Latent hypoxia is a condition where the oxygen content of the lungs and arterial blood is sufficient to maintain consciousness at a raised ambient pressure, but not when the pressure is reduced to normal atmospheric pressure. It usually occurs when a diver at depth has a lung gas and blood oxygen concentration that is sufficient to support consciousness at the pressure at that depth, but would be insufficient at surface pressure. This problem is associated with freediving blackout and the presence of hypoxic breathing gas mixtures in underwater breathing apparatus, particularly in diving rebreathers. The term latent hypoxia strictly refers to the situation while the potential victim is at depth, still conscious, and not yet hypoxic, but is also loosely applied to the consequential blackout, which is a form of hypoxic blackout also referred to as blackout of ascent or deep water blackout, though deep water blackout is also used to refer to the final stage of nitrogen narcosis. Mechanism The minimum tissue and vascular partial pressure of oxygen which will maintain consciousness is about . This is equivalent to approximately in the lungs. Approximately 46 ml/min oxygen is required for brain function. This equates to a minimum arterial oxygen partial pressure (PaO2) of at 868 ml/min cerebral flow. An ascent blackout, or deep water blackout, is a loss of consciousness caused by cerebral hypoxia on ascending from a deep freedive (breath-hold dive), typically of ten metres or more when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it, or from a dive using underwater breathing apparatus while using a breathing gas which has too low an oxygen fraction to support consciousness at the surface. Breath-hold victims typically black out close to the surface, sometimes even as they break surface, and have been seen to approach the surface without apparent distress only to sink away. Breath-hold victims are usually established practitioners of deep breath-hold diving, are fit, strong swimmers and have not experienced problems before. Blackout by this mechanism may occur even after surfacing from depth and breathing has commenced if the inhaled oxygen has not yet reached the brain and may be referred to as a surface blackout. Divers ascending using breathing apparatus typically ascend at slower ascent rates to avoid decompression sickness, and the depth at which consciousness is lost tends to follow the