CO2-controlled sampling of alveolar gas in mechanically ventilated patients

A newly designed gas-sampling device using end-tidal CO2 to separate dead s pace gas from alveolar gas was evaluated in 12 mechanically ventilated pati ents. For that purpose, CO2-controlled sampling was compared with mixed exp iratory sampling. Alveolar sampling valves were easily controlled via CO2 c oncentration. Concentrations of four volatile substances were determined in the expired and inspired gas. Isoflurane and isoprene, which did not occur in the inspired air, had ratios of end-tidal to mixed expired concentratio ns of 1.75 and 1.81, respectively. Acetone and pentane, found in both the i nspired and expired air, showed ratios of 0.96 and 1.0, respectively. Preci sion of concentration measurements was between 2.4% (isoprene) and 11.2% (i soflurane); reproducibility las coefficient of variation) was 5%. Because t he only possible source of isoflurane and isoprene in this setting was pati ents' blood, selective enrichment of alveolar gas was demonstrated. By usin g the new sampling technique, sensitivity of breath analysis was nearly dou bled.