Oxygen therapy plays a role in various medical conditions, delivering supplemental oxygen to individuals who require it. Understanding how oxygen delivery devices work and the concentration of oxygen they provide is important for effective treatment. Two fundamental terms in this context are FiO2 and liters per minute (L/min).
FiO2, or Fraction of Inspired Oxygen, represents the concentration of oxygen a person inhales. Room air naturally contains about 21% oxygen, meaning its FiO2 is 0.21. When supplemental oxygen is used, the FiO2 increases above this baseline. Liters per minute (L/min) refers to the flow rate of supplemental oxygen delivered by a device. The relationship between FiO2 and L/min is important for ensuring individuals receive the appropriate amount of oxygen for their needs.
Calculating FiO2 from Liters
To determine the approximate FiO2 delivered by a nasal cannula at a specific flow rate, a general guideline is used. For individuals receiving supplemental oxygen via a nasal cannula, the FiO2 increases with each additional liter per minute of oxygen flow.
A common clinical estimation states that for every 1 L/min increase in oxygen flow through a nasal cannula, the FiO2 increases by about 4%. Applying this rule to a flow rate of 2 L/min, the calculation begins with the room air FiO2 of 21%. Adding the increase from 2 L/min (2 L/min 4% per liter = 8%), the estimated FiO2 becomes approximately 29% (21% + 8% = 29%). This approximation is widely used in clinical settings for quick estimation of oxygen concentration delivered with low-flow nasal cannula therapy.
Variables Affecting Delivered Oxygen
While the 4% rule provides a helpful estimate, the actual FiO2 a patient receives can vary due to several factors. A patient’s breathing pattern significantly influences the amount of room air mixed with the supplemental oxygen.
For instance, a faster or deeper breathing pattern can lead to the patient inhaling more room air, which dilutes the concentration of supplemental oxygen, potentially lowering the effective FiO2. Conversely, slower, shallower breaths might allow for a higher oxygen concentration to be inhaled.
Mouth breathing can also decrease the FiO2 delivered by a nasal cannula, as it allows more ambient air to mix with the oxygen flow. The type of oxygen delivery device also plays a significant role in the delivered FiO2. Devices like simple face masks, Venturi masks, and non-rebreather masks deliver different FiO2 ranges compared to a nasal cannula. For example, a simple face mask typically delivers 35-55% FiO2, while a non-rebreather mask can deliver 60-90% FiO2.
Why Accurate Oxygen Matters
Oxygen is considered a medication, and its precise delivery is important for patient outcomes. Like any medication, too little or too much oxygen can have unwanted effects.
Delivering insufficient oxygen can lead to hypoxemia, or low blood oxygen levels. This can cause symptoms such as headaches, difficulty breathing, and a rapid heart rate, and if prolonged, it can result in organ damage.
Conversely, providing too much oxygen, known as hyperoxemia, can also be harmful. Excessive oxygen can lead to oxygen toxicity, which may damage the lungs and central nervous system. Symptoms can include chest pain, coughing, or seizures. Healthcare professionals carefully determine the appropriate FiO2 based on individual patient needs to ensure effective treatment while minimizing potential risks.