What is fio2
FiO2 (Fraction of Inspired Oxygen) is the proportion of oxygen in the gas mixture being inhaled. Room air has an FiO2 of 0.21 (21%). When supplemental oxygen is given, FiO2 increases up to 1.0 (100%). It is a key parameter in respiratory medicine and mechanical ventilation.
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FiO2 fraction of inspired oxygen clinical significance
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Here is a thorough explanation of FiO2:
FiO2 - Fraction of Inspired Oxygen
FiO2 stands for Fraction of Inspired Oxygen. It is the proportion (or percentage) of oxygen in the gas mixture being inhaled by a patient. It is expressed as a decimal (0.0 to 1.0) or as a percentage (0-100%).
Normal Value
- Room air FiO2 = 0.21 (21%) - the atmospheric oxygen concentration at sea level
- When supplemental oxygen is given, FiO2 ranges from 0.21 up to 1.0 (100%)
- An FiO2 below 0.16 (16%) causes air hunger, tachypnea, and altered consciousness; below 0.10 causes loss of consciousness and seizures (Tintinalli's Emergency Medicine)
FiO2 by Oxygen Delivery Device
| Device | Approximate FiO2 |
|---|---|
| Room air | 21% (0.21) |
| Nasal cannula (1-6 L/min) | ~24-44% (each 1 L/min adds ~4%) |
| Simple face mask | ~35-55% |
| Venturi mask | 24-60% (precisely controlled) |
| Non-rebreather mask | Up to ~90% |
| High-flow nasal cannula (HFNC) | Up to >90% |
| Mechanical ventilation | Dial 21-100% precisely |
As noted by Miller's Anesthesia and Tintinalli's Emergency Medicine, each liter per minute of oxygen via nasal cannula increases FiO2 by approximately 4%. Standard nasal cannulae are generally limited to 6 L/min (provides about 28% FiO2 max) because higher flows are uncomfortable.
Clinical Uses of FiO2
1. Monitoring and titration of oxygen therapy
- Modern anesthesia machines and gas analyzers continuously measure FiO2, with alarm thresholds to prevent dangerously low oxygen delivery (Morgan & Mikhail's Clinical Anesthesiology)
- Oxygen therapy is titrated (increased or decreased) based on SpO2 (pulse oximetry) and arterial blood gas (ABG) results
2. The P/F Ratio (PaO2/FiO2)
- One of the most important uses of FiO2 clinically
- P/F ratio = PaO2 (mmHg) ÷ FiO2
- Example: PaO2 of 240 mmHg on FiO2 of 0.80 → P/F ratio = 300
- Used to classify the severity of Acute Respiratory Distress Syndrome (ARDS) (Goldman-Cecil Medicine, Fishman's Pulmonary Diseases):
- Mild ARDS: P/F 200-300 mmHg
- Moderate ARDS: P/F 100-200 mmHg
- Severe ARDS: P/F < 100 mmHg
- Also used to classify severity of hypoxemia in sepsis/shock (requires PEEP ≥5 cmH2O per Berlin definition)
3. Cardiac arrest resuscitation
- 100% FiO2 (FiO2 = 1.0) is recommended during active cardiac arrest, then titrated down to maintain SpO2 > 94% after return of spontaneous circulation (ROSC) (Tintinalli's Emergency Medicine)
4. Surgical and anesthetic management
- During one-lung ventilation, FiO2 is a key parameter to prevent desaturation (Miller's Anesthesia)
- In acute air embolism, FiO2 is set to 1.0 and nitrous oxide (N2O) is discontinued
5. Detecting anatomic shunt
- In a pure intracardiac or intrapulmonary shunt, PaO2 does NOT improve despite increasing FiO2 - this is a diagnostic clue to anatomic shunt (Frameworks for Internal Medicine)
Oxygen Toxicity Risk
At very high FiO2 (> 0.6), prolonged exposure can cause oxygen toxicity - free radical damage to the lungs (absorption atelectasis, pulmonary oxygen toxicity). This is why clinicians aim to wean FiO2 to < 0.6 when possible, especially in ventilated patients.
Summary
FiO2 is a fundamental parameter in respiratory medicine. It defines how much oxygen a patient is breathing, guides oxygen therapy delivery devices, and is used in key calculations like the P/F ratio to assess the severity of respiratory failure and ARDS.
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