the valve allowing air to flow to the second stage. Due to their design and their particular advantages, each of them has become the favorites of different groups in the diving community. First stages can be further classified between CLASSIC DOWNSTREAM and BALANCED, affecting function and performance of the first stage related to pressure changes in the tank.

THE PISTON TECHNOLOGY
The general advantage of a piston-based first stage lays in its reliability and reduced maintenance requirements, due to an effective but simpler mechanism with less moving parts. More importantly, no other design can reach the high air delivery rate of a balanced piston controlled first stage.

Balanced-piston first stages are the first choice of demanding deep sport divers and professionals. First stages with air balanced pistons deliver significantly more air to the second stage than any other first stage, while their performance is totally unaffected by changing tank pressure and depth. 

AIR BALANCED FLOW THROUGH PISTON 
First stages with air balanced pistons deliver significantly more air to the second stage than any other first stage, while their performance is totally unaffected by the changing tank pressure. A balanced piston allows the use of lighter and more sensitive components, resulting in ultra fast breathing response, instant delivery of air on demand and extra high air flow, especially in low tank pressure ranges. The tired diver benefits from of a smoother breathing regulator during the ascent or deco stop. Balanced piston first stages are the first choice of demanding sport divers and professionals. A balanced piston performs equally in both warm and cold water environments.

CLASSIC DOWNSTREAM PISTON
This is the best example of bulletproof reliability and trouble-free, minimal maintenance regulators. The conventional downstream piston configuration is the simplest mechanism that exists to control the pressure drop from a tank to feed the 2nd stage. The classic downstream valve is the first choice of diving centers and rental facilities worldwide for warm and moderate water temperatures.

This is the best example of bulletproof reliability and trouble-free, minimal maintenance regulators. A classic downstream piston does not compensate for the minor changes in pressure delivered to the second stage as tank air is consumed, but still guarantees solid performance.

DIAPHRAGM TECHNOLOGY
Diaphragm based first stages are environmentally sealed so that water cannot enter the inner mechanism. Bearing in mind that regulators generate temperatures up to minus 30 °C due to the incredibly fast moving air and the high pressures involved, it is imperative that sensitive moving metal parts avoid contact with extremely cold water.
Therefore, diaphragm first stages have been the favored choice of cold-water divers and those working in contaminated or muddy water.
All SCUBAPRO diaphragm first stages feature a balanced technology and are packed with patented features resulting in ultra fast flow to the second stage upon request and a fantastic overall performance and are my personal favorite of the diaphragms. The balanced diaphragm technology provides consistent performance at all cylinder pressures, at any depth and optimizes the second stage performance, thus allowing for effortless breathing.

Diaphragm first stages featuring a dry balancing chamber so that the water cannot enter the first stage, means it's resistance to freezing is unmatched even in extremely cold waters.

Diaphragm first stages with the same structure but without a dry chamber are the perfect choice for the recreational diver who wants the advantage of a diaphragm first stage for use in temperate waters.

 ~More details below~

BALANCED DIAPHRAGM (Dry Chamber)

Diaphragm based first stages are more complex and have more moving parts than piston first stages. With a dry chamber, they have an environmentally sealed design so that water cannot enter the inner mechanism.

Diaphragm first stages have been the favored choice of cold-water divers because of their better resistance to freezing. They are also recommended for divers working in water containing a high degree of suspended particles, silt, or other contaminating materials.

BALANCED DIAPHRAGM

Diaphragm first stages have been the favored choice of cold-water divers because of their better resistance to freezing.

Diaphragm first stages are also recommended for those working in water containing a high degree of suspended particles, silt, or other contaminating materials.

Second Stages

A second stage regulator must translate natural breathing behavior into mechanical reality. A high quality second stage can significantly reduce stress and enhance diver safety by providing smooth, low-effort breathing response, resulting in ample yet controllable quantities of air. The designation "balanced" and "classic downstream" are also distinguishing marks for second stages and imply the same general attributes like first stages, i.e. ultimate flow performance and incredible inter-stage pressure stability in addition to reliability, simplicity and solid performance

AIR BALANCED VALVE TECHNOLOGY
The air balanced valve technology of SCUBAPRO second stages features a balancing chamber in the second stage mechanism, slightly offsetting the force of the downstream air entering from the first stage. This allows reduced spring tension and decreases the inhalation resistance to the lowest possible level. The result is an ultra-high airflow that remains exceptionally stable under all breathing conditions. The air-balanced valve technology is featured in our A700 and S600 second stages. For optimum results they should be combined with balanced first stages, like the MK25 or the MK17. These are the combonations Coleen and I dive. 

OPTIMAL FLOW DESIGN TECHNOLOGY (OFD)

The Optimal Flow Design valve is the ultimate evolution of the classic downstream technology. Unlike this one the OFD valve features a full length piston shaped to obtain the optimal aerodynamic performance. The gas flows freely over the piston surface and provides the diver with the best breathing experience. No regulator part slows down the airflow, and even the valve spring is shifted to the back side of the piston so that the air doesn't have to flow through the spring coils. The airflow is huge and rich but always under control. This is an intermediate worry free regulator with high end performance.

CLASSIC DOWNSTREAM TECHNOLOGY
Classic downstream valves are particularly noted for their legendary safety and reliability. When in use, the downstream valve opens in the same direction as the incoming airflow. To close the valve and stop the airflow, a spring counteracts the force of the incoming air. Therefore, a certain inhalation effort is always required to overcome the spring tension and open the valve. The classic downstream valve is a popular design for alternate air sources such as an octopus or AIR 2. Technical divers will often use a Classic downstream for stage bottles.

Decompression, Part Three: Putting it All Together

Divers should be aware that they have control over nucleating events and they should minimize pre and post dive activity (including underwater activity).  Minimizing activity may help reduce decompression risk and performing activity will certainly increase it.  Of course dives should be planned in a conservative fashion since factors such as age and body fat can also increase decompression risk.  But putting this aside, some individuals are simply more susceptible to DCS than others.  Since most of these divers do not know who they are until it is possibly too late, dives should always be planned in a conservative fashion.  Very mild muscle movement can also be done to eliminate gas faster during decompression.  But if this is started too deep, there is a possibility that it will also result in more gas being taken in.  Divers can also remain on their final decompression gas while at the surface to increase the gas elimination rate before the gas can enter bubbles leading to bubble expansion.

Decompression Strategies, Part Two: The Bad Assumptions, Conditioning & Decompression Tactics

The (bad) assumptions

A model is only as valid as the assumptions behind it.  Most models are based upon certain "knowns" such as accepted no-stop time limits for the different depths.  But these time limits/tables are only valid if the diver performs the dive in the same manner the tables were tested.  Most tables were tested with only mild to no exercise being performed before, during or after the dive ... so their time limits would give a lower incidence of bubbling than what the diving public would probably experience.  So always dive conservative since you are more than likely to have more micronuclei and thus more bubbling after a dive than the acceptable limits of the table when doing the same dive as a table test subject (who was probably sitting around with very little activity waiting his turn to get in the chamber for his test dive).

The conditioning

As much as many of us would hate to admit it, the condition of our bodies do make a difference.  Age has been shown to affect the degree of bubbling from a dive.  Older divers have more bubbles from a dive than younger divers.  The reason is not exactly known but may be accounted for by having more adipose tissue.  Adipose tissue can act as a gas storage area and could result in an overload and release of gas later.

The physical shape of a diver is also important.  It has also been shown that the greater the body uses oxygen, the less risk one has for DCS.  The usage of oxygen simply shows  the oxygen used by the tissues which is an indication of blood flow rates.  Obviously, the better the blood flow the better the off-gassing during decompression or after a dive.

Decompression Tactics

All divers need to decompress properly.  This includes recreational divers doing deep safety stops and ascending slowly.  The benefits of a slow depressurization can be seen by a simple experiment.  Take two soda bottles side by side and open one very slowly and the other one rapidly.  Notice that the one with the rapid decompression results in more bubbling.

In addition to the rate of decompression through the stops and making sure enough pressure is kept on the diver during decompression, divers can also speed up their off-gassing (as well as in-gassing if not careful at depth) by doing very super mild movements.  We hesitate to call it exercise because that makes it sound like work must be performed ... which is exactly what a diver does not want to do.  Performing work or exercise will generate micronuclei and make decompression worse.  Instead, very slow and ultra gentle movements are desired, such as bending the arms and legs slowly.  It is the change in muscle form that will open up capillaries and make blood flow occur and result in better off-gassing due to the increase in blood flow.  But again, anything more that amounts to activity can generate micronuclei and make things worse ... so always error on the side of caution by doing ultra gentle movements   The exercise that some divers think is helpful during decompression is a big no-no. 

Another procedure a diver can perform to reduce decompression risk is to obviously spend more time decompressing.  But the point of this article is to suggest and explain procedures that a diver may not have thought about.  Another of these procedures is for a diver to remain on their final decompression gas for a period of time after surfacing (in addition to avoiding anything other than mild activity).  This will assist with the elimination of gas whether bubbles are present or not.  But assuming a proper conservative decompression was done, bubbles should not yet really be present.  Bubbles take time to generate after a dive (but this may differ with mandatory decompression stop dives).  The reason for this is that it takes awhile for gas leaving the tissues to randomly bump into a micronuclei.  As the micronuclei grow and provide a larger surface area, the rate of gas bumping into the micronuclei increase.  If bubbles are avoided upon the initial surfacing, then there will be a window of time in which the diver can increase gas elimination by breathing a decompression mix at the surface before the bubbles grow - which in turn will reduce decompression risk.  But again, even if bubbles are present, this procedure will still assist in a diver eliminating gas.

Decompression Strategies: Part One

Here is a little information on Decompression Strategies for those of you who cannot get enough of this stuff. I am planning on planning on making 5 entries in the weeks:

1. Micronuclei
2. Bad Assumptions
3. Conditioning
4. Decompression Tactics
5. Putting it Togerther

MICRONUCLEI

The starting point is to understand more about what causes DCS.  Everyone understands that excessive bubbles will cause DCS.  But what a lot of divers don't realize is how these bubbles start ... which will help a diver to understand how to decrease their risk.  When a diver goes underwater, the diver's tissues start taking in inert gas due to the pressure.  So when the diver surfaces, the tissues have an excess of inert gas in them.  But, contrary to what divers believe, this is not the problem.  A diver could take up a very large amount of inert gas and never get DCS if it wasn't for one thing ... the presence of micronuclei ... or bubble seeds.  In other words, a diver could dive to very deep depths for very long times without ever needing to decompress if it wasn't for the micronuclei.  These micronuclei act as a source for bubbles to start occurring.  They may be viewed as very tiny bubbles themselves.  What happens while ascending is that the gas built up in the tissues from the dive are now in a supersaturated and high pressure state.  This gas wants to start leaving the tissue and escaping.  This gas will go wherever possible.  One route is for it to enter the blood and exit that way.  Another possible route is that the gas leaving randomly bumps into and enters a nuclei.  This nuclei will continue to grow as more gas enters it and/or the diver ascends towards the surface causing it to expand according to Boyle's law.  If this nuclei gets too big, a bubble results.  So why is this being mentioned?  In addition to the obvious point of slowly ascending/decompressing through the water column towards the surface, the control of nuclei is a topic that should be discussed.  It has been shown that activity increases resulting bubbling from a decompression.  Since micronuclei can't be seen, they are postulated due to the known increase in bubbling from activity.  Most activity generates micronuclei, but the activity performed by divers are especially bad for generating micronuclei such as hauling gear to and from the water or climbing up the ladder onto a boat.  The greater the activity, the more nucleation that will occur.  It doesn't matter when the nucleation occurs.  It can occur from pre-dive activity or post-dive activity.  As long as there is an excess of gas in the tissues from the dive, nucleation will generate more places for gas to enter while trying to leave the tissues ... and the more places gas has to enter, the greater the number of resulting bubbles ... and the greater the risk of decompression sickness.  So the moral of the story is not to believe that a model is responsible for your safety, but instead understand that your own activity can be too and be aware that pre and post dive activity (as well as that during the dive itself) can substantially increase the risk of getting decompression sickness.

Nitrox Notes

Lance’s Nitrox Notes

What is Nitrox and how is it different from air?

Air: 21% O₂ 79%N. Therefore air is Nitrox (NO₂). But for our purposes diving Nitrox has between 22% and 40% O_2.

O₂ is actively metabolized by the body while N is inert and always being absorbed by the tissues of the body (blood is a tissue). More N gets absorbed during diving and this is directly related to the risk of DCS and is a critical factor in determine dive depths and times.

Advantages of Nitrox

Due to the depleted N:

·         Nitrox may allow for extended bottoms times compared to air at the same depth

·         Nitrox may provide decreased surface interval times allowing for more dives to be completed in the same time period compared to air.

·         Nitrox may decrease the risk of DCS due to the decrease N in the breathing gas.

·         Nitrox may decrease the no fly time incurred by diving air.

Other Names

·         NO₂

·         EAN 32 or EAN 36 (The two most commonly used blends.)

·         NOAA I and NOAA II

·         De-Nitogenated  Air

·         Safe Air

·         Oxygen Enriched Air or Enriched Air

(Experiments with Nitrox started in the 1800s but it was NOAA and the US Navy which made is a useful dive tool. It was not until 1995 that is Nitrox was widely accepted by recreational training organizations.)

Principals of Pressure

1.      Ambient Pressure is the force pressing on an object, diver or gas. At the surface of the ocean we have the weight of the atmosphere pressing downward on us. It is measured in the metric system as 1 bar equivalent to 1 Kilogram per sq cm. The Imperial System abbreviates it as 1 atm equivalent to 14.7 psi. For practical purposes these are considered equal.

While at depth the diver must take into account both the weight of the atmosphere and the water. Because water is much denser than air even very minor adjustments in depth will make significant changes in pressure. The total weight of the atmosphere and water are known as absolute pressure or ambient pressure.

Rather than thinking in depth it is advantageous to think in pressure. (All depths represent depth in seawater.)

Depth-Pressure Relationship (Increments based on Pressure)
Depth		Pressure
Metric	Imperial	bar/atm
0 m	O ft	1/1
10 m	33 ft	2/2
20 m	66 ft	3/3
30 m	99 ft	4/4
40 m	132 ft	5/5

The above table is useful for increments equal to one atm. But we do not dive at these preset depths.

Depth to Pressure Calculations

As we are used to Imperial Calculations I will present in that format.

Remember 33 feet of sea water = to 1 atm PLUS the atmosphere itself. So simply divide the depth by 33 and add one for the atmosphere.

P = (D/33) + 1

P = (99/33) +1 = 4 atm

Pressure to Depth Calculations

Pressure to Depth Calculations are just as straight forward; just take a way 1 from your pressure and multiply by 33 to find your depth.

D = (P-1) x 33

D = (4-1) x 33 = 99 feet

Boyle’s Law

This is the first law that a Nitrox diver should understand. Boyle’s Law describes the affect of ambient pressure on a diver’s breathing gas. Simply stated; the volume of a gas in a flexible container is inversely proportional to the pressure being exerted on the container. It is important to note here the amount of gas molecules remains constant in the container.

A simple example of Boyle’s Law may be demonstrated like this: If the pressure on a flexible container is doubled the volume will be halved. Conversely, if the pressure is halved, the volume will double. Divers usually think of this in terms of over expansion injuries. Nitrox divers need to think about Boyle’s Law in terms of on and off gassing (which is pressure dependent).

Boyle’s Law can be mathematically expressed as:

P₁V₁=P₂V₂

Where the subscript is used to designate the beginning and ending values of Pressure and Volume.

Pressure-Volume-Density Relationship
Depth		Pressure	Volume	Density
Metric	Imperial	Bar/atm
0 m	0 ft	1/1	1	x1
10 m	33 ft	2/2	1/2	x2
20 m	66 ft	3/3	1/3	x3
30 m	99 ft	4/4	1/4	x4
40 m	132 ft	5/5	1/5	x5

An important fact that is being demonstrated is; in a gas filled flexible container, the pressure of the gas inside the container is equal to the ambient pressure outside the container. It is this homeostatic nature that allows gas to expand and contract. And of course, Bole’s Law has no effect in a ridged container, such as a SCUBA cylinder.

The tissues of the body are largely a non-compressible liquid, and are not directly impacted by Boyle’s Law. The lungs and connected spaces are a flexible container. The instant the gas leaves the regulator it becomes subject to Boyle’s Law. For Nitrox divers, the most important issue is the gas inside the diver’s lung will be equal to the ambient pressure surrounding the diver.

Dalton’s Law

For the Nitrox diver, Dalton’s law tells us each individual gas in a mixture has its own specific pressure. This is called the Partial Pressure and for oxygen is abbreviated PO₂ and for nitrogen PN_2.  Pressure is expressed in terms of atmospheric pressure as bar or atm.

Dalton demonstrated the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures exerted by the sum of the total gases in the mixture. And each component gas accounts for its fraction of the total pressure, in direct proportion to the fraction of that gas present in the total mixture.  In other words; in a Nitrox mixture of 32% oxygen and 68% nitrogen the oxygen exerts 32% of the pressure and nitrogen exerts 68%of the pressure. In a SCUBA cylinder of EAN 32 at 3,000 psi oxygen exerts 320 psi and nitrogen 680 psi. This is expressed as FO2 and PN2.

The other important part of Dalton’s Law for divers is; gases will move to an even pressure throughout a space. That is they move from an area of higher partial pressure to equalize with an area of lower partial pressure until the gases are equally co-mingled.

Understanding these two aspects is important for the discussion on Henry’s Law and membrane pressure gradients.

Partial Pressures of EAN 32
Depth		Pressure	PO2	PN2
Metric	Imperial	Bar/atm	Bar/atm	Far/atm
0 m	0 ft	1	.32	.68
10 m	33 ft	2	.64	1.36
20 m	66 ft	3	.96	2.04
30 m	99 ft	4	1.28	2.72
40 m	132 ft	5	1.60	3.40

While the fraction of a gas in a mixture remains constant in a mixture, its partial pressure varies dramatically.

·         As you can see in the table above partial pressure of oxygen, in a flexible container, at 3 atm is roughly equivalent to breathing 100%oxygen at the surface when using EAN 32.

·         Breathing EAN 32 at 3 atm is approximately doubling the amount of nitrogen compared to the surface.

If we were to look at air on the same table we would see that the oxygen exposure reaches an approximate equivalent of 100% at approximately 5 atm. Nitrogen exposure from air at 3 atm is roughly equivalent to the nitrogen exposure of EAN 32 at 4 atm.

This is the key to the advantages of using Nitrox. There is simply less nitrogen exposure at depth with Nitrox than with air. Therefore the physiological effect of nitrogen at depth is diminished.

Determining the Partial Pressure of a Gas

Simply multiply the total partial by the fraction of the gas. (G represents the gas e.g. oxygen or nitrogen.)

PO2 = P x FO2 ~or~ PN2 = P x PN2

Henry’s Law

Henry’s Law illustrates the mechanisms by which gas moves in and out of tissues of a diver’s body. Henry discovered the specific quantity of gas that will dissolves into a liquid is dependent on two factors:

1.      The partial Pressure of the gas

2.      The coefficient of that gas in a particular liquid (This coefficient is a mathematical variable that demonstrates different liquids absorb the same gas, into solution at different rates and in different quantities.)

According to Henry’s Law, when a partial pressure of a gas is increased, additional gas will be dissolved into the liquid. This happens where the gas comes into contact with the liquid, normally with the surface of the liquid or through a membrane such as an alveolar wall. This takes time. Where there is a chemical interaction the time can be significantly shorter. The actual rate will vary depending on the pressure gradient. (This is the difference in partial pressure in the gaseous gas and the dissolved gas.)

As long as the total partial pressure remains constant, any sudden change in the partial pressure of an individual gas will merely accelerate the on-gassing or off-gassing. However, if there is a sudden decrease in the total pressure exerted upon a liquid, it may cause some of the dissolved inert gas to come out of solution while still within the liquid and from bubbles.

The solubility of any gas in liquid is directly proportional to the pressure exerted on it. When the pressure is doubled, the amount of gas that can be dissolved is also doubled.

Gas Solubility in a Liquid
Partial Pressure of Gas	Maximum Quantity of Gas
bar/atm	In solution
1	x1
2	x2
3	x3
4	x4
5	x5

Gas Dynamics

Gases continually travel between our lungs and tissues, transported by the blood, moving from areas of higher partial pressure to lower partial pressure. They are effectually following the laws of physics seeking to equalize their own partial pressure.

The gases enter the blood stream via the lung’s alveoli, and are transported to tissues where the lower partial pressure of the gas “draws” the newly arriving gas in. When the partial pressure of the gas in the blood is less than that of the tissues the gas is drawn into the blood and exits the body via the same alveolar pathway. This off gassing will begin to occur at some point during the diver’s accent to the surface and will continue until there is no longer a pressure gradient between the diver’s tissues and the ambient partial pressure of the gas.

Nitrogen Dynamics

Nitrogen is not used by the body in any fashion. It is merely absorbed by tissues according to the laws of physics. When a diver spends sufficient time at sea level (1atm) the partial pressure of nitrogen in the his tissues will equalize to 1 atm. This is referred to as “saturated”. That is, the tissues have absorbed all the nitrogen possible under these circumstances.

When the diver descends, the partial pressure of nitrogen will increase with depth. It will be greater in the lung space than the blood creating a pressure gradient. Now the “on gassing” process will begin. The amount of on gassing will depend on the depth and time of the dive. If the diver was to stay a given depth sufficiently long, the partial pressure of nitrogen in his tissues will equalize with the ambient partial pressure of nitrogen, and again, he will be “saturated”. In the case of tissue saturation, off gassing will immediately begin when the ambient partial pressure of nitrogen is less than the partial pressure in the diver’s tissues. This is why there is a “no fly time” after diving.  Most aircraft are pressurized to equal an 8,000 foot elevation. That is approximately equal to ¼ atm or 8.25 feet of sea water. (It is a difficult to be exact because 3/4of the earth’s atmospheric mass is within 36,000 feet while the upper boundaries of the atmosphere are approximately 400 miles with no definite boundary with space!)

Decompression Sickness

At the end of every dive the diver is “super-saturated”. That is, he is carrying more nitrogen in his tissues than the ambient partial pressure. … He is not done off gassing. There is a limit to the body’s ability to carry this excess nitrogen in tissues. This is where the dive tables and computers come in. Generally, staying within these parameters will prevent DCS.

DCS occurs when the partial pressure of nitrogen absorbed nitrogen is suddenly greater than the ambient partial pressure of nitrogen. The time for nitrogen to cross membranes and exit the body is insufficient for the partial pressures to equalize. Now the laws of physics cause the nitrogen to come out of solution into its gaseous state. At first, the smallest amounts of gaseous nitrogen begin to form micronuclei. As the micronuclei bump into each other they eventually form bubbles. These bubbles cannot escape the tissues and become lodge therein. These bubbles may interfere with bodily functions causing symptoms.  This is Decompression Sickness.

There are predisposing factors to DCS which make predicting it very difficult. They include: body fat (not just obesity), dehydration (your body needs fluids to carry the gases), elevated Carbon dioxide levels (from working hard on a dive or poor lung function from smoking or other lung disease), old age and diminished fitness.

Oxygen Dynamics

The movement of oxygen from your lungs to your tissues is completely dependent on pressure gradients created by partial pressures. Oxygen is readily metabolized by your body and is completely vital. It is accordingly very difficult to saturate your tissues with oxygen. Because the byproducts of these metabolic process are CO₂ and H₂O,  pressure gradient for oxygen is towards the tissues. Due to the lack of CO₂ in our breathing gas, its pressure gradient carries it out of the body.

Haldane’s Theory

The British scientist was approached by the Royal Navy to come up with depth and time limits for the Royal “Hard Hat” divers. Haldane theorized nitrogen loading by creating multiple “compartments” that load and unload (on gas and off gas) in half times. A half time here is the time, in minuets, it takes for a compartment to go for the starting saturation half way to a new saturation based on the new depth. This is an exponential progressing. Haldane used algorithms mimic the tissues absorption. As it turns out he was pretty darn accurate. His work is the bases today for all decompression theory, dive tables and computers. I have included the tables showing his original half life theory.

60 Minuet Compartment On Gassing
Elapsed Time	On Gassing Completed
Start	0%
1 Hr	50%
2 Hrs	75%
3 Hrs	87.5%
4 Hrs	93.8%
5Hrs	96.9%
6 Hrs	98.5%

5 Minuet Compartment On Gassing
Elapsed Time	On Gassing Completed
Start	0%
5 Minuets	50%
15 Minuets	75%
20 Minuets	87.5%
25 Minuets	93.8%
30 Minuets	96.9%
35 Minuets	98.5%

Maximum Operating Depth: MOD

(Calculation for MOD)

For CNS exposure of 1.5 max on EAN 32

Oxygen Partial Pressure ÷ FO2 =
Pressure in ATA (Bar)
1.5 ÷ 0.32 = 4.6875 ata (bar)

(Pressure in ATA - 1) × 33ft = MOD in feet
(4.6875 - 1) × 33 = 122 fsw

(Pressure in Bar - 1) × 10m = MOD in meters
(4.6875 - 1) × 10 = 37 msw