Metabolic gas effect on phase III slopes of single-breath nitrogen tests

This study was designed to estimate the relative contribution of gas exchange to the slope of phase III and the mean respiratory quotient (R) during this maneuver. With eight normal volunteers, we studied single-breath nitrogen tests and single-breath reversed tests (SB-N2, SB-R) without and with CO2 added to the test inspiration (SB-N2CO2, SB-RCO2). With an inspired CO2 of about 6% the slope of phase III in SB-N2 and SB-N2CO2 increased from 0.87 +/- 0.24 (mean +/- SD) to 1.03 +/- 0.21% N2.l-1 (P less than 0.01); however, in SB-R an SB-RCO2 the steepness of the slope of phase III decreased from 0.65 +/- 0.13% to 0.50 +/- 0.14% N2.l-1 (P less than 0.01). From these data we can calculate that gas exchange accounted for 13% of the slope of phase III in SB-N2 and SB-R. The mean R value during the slope of phase III for this effect was between 0.6 and 0.7. R was further decreased when CO2 was added, and its influence was increased to 26% of the slope of phase III.