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Respiratory Therapist/Nasal cannula FiO2 calculation


QUESTION: The calculation is based on a set volume and inspiratory time/ flow rate. Can you provide those numbers? Thanks, Donn

ANSWER: Hi Donn,

This is perhaps the easiest question I have answered in awhile, because it is a pre-calculated percentage and there is not really and guesswork or estimating to it. :-)

That being said, Room Air, as in the air that we breathe day in and day out, is 21% oxygen and basically 78% nitrogen. However, going from Room Air to 1 liter of oxygen via a nasal cannula raises the Fi02 (fraction of inspired oxygen) to 24%. And then for every 1 liter beyond that, you simply add 4% to each additional liter. As in:

Room Air ... 21%
1 Liter ... 24%
2 Liters .. 28%
3 Liters .. 32%
4 Liters .. 36%
5 Liters .. 40%
6 Liters .. 44%

And so on... I hope this helps.


Larry, RRT

---------- FOLLOW-UP ----------

QUESTION: What I meant to ask is the calculated FiO2 for the nasal cannula dependent upon a set inspiratory tidal volume and inspiratory time of a spontaneously breathing patient?  In other words, will an infant have the same tracheal FiO2 as an adult when both are on a nasal cannula at 1 LPM O2? Can a patient with respiratory depression and shallow breathing have an increased tracheal concentration of O2 while on a nasal cannula at 2 LPM O2 due to less mixing of the O2 and room air? What if a patient has half the minute ventilation that they should have? Of course the PaCO2 will rise, but could the inspired O2 concentration double or triple?

Thanks for helping me out, Donn


The FiO2 equation is a fixed measurement, not really a "fluid" variable that changes based on flow, patient age, tidal volume, etc. A person can be better oxygenated with additional pressure beyond the physiological pressure that already exists in the airways (5 centimeters of water pressure), but minute ventilation should not directly affect oxygenation.

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Larry W. Wical, BA-RRT


I CAN answer: ALL questions and queries related to the following topics... - Oxygen - Asthma - COPD - Bronchitis - Emphysema - Pneumonia (Viral/Bacterial) - Tuberculosis (TB) - SARS - Influenza (Flu) - Vaccines - Pulmonary Embolism - Pleural Effusion - Atelectasis - Inhalation injuries (burns, chemicals, etc.) - PFTs - Cardiovascular health - Sleep Apnea - BiPap/CPAP - Ventilators ("Respirators") - Aspiration injuries - Thoracic injuries - Lung contusions - Tracheal injuries - Artificial Tracheostomy - Secretions - Prolotherapy/Regenerative Injection Therapy (RIT): A patient's experience/perspective - General health and fitness I CANNOT answer: Questions that vary too far from my primary scope of pulmonary and cardiovascular care and fitness. I promise to be open and honest about my knowledge of submitted topics, and will always openly provide my personal as well as professional feedback as it relates.


Registered Respiratory Therapist (RRT, RCP) since 2005. I have worked primarily in the acute care, critical care, burn care and home care settings.

NBRC - National Board of Respiratory Care AARC - American Association of Respiratory Care

-All About Kids Magazine -The Clermont Sun -Cincy Sports & Fitness Magazine -Many online Fitness and Health blogs and "webazines"

- B.A. in Communication (1997) - A.A.S. in Respiratory Science (2005) - RRT license (state of OH, KY and IN) - Basic Life Saving (BLS) - Advanced Cardiovascular Life Support (ACLS) - Advanced Burn Life Support (ABLS)

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Currently work in the city's largest academic/research hospital.

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