Since we are, sigh, once again back on the topic of skip breathing, let's look at the physiology.
1) you have a required "minute ventilation" (volume breathed per minute) that is a function of metabolic rate, body temperature and stimuli like drugs and CO2;
2) skip breathers do not have lower RMVs because they are "breathing less";
3) skip breathers may have fewer "wasted" breaths; wasted breaths being ventilation that wasn't really necessary for oxygenation or CO2 exchange, but because "you were thinking about your breathing." (See
this post)
When you take an "extra" breath, a couple of things happen. You waste that volume of gas, and for a brief time, your alveolar CO2 drops because you have brought in fresh carbon-dioxide-free gas that has yet to exchange with the capillaries. That means that your body wants to breathe a little less in the minute that follows, to preserve CO2 and acid-base balance. At the same time, you used a bit more gas from your tank.
Perhaps sensing that you were breathing more than you really needed to, you try to "breathe less". This usually translates to smaller breaths. Smaller breaths work to preserve homeostasis, because with a larger dead space fraction, those breaths don't keep lowering your CO2, and your body gets back in balance (this is all very subtle - you can't feel it). But with the shallow breaths, you continue to waste gas and your RMV stays higher than physiologically required.
Skip breathers, on the other hand, actually try to breathe less.
Of course, it's not possible. Their bodies drives to respiration (increasing CO2 and decreasing oxygen) mandate that they have the same minute ventilation that they would without skip breathing. But what IS happening is in two parts: acute and chronic.
Acutely, as a skip breather tries to breathe less, his RMV is lower. But his CO2 is rising. His oxygen isn't falling because of all the extra oxygen molecules packed into a breath at depth.
As a result, he'll soon have to breathe MORE to lower his CO2 back to where it belongs.
OR...he/she can become acclimatized to slightly higher pCO2's. But at that point we switch to the chronic state.
At a chronic state of higher pCO2, his metabolic demands haven't changed, and he needs to have exactly the same minute ventilation that he'd required before he started skip breathing.
But at a higher pCO2. And that is where the risk lies. Diving at a higher chronic pCO2 for the benefit of that first couple of minutes when your RMV was briefly lower comes at the price of lower margin when gas density, workload and higher metabolic production of CO2 combine to make you hypercarbic. The skip breather is that much closer to a CO2 "hit" than a normal breather, and DOESN'T have a lower RMV. Gas density, CO2 production and your particular equipment may combine to result in unrecoverable hypercarbia and death.
Learning to breathe efficiently is the key. A brief pause at end inspiration to maximize exchange should be followed by a slow deep exhalation to exhaust the extra molecules of CO2 that accumulated during the pause. This is not a skip. It's just an altered pattern. The required minute ventilation is fixed. It may increase with exercise, and may decrease as a new diver gains experience, calmness and "Zen", but it won't change by breathing less or more.
At least I did. So, I do no more skip breathing after that.
