Pattern of inspiratory gas delivery affects CO2 elimination in health and after acute lung injury

Abstract

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.

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

“…Our results agree with previous findings that a postinspiratory pause enhances CO 2 elimination in healthy or surfactantdepleted animals (4,10,11,20) and with those obtained in surfactant-depleted pigs in which inspiration at pressure-controlled ventilation was prolonged (13). They also agree with results in patients with ALI/ARDS (2,8,12,14).…”

“…To limit confounding factors due to spontaneous changes of metabolism and hemodynamic and other physiological parameters, the study periods were limited to 30 min as a compromise against a desirable steady state during each period. Aström (4). In the present study this fraction was only 10/28, i.e., 0.36.…”

“…At high rates, MDT becomes shorter. Aström showed a steep decline in CO 2 exchange at low MDT values (4). At high respiratory rates, it may therefore be important to use a pause to adequately prolong MDT.…”

“…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.…”

Gas exchange with a reflecting system for inhalational anaesthesia

“…77 Airway dead space can also be calculated by determining the volume expired at the steepest increase in the CO 2 fraction during expiration. 83 This method was chosen in paper II, III and IV for our test lung system as it was stable in its performance and gave reproducible results. In paper III and IV, V Daw in patients were calculated based on the principle suggested by Wolff and Brunner.…”