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Text Box: Decompression Theory in a Nutshell
One of the risks of scuba diving is getting Decompression Sickness (DCS), or the Bends. The term “Bends” comes from observing victims curling and bending their bodies in pain. It surely is not a cheerful infliction and definitely a condition to avoid.
The Decompression Theory can be bewildering, and when the experts start to debate different aspects, it can be much more than just confusing. 
After years of teaching Tec Divers I managed to condense the Decompression Theory into two important components. That makes it easy to understand and enable divers to be more attentive of DCS and the prevention thereof.
Thus Decompression Theory in a Nutshell, is a simple understanding of “Gas going INTO”, your body or ON-GASSING, and “Gas going OUT of your body” or OFF-GASSING.







OFF GASSING
So why do we “OFF-GAS” and “ON-GAS” in the first place? 
The atmosphere around us is always at pressure, at sea level the pressure is about 1 Bar, or 1 Atmosphere. If we go up a mountain the pressure will drop, and the higher we go up the pressure will continue to drop, until eventually we end up in Outer Space where the pressure is zero.
So every time the atmospheric pressure is reduced the gas that is in a dissolved state in our bodies will come out of our bodies and we refer to the process as “Off-Gassing”.
When do we stop to Off-Gas? 
Let’s introduce the concept of partial pressure. If you have done a Nitrox dive course you are most probably already familiar with the concept. The partial pressure of a gas is the percentage of a gas multiplied with the pressure.
Let’s take a look at Air. Air consists of 21% Oxygen and roughly 79% Nitrogen. At sea level the atmospheric pressure is about 1 Bar. Thus the partial pressure of Oxygen is 1Bar x 21% = 0.21. The partial pressure of Nitrogen is 1 Bar x 79% = 0.79.
(A percentage is actually the number divided by 100. For example 21% is actually 21/100 = 0.21)
If we stay at sea level for long enough the partial pressure of the gas in the air around us will equalize with the gas dissolved in our bodies. Thus, if the partial pressure of the Nitrogen in the air is 0.79, then after some time, the partial pressure of Nitrogen in our bodies will also be 0.79.
If we go up a mountain and say the atmospheric pressure drops from 1 Bar to 0.8 Bar, the partial pressure of the Nitrogen will drop to 79% x 0.8 Bar = 0.63. The Nitrogen dissolved in the body will come out of the body, “Off Gas”, until the partial pressure of the Nitrogen in our bodies is also 0.63.
The big lesson we need to learn from this, is that the partial pressure of the gas we breathe always strives to be at equilibrium with the gas dissolved in our bodies.
WHICH GASSES ARE WE TALKING ABOUT?
As mentioned already Air consists of 21% Oxygen and about 79% Nitrogen. For the purposes of scuba diving we can separate gasses into two groups. Oxygen and the rest, being Inert gas. Oxygen is metabolized and for all practical purposes we can assume it does not follow the same on-gassing and off-gassing cycles. All gasses other than Oxygen can be referred to as Inert gasses. 
(Technically these statements are not strictly correct, but adequate for our purposes to understand the essence of the Decompression Theory).
Thus for a scuba diver the importance is to assess the Inert Gasses, because that will affect the on-gassing and off-gassing processes.
ON GASSING
When we scuba dive the water pressure increases the deeper we descend. Scuba equipment is designed to provide us with a breathable gas, at ambient pressure, no matter what the water pressure is. 
At a depth of 10m the water pressure is about 2 Bar. Thus the partial pressure of Nitrogen from an Air tank will be 79% x 2 Bar = 1.58 Bar.
The process of On-Gassing will start as soon as the water pressure starts to increase, and will continue to do so until it can reach a state of equilibrium.
The deeper we go, the higher the water pressure, the higher the partial pressure of the Inert Gas, the faster the process of on-gassing occurs. 
When we go shallower, the process of on-gassing will start to reverse and on-gassing will start. However, not all tissues in our bodies will on-gas and off-gas at the same speed. So even if we start to ascend and the water pressure starts to reduce, the on-gassing in certain slower tissues will still continue, while in the faster tissues the off-gassing will also be on-going.
The result is a massive transportation of inert gas going in and out of the body tissues. On-Gassing during descend, and Off-Gassing AND On-Gassing during ascend.
Only when we reach the surface the Off-Gassing process is in full swing and On-Gassing has stopped.
HOW DO WE GET THE BENDS?
It is important to understand that On-Gassing and Off-Gassing happens every day as atmospheric pressure changes. Even when the weather changes, the atmospheric pressure fluctuates, and Inert Gas will go through the process of On-Gassing and Off-Gassing in our bodies.
However, there happens to be a “speed limit” at which cells can transfer gas in and out through the cell walls. This “speed limit” also varies according to the different types of cells. 
If we ascend from a scuba dive at a rate faster than what the Inert Gas can transfer out of the cell, it has no option but to come out of the dissolved state into a gas state. These gas bubbles will also continue to expand in size as the pressure decreases. 
The gas bubbles that form in this way get trapped in certain areas and if it happens to be in a joint, the pain will be excruciating. Hence the word “Bends”.
If the bubbles happen to form in the spinal cord, the nerves can be crushed and even permanent nerve damage can result. The results of serious bubble formation can be debilitating and even life threatening.
HOW CAN WE PREVENT GETTING THE BENDS?
Unfortunately nobody can guarantee that we will never get the Bends during scuba diving, we can follow guidelines and established limits that has proven to be safe for millions of dives done every year.
The truth is actually that during every dive small micro bubbles do form while ascending. These micro bubbles are also referred to as silent bubbles. Our bodies seem to be able to deal with the micro/silent bubbles and are normally not a cause for concern.
The problem occurs when the micro bubbles “stick” together and form larger bubbles that block circulation and reduce the rate of Off-Gassing. The bubble formation then starts to escalate until DCS is established.
There are many factors that can affect our ability to Off-Gas, which is outside the scope of this article, but one item I do want to expand on is the style of diving.
Many scuba divers often follow a yo-yo profile without realizing the negative affect it has on their bodies. Changing the pressure rapidly from high to low, and then descend again to increase the pressure, and then ascend again to decrease pressure. This up-down, up-down profile causes the accumulation of micro bubbles.
As we ascend micro bubbles form, this is normal. Rapidly descending again, will stop the micro bubbles from being transported out of the body, but the increased pressure is also not enough to cause the micro bubbles to go back into a dissolved state. Ascending again will cause more micro bubbles, which will now adhere to the micro bubbles previously formed. This process of micro bubble accumulation will continue with every yo-yo profile and if continued, will result into DCS.
To reduce the risk of DCS, follow established safe diving guideline & practices. 
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Theuns van Niekerk

Mixed Gas Rebreather Instructor Trainer

 

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