[sciencemike]

All this is based upon confused terminology. Some people call a BO when its shallow a SWB. Some people call DWB a black out in shallow water when you resurface from a deep dive. But i tend to stick with the definition i was taught that a SWB is when you come up from a deep dive where your blood PP02 and you lung PP02 are low, and upon entering the shallows again the PP02 in your lungs negatively and DRAMATICALLY changes, decreasing the pulmonary vein blood PP02 which will then immediately be fed from the heart to the brain, therefore causing a black out. Im sure there are better definitions out there but this is the one that i use.

As for a SWB happening in shallows without the concomitant deep dive experience. And most likely it can happen even in shallow water as i illustrate at the end of the post. You are also correct that a the PP02 shift is the greatest/meter near the surface. Changing that last 10m changes the PP02 by 50%, but if surfacing from 20 meters it will change it by 66% and from 30m by 75%. So although the change in PP02 is the greates, its not the greatest PP02. Now im not sure what the diffusion rate is for the 02 across the PCB, but you must remember that its not just the shifting PP02. There are other factors that amplify this change in PP02 to make it cause a SWB.

First, 02 must diffuse across the pulmonary capillary bed (PCB) and into the lungs. We find that higher differences in PP02 result in much higher diffusions. Secondly, diffusion is also a time dependent function so that a faster diffusion is also multiplied by the longer time per ascent So we see a faster rate of diffusion as well as a longer diffusion time resulting in a much greater dPP02 and therefore the greater chance of SWB from deep water. But in reality i can definitely agree with you and see that its really a function of the initial PP02 in the lungs and blood if they are in that danger zone.

Example.....if you are about to black out at -15m, a SWB is more likely vertically than if you were doing that last 15m horizontally in a pool at 2m. But in this example the person is about to black out any ways so it would probably be called a BO vs. a SWB.

Please if anyone sees anything wrong with this let me know, so i can update my knowledge, and change the post so others dont follow the blind.