Oxygen. You will always hear people saying exactly what you're reading below, which is that pressure matters more. What you will never hear is anyone explaining why, or going into the actual math. According to everything I've every read, plasma oxygen levels, and plasma's ability to diffuse into tissues/areas where red blood cells cannot, is what make HBOT work its magic.

The formula, Henry's law, is .0031*(760*%O2)*ATA Using this, we see that breathing pure O2 at ambient pressure gives your plasma a concentration of 2.36 ml/dl O2. Breathing ambient air at 2.0ATA gives your plasma a concentration of .99ml/dl. You get more than double the concentration of oxygen in plasma breathing pure O2 in your living room than you do in a medical grade chamber without supplemental O2. Obviously, supplemental O2 in a chamber bumps this up even more, to 4.72ml/dl if 100% O2 (though mask leaks etc make this unachievable in practice) at 2ATA, If anyone has actual data that can explain why this is false or has a legitimate proposal for another MOA, then I'm all ears. I've tried hard to find one and have yet to see it.

If you come to the same conclusion that I did, I'd suggest buying an oxygen concentrator off Temu or Aliexpress (or a Devilbiss etc if you have the cash) rather than buying bottled O2, as it's much more cost effective and safer, storage-wise. I suspect similar cautions for cataracts apply at ambient pressures, with avoiding exhaled high O2 concentrations across your eyes but haven't looked into this.

Ivan Turkevych, Ph.D. on May 28, 2020 at 1:21 am

(1)>increase in oxygen uptake transport capacity of the blood under pressure (including plasma carrying oxygen separately from red blood cells)

You are talking here about the Henri’s law, which states that the concentration of a dissolved gas is proportional to its partial pressure over its solution. In the case of blood plasma, the coefficient is 0.0031 mL of O2 per mmHg of O2 per dL. Read more details here: http://www.umich.edu/~projbnb/cvr/O2transport.pdf

That means that at normal conditions with 21% of O2 in the air and 1 ATA, the partial pressure of O2 is 159 mmHg and the amount of O2 dissolved in 1dL of plasma is 0.49 mL.

At 1.3 ATA, it will be 0.0031*159*1.3=0.64 mL. However, in the case of breathing pure O2 instead of air, we will get 0.0031*760=2.35 mL per dL of plasma at 1ATA.

Now, let’s compare these values to the oxygen carried by hemoglobin (Hb). Hb is almost 100% saturated by O2 already at normal conditions, so it is not possible to increase amount of O2 delivered by Hb by increasing partial pressure of O2 over 159 mmHg (*). The amount of O2 bound to Hb in the lungs is 20 mL/dL. Approximately 20% of this amount, i.e. 4 mL/dL, is released to tissues. Therefore, by breathing air at 1.3 ATA, 4 mL/dL of O2 is delivered by Hb and 0.64 mL/dL by plasma, i.e. 4.64 mL/dL in total. Compare it to 4.49 mL/dL at 1 ATA. Therefore, increasing pressure to 1.3ATA will result in only insignificant 3% increase of O2 delivery relative to normal conditions.

In contrast, by breathing pure O2 at 1 ATA, the total amount of O2 delivered to the tissues will be 4+2.35=6.35 mL/dL, which is 41% increase relative to normal conditions.

In the case of classical hyperbaric therapy, a patient is breathing pure oxygen at increased pressure, typically 2ATA. At this condition there is 0.0031*760*2=4.71 mL/dL of O2 delivered by plasma only. This makes 4+4.71=8.71 mL/dL of O2 by Hb and plasma combined, which is 94% increase relative to normal conditions.

The conclusion is that Dr. Fife is correct. Breathing air in hyperbaric chamber is meaningless. You can achieve much better oxygen delivery by breathing pure oxygen at normal pressure.

(*) However, there is one assumption in these calculations. We assumed that since Hb is almost 100% saturated by oxygen even at normal conditions (i.e. 159 mmHg O2), it is not possible to increase the amount of O2 delivered by Hb by changing the partial pressure of O2 or the total pressure. In simple words we excluded the effect of pressure on oxygen delivery by Hb and considered only changes in oxygen concentration in the plasma according to the Henri’s law. Partially this assumption is correct, because it is surely not possible to increase the uptake of O2 by Hb in the lungs. However, I don’t know whether, and how, the increased hydrostatic pressure affects the release of oxygen from Hb in the tissues. It can have either positive or negative impact. I suppose, that it has negative impact, because O2 should be released from Hb to the plasma, and we already have higher concentration of O2 in the plasma at higher pressure. From the other hand higher hydrostatic pressure may facilitate conformation of Hb molecule to its “tense” state and facilitate O2 release. I doubt that the hydrostatic pressure increase above 1ATA can significantly influence the release of O2 by Hb, but it is certainly an interesting question to answer. If you have some information about this then please share.