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COMPOSITION OF AMBIENT AIR

Category: Airway

Topic: Oxygen Delivery

Level: EMT

8 minute read

Composition of Ambient Air

Ambient air contains approximately 21% oxygen, 78% nitrogen, with minute amounts of carbon dioxide and other gases. Supplemental oxygen therapy replaces the inert gas (e.g. nitrogen) with oxygen and can improve respiration due to the increased relative percentage of oxygen (30% to 99%) in the air that reaches the alveoli.

Beyond the relative percentage of oxygen in the air, the total pressure of the air is also important. At sea level, the air pressure is 760mmHg, at the top of Mount Everest the air pressure is 253mmHg. The percentage of oxygen at BOTH is around 21%. But the total amount of oxygen your lungs have access to is much lower on mt Everest given the lower total air pressure. This is a highly tested concept as variations in altitude even between major U.S. cities (such as San Diego CA and Denver CO) can have dramatic effects on patients. 

Fraction of Inspired Oxygen (FiO2) is a decimal number (often reported as a percentage, e.g. 0.21 = 21%) that defines the relative fraction of oxygen that is present in the inhaled air. For normal air, this is 0.21. If a patient is intubated and on 100% O2 this is 1.00. For a patient on partial oxygen therapy, such as through a nasal cannula, the FIO2 is often 0.3 to 0.4. 

Fraction of Delivered Oxygen (FdO2) describes the actual amount of oxygen reaching the capillary beds in the body and perfusing the cells. It is rarely used and is only included here for completeness' sake.

 

Flow Rates and Corresponding FiO2s

The "flow rate" generally refers to the amount of oxygen being delivered to the patient each minute, it is reported in liters per minute (LPM, or L/M, or commonly just L). A given LPM of oxygen will result in a different FiO2 depending upon which delivery device is used.  See below for more details and a breakdown of each device. Remember that normal air has 21% (0.21) FiO2 when reading these charts.

Nasal Cannula

Rule of thumb: 24% FiO2 for 1 Liter/minute. Thereafter, an additional 4% FiO2 for each liter of flow:

1L 24% (24% + 0x4%)
2L 28% (24% + 1x4%)
3L 32% (24% + 2x4%)
4L 36% (24% + 3x4%)
5L 40% (24% + 4x4%)
6L 44% (24% + 5x4%)

 

Simple Masks (40-50% FiO2)

Simple masks/Tracheal masks deliver a FiO2 of 40-50% with a minimum oxygen flow of 5-12 LPM, they differ from nasal cannulas in that they are not used with lower flows, this is because a minimum flow of 5 LPM is needed to flush the expired air out fo the mask before the next breath.

 

Non-Rebreather Masks (NRBs) (60-90% FiO2)

Non-rebreather masks deliver FiO2 of 60-90% if fit correctly to the patient's face. These masks have a bag that fills with O2 and a one-way valve for exhalation that ensures that each breath is taken from the contents of the oxygen-filled bag. These masks are always used with the oxygen set "wide open" as the bag must inflate for the patient to receive a full breath of oxygen.