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Wrong! The higher the oxygen flow of the nasal oxygen cannula, the greater the oxygen concentration!

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Author : Joe Wong
Update time : 2021-02-26 14:18:21
Wrong! The higher the oxygen flow of the nasal oxygen cannula, the greater the oxygen concentration!

Regarding the calculation formula of oxygen concentration,

Inhaled oxygen concentration (FiO2)% = 21+ 4 x oxygen inhalation flow rate (L/min)

Take a normal person's ideal breathing pattern as an example, namely: tidal volume 500 mL, respiratory frequency 20 times/min, inhalation time 1 second, exhalation time 2 seconds, and oronasopharyngeal anatomical dead space 50 mL.

If the oxygen flow rate of the nasal cannula is 6L/min (100 mL/s), assuming that the exhalation is completed 1.5 s before the expiration time, there will be almost no air exhalation in the last 0.5 seconds. 6L/min, or 100 mL/s) will fill the oronasopharyngeal anatomical dead space in 0.5 seconds.

Then, within 1 second of inspiratory time, if there is 220 mL of pure oxygen (=50 mL+100 mL+70 mL) in the 500 mL inspiratory tidal volume, the inhaled oxygen concentration is 44% (220 mL/500 mL).

Inspiratory tidal volume 500 mL

Oral, nasopharyngeal anatomy dead space: 50 mL pure oxygen

Nasal cannula: 100 mL pure oxygen

Additional inhalation: 350 mL of air

(Oxygen concentration 21%, oxygen 70 mL)

In an ideal situation, calculated according to the formula, inhaled oxygen concentration (FiO2)% = 21+ 4 x inhaled oxygen flow (L/min), when the nasal cannula inhales oxygen with a flow rate of 6L/min, the inhaled oxygen concentration (FiO2) is approximately 45%

Under other conditions unchanged, if the oxygen flow rate is gradually increased from 1L/min to 6L/min, every time the oxygen flow rate changes by 1L/min, FiO2 will change approximately 4% accordingly. This is the calculation basis of the formula (y = ax +b, b is 21, because the air oxygen concentration is 21%).

Reminder: The oxygen inhalation formula (21+4Q)% is only a formula for simply calculating oxygen inhalation through the nasal cannula. If the oxygen inhalation device is different, such as when using a common mask/oxygen storage mask to inhale oxygen, there is no corresponding relationship between the oxygen inhalation flow and the oxygen concentration. Therefore, this formula is not applicable to the oxygen concentration calculation of other oxygen inhalation devices.

In the textbook, the description of the flow adjustment of the oxygen inhalation of the bilateral nasal cannula: 1~2 L/min for mild hypoxia, 2~4 L/min for moderate hypoxia, 4~6 L/min for severe hypoxia, 1~ for children 2 L/min.

Why is the flow rate limited to 6 L/min for nasal cannula oxygen inhalation?

The reason is:

The anatomical dead space of the nasopharynx is roughly estimated to be 50 mL, and when the oxygen flow rate of the nasal cannula is 6L/min, the dead space has been completely prefilled with oxygen. At this time, the oxygen flow rate is increased to 7L/min or 10L/min, etc. It is impossible to further increase the inhaled oxygen concentration, that is to say, theoretically, the highest oxygen concentration of oxygen inhaled by the nasal ox
ygen tube is about 45%.