Measuring exhaled nitric oxide at high altitude.

The altitude performance of two NO analysers using different NO detectors was studied. The analysers and their flow regulators were tested with simulated exhalations of reference gases. At 4000 m, volume flow was +35% and mass flow -24% of nominal in both instruments. The reduced mass flow increased the exhaled NO fraction by 26% for a given rate of NO excretion. Furthermore, the electrochemical NO detector in one analyser showed an increased signal level for a given partial pressure of test gas. Taken together, these two effects increased the signal output by 60% in comparison to the NO partial pressure. To avoid the above errors, it is proposed that the flow regulator should be readjusted to give a volume flow of 50 ml s(-1) at the altitude of interest and that the analyser should be recalibrated to the operational altitude. Finally, it is recommended that exhaled NO should always be reported as partial pressure and not as volume fraction, in order to compare measurements at any altitude.