A detailed reading of the O2 index algorithm

In clinical terms, we hear a lot of questions from our superior doctors: What is the oxygen pressure of this patient? What's the oxygen index? Oxygen fractional pressure can be analyzed for blood and blood by extracting artery blood. What about the oxygen index?

In clinical terms, we often hear the superior doctor ask, "What is this patient's aerobic pressure?" What's the oxygen index? The Oxygen Index reflects the oxygenation of the patient.

Many people use this formula to calculate:

Oxygen = aerobic fraction pressure (PaO2)/inhalation oxygen fraction (FiO2) = PaO2 (21+4*V) %.

V represents inhalation of oxygen flow (L/min). 21 represents oxygen fractions in sea-level air.

When we see a patient with a reduced aerobic pressure, the patient is given 5L/min pure oxygen inhalation, the patient ' s oxygen pressure is not increased significantly, and the flow of oxygen is raised to 8L/min or even 10L/min in the hope of increasing the patient ' s oxygen index.

Is that right? To answer that question, we should first look at how this formula came from: In this formula, the only thing we need to calculate is the oxygen intake score. Our normal human flow is about 500 mL, and the frequency of breathing is around 20 per cent, i.e. once every 3s, in which the breathing time is 1 s and the breathing time is 2s (1.5s can be completed). The cavity of the mouth and nose is about 50 mL (this value is very important!

When the patient takes oxygen from the nose catheter, the flow is 6L/min, let's calculate the patient's oxygen index. The patient spent 1.5s to complete the exhale and 0.5s before the next inhaling started, during which time the oxygen in the nostrils fills the dead catheter. Filling capacity is 100 mL/s*0.5s = 50 mL. Just enough to fill the dead. When the patient breathes next, the inhalation is made up of several components: (1) 50 mL pure oxygen in the dead mouth and nose; (2) pure oxygen in the continuous supply of the nostrils, 100 mL/s*1s = 100 mL; (3) tidal mass, 500 mL, has been inhaled, 150 mL pure oxygen, and 350 mL gases need to be replenished by air, with an oxygen concentration of 21% in air, assuming 20%, 350 mL*20% = 70 mL. Patient inhaled at once with an oxygen fraction (FiO2) of 50mL+100mL+70mL/500mL=44%.

These figures are listed for ease of understanding, and let's look at the general situation:

Inhalation pure oxygen: 0.5*V (Oxygen in the hypothalamus before inhalation) + 1*V (Purple oxygen in inhalation) + (500-0.5*V-1*V)*20% = 1.2*V+100.

FiO2=(1.2*V+100)/500=0.0024*V+0.2. The unit here is mL/s. We can look at the oxygen fractions of inhalation at different oxygen flows:

1L/min（16.67mL/s）：0.24

2L/min （33.34mL/s）：0.28

3L/min （50.00mL/s）：0.32

4L/min （66.68mL/s）：0.36

5L/min （83.35mL/s）：0.40

6L/min （100.00mL/s）：0.44

As can be seen, when the flow of oxygen is between 1 and 6 L/min, the flow of oxygen increases by 1 L/min and the intake fraction increases by 0.04, which is the origin of 4 in the article starting formula, so that the calculation of the inhalation oxygen fraction is consistent with that calculated in the article beginning formula.

The application of this formula is based on several premises: a cavity volume of 50 mL (the maximum value of 0.5*V) at the mouth and nose, fixed at 0.5*V when the oxygen flow exceeds 6 L/min (100 mL/s); a breathing frequency of 20 times/min, which cannot be calculated when the breathing frequency is too high or too slow; and an oxygen concentration of 20 per cent in air, which can be reduced or not applied to the formula.

In summary, the formulas used to calculate the oxidation index, which we normally use, will be relatively accurate only if oxygen is inhaled with a nose tube, oxygen flow is below 6L/min, breathing frequency is normal, sea level conditions are normal. Inaccuracies in calculations occur when masked oxygen is used (increased cavity volume), breathing too fast or too slow, and altitude is too high, which requires recalculation based on the principle of inhaling oxygen fractions.