CO2 elimination at varying inspiratory pause in acute lung injury.

Previous studies have indicated that, during mechanical ventilation, an inspiratory pause enhances gas exchange. This has been attributed to prolonged time during which fresh gas of the tidal volume is present in the respiratory zone and is available for distribution in the lung periphery. The mean distribution time of inspired gas (MDT) is the mean time during which fractions of fresh gas are present in the respiratory zone. All ventilators allow setting of pause time, T(P), which is a determinant of MDT. The objective of the present study was to test in patients the hypothesis that the volume of CO(2) eliminated per breath, V(T)CO(2), is correlated to the logarithm of MDT as previously found in animal models. Eleven patients with acute lung injury were studied. When T(P) increased from 0% to 30%, MDT increased fourfold. A change of T(P) from 10% to 0% reduced V(T)CO(2) by 14%, while a change to 30% increased V(T)CO(2) by 19%. The relationship between V(T)CO(2) and MDT was in accordance with the logarithmic hypothesis. The change in V(T)CO(2) reflected to equal extent changes in airway dead space and alveolar PCO(2) read from the alveolar plateau of the single breath test for CO(2). By varying T(P), effects are observed on V(T)CO(2), airway dead space and alveolar PCO(2). These effects depend on perfusion, gas distribution and diffusion in the lung periphery, which need to be further elucidated.