Alveolar VD is large and does not vary systematically with PEEP in patients with various degrees of ALI. Individual measurements show a diverse response to PEEP. Respiratory mechanics were of no help in optimising PEEP with regard to gas exchange.

Objective: To avoid ventilator induced lung injury, tidal volume should be low in acute lung injury (ALI). Reducing dead space may be useful for example by using a pattern of inspiration that prolongs the time available for gas distribution and diffusion within the respiratory zone, the mean distribution time (MDT). A study was conducted to investigate how MDT affects CO 2 elimination in pigs at health and after ALI.Design and setting: Randomised crossover study in the animal laboratory of Lund University Biomedical Center. Subjects and intervention:Healthy pigs and pigs with ALI, caused by surfactant perturbation and lungdamaging ventilation were ventilated with a computer-controlled ventilator. With this device each breath could be tailored with respect to insufflation time and pause time (T I and T P ) as well as flow shape (square, increasing or decreasing flow). Measurements and results:The single-breath test for CO 2 allowed analysis of the volume of expired CO 2 and the volume of CO 2 re-inspired from Y-piece and tubes. With a long MDT caused by long T I or T P , the expired volume of CO 2 increased markedly in accordance with the MDT concept in both healthy and ALI pigs. High initial inspiratory flow caused by a short T I or decreasing flow increased the re-inspired volume of CO 2 . Arterial CO 2 increased during a longer period of short MDT and decreased again when MDT was prolonged.Conclusions: CO 2 elimination can be enhanced by a pattern of ventilation that prolongs MDT. Positive effects of prolonged MDT caused by short T I and decreasing flow were attenuated by high initial inspiratory flow.

Devaquet J, Jonson B, Niklason L, Si Larbi A-G, Uttman L, Aboab J, Brochard L. Effects of inspiratory pause on CO 2 elimination and arterial PCO2 in acute lung injury. J Appl Physiol 105: 1944 -1949, 2008. First published 18 September 2008 doi:10.1152/japplphysiol.90682.2008.-A high respiratory rate associated with the use of small tidal volumes, recommended for acute lung injury (ALI), shortens time for gas diffusion in the alveoli. This may decrease CO 2 elimination. We hypothesized that a postinspiratory pause could enhance CO 2 elimination and reduce PaCO 2 by reducing dead space in ALI. In 15 mechanically ventilated patients with ALI and hypercapnia, a 20% postinspiratory pause (Tp20) was applied during a period of 30 min between two ventilation periods without postinspiratory pause (Tp0). Other parameters were kept unchanged. The single breath test for CO 2 was recorded every 5 min to measure tidal CO 2 elimination (VtCO2), airway dead space (VDaw), and slope of the alveolar plateau. Pa O 2 , PaCO 2 , and physiological and alveolar dead space (V Dphys, VDalv) were determined at the end of each 30-min period. The postinspiratory pause, 0.7 Ϯ 0.2 s, induced on average Ͻ0.5 cmH 2O of intrinsic positive end-expiratory pressure (PEEP). During Tp20, VtCO 2 increased immediately by 28 Ϯ 10% (14 Ϯ 5 ml per breath compared with 11 Ϯ 4 for Tp0) and then decreased without reaching the initial value within 30 min. The addition of a postinspiratory pause significantly decreased V Daw by 14% and V Dphys by 11% with no change in VDalv. During Tp20, the slope of the alveolar plateau initially fell to 65 Ϯ 10% of baseline value and continued to decrease. Tp20 induced a 10 Ϯ 3% decrease in Pa CO 2 at 30 min (from 55 Ϯ 10 to 49 Ϯ 9 mmHg, P Ͻ 0.001) with no significant variation in Pa O 2 . Postinspiratory pause has a significant influence on CO 2 elimination when small tidal volumes are used during mechanical ventilation for ALI. gas exchange; dead space; mechanical ventilation; ARDS AFTER TRANSPORT of inspired gas through conducting airways, gas mixing in the respiratory zone by diffusion is time dependent. Therefore, a pause following gas insufflation may enhance gas exchange. Mechanical ventilators allow setting of a postinspiratory pause time (Tp), often in percent of the breathing cycle.During mechanical ventilation, prolonged Tp has been shown to enhance CO 2 elimination (8, 10 -12, 14, 21). In healthy pigs, a prolonged Tp increases CO 2 elimination per tidal breath (VtCO 2 ) by decreasing airway dead space (V Daw ) (20). 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).In pigs at health and with acute lung injury (ALI), Aström et al. recently found that a certain prolongation of MDT achieved with a longer Tp or with a longer inspiratory insufflation ti...

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.

SummaryBackground: CO 2 elimination per breath (V CO 2 , T ) depends primarily on tidal volume (V T ). The time course of flow during inspiration influences distribution and diffusive mixing of V T and is therefore a secondary factor determining gas exchange. To study the effect of a postinspiratory pause we defined Ômean distribution timeÕ (MDT) as the mean time given to inspired gas for distribution and diffusive mixing within the lungs. The objective was to quantify changes in airway dead space (V Daw ), slope of the alveolar plateau (SLOPE) and V CO 2 , T as a function of MDT in healthy pigs. Methods: Ten healthy pigs were mechanically ventilated. Airway pressure, flow and partial pressure of CO 2 were recorded during resetting of the postinspiratory pause from 10% (baseline) to, in random order, 0, 5, 20 and 30% of the respiratory cycle. The immediate changes in V Daw , SLOPE, V CO 2 , T , and MDT after resetting were analyzed. Results: V Daw in percent of V T decreased from 29 to 22%, SLOPE from 0AE35 to 0AE16 kPa per 100 ml as MDT increased from 0AE51 to 1AE39 s. Over the same MDT range, V CO 2 ,T increased by 10%. All these changes were statistically significant. Conclusion: MDT allows comparison of different patterns of inspiration on V Daw and gas exchange. Estimation of the effects of an altered ventilator setting on exchange of CO 2 can be done only after about 30 minutes, while the transient changes in V CO 2 ,T may give immediate information. MDT affects gas exchange to an important extent. Further studies in human subjects in health and in disease are needed.