In this article "constant pressure blackout" and "shallow water blackout" refers to blackouts in shallow water following hyperventilation and "ascent blackout" and "deep water blackout" refers to blackout on ascent from depth. Some free divers consider blackout on ascent to be a special condition or subset of shallow water blackout but the primary underlying mechanisms differ. This confusion is exacerbated by the fact that in the case of blackout on ascent, hyperventilation induced hypocapnia also may be a contributory factor even if depressurisation on ascent is the actual precipitator.