The Most Important Deco Stop

There has become a significant gap in what is taught for decompression theory in trimix classes. When I took my trimix class, I was taught to plug my dive into deco-planner or V-Planner. There was no discussion about how to modify the deco curve. When you start to do multi-bottle deco dives, you should know that the 30 foot decompression stop becomes the most critical stop of your deco strategy.

So why is the 30 foot stop so important? Before I get into that, first let me setup some dive parameters. I’m talking about doing two deco gas type of dives with 50% Nitrox and Oxygen as the deco gases of choice, but we’ll get into why in a minute.

If you plug a 200 foot dive into you’re favorite dive planning software, it will spit out whatever deco schedule the algorithm comes up with. You’ll see that it will call for a standard gas switch off 50% Nitrox to Oxygen at 20 feet. If you look at the partial pressure of 50% at 30 feet, you’ll see it is 1.0. The goal of every decompression strategy is to optimize the deco. When the lungs are exposed to a high PO2 for long durations, divers can get pulmonary oxygen toxicity. You should have learned this in your Nitrox class and certainly in more detail in your initial technical diving class. Prolonged or very high oxygen concentrations can cause oxidative damage to cell membranes and the collapse of the alveoli in the lungs. This damage to the lungs causes inflammation of the lungs. Resulting in coughing and difficulty breathing. Vital capacity of the lungs also becomes reduced. What does all of this mean? Simply put, if the lungs are not working properly, the gas exchange that goes on in the lungs becomes reduced. In other words, it becomes more difficult for the deco gas to enter the tissues to drive out the nitrogen and helium.

If you’ve been trained to breathe pure oxygen, you should have learned what a gas break is. If not, a gas break is a period of time that a diver switches off a high PO2 deco gas onto a low PO2 gas in order to give the lungs a rest. Gas breaks typically happen while breathing oxygen at 20 feet. The 30 foot stop becomes the place to switch off a high PO2 onto a low PO2 deco gas. In the case of using 50% and oxygen for deco gases, a diver will switch off 50% onto back gas as that has the lowest PO2 of the three gases being carried (remember, 2 deco gas dive). The theory behind this is that if you give the lungs a break to the lowest PO2 possible, the lungs will not become inflamed as quickly when at 20 feet breathing oxygen at a 1.6 PO2. With no or little inflammation in the lungs present, when a diver switches to oxygen at 20 feet, the lungs will be fully ready to handle the high PO2. The 30 foot stop on back gas prepares the lungs for the switch to a very high PO2.

For the 200 foot dives that I’ve been doing, my 30 foot stop is around five minutes. So I do the full five minutes on back gas. If my 30 foot stop was to be longer, I’d do the last three to five minutes of my required 30 foot stop time on back gas. For example, if I had 15 minutes of deco at 30 feet, I’d do 10 minutes on 50% and the last five minutes on back gas. This way, when I get to 20 feet on oxygen, my lungs are fully ready to handle the high PO2. If my deco up to the 30 foot stop was stressful or extended, I may do a little more time on back gas at 30 feet.

You’re probably thinking that not doing the required deco as spit out by your computer could get you bent. Remember, I’m not suggesting that you do ALL of your 30 foot stop on back gas. Just the last five or so minutes of the stop. You will still be off gassing as we can take advantage of the dissolved gas model where it takes a great reduction in ambient pressure to force dissolved gas out of the tissues.

Before, I mentioned that this is best when using 50% Nitrox and oxygen as deco gases. The reason why is because 50% at 30 feet still gives a rather high PO2. If your first deco gas was something else like 36% or 32%, the PO2 at 30 feet is already pretty low, but you are not taking full advantage of the higher PO2 at the shallower deco stops you get with 50%. If you are not using oxygen as your final deco gas, again you’re not being exposed to a high PO2 and decompression will not be fully optimized.

I have found that this strategy has made me feel really good after deco. During my trimix training and subsequent trimix dives, I had symptoms of pulmonary oxygen toxicity after the dives. I was coughing and had a feeling of being tired. Probably a little symptomatic of DCS. If you are intrigued, try adding a minute or two on back gas to your 30 foot stops. See how you feel. I think you might just like it.

References:
Bean, JW; Johnson, PC (1954). “Adrenocortical response to single and repeated exposure to oxygen at high pressure”. American Journal of Physiology 179 (3)

Edstrom, JE; Rockert, H (1962). “The effect of oxygen at high pressure on the histology of the central nervous system and sympathetic and endocrine cells”. Acta Physiologica Scandinavica 55: 255–63