Supplemental oxygen therapy is a common medical intervention used when a patient’s lungs cannot pull enough oxygen from the surrounding air to meet the body’s needs. The most frequent and least invasive method for administering this extra oxygen is through a nasal cannula, a flexible tube with two small prongs that sit just inside the nostrils.
Healthcare providers measure oxygen delivery in two distinct ways: the flow rate and the concentration of the gas being inhaled. Understanding the difference between these concepts is necessary to grasp how much oxygen a patient is truly receiving.
Understanding the Basics: FiO2 and Flow Rate
The flow rate of oxygen is measured in Liters Per Minute (LPM), representing the volume of gas moving from the oxygen source through the delivery device. This LPM setting dictates the quantity of oxygen being pushed toward the patient.
The other measurement, the Fraction of Inspired Oxygen (\(\text{FiO}_2\)), represents the actual percentage of oxygen the patient is breathing in. This is the true concentration of oxygen reaching the lungs. Ambient or room air naturally contains an \(\text{FiO}_2\) of approximately 21%.
Supplemental oxygen devices increase this baseline 21% concentration. The flow rate (LPM) and the concentration (\(\text{FiO}_2\)) are not the same because the oxygen flow is always mixed with the patient’s inhalation of room air. Consequently, the final \(\text{FiO}_2\) is always lower than the 100% pure oxygen coming from the source tank.
The Standard Calculation Rule for Nasal Cannulas
Healthcare professionals use a simple, standardized method to quickly estimate the \(\text{FiO}_2\) delivered by a nasal cannula. This estimation is specific to low-flow oxygen delivery systems, typically set between 1 and 6 LPM, and is often called the “rule of thumb” in clinical practice.
The standard formula states that for every additional liter per minute of oxygen flow, the \(\text{FiO}_2\) increases by about 4%. This 4% increase is added to the 21% oxygen concentration found in room air. The mathematical representation is: Estimated \(\text{FiO}_2\) (%) = (LPM \(\times\) 4) + 21%.
The 4% multiplier relates to the volume of the nasal cavity, which acts as a small reservoir for the oxygen gas. As the patient inhales, the oxygen is drawn from this reservoir and blended with room air. This calculation provides a practical way to approximate the oxygen concentration for a stable patient.
The Answer for 6 Liters per Minute
Applying the standard calculation rule directly answers the question of how much oxygen is delivered at 6 LPM. Using the formula (LPM \(\times\) 4) + 21%, multiplying the flow rate of 6 LPM by 4% yields a 24% increase in oxygen concentration.
Adding this 24% increase to the 21% of oxygen present in room air results in a total estimated \(\text{FiO}_2\) of 45%. Therefore, 6 liters per minute of oxygen delivered through a nasal cannula provides an estimated inhaled oxygen concentration of 45%.
Six LPM is considered the maximum flow rate for a standard low-flow nasal cannula. Flow rates above 6 LPM are discouraged because they can cause discomfort, nasal dryness, and irritation. Furthermore, higher flow rates often become inefficient, as the excess gas escapes into the surrounding air instead of being inhaled.
Why Actual Oxygen Delivery Varies
The estimated 45% \(\text{FiO}_2\) is a useful starting point, but the actual concentration inhaled by the patient can fluctuate. This variability occurs because the nasal cannula is an open system, meaning the supplemental oxygen constantly mixes with the surrounding room air. The patient’s own physiology is the biggest factor influencing the final concentration.
A patient’s breathing pattern, termed minute ventilation, determines how much room air is mixed with the supplemental oxygen. If a person is breathing quickly and shallowly, they have a high respiratory rate. This rapid breathing pulls in a larger volume of room air, diluting the supplemental oxygen and lowering the actual \(\text{FiO}_2\) below the 45% estimate.
Conversely, a patient breathing slowly and deeply will entrain less room air with each breath. This slower, deeper pattern allows more supplemental oxygen to be inhaled, potentially resulting in an actual \(\text{FiO}_2\) closer to or slightly higher than the calculation suggests. The peak inspiratory flow (how fast the patient inhales) further affects how much room air is pulled in.
For these reasons, healthcare providers rarely rely solely on the LPM setting and the estimated \(\text{FiO}_2\) calculation. Instead, the patient’s blood oxygen saturation is continuously monitored using a pulse oximeter. This direct measurement of blood oxygen levels is the most reliable way to assess the effectiveness of the oxygen therapy.