i am no expert but one thing that has not been mentioned here, that i seem to have heard something about is that O2 is exchanged most effectively when air is moving in the lungs. (i am not 100% sure about this) but if this is true then during a cesa extra air is leaving the lungs, but the air that is in the lungs is not moving around that much to facilitate the transfer of O2 and CO2 in and out of the blood stream. also at pressure the lungs are not quite as efficient as at 1ata. combine these things plus the physical stress of having to do an extra long assent and it could lead to possible complications that might show symptoms like blacking out
One reason why everyone is confused is because we are talking about several different processes here.
Ambient pressures and partial gas pressures matter only when one is breathing. It matters then because these factors determine what is available IN the lungs to exchange with the blood. If one is not breathing, then what gets absorbed into the blood depends upon what was available in the lungs when the last breath was taken.
Usable oxygen is transferred from the alveoli to hemoglobin in the pulmonary capillaries. Oxygen is then carried to other tissues where the hemoglobin unloads the oxygen, making it available for use by other cells. Breathing higher concentrations or higher pressures of oxygen does not necessarily provide more oxygen for the body because hemoglobin can only hold so much, and in a normal person, it is almost completely saturated all the time anyway. The oxygen that is NOT bound to hemoglobin, and which is simply dissolved in the blood itself, is not really usable, (although it can become toxic).
If one is NOT breathing, then the oxygen in the alveoli eventually gets used up -- or, at least, all that can be exchanged without breathing gets exchanged.
Carbon dioxide (CO2) is different. CO2 is not dependent upon hemoglobin. It's handling is a bit complicated and is controlled by the breathing rate and by the kidneys. If we assume that the kidneys are working right, then the simple way to think of it is that if you are breathing, then you are eliminating CO2. If you are not, then it is accumulating in the body. Once it reaches a certain point, an irresistible urge to breathe occurs. Normally, it is the CO2 level that controls the breathing rate.
Shallow water blackout occurs when a person intentionally lowers his CO2 levels through hyperventilation so that the urge to breathe is decreased. If his brain runs out of oxygen before he gets the urge to breathe, then he passes out and drowns.
A CESA is different. For one thing, the normal diver has not intentionally hyperventilated to artificially lower CO2 levels. Second, the scuba diver has been breathing at depth, so he has more oxygen molecules available for exchange. And even though the pressures in the lungs are changing during the ascent, those changes do not affect the oxygen that is attached to hemoglobin which, in turn, is still carrying oxygen to the other tissues.
In short, comparing CESA to free diving is apples and oranges.
