The BENDs: Decompression Illness 101
First Employee Jul 06, 2017
Over the years I have talked many divers that do not understand what is happening in their body during a dive. So I thought I would write a blog to get you thinking about it. This is in no way a comprehensive article that will give you a expert knowledge of DCI (decompression illness) but will hopefully cause you to think. Also, i am not a medical doctor so do not take this as medical advice. If you suspect you have DCI contact Divers Alert Network and medical services immediately.
Although we still consider a good deal of what we know about diving physics to be theory, much of what we understand today is testable and repeatable. For instance, if I descend on SCUBA without equalizing my ears I will experience pain and possibly rupture my ear drum. And if I hold my breath on SCUBA and ascend, the air trapped in my lungs will over expand and rupture. Obviously, neither of these are good things. They are both painful and can result in serious injury or death.
But what about things happening in our body that we don't feel and may not feel until hours after a dive?
?Decompression illness, also know as the bends, is a very real thing and people do die from this malady. The cause of it is breathing gas (air, nitrox, trimix) under pressure and coming up before you body has time to properly offgass. This can look like coming up too fast or staying down for too long then surfacing. It is a having much more pressure in your body's tissues that the pressure that surrounds your body. We are not talking about pressure from the air compressed inside the scuba cylinder, but the atmospheric pressure that the water creates around us at different depths.
From sea level to roughly 60 miles straight up towards space, our atmosphere that we live in creates 14.7 psi or 1 atmosphere of pressure. This is the weight of air at sea level. The higher we go in altitude the less air pressure there is. When we go diving the atmospheric pressure is doubled just at 33ft deep and pressure builds the deeper we go.
So what's the big deal? Why does this matter?
Well, look at it this way. If you haven't dived in the last 24 hours, the pressure of the gasses inside your body is considered to be at equilibrium with the atmosphere. In other words, the pressure of the gasses inside your body is equal to the pressure of them around you.
When you dive on SCUBA and breathe at depth, your body's natural response is to equalize to the atmospheric pressure around you. If you are at 33ft or 2 atmospheres, twice the pressure of earths atomsphere. At that time you are breathing in twice the amount of the gas molecules with every breath than you would at the surface. You body begins to store the excess gases into your bodies tissues to try and achieve equilibrium at that atmospheric pressure. We call this ingassing. At 132ft or 5 atmospheres you would be ingassing 5 times the amount of gas molecules with every breath than you would at the surface. This should help explain why the deeper you go the less time you have.
Our primary concern in recreational air diving is the gas Nitrogen (N2). This is the gas that limits our time at depth and causes DCI. There are other forms of DCI with helium based mixes and oxygen toxicity issues with nitrox but that conversation is for another time. Dive tables and dive computers helps track the intake of these gases in your body and give us No Decompression Limits (NDLs) which are time limits that we can stay underwater without required decompression stops. These NDLs help to keep us safe and avoid DCI if we maintain safe diving practice such as, slow controlled ascents, 3-5 minute safety stops, proper hydration, and etc. Now there is no guarantee that if you do everything correctly that you will not get DCI but it is unlikely. Most DCI occurrences are diver error.
You need to develop the mind set that every dive you do is a decompression dive. When you go down and breathe gas under pressure you are ingassing excess gases. When you ascend you begin the first stages of offgassing the excess gasses. During the ascent it is important that you control your ascent rate. The current recommend ascent rate is 30ft per minute. That's 1ft every 2 seconds. This gives time for the gas to safely come out of your system which in turn reduces the risk of DCI and lung over expansion injuries.
You are considered totally offgassed 24 hours after you last dive. While we ingass rapidly at depth, it takes time to offgass. You should notice that when you are planning multiple dives in a day that your NDLs are less for your 2nd & 3rd dives. Some divers call this the nitrogen penalty. I don't see it as a penalty because these NDLs are greatly reducing the risk of DCI.
I wrote this after talking to a group of divers recently who was taught to follow their bubbles up and let the bubbles beat them to the surface and judge if they should go back down according to how their body felt after the dive. When i asked them which bubble do I follow? The big ones, little ones, medium ones?? None could answer me. This DCI we are discussing is happing in our bodies on a molecular level. We can't see and generally don't feel symptoms to long after the dive. Some cases reported no DCI symptoms until the next day. So don't base your diving plan on following bubbles and "I feel fine after the dive". Based your plans on what doctors and scientist have been studying and developing over years of research. Use your dive computer or table to plan dives. Read and research this stuff as much as you can... I believe the more you know the safer you can be. It is arrogant to think it can't happent to you.... it's science!
Here is a link to signs and symptoms of DCI
Here is a link to the treatment of DCI
Check out this video from TED Ed on decompression illness. It is a great visualization of what is happening inside your body when you dive.
