FiO2

FiO2 stands for the fraction or percentage of inspired oxygen in the gas that a person breathes. It is a measure of how much oxygen is present in the inhaled air. In ambient room air, FiO2 is about 0.21, meaning roughly 21 percent of the gas is oxygen.

In clinical practice, FiO2 is controlled and adjusted to treat hypoxemia and maintain adequate blood oxygen levels. It is commonly expressed as a decimal (for example 0.40) or as a percentage (40%). Devices that deliver graded oxygen concentrations include nasal cannulas, simple face masks, non-rebreather masks, and more advanced systems such as ventilators, continuous positive airway pressure (CPAP), bilevel positive airway pressure (BiPAP), and high-flow nasal therapy. The actual FiO2 delivered to a patient can vary from the setting due to factors such as mask fit, leaks, mouth breathing, and patient anatomy.

Typical FiO2 ranges used clinically vary with the device and patient needs. Room air is FiO2 0.21; with supplemental oxygen, FiO2 can range from about 0.24 to 0.60 with standard methods, while devices like non-rebreather masks and high-flow systems can deliver much higher concentrations, up to near 1.0 in some cases. Clinicians titrate FiO2 to keep arterial oxygen saturation within a target range (often around 92–96%), while monitoring for potential oxygen toxicity during prolonged exposure to high FiO2.

FiO2 is a fundamental parameter in respiratory physiology and critical care, used alongside oxygen saturation, blood gases, and calculations such as the alveolar gas equation to assess and manage respiratory support.