Effect of barometric pressure on single-breath carbon monoxide diffusing capacity

Single-breath diffusing capacity for carbon monoxide (D_LCO) quantifies gas transfer in the lungs. D_LCO measurement is affected by barometric pressure (Pb) and alveolar partial pressure of oxygen (PAO₂). The current equations for adjusting D_LCO for Pb and PAO₂ may not be accurate given advances in test performance and technology. We quantify changes in D_LCO with alterations in Pb in normal and COPD subjects, determine the accuracy of the current Pb and PAO₂ adjustment equations and develop updated adjustment equations.

We measured D_LCO in 13 normal and 10 COPD subjects at 1330 m altitude and in a hypobaric/hyperbaric chamber at altitudes of sea-level and 2500 m; six normal subjects were tested at 3600 m. We determined if there were significant differences in D_LCO between altitudes. We developed an equation for adjusting D_LCO for changes in Pb from sea-level. We compared this equation with the existing Pb adjustment equation in normal and COPD subjects. We determined the accuracy of the current PAO₂ adjustment equation and developed a new PAO₂ adjustment equation.

D_LCO significantly increased with decreasing Pb. We developed a Pb adjustment equation that adjusts D_LCO measured at altitudes between 1330 m and 3600 m to sea-level values. This Pb adjustment equation yields D_LCO results that are not significantly different than the currently recommended equation. We developed a more accurate PAO₂ adjustment equation.

D_LCO measurement is significantly affected by altitude. We developed equations that accurately adjust D_LCO for changes in Pb and PAO₂ in normal and COPD subjects.