High flow does not mean high concentration. High concentration oxygen therapy is not suitable for most patients, while high flow provides a steady flow of oxygen. Next, let's discuss how does a Venturi Oxygen Mask works?

1.What Is the Venturi Effect？

1.1 Venturi Mask Mechanism

The venturi effect involves the continuity equation and Bernoulli's principle in fluid physics. When a certain flow rate of oxygen passes through a small cross-sectional area of the jet (see figure1.1), the flow rate increases to form a high-velocity airflow, which generates a certain negative pressure. And surrounding air drawn in and finally forms a high flow air oxygen mixture.

This effect is similar to when a high-speed train is passing, the airflow around it will "suck" the leaves on the roadside toward the train.

2.What Are the Factors That Affect the Inhalation Oxygen Concentration?

The correspondence between the oxygen concentration after mixing and the oxygen flow rate (LMP) has been marked on the venturi valve (figure2.1). According to its effect, the inhalation oxygen concentration is related to the jet hole diameter, air inflow diameter, and oxygen flow rate.

2.1 Venturi Color Code

When we decrease the jet diameters and oxygen flow rate or increase the air inflow diameter, the oxygen concentration decreases, and vice versa.

3.Why Does a Venturi Mask Not Cause Carbon Dioxide Retention?

The oxygen inhalation concentration of a normal oxygen mask is between 50% and 60%. If the oxygen flow rate is too low when using the mask, not only does the oxygen inhalation concentration decreases, but also the exhaled carbon dioxide accumulates in the mask, leading to repeated inhalation of carbon dioxide.

In contrast, the airflow injected into the venturi mask is greater than the highest flow rate and tidal volume when the patient is inhaling, so the inspiratory oxygen concentration is constant. Because the high flow rate constantly flushes the inside of the mask, it is difficult for the exhaled air to remain in the mask, so there is no repeat breathing.

In the treatment of hypoxemic patients with hypercapnia, a venturi mask is needed to accurately control the oxygen concentration.

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