Why using 15% FiO2 in the fitness-to-fly test?

Maria Pia De Carolis, MD,

Other Contributors:

November 24, 2011

Dear Editor,

we read with interest the paper by C J Bossley et al (1) in which the authors performed fitness-to-fly tests to understand if preterm infants with broncopulmonary dysplasia might require supplemental oxygen during high-altitude flight. In the opening sentence of abstract it is stated that "during air flight cabin pressurisation produces an effective fraction of inspired oxygen of 15%", but this sentence, in our opinion, may be misleading or, at least, needs to be explained in more details. For this reason we would like just to point out the difference between fraction of inspired oxygen and partial pressure of oxygen. The fraction of inspired O2, or FiO2, also known as ambient O2 concentration, indicates the proportion of O2 in the inspired air. In standard air condition FiO2 is 21%. This parameter is not directly related to the absolute number of O2 molecules per gas volume, but specifies the proportion of oxygen in the air mixture. The effective number of O2 particles depends rather on the specific air thermodynamic state (its temperature and pressure). The partial pressure of O2 in alveolar gas, or PAO2, instead is directly proportional to the amount of O2 present in the gas mixture (at a specific temperature) and therefore O2 exchange at the alveoli level is degraded as the air pressure is reduced. In other words PAO2 relates directly to the actual number of O2 molecules and consequently to the alveolar oxygen tension which in turn affects the diffusion into the pulmonary capillary blood. The alveolar O2 exchange can be improved by increasing both the FiO2 (changing the air mixture proportions) and the air thermodynamic pressure (augmenting the PAO2). The PAO2 is dependent on barometric pressure which at sea level is considered equal to 760 mmHg (with 15?C temperature). Therefore at sea level PAO2 is approximately 150 mmHg. Considering that barometric pressure varies with altitude, PAO2 consequently decreases while FiO2 remains constant at 21%. Because aircrafts use to fly at high altitudes, the common practice is then to pressurise the cabin thus maintaining an adequate PAO2. The pressure value into the cabin is fixed to a minimum of 75.3 kPa, or 564 mmHg, corresponding to an altitude of 8000 ft, or 2438 m, the so-called physiological zone. However, at 2438 m (8000 ft) the PAO2 is less than at sea level and thus the O2 exchange is degraded. Specifically such condition is equivalent of breathing an air mixture with 15% oxygen at sea level. Consequently, to simulate the situation of being at 2438m, the fitness-to-fly test is performed, following the British Thoracic Society Recommendations (2), with the oxygen level inside the box reduced to 15% by adding nitrogen into the chamber. We suggest, in order to avoid confusion among not specialized readers, to specifically remark that during high-altitude flights the FiO2 in the cabin remains 21%, while the PAO2 changes. The value of PAO2 in pressurised cabin is actually equivalent to the value detectable at sea level when the FiO2 is artificially decreased to 15%.

References

1. Bossley CJ, Cramer D, Mason B, Hayward A, Smyth J, McKee A, Biddulph R, Ogundipe E, Jaff? A, Balfour-Lynn IM. Fitness to fly testing in term and ex-preterm babies without bronchopulmonary dysplasia. Arch Dis Child Fetal Neonatal Ed. 2011 Sep 13. 2. www.brit- thoracic.org.uk/Portals/0/Clinical%20Information/Air%20Travel/Guidelines/FlightRevision04.pdf Brithish Thoracic Society Standards of care Committee.Managing Passengers with Respiratory Disease Planning and Air Travel. (accessed 13 Jul 2011).

Conflict of Interest:

None declared

Conflict of Interest

None declared