[trux]

Well, there are always several factors leading to the hypoxic blackout - at an ascent it is the depressurizing effect, and on the surface it is the sudden exhale, forgetting to breath up properly, laryngospasm due to inhaling water from a snorkel, or samba. You may get lightly hypoxic on the ascent, get a light LMC which then can cause the loss of control of respiratory muscles, and you black out in consequence of it.