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Categories of Oxygen Therapy: Low-Flow vs High-Flow Systems

Apr 20, 2023

categories of oxygen therapy low flow high flow systems

Oxygen therapy is a treatment where the patient is provided with extra oxygen to breathe. Also called supplemental oxygen, it is provided to people whose bodies are incapable of taking in the required amount of oxygen.

Oxygen therapy is an important topic for Anesthesia. Read this blog post till the end and ace your NEET PG preparation.

ENT Residency

Categories of Oxygen Therapy 

  • Depending on this, there are two categories of oxygen therapy.
    • Low-flow oxygen therapy
    • High-flow oxygen therapy
  • These therapies are being given because the patient is not able to maintain their saturation in the natural atmosphere.  Oxygen therapy is therefore given to the patient if their O2 saturation is low due to any kind of illness or injury, be it at a cellular or a bigger level.  It is administered at a concentration of pressure greater than that found in the atmosphere. 

Low Flow System Vs High Flow System

Low flow system/ Variable performance

High flow system/ Fixed performance

  • Contribute partially to inspired gases FiO2 is dependent of TV, RR 
  • Delivers Specific and constant % of O2


  1. Nasal Cannula 
  2. Simple mask / Hudson mask
  3. Non rebreakable mask 
  4. Partial rebreaking Mask 


  1. Venturi mask 
  2. High flow nasal cannula 

Low Flow System

Nasal Cannula

  • Used in low to medium concentration of O2 (22-44%)
  • Amount delivered-
    • 1 l/min – 24% 
    • 2 l/min -28%
    • 3 l/min – 32%
    • 4 l/min – 36%
    • 5 l/min – 40%
    • 6 l/min – 44%
  • This device is used for providing preliminary oxygen to the patient. These can deliver oxygen with a concentration of 22 to 44%. 
  • When these are attached to the flow metre which is connected to the central pipeline supply of an ICU or cylinders and is turned on at 1L per minute, these deliver oxygen of about 24%.  Atmospheric oxygen is from 20 to 21%.  When the concentration of oxygen is increased, it leads to an increase in the concentration of FiO2.  This device can only deliver up to 6L per minute. Every litre corresponds to its subsequent FiO2. This device is not beneficial for a capacity of > 6 litres.


  • Patients are able to talk and eat with O2 support when using a nasal cannula. A nasal cannula can be used in home settings.


  • It may cause irritation of nasal and pharyngeal mucosa. If the flow is given for more than 6L, the FiO2 may vary. 

Face Mask

  • It is a simple O2 mask.  There are partial rebreather masks and non rebreather masks.  These face masks are used when the oxygen requirement of the patient increases. These have a flow rate of 6 to 10 litres per minute. These supply 35 to 60% O2.


  • An increased delivery of oxygen can be made but for a shorter duration due to damage to the skin surface from the mask.


  • Tight seals are required, which can make many patients feel claustrophobic. There lies a potential for skin breaching. It can be uncomfortable for the patient while eating or talking.

Partial Rebreathing Mask

  • This consists of a mask and a reservoir bag. The bag should be inflated all the time during inspiration and expiration to avoid any rebreathing.  The flow rate is from 6 to 8 L per minute, with the FiO2 being at 80%. 


  • The patient can inhale room air through the opening of the oxygen supply is cut.


  • This device has a tight seal which can be uncomfortable for patients.

Non-Breathing Mask

  • These are instruments through which oxygen with a concentration of 95 to 100% (0.9 -1% FiO2) is delivered. This oxygen percentage is the maximum among all instruments.  The flow rate is 10 to 15 L/m in this case. 


  • These instruments deliver the highest possible oxygen concentration for patients suffering from severe hypoxemia.


  • This instrument is not good for long-term oxygen use. It can cause suffocation. It is expensive.

Fixed-Performance Devices

  • If the patient’s tidal volume or respiratory rate is not normal, fixed performance devices provide the accurate volume of oxygen to the patient.

 Venturi Mask

  • Oxygen is delivered at a concentration of 24 to 60% with the help of a Venturi Mask. The flow rate is 4 to 15 litres per minute.  A constant amount is delivered through this mask which is not dependent on the respiratory rate and the tidal volume of the patient.  Colour coding is done on these masks, according to which they deliver a specific percentage of oxygen to the patient.
Colour FlowrateOxygen Concentration
Blue           2L - 24%
White         4L - 28%
Yellow        6L - 35%
Red           8L - 40%
Green      12L - 60%


  • A very precise concentration of oxygen is provided. It does not dry out the mucosa and does not cause irritation.


  • It might be uncomfortable. It might cause skin irritation.

High-Flow Nasal Cannula (HFNC)

  • These have humidifiers, heated tubes, a nasal cannula, and an oxygen blender. 


  • There is Peak End Expiratory Pressure (PEEP). At the end of respiration, the alveoli collapse. And in order to avoid a complete collapse, Peak End Expiratory pressure has to be present.  There is the delivery of constant FiO2 at 100%. These decrease the anatomical dead space and provide the maximum amount of oxygen possible to the patient.

PEEP (peak end expiratory pressure)

To avoid the collapse of alveoli PEEP is needed in these devices. The patient can inhale the air in the room till the time the oxygen supply is replenished. 

And that is everything you need to know about Oxygen Therapy for your NEET PG preparation. For more informative and interesting posts like this, download the PrepLadder App and keep following our blog! 

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