CO₂ elimination at varying inspiratory pause in acute lung injury

Abstract

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.

Effects of inspiratory pause on CO2 elimination and arterial Pco2 in acute lung injury

“…Therefore, MDT is the interval from mean time of arrival of partitions of tidal volume in the respiratory zone until start of expiration. It expresses the time available for enhanced diffusion between inhaled tidal volume and resident alveolar gas, as mathematically described by Aboab et al (2).…”

“…It was suggested that a prolonged Tp increased the mean distribution time (MDT) of inspired gas, so as to allow more time for diffusion of CO 2 towards more central airways (7). MDT, further explained below, expresses the time available for enhanced diffusion between inhaled tidal volume and resident alveolar gas (2).…”

“…Recently Aboab et al showed that a longer Tp enhances CO 2 exchange evaluated from volumetric capnography in ALI and acute respiratory distress syndrome (ARDS) (2). Positive effects were observed both with regards to a reduced V Daw and an elevated alveolar plateau.…”

The Physiology of Ventilation

“…This option is especially interesting when lung-protective ventilation results in hypercapnia. In particular, doubling the proportion of the inspiratory cycle from 20 to 40% (without creating auto-PEEP), 59 increasing end-inspiratory pause up to 30% of the inspiratory cycle, 58 or both 60 markedly reduced P aCO 2 and physiologic V D /V T , allowing the use of protective ventilation with low V T and enhancing lung protection.…”

“…The mean distribution time of inspired gas is the mean time during which fractions of fresh gas are present in the respiratory zone. 19,58,59 It was recently proposed that setting the ventilator to a pattern that enhances CO 2 exchange can reduce dead space and significantly increase CO 2 elimination or alternatively reduce V T . This option is especially interesting when lung-protective ventilation results in hypercapnia.…”