How can I adjust FiO2 (fraction of inspired oxygen) for a 62-year-old female patient with an incomplete spinal cord injury and hypertension when the anesthesia machine cannot incorporate room air?

Adjusting FiO2 on Anesthesia Machines Without Room Air Entrainment

When your anesthesia machine cannot incorporate room air to reduce FiO2, you must use an oxygen-air blender system or adjust the oxygen/air flow meter ratios directly on the machine to achieve your desired FiO2, as modern anesthesia machines deliver precise oxygen concentrations through controlled gas mixing rather than room air entrainment. 1

Understanding the Problem

Modern anesthesia machines are closed-circuit systems that do not rely on room air entrainment—they blend medical-grade oxygen and air (or nitrous oxide) through separate flow meters to achieve the desired FiO2. 2 This is fundamentally different from devices like nasal cannulas or simple masks that dilute oxygen with ambient air.

Immediate Solution: Use the Air Flow Meter

• Locate your air flow meter on the anesthesia machine (separate from the oxygen flow meter) 1
• Calculate the required oxygen-to-air ratio using this formula:
  • For 50% FiO2: Equal flows of oxygen and air (e.g., 1 L/min O2 + 1 L/min air)
  • For 40% FiO2: 1 part oxygen to 3 parts air (e.g., 1 L/min O2 + 3 L/min air)
  • For 30% FiO2: 1 part oxygen to 7 parts air (e.g., 0.5 L/min O2 + 3.5 L/min air) 1

Step-by-Step Adjustment Protocol

For Mechanically Ventilated Patients

1. Maintain total fresh gas flow at your desired minute ventilation (typically 4-6 L/min for adults) 1
2. Adjust the oxygen flow meter downward while simultaneously increasing the air flow meter to maintain total flow 1
3. Monitor the FiO2 analyzer on your anesthesia machine display—all modern machines have built-in oxygen analyzers 2
4. Target SpO2 of 94-98% for patients without risk of hypercapnia, or 88-92% for those with COPD or chronic respiratory failure 3, 4

Critical Safety Considerations

• Never reduce FiO2 below what maintains adequate oxygenation (SpO2 ≥94% in most patients) 1
• Reducing FiO2 takes time: When decreasing from 100% to 30% oxygen, it can take 2-5 minutes for the circuit oxygen concentration to equilibrate, depending on fresh gas flow rate and circuit volume 2
• Monitor both inspired AND expired oxygen concentrations: The expired oxygen may remain elevated for several minutes after adjusting inspired oxygen, which is particularly important if using electrocautery near the airway 2

Special Considerations for Your Patient

For a 62-year-old with spinal cord injury and hypertension:

• Start with FiO2 1.0 initially if the patient required intubation, then titrate down once stable 1
• Target SpO2 94-98% as there's no indication of chronic hypercapnic respiratory failure 3, 4
• Use pressure or volume control ventilation with tidal volumes of 4-6 mL/kg ideal body weight 1
• Adjust FiO2 gradually (in 10% decrements) while monitoring SpO2 continuously 1

Common Pitfalls to Avoid

• Do not assume the machine is broken: If you cannot reduce FiO2, you likely need to increase the air flow meter rather than just decreasing oxygen 1
• Do not rely on oxygen flow alone: A 2 L/min oxygen flow with no air flow still delivers 100% FiO2 in a closed circuit 2
• Do not make rapid changes: Allow 3-5 minutes for circuit equilibration after each adjustment, especially at lower fresh gas flows 2
• Do not forget to check your air supply: Ensure the air cylinder or pipeline supply is connected and pressurized 1

If Air Supply Is Unavailable

If your machine truly has no air source available:

• Use the lowest oxygen flow rate that maintains adequate minute ventilation 1
• Consider switching to a different ventilation mode or oxygen delivery system (e.g., Venturi mask post-extubation) that allows precise FiO2 control 3
• Contact biomedical engineering immediately, as a functioning air supply is essential for safe anesthesia machine operation 1