THE PHYSIOLOGICAL-EFFECTS OF INVERSE RATIO VENTILATION

Study objectives: The efficacy of inverse ratio ventilation in ARDS is not clear. Furthermore, the mechanism responsible has not been determ ined. We designed an animal study to determine if inverse ratio ventil ation improves gas exchange and by what mechanism. Design: Prospective randomized, controlled design was used. Setting: University of Missou ri Pulmonary/Critical Care Animal Laboratory. Participants: Nine dogs with oleic acid-induced lung injury as control animals to assess stabi lity of the model, nine in the experimental model. Interventions: Conv entional ventilation with full recruitment extrinsic positive end-expi ratory pressure (PEEP) was compared with two other modes of ventilatio n. One was inverse ratio with extrinsic PEEP and the second was invers e ratio with intrinsic PEEP equal to full recruitment PEEP. Full recru itment levels of PEEP were defined by optimizing compliance, then incr easing PEEP by 2.5 cm/H2O. Each type of ventilation was maintained for 45 min after the edema had stabilized. Comparison of lung injury over time requires stability of the model over time. Therefore, we also as sessed the stability of the preparation over time by examining complia nce, extravascular lung water, and venous admixture in nine control do gs with equivalent lung injury over the same time span. Measurements a nd results: Mean airway pressure was increased by both types of invers e ratio ventilation, while compliance remained stable. Venous admixtur e was reduced (conv=0.32+/-0.12, inverse ratio with extrinsic PEEP=0.2 4+/-0.10, inverse ratio with intrinsic PEEP=0.28+/-0.11) with inverse ratio with extrinsic PEEP, but the improvement was less with inverse r atio with intrinsic PEEP, even though the mean airway pressure was hig her. Conclusions: We conclude that increasing mean airway pressure by prolongation of the inspiratory time improves gas exchange in our mode l of ARDS, but when mean airway pressure is increased further, allowin g the development of intrinsic PEEP, the beneficial effects on gas exc hange are less, increasing mean airway pressure with intrinsic PEEP is not equivalent to other methods of increasing mean airway pressure.