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

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 CO2 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 lung-damaging 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 CO2 allowed analysis of the volume of expired CO2 and the volume of CO2 re-inspired from Y-piece and tubes. With a long MDT caused by long T-I or T-P, the expired volume of CO2 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 CO2. Arterial CO2 increased during a longer period of short MDT and decreased again when MDT was prolonged. Conclusions: CO2 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.