Controlled Emergency Swimming Ascent (CESA) (aka "Free Ascent" or "blow and go") is a technique used by scuba divers as a safety measure when a diver has run out of breathing gas in shallow water and must return to the surface.[1]
The technique involves simply ascending at a controlled pace, typically about 18 meters (60 feet) per minute while exhaling slowly or vocalizing a continuous sound. As the diver ascends, the pressure in the lungs increases relative to surrounding water pressure. Exhaling serves to equalize this difference. Hence, the diver can continue exhaling throughout the ascent, and still have air in their lungs at the surface. For the same reason, if the diver fails to exhale during the ascent, lung over expansion injury is likely to occur. As the diver swims to the surface (making the ahhhhh sound) they will feel as if they are not running out of breath; this is because the compressed air they inhaled at depth is expanding as they ascend.
While in a practical sense there is little difference between a CESA and a "Free Ascent" (aka Emergency Swimming Ascent or ESA), the technical difference between the two is that in a CESA the regulator second stage is retained in the mouth and the diver exhales through it (in case gas becomes available due to the drop in ambient pressure) while in an ESA, the regulator is not retained and the diver exhales through pursed lips.