Introduction

The FiO2 at altitude in commercial aircrafts is 0.16. A hypoxic challenge test (HCT) is typically conducted using nitrogen as the driving gas via a Venturi mask (8l/min flow, 24% mask). 

Patients who are dependent on long-term non-invasive ventilation (NIV), present a challenge for clinicians undertaking a HCT as the Venturi device cannot be used in an NIV circuit.

Aim

To establish whether it is possible to conduct an HCT on individuals using NIV at the time of the test, and whether this provides an accurate simulation of the hypoxic conditions during flight.

Methods

The bench test involved one healthy subject.  Nitrogen was added at the ventilator outlet.  Flow of nitrogen was increased incrementally from 4 l/min to the maximum of 15 l/min.  Maximum and mean gas flow and FiO2 were measured.  NIV settings were adjusted incrementally between 5cmH2O and 20cmH2O.  Oxygen saturations and transcutaneous CO2 were monitored.

Results

We were able to deliver an FiO2 of 0.16 via NIV at all pressures and gas flows. Inspiratory flow increased as ventilatory pressures increased which diluted the nitrogen delivered and increased the FiO2.  Desaturation only occurred with low FiO2 during NIV with low ventilatory pressures.

Discussion

We have demonstrated that we can replicate the FiO2 during commercial flights (0.16) in a healthy subject receiving NIV across a range of pressures.  This informs clinical recommendations about flying with NIV but without oxygen.

Conclusion

We have shown how to replicate an HCT during NIV.  Patients receiving long term NIV should be assessed individually and the FiO2 measured.  It may be preferable to use NIV rather than unopposed oxygen.