Wednesday, July 16, 2008

Decompression the Basics

It occurred to me that I take for granted that every divers knows about decompression. What I forget is that most agencies do not teach decompression diving from the get go (CMAS does which is where I started). Most sport agencies rely on NDL, no decompression limit diving to keep their divers safe. It certainly does keep things simple and reduce the entry level barrier to new divers, but as you no doubt know from some of my other blogs, I believe that it is a false economy as divers learn diving without learning the real danger of diving and so end up at 60 meters and wondering why they are bending. So, here are some of the basics behind decompression theory.
Decompression sickness or DCS is the body’s reaction to bubbles. These bubbles can lodge anywhere in your system (brain, spine, lungs, heart, muscles) and tend to be made of either nitrogen or helium (this obviously depends on what you are breathing when you are diving, nitrogen and or helium). The gases that you breathe are transported from your lungs to all areas of your body (tissue) in the blood along with the oxygen you need to survive. The deeper you go, the more ‘thirsty’ your tissues are for these gases, so the faster they absorb them (a process called in-gassing). This absorption process occurs until the amount of gas outside the tissue is exactly the same as inside (as measured by the partial pressure of the gas). When you change depth and ascend the tissue suddenly has more gas than blood and so releases the gas, a process called out gassing. This is the simplest picture I can paint…and so has its own set of inaccuracies, but you get the idea.
Not all tissues have the same reaction to an increase in the amount of gas available. Some are thirstier than others. When a tissue is greedy and absorbs gas fast it is called a fast tissues and when it takes its time to take on board the new gas, it is called a slow tissue. The speed at which tissues absorb gas is used to classify every tissue in the body. To make it easier tissues are further grouped together into compartments that behave in more or less the same way, absorbing and releasing gases at the same rate (Buehlmann created 16 compartments). The compartments are important for decompression programs as the maths then treats all tissues in that compartment as having the same absorption features.
When discussing tissues the phrase tension is often used (tissue tension) which is simply another way of saying the amount of gas (nitrogen or helium) that is absorbed by that tissue (or put yet another way, the gas pressure). The tables ensure that the tension of the gas in the tissue does not drastically exceed the outside tension. When it does.. bubbles result.
The amount of time it takes for a tissue to release (or outgas) its nitrogen or helium is referred to as that tissues half time. This is a standardised unit that measures the amount of time it takes for a tissue to halve its gas tension (halve the amount of gas it has absorbed).
There are a number of different factors that affect whether or not a tissues is fast or slow, these include the degree of blood flow to that tissue (perfusion), the amount of blood vessels within the tissue and so the distribution of the blood to a tissue and the solubility of the tissue (i.e. how easily it absorbs nitrogen and helium.) This means that areas that have a lot of blood vessels (good perfusion) tend to be faster tissues (lungs and abdominal organs). Slower tissues are normally fat and joints. Fat also holds onto nitrogen better (incidentally this is believed to be one of the reasons we are susceptible to narcosis, our neurons are sheathed in fat and fat likes nitrogen).
One common misconception is that as you ascend all your tissues will be outgassing. But not all the tissues find themselves in a situation where the pressure outside is less than the pressure inside… which means that they are in fact still ongassing (the tension of the tissue is less than the ambient pressure rather than greater than). This is more often the case on deeper dives and has been used as an argument AGAINST deep stops ( personally believe the value of the deep stop is to outgas your fast tissues effectively and stop these bubbling and causing problems at shallower levels. My philosophy is to let your slower tissues absorb, I will deal with them when I get to them J , i.e shallower)
A tissue is deemed to be saturated when the tension outside and inside is the same. The tissue is in a state of equilibrium, it neither takes up gas nor does it let it go. Supersaturation is when the tissues have more gas than outside (ambient pressure outside is less than the pressure of the tissue). Bubbles form when based on how high this supersaturation is (i.e. how great the pressure difference) and how long this state remains true. This is termed critical supersaturation.
Another element often referred to in tables is M-values. These are the maximum nitrogen tensions for a tissue after which bubbles form (or the supersaturation point). Fast tissues tolerate higher supersaturation rates than slower tissues and so have higher M-values.
Tables and decompression programs put all this together and use maths to calculate the times you need to stop to allow the tension in your tissues to subside (i.e. gas to be released into your blood, move to your lungs and then be breathed out). Most tables are parallel models in that they assume that all tissues are exposed to the effects of the gas at any one time…as opposed to serial models where each compartment reacts one after the other (which is obviously not true).
Simple right ? J