to the loop does not adequately compensate for partial pressure reduction due to decreasing ambient pressure, the oxygen concentration in the breathing loop can drop below the level required to support consciousness. Freediving During breath-hold diving there is no additional breathing gas available during the ascent. If the diver stays down long enough to use up the available oxygen to the extent where tissue concentration has dropped below a level sufficient to support consciousness at surface pressure, there is a very high risk of blackout before the surface can be reached. Management Avoidance In open circuit diving using underwater breathing apparatus, the diver must switch to a breathing mixture with sufficient oxygen fraction to maintain an acceptable partial pressure during the next stage of ascent. This adjustment may be done as many times as is necessary and should be scheduled in the dive plan. Semi-closed circuit rebreathers may be flushed to increase oxygen fraction in the loop before and during ascent. This will ensure that the gas in the loop has not been partially deoxygenated by rebreathing, and has the highest available oxygen content. This is particularly relevant in passive addition rebreathers, as the expansion of loop gas during the ascent may overwhelm the capacity of the system to dump the appropriate amount of rebreathed gas from the loop. Closed circuit rebreathers can have oxygen added to the loop either manually or automatically, to maintain the desired oxygen partial pressure set-point during ascent. Freedivers can only avoid getting into a situation where latent hypoxia occurs. Once oxygen levels reach this point they will probably lose consciousness during the ascent and are at a high risk of drowning. Ensuring that they are positively buoyant in the event of blackout will give them a chance of recovery if they reach the surface. Rescue The diver who has lost consciousness due to latent hypoxia is already hypoxic, and should be brought to the surface as soon as possible, while protecting the airway. There are no contraindications to immediate surfacing for a breath-hold diver, and the mouth and nose may be blocked to prevent involuntary aspiration of water. First aid and medical treatment The first priority is ventilation at the earliest possible opportunity, and in many cases this may be sufficient. If ventilation with sufficient oxygenation is achieved promptly, and there has been no aspiration of water it is quite possible that no further