FiO₂ (Fraction of Inspired Oxygen) is a critical parameter in respiratory care that defines the percentage of oxygen delivered to a patient. In both oxygen therapy and mechanical ventilation, FiO₂ directly influences arterial oxygenation and overall gas exchange. While room air contains 21% oxygen (FiO₂ 0.21), this value can be adjusted up to 100% in critically ill patients requiring respiratory support.
Understanding FiO₂ is essential for clinicians managing acute respiratory failure, hypoxemia, and critical care ventilation, as it helps optimize oxygenation while minimizing the risk of oxygen toxicity and lung injury.
What Does FiO₂ Mean?
FiO₂ stands for Fraction of Inspired Oxygen. It represents the oxygen concentration delivered to a patient during spontaneous breathing, oxygen therapy, or mechanical ventilation.
- Room air FiO₂: 0.21 (21%)
- Adjustable range in ventilator oxygen settings: 0.21–1.00
- Used to evaluate oxygen delivery and respiratory efficiency
As FiO₂ increases, arterial oxygenation (PaO₂) generally increases. For this reason, FiO₂ adjustment is one of the most frequently modified ventilator parameters in critical care practice.
FiO₂ in Mechanical Ventilation
In mechanical ventilation, FiO₂ describes the oxygen concentration delivered by the ventilator. It is adjusted to achieve adequate arterial oxygenation based on:
- Target SpO₂ values
- Measured PaO₂ levels
- The patient’s clinical condition
- Oxygenation targets in ICU settings
FiO₂ is often set at a higher level initially in cases of acute respiratory failure and then reduced through careful titration. The goal is clear: maintain sufficient oxygenation using the lowest safe FiO₂.
Prolonged exposure to high oxygen concentration levels increases the risk of oxygen toxicity and ventilator-associated lung injury. Therefore, FiO₂ management is typically combined with lung-protective ventilation strategies.
Typical FiO₂ Ranges
The FiO₂ of atmospheric air is 0.21 (21%).
Different oxygen delivery systems provide varying oxygen concentration levels:
- Nasal cannula: 0.24–0.44
- Simple face mask: 0.35–0.60
- Reservoir mask: 0.60–0.90
- Mechanical ventilation: 0.21–1.00
In hypoxemia management, the primary clinical objective is to achieve adequate oxygenation using the lowest effective FiO₂.
Risks of High FiO₂
High FiO₂ levels may lead to oxygen toxicity, particularly when used for prolonged periods in critical care ventilation.
Potential risks include:
- Alveolar damage
- Reabsorption atelectasis
- Increased production of reactive oxygen species
- Worsening lung injury in ARDS
- Impaired gas exchange
Excess oxygen exposure can contribute to lung inflammation and structural damage. For this reason, FiO₂ should always be maintained at the lowest effective level that ensures adequate oxygenation.
Balancing FiO₂ with PEEP
FiO₂ increases oxygen concentration, while PEEP (Positive End-Expiratory Pressure) helps maintain alveolar patency and improve gas exchange.
As PEEP increases, adequate oxygenation can often be achieved with a lower FiO₂. This balance is a core component of lung-protective ventilation and ARDS management.
The goal is to reach target PaO₂ and SpO₂ values using:
- The lowest effective FiO₂
- Appropriate PEEP titration
- Careful monitoring of ventilator parameters
In patients with severe hypoxemia and ARDS, the FiO₂–PEEP balance is especially critical to prevent further lung injury.
Frequently Asked Questions
What is normal FiO₂?
Normal ambient air has an FiO₂ of 0.21 (21%). This is considered the baseline oxygen concentration for healthy spontaneous breathing.
Why should FiO₂ be kept as low as possible?
High FiO₂ increases the risk of oxygen toxicity and lung injury. Adequate oxygenation should therefore be achieved using the lowest effective FiO₂.
Why can FiO₂ be reduced when PEEP is increased?
PEEP improves alveolar recruitment and gas exchange efficiency. As lung units remain open, the same oxygenation level can often be maintained with a lower FiO₂.
In which patients is FiO₂ titration most critical?
FiO₂ titration is particularly important in patients with ARDS, acute respiratory failure, and severe hypoxemia, where improper oxygen settings may worsen lung injury.
References
- StatPearls Publishing. (2023). Fraction of Inspired Oxygen (FiO₂).
- TÜSAD – Türk Toraks Derneği. Mechanical ventilation and respiratory support training materials.
- Tobin, M. J. (2013). Principles and Practice of Mechanical Ventilation (3rd ed.). McGraw-Hill Education.
- ARDS Network. (2000). Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and ARDS. New England Journal of Medicine, 342(18), 1301–1308.