What is the recommended oxygen (O2) therapy protocol for nitrogen washout?

Oxygen Therapy Protocol for Nitrogen Washout

The recommended oxygen therapy protocol for nitrogen washout requires 100% oxygen delivered at a constant bias flow of 10 L/min for patients weighing >5 kg, or 5 L/min for infants weighing <5 kg. 1

Equipment Setup

• Delivery System:

  • Clear face mask with minimal dead space
  • Three-way switching valve with two inlet/outlet ports and a mask port
  • T-piece
  • Pneumotachometer
  • Nitrogen analyzer and mixing chamber
  • Calibrating syringe

• Optional Component:

  • Collapsible breathing bag (0.5 L) incorporated via a second T-connection between the patient and O₂ source to enhance reproducibility by minimizing flow swings 1

Procedure Protocol

1. Patient Position:

   • Upright seated position 1
   • For infants: supine position with head in midline

2. Oxygen Flow Rate:

   • 10 L/min for patients weighing >5 kg
   • 5 L/min for infants weighing <5 kg 1

3. Switching Mechanism:

   • Switch from room air to 100% oxygen at end-expiration
   • Use either:
     • Automatic switching (software monitors stability of tidal volume)
     • Manual switching by an experienced operator (preferred for infants with irregular breathing) 1

4. Washout Duration:

   • Continue until nitrogen concentration in the mixing chamber reaches 0.0065 (corresponding to end-tidal FN₂ of 0.02) 1
   • Ensure regular breathing pattern before ending the test

Monitoring During Procedure

• Continuous monitoring of nitrogen concentration
• Observation of breathing pattern through the collapsible bag if used
• Monitor for signs of oxygen toxicity, especially in premature infants

Special Considerations

• Premature Infants: Consider using heliox (79% helium, 21% oxygen) instead of 100% oxygen to reduce risk of retinopathy of prematurity 2

• Patients on Supplemental Oxygen:

  • For patients already requiring oxygen, calibration should be performed with gas volumes of the same FN₂ as the patient is breathing
  • Alternative: calibrate with room air and apply a correction factor 1

• Ventilated Patients:

  • Different protocols apply for mechanically ventilated patients 1

Common Pitfalls and Caveats

• Equipment-Related:

  • Ensure no leaks in the system as this will lead to inaccurate measurements
  • Properly align flow and gas concentration signals in time
  • Ensure adequate dynamic response of the gas analyzer 1

• Procedure-Related:

  • External dead space (including mask) should ideally be less than 1.0 mL/kg and not exceed 2.0 mL/kg 1
  • Ensure stable end-expiratory level before switching to oxygen
  • Avoid excessive exposure to 100% oxygen in premature infants due to risk of retinopathy 2

• Interpretation-Related:

  • Account for switching error above FRC
  • Apply BTPS (body temperature, pressure, saturated) corrections 1

The nitrogen washout technique using 100% oxygen remains the most commonly used approach for measuring functional residual capacity and assessing ventilation distribution in the lungs due to its relative simplicity and low cost compared to alternative methods 1.