It's a good question. You need about 16% O2 at the surface, or about 2.5 psi, to maintain consciousness. Once you stop exchanging gas, the oxygen content of the gas in your lungs will begin to drop -- but how fast it drops will depend on how cold you are and what you are doing. Obviously, trained free divers can spend several useful minutes underwater, just on the gas they took down from the surface in their lungs -- but part of how they do it is learning to be very efficient. In fact, CO2 probably limits the time you can be underwater and conscious as much as O2 does. CO2 is what will force you to inhale, long before you black out from hypoxia.
A completely lack of oxygen delivery to the brain gives you about four minutes to significant brain damage, at normal body temperature; at lower core temperatures, that period is longer. But the brain has a lot of mechanisms to stay warm, so the cases where people have survived immersion in cold water are generally cases where they were IN the water for a long time before going UNDER the water -- long enough to allow core temperature to fall significantly. A person who falls off a boat into cold water and is knocked unconscious is going to have brain damage in about the four minute time.
When we do intubations, though, we have the person breathe 100% O2 (1ATA) to preoxygenate -- and in that circumstance, we may have as much as 8 minutes before the oxygen level in the blood even falls significantly. 1 ATA is essentially the ppO2 of someone at 60 feet on 32% -- so someone deprived of breathing gas at that depth, who has been breathing that mix, has quite a long time before they'll desaturate enough to pass out, assuming they are not flailing.
There are a lot of variables in this question.
A completely lack of oxygen delivery to the brain gives you about four minutes to significant brain damage, at normal body temperature; at lower core temperatures, that period is longer. But the brain has a lot of mechanisms to stay warm, so the cases where people have survived immersion in cold water are generally cases where they were IN the water for a long time before going UNDER the water -- long enough to allow core temperature to fall significantly. A person who falls off a boat into cold water and is knocked unconscious is going to have brain damage in about the four minute time.
When we do intubations, though, we have the person breathe 100% O2 (1ATA) to preoxygenate -- and in that circumstance, we may have as much as 8 minutes before the oxygen level in the blood even falls significantly. 1 ATA is essentially the ppO2 of someone at 60 feet on 32% -- so someone deprived of breathing gas at that depth, who has been breathing that mix, has quite a long time before they'll desaturate enough to pass out, assuming they are not flailing.
There are a lot of variables in this question.
