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Why is the oxygen flow rate up to be maximum 6 L/min when inhaling oxygen through the nasal oxygen cannula?

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Author : Joe Wong
Update time : 2021-03-29 10:49:38

Why is the oxygen flow rate up to be maximum 6 L/min when inhaling oxygen through the nasal oxygen cannula?

Everyone knows that there is only a difference in the oxygen flow rate between high-flow or low-flow oxygen inhalation. You can inhale oxygen at a high flow rate with a low concentration, or you can inhale oxygen at a low flow rate with a high concentration.

The ordinary nasal oxygen cannula, ordinary PVC oxygen masks, and venturi masks used in our daily work are commonly used low-flow oxygen therapy equipment.When the oxygen is heated and humidified, the oxygen concentration can be directly adjusted, which is an ideal way of inhaling oxygen.

Some people think that when oxygen is inhaled through the nasal cannula, the oxygen concentration will no longer increase when the oxygen flow rate is greater than 6 L/min, so the maximum flow rate of oxygen inhalation through the nasal cannula is set to be maximum of 6 L/min. Here we will discuss the relationship between oxygen concentration and oxygen flow in a low-flow oxygen inhalation device, and how much oxygen flow is more appropriate when inhaling oxygen through the nasal cannula.

From textbooks you will see the relationship between oxygen concentration and oxygen flow through nasal cannula

For the calculation formula of oxygen concentration during oxygen inhalation by nasal cannula, please refer to the "Basic Nursing Science" published by Renwei:

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: normal person’s tidal volume is 500 mL, breathing rate is 20 times/min, inhalation time is 1 second, exhalation time is 2 seconds, and the oronasopharyngeal physiological anatomical dead space is 50 mL calculation.

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

Then, within 1 second of inhalation time, 500 mL of the inspiratory tidal volume contains 50 mL of pure oxygen in the oronasopharynx physiological dead space + 100 mL of pure oxygen in the nasal catheter (the flow of the nasal catheter is 6L/min, which is 100 mL/min). s, the flow rate in one second is 100 mL) + 70 mL pure oxygen (tidal volume 500 mL-50 mL-100 mL) x 21%), the calculated inhaled pure oxygen is 220 mL, then the inhaled oxygen concentration is (220 mL/ 500 mL = 44%).

Inspiratory tidal volume

500 mL

Oral, nasopharyngeal anatomy dead space: 50 mL pure oxygen

Nasal cannula: 100 mL pure oxygen (one second flow rate is 100 mL/min x 1 second)

Additional air intake: 500 mL-50 mL-100 mL = 350 mL

(Oxygen concentration 21%, 350 mL x 21% = pure oxygen 70 mL)

In an ideal situation, the oxygen absorption concentration calculated according to the oxygen absorption concentration formula is as follows:

Inhaled oxygen concentration (FiO2)% = 21+ 4 x oxygen inhalation flow rate (L/min). When the oxygen flow rate is 6L/min through the nasal cannula, the inhaled oxygen concentration (FiO2) is about 45%. It can be seen that the oxygen concentration calculated according to the formula (45%) is almost equal to the oxygen concentration (44%) that we inferred from the ideal breathing pattern of normal people.

Therefore, we can use the calculation formula of oxygen inhalation concentration to estimate the oxygen inhalation concentration when inhaling oxygen at different oxygen flow rates. When the oxygen flow rate gradually increases from 1L/min, according to the formula: Inhaled oxygen concentration (FiO2)% = 21+ 4 x oxygen inhalation flow rate (L/min), it can be seen that FiO2 increases by about 4% for every 1L/min increase in oxygen flow rate. This is the calculation basis of the formula (y = ax+b, b is 21, because the air oxygen concentration is 21%).

According to the textbook, according to the degree of hypoxia, the flow rate of oxygen inhalation with bilateral nasal cannula is adjusted as follows:

1 to 2 L/min for mild hypoxia, 2 to 4 L/min for moderate hypoxia, 4 to 6 L/min for severe hypoxia, and 1 to 2 L/min for children.

We can see that for nasal cannula oxygen inhalation, the maximum oxygen flow rate in severe hypoxia is set to 6 L/min.

Layer by layer analysis: Why is the maximum oxygen flow rate set to 6 L/min?

The author saw comrades-in-arms @martin937 in the Dingxiangyuan forum about the reason why the maximum flow rate of oxygen inhalation through the nasal cannula is 6 L/min:

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 the end of respiration. At this time, the oxygen flow is increased to 7L/min or 10L /min etc. cannot increase the oxygen content of the anatomical dead space of the nasopharynx, and it is impossible to further increase the inhaled oxygen concentration.

I think the oxygen concentration will increase when the nasal cannula inhales more than 6 L/min.

First, let’s take a look at the calculation method of inhaling oxygen concentration according to the ideal breathing pattern of normal people: [Oral, nasopharynx, physiological dead space, pure oxygen 50 mL + nasal catheter flow rate 100 mL, pure oxygen (nasal catheter flow rate 6L/min, which is 100 mL/s , The flow rate in one second is 100 mL) + 70 mL pure oxygen content in inhaled air]/tidal volume = oxygen concentration.

It can be seen that the oxygen concentration is determined by four factors: the size of the physiological dead space of the nasopharynx, the flow rate of the nasal catheter, the amount of inhaled air and the tidal volume. When the patient's oxygen flow increases to 7L/min, although the oronasopharynx is physiologically dead The oxygen storage in the cavity no longer increases, but the oxygen inhaled through the nasal catheter will increase (7L/min, equivalent to 117 mL/s). The inhalation time is 1s, the pure oxygen inhaled through the nasal catheter is 117 mL, and the inhaled air volume is 500 mL-50 mL-117 mL = 333 mL, inhaled air oxygen content is 333 mL x 21%≈70 mL, inhaled oxygen concentration is (50 mL+117 mL+70 mL)/500 mL = 47.4%, inhaled oxygen concentration This is an increase from 45% at 6L/min. 

Ideal breathing pattern for normal people (according to textbook): tidal volume is 500 mL, breathing rate is 20 times/min, inhalation time is 1 second, and expiration time is calculated as 2 seconds;

Oxygen flow

Oxygen inhalation concentration (calculated according to oxygen inhalation formula)

Oxygen concentration (calculated according to the ideal breathing pattern of normal people)

6L/min

21+4x 6 = 45%

(50+100+73.5)/500 = 44.7%

7L/min

21+4x 7 = 49%

(50+117+70)/500 = 47.4%

8L/min

21+4x 8 = 53%

(50+133+66.5)/500 = 50%

It can be seen from the above table that according to the ideal breathing pattern of a normal person, as the flow increases (from 6L/min to 8L/min), the patient's inhaled oxygen concentration increases accordingly. It can be seen that when the oxygen flow rate of the nasal cannula is greater than 6L/min, although the 50 mL physiological dead space of the oronasopharyngeal cavity no longer increases, within 1 second of the patient's inhalation phase, due to the increase in oxygen flow, the pure inhalation is Oxygen will increase accordingly, and the inhaled oxygen concentration will increase accordingly.

However, as the oxygen flow rate increases, the gap between the oxygen concentration calculated by the oxygen inhalation formula and the actual oxygen concentration increases. Therefore, the author believes that the oxygen inhalation concentration does not increase when the oxygen inhalation flow rate is greater than 6L/min, so it is not accurate to set the maximum oxygen inhalation flow rate through the nasal catheter to 6L/min. So why do textbooks set the oxygen flow rate through the nasal catheter to a maximum of 6L/min?

The author believes that when oxygen is inhaled through a nasal catheter and the flow rate is greater than 6L/min, although the oxygen concentration will increase, due to insufficient heating and humidification, dry and cold high-flow ventilation will cause the patient’s respiratory mucosa to become dry and uncomfortable. Symptoms increase, leading to increased sputum viscosity and reduced tracheal mucosal ciliary motor function, which is not conducive to sputum discharge, or leads to complications such as frontal sinus pain and dry bleeding of the nasal mucosa. Therefore, weighing the pros and cons, it is believed that the maximum flow of oxygen inhalation through the nasal catheter should not be higher than 6L/min.