Assessment of fractional expired gases and air flow by an ambulatory metabolic analyzer

Fractional expired oxygen (F_Eo₂) and carbon dioxide (F_Eco₂), and air volume flow were compared using an ambulatory metabolic analyzer (KB1-C, Aerosport, Inc.) and a laboratory metabolic analyzer (Horizon MMC, Sensormedics, Inc.). Ten subjects with paraplegia exercised to exhaustion using their arms, and ten nondisabled subjects ran to exhaustion on a treadmill. Simultaneous F_Eo ₂ and F_Eco₂ measurements were taken during exercise by inserting the KB1-C pneumotachometer in series with the MMC. Measurement accuracy of air flow was examined by sending known air volumes through the KB1-C at three pneumotachometer flow settings (low = 3 to 30 L/min; medium = 3 to 120 L/min; and high = 3 to 200 L/min). Results showed a significant positive association for F_Eo₂ (R² = 0.94) with minimal but statistically significant absolute differences (SE _E = 0.26%, p>0.001). The R² for F_Eco ₂ was 0.89, although average KB1-C measurements were slightly lower (SE_E = 0.28%, p>0.001). Differences from the reference standard for the low flow pneumotachometer were 12.3 and 15.5% at 3 L/min and 6 L/min, respectively, but averaged between 2.5 and 4.0% at 9-45 L/min. For the medium flow setting, average differences from the reference standard ranged from 0.5% at 72 L/min to 4.6% at 117 L/min. For the high flow setting, measurements varied dramatically from the reference standard at flow rates up to nine L/min, and by 7.0 - 8.8% at 27 to 54 L/min. Otherwise, differences of less than 2% were observed at rates of 99-198 L/min. This study observed similar measurements for F_Eo₂ and F_Eco₂ when measured by an ambulatory metabolic analyzer and a laboratory standard, and found that accurate measurement of air flow depended upon careful matching of the pneumotachometer setting with the expected air volume ranges.