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Flow Devices: Nasal Cannula, Aerosol, Venturi and the Rebreathing Mask

Flow Devices

Flow devices that are used for oxygen therapy are classified into high-flow and low-flow devices. The high flow devices provide a higher supply of airflow while the low flow devices provide a lower supply of airflow. Some of the common examples of these flow devices are nasal cannula, aerosol mask, rebreathing mask, venturi mask, etc.

Differences between high flow and low flow devices

(High Flow Vs Low Flow Devices)

  • Flow Rate:- Flow rate for low flow devices is lower than the normal inspiratory flow rate of the patient which is usually from 20 liters/min to 30 liters/min. The patient gets the remaining portion of the air from the atmosphere. On the other hand, high-flow devices supply more air than the required inspiratory flow rate of the patient.
  • Fraction of Inspired Oxygen (FiO2):- The value of FiO2 remains constant for the high-flow devices. However, for the low-flow devices, the value of FiO2 cannot remain fixed.

  • Temperature:- Air is supplied at a normal atmospheric temperature in the case of low flow devices. But, in the case of high-flow devices,  it supplies air that is generally warm and humid. This allows maximizing the tolerance of the patient.
  • Examples:- Some of the common examples of high flow devices include venturi mask, jet nebulizer, high flow nasal cannula, etc. On the other hand, low flow devices include nasal cannula, partial rebreathing mask,  normal (aerosol) mask, etc.

Why does the value of FiO2 vary for the low-flow devices?

  1. Breathing rate:- Breathing rate differs from person to person. For a fixed supply of oxygen, the value of FiO2 is inversely proportional to the breathing rate. This is because the dilution of pure oxygen by atmospheric gases increases. So, FiO2 decreases.
  2. Supply flow rate:- If the supply flow rate increases then the value of FiO2 will also increase.
  3. The purity of supplied oxygen:- The purity of supplied oxygen from oxygen sources is not always 100%.  It may differ from machine to machine. For example, an oxygen concentrator supplies variable oxygen, ranging from 90 to 96 percent purity.

Due to these reasons, the value of the fraction of inspired oxygen can vary. However, the high-flow devices are the closed systems. Thus, they can control the value of FiO2.

 

1. Nasal Cannula

Nasal cannula
Image Source:- https://www.directhomemedical.com/1820-7foot-softech-nasal-cannula.html

The nasal cannula is a type of oxygen delivery device. It is made of plastic. One end of the cannula consists of two prongs that go into the nostrils. The other end connects to the oxygen source. The oxygen source may be an oxygen concentrator, an oxygen pipe system in the hospital, or the oxygen cylinders.

Types  of Nasal Cannula

  • Low flow nasal cannula
  • High flow nasal cannula (HFNC)

Both of them can be used by patients while eating and talking. The cannulae are light, portable, and easy to use.

Low Flow Nasal Cannula High Flow Nasal Cannula
Low flow device High flow device
The supply flow rate ranges from 1 to 6 liters/min The supply flow rate ranges up to 50 or 60 liters/min based on the design of the machines.
The value of FiO2 ranges from 21% to 41%. The value of FiO2 ranges from 21% to 100%.
No need for humidification

 

Humidification is needed.
The value of FiO2 cannot be controlled. It can control the value of FiO2. The oxygen sensor measures its value. Similarly, the rotameter controls the flow rate of oxygen.
There is no variation in the temperature of the supplied air Air is generally heated up to 370C prior to the administration to the patient.
Best suited for a stable and minor patient. Best suited for patients with acute respiratory failures.
Applicable for a longer period of time. Applicable for a shorter period of time.
The tubes are narrow The tubes are wider to sustain a higher velocity of air.
Cheaper Quite expensive

Handling process of Nasal Cannula

  • Check the condition of the device. If there is an oxygen sensor then check it. Expose it to the atmosphere and see the value of FiO2. It should show about 21% value. If this is not the case then you may need to do the calibration.
  • Do not bring flammable objects near the machine. Put it away from the risk of electric sparks. Oxygen acts as a catalyst for a burning fire.
  • Check for leakage in the cannula tubes.
  • Always use distilled water in the humidifier.

 

2. Aerosol Mask

Aerosol Mask
Image Source:- https://www.healthproductsforyou.com/p-drive-aerosol-mask.html

An aerosol mask is one of the oxygen delivery devices. In other words, it is a mask that is used to supply oxygen to the patient.

An aerosol mask is a simple respiratory face mask which consists of holes on two sides for breathing out the air. The mask covers both mouth and nose. An elastic connects to both ends of the mask which goes back of the head.

Since the mask is light, durable, and suitable for both the mouth and nose breathers, it is widely used for the treatment of patients; in both hospitals and houses.

Features of Aerosol Mask

  • The value of FiO2 ranges from 40% to 60%.
  • Flow rate normally ranges from 6 to 10 liters per minute. If you set it below 6 liters/min there is a chance that the atmospheric gas will enter into the nostril through the holes on both sides of the mask.
  • It is a low-flow device.
  • Humidification may or may not be needed.
  • It has got a reservoir capacity of about 100 ml to 250 ml.
  • An aerosol mask should be used only for a short period of time and should later be replaced with a nasal cannula. Exposure to high oxygen concentration is toxic to the human body.

Disadvantages of Aerosol Mask

  • Patients will not be able to eat or talk by using this mask
  • Difficult to fix this aerosol mask in the face for a longer period of time. Due to this reason, there is a high chance of leakage of the air from the mask.
  • If the patient uses the mask for a long period of time, the elastic can cause pain and red marks around the ear loop.
  • There is also a high chance of rebreathing the exhaled air.

Life span of Aerosol Mask

These respiratory masks can last for over a year. But, I recommend you, replace it between 6 months to 9 months of use.

Cleaning process of Aerosol Mask

  • You can use soap water to clean the mask and the connecting tube.
  • Vinegar can also be used for cleaning but it will give an unpleasant smell once the mask has been dried out.
  • Do not use bleaching powder or alcohol for cleaning. They are highly corrosive in nature.

 

3. Rebreathing Mask

A Rebreathing mask is one of the oxygen delivery devices. It is a kind of aerosol mask that consists of a reservoir bag at the bottom. These reservoir bags have a volume of about 1 liter to store pure oxygen.  Thus the bag should always be at least 1/3 or 1/2 inflated with oxygen.

Drawbacks of Rebreathing Mask

  • Possibility of neither eating nor talking using this mask
  • Chances of skin irritation around the nose and ear
  • Chances of air leakage from the mask
  • Might need to adjust the mask from time to time

Types of Rebreathing Mask

Aerosol Mask
Partial Rebreathing Mask vs Non-Rebreathing Mask

Partial Rebreathing Mask vs Non-Rebreathing Mask

Two types of rebreathing masks are available in the market. Major differences among those masks are as follows.

Partial Rebreathing Mask Non- Rebreathing Mask
Since there is no valve in the mask, there is a partial chance of exhaled air being inspired again. There may be one, two, or more valves attached to the mask and tube connecting the reservoir bag. These valves allow the one-way flow of the gas and thus there is no chance of inhaling the exhaled gas.
The fraction of inspired oxygen (FiO2) ranges from 40% to 70%. FiO2 ranges from 60% to 80% for a single valve mask. In the case of a double valve mask, it can go up to 100%.
It is a low-flow device. It may either be a low-flow or high-flow device.
Flow rate ranges from 8 to 15 liters per minute. Flow rate ranges from 10 to 15 liters per minute.
There is no risk for the suffocation of the patient. If the valve gets damaged then the patient will experience suffocation. So, do not leave the patient alone while using this mask.
Generally, used for a longer period of time. Generally, used for a short period of time only. For example, during transportation of the patient.

Risk of Non-Rebreathing Mask

Some of the risks of non-rebreathing masks are as follows:

  • Suffocation of the patient
  • Lack of humidity
  • High chance of hyperoxygenation

 

4. Venturi Mask

Venturi mask depends upon the principle of venturi. It is one of the oxygen delivery devices.

Components of Venturi Mask

  • Aerosol mask
  • Corrugated tubing
  • Protective case
  • Adaptors
  • Oxygen tubing

Features of Venturi Mask

  • FiO2 ranges from 24% to 60%.  Over 60% is difficult to maintain by the venturi mask.
  • Flow rate ranges from 24 LPM (FiO2 = 60%)  to 104 LPM (FiO2= 24%).
  • High flow device

Advantages of Venturi Mask

  • Helps to provide the precise level of FiO2
  • Humidification is not needed.

Disadvantages of Venturi Mask

(Complication of Venturi Mask)

  • Not possible to eat and talk while using the mask
  • Uncomfortable to use
  • Chance of air leakage from the face
  • Skin/ Eye irritation

Venturi Mask Principle

Oxygen comes from the oxygen source to the mask via oxygen tubing. The tubing is in connection with the adaptor. Adaptors are found in different colors. They adapt the principle of Venturi which alters the value of FiO2 and air entrainment. A protective case protects the adaptors. Oxygenated air then goes to the patient through an aerosol mask.

Types of adaptors

Different manufacturing companies have developed different designs of adaptors for the venturi mask. Some of them are as follows.

1. Fixed adaptors (valves)

(Venturi Mask Flow Rate)

(Venturi Mask FiO2

Venturi Mask
Image Source: https://www.hensomed.com/products/venturi-mask-six-diluters-oxygen-therapy/

There are 5-6 different adaptors available with the venturi mask.

Adaptors

FiO2 (%)

The flow rate of oxygen (LPM)

Blue

24

2

White

28

4

Orange

31

6

Yellow

35

8

Red

40

10

Green

60

15

 

These ratings are given in the adaptors. You need to read the values and adjust the output of the oxygen source.

 2.  Adjustable adaptor (valve)
Venturi Mask
Image Source: http://www.respiratoryupdate.com/members/Air_Entrainment_Mask_Venturi.cfm

You may even find an adaptor (valve) with adjustable settings. It is done by rotating the adaptor. In other words, you just need a single adjustable adaptor to set the values of FiO2. This kind of adaptor is quite expensive in comparison to the traditional one.

Calculation of the total flow of oxygenated air

An entrainment ratio is defined as the ratio of oxygen flow to atmospheric airflow from the adaptor to the aerosol mask.

FiO2

Entrainment Ratio

Total Flow (liters per minute)

24%

1:25

(1+25)*4= 104

28%

1:10

(1+10)*4= 44

31%

1:7

(1+7)*6 = 48

35%

1:5

(1+5)*8= 48

40%

1:3

(1+3)*8 = 32

50%

1:1.7

(1+1.7)*12= 32.4

60%

1:1

(1+1)*12= 24

70%

1:0.6

(1+0.6)*12= 19.2

In the table above, we can see that the minimum value of FiO2 is 24% for the total airflow of 104 LPM. For 60% FiO2, total airflow is equal to 24 LPM which may be for a low flow device. Since the venturi mask is a high-flow device, it is rarely useful for FiO2 greater than 50%.

 

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