Effect of altitude on respiratory oscillometry in healthy adults

Respiratory oscillometry is a sensitive, non-invasive pulmonary function test that does not require forced breathing maneuvers. Although the density of the inspired gas is known to influence the measurement, its effect within the same subject at different altitudes has been little explored. We investigated whether geographic altitude systematically modifies respiratory impedance measured by oscillometry in healthy adults. In a prospective, repeated-measures study, 48 healthy adults residing in Mexico City were evaluated by oscillometry at two altitudes: Mexico City (2240 m) and Acapulco (30 m). The effect of altitude was estimated as the paired difference between sites, using the paired t-test or, for non-normally distributed differences, the Wilcoxon test with the Hodges–Lehmann estimator. Total resistance (R5), central resistance (R20) and reactance at 20 Hz (X20) were higher at sea level (all p<0.001), whereas resonant frequency, reactance area and the indices of peripheral resistance were higher at altitude; reactance at 5 Hz did not differ between sites. Within-subject z-scores showed that about half of the participants would be classified as normal or abnormal depending on the altitude of testing. Altitude systematically modifies respiratory impedance, mainly as an effect of air density on the measurement rather than a change in the participant's respiratory mechanics. Barometric pressure must therefore be considered when interpreting oscillometry, and reference equations should be adjusted for altitude.