dteubner

O2 does not contribute significantly to narcosis because the partial pressure of oxygen in the neurons doesn't get very high because oxygen is used up.

Moderately complicated explaination follows.

Oxygen is carried in blood in 2 ways, bound to haemoglobin and dissolved in the blood. The haemoglobin in 100mls of normal blood carries about 20ml of oxygen and the dissolved component is 0.003ml per mmHg O2 per 100mls of blood.

You use the dissolved component first. Your brain uses about a quarter of the available oxygen breathing normobaric air, so you have about 15mls of oxygen per 100mls blood in the veins which equates to a partial pressure of oxygen of about 40mmHg. The venous and tissue partial pressures are roughly the same

If you are diving at a setpoint of 1.4, each 100mls of your blood contains a bit more oxygen than usual (20mls bound to haemoglobin and about 3.2 mls dissolved). If you are using the same amount of oxygen then there will be about 18mls O2/100mls blood, which means the haemoglobin is about 90% saturated and the partial pressure of O2 is about 75mmHg.

If oxygen was as narcotic as nitrogen then you can see why the contribution fo oxygen to the narcosis is negligable, and why narcosis worsens as the PO2 falls.

Say you are at 40m with air dil and a setpoint of 1.4. The partial pressure of nitrogen in the brain is 3.6 ATA (or 2736mmHg) and the partial pressure of oxygen in the brain is about .1 ATA (75mmHg) so the total "narcotic load" is 3.7 ATA.

Now say your setpoint changes to 0.2 (for whatever reason) The partial pressure of nitrogen in the brain is now 4.8 ATA (or 3648mmHg) and the partial pressure of oxygen in the brain is about .05ATA (40mmHg) so the total "narcotic load" is 4.85 ATA - much worse.

Dave T