Fraction of Inspired Oxygen (FiO2) is a fundamental concept describing the concentration of oxygen in the air a person inhales. Understanding FiO2 is essential for comprehending respiratory processes and the delivery of supplemental oxygen when needed.
Understanding FiO2
The term ‘Fraction of Inspired Oxygen’ refers to the air drawn into the lungs during each breath. The ‘fraction’ quantifies the proportion of oxygen within this inhaled air, typically expressed as a decimal or percentage.
Ambient air, or room air, consistently contains about 0.21, or 21%, oxygen. Regardless of altitude, this concentration indicates the oxygen available for gas exchange in the lungs. Even if a person breathes more deeply or rapidly, the FiO2 of room air remains 0.21 unless supplemental oxygen is added. This concentration determines how much oxygen transfers into the bloodstream.
Delivering Supplemental Oxygen
When the body requires more oxygen than room air provides, supplemental oxygen is administered to increase the FiO2 above 0.21. Various devices deliver different oxygen concentrations. The choice of device often depends on the patient’s specific needs and the required increase in oxygen concentration.
Nasal cannulas are a common method, delivering oxygen through prongs placed in the nostrils. These can increase FiO2 from approximately 0.24 (24%) at 1 liter per minute (LPM) up to 0.44 (44%) at 6 LPM. For every additional LPM, the FiO2 generally increases by about 4%. Simple face masks cover the nose and mouth, providing a higher FiO2 range, typically between 0.35 (35%) and 0.50 (50%) with flow rates from 5 to 10 LPM.
For very high oxygen concentrations, non-rebreather masks are used. These masks feature a reservoir bag and one-way valves that prevent exhaled air from mixing with the fresh oxygen supply. Non-rebreather masks can deliver an FiO2 ranging from 0.60 (60%) to 0.90 (90%) or even up to 1.00 (100%) at flow rates of 10-15 LPM. More precise and controlled oxygen delivery is achieved with equipment like mechanical ventilators, which can be set from 0.21 up to 1.00 FiO2.
Clinical Importance of FiO2
Monitoring and adjusting FiO2 is important in healthcare. The objective is to maintain adequate oxygen levels in a patient’s blood to ensure tissues and organs receive sufficient oxygen. Insufficient oxygen, known as hypoxemia or hypoxia, can lead to impaired organ function and potentially severe health complications, including damage to the brain and heart.
Conversely, too much oxygen can also be harmful, leading to oxygen toxicity. Oxygen toxicity can damage lung tissue, causing symptoms such as coughing, chest pain, and difficulty breathing. Healthcare professionals assess oxygen saturation levels, often using a pulse oximeter, and adjust the FiO2 delivered. This management ensures patients receive enough oxygen while minimizing risks from both too little and too much.