Am. J. Respir. Crit. Care Med., Volume 161, Number 5, May 2000, 1537-1545

Effects of Inverse Ratio Ventilation and Positive End-Expiratory Pressure in Oleic Acid-Induced Lung Injury

PETER NEUMANN, JAN E. BERGLUND, LARS G. ANDERSSON, ENN MARIPU, ANDERS MAGNUSSON, and GÖRAN HEDENSTIERNA

Departments of Clinical Physiology, Anesthesiology, Hospital Physics, and Diagnostic Radiology, University of Uppsala, Uppsala, Sweden

Continuous as well as cyclic (with each expiration) lung collapse in acute respiratory failure can be reduced by positive end-expiratory pressure (PEEP) or short expiration times, as in inverse ratio ventilation (IRV). In 20 pigs with oleic acid-induced lung edema, we compared the effects of a PEEP of 20 cm H₂O with IRV, using an inspiratory-to-expiratory ratio of 3:1 without external PEEP. During IRV, expiration times of 0.5 or 1.0 s were obtained with respiratory rates of 30 breaths/min or 15 breaths/min, respectively. In 15 animals, ventilation-perfusion relationships were studied through the multiple inert gas elimination technique, and lung morphology was studied with computed tomography. In another five pigs, blood flow distribution was studied with perfusion scintigraphy. All three ventilatory modes had similar effects on mean arterial blood pressure, cardiac output, oxygen delivery, and mean airway pressure. PEEP reduced shunt and improved oxygenation to a greater extent than the two modes of IRV, although there was a large variation within each group. The improvement, irrespective of which ventilatory mode was superior in a particular pig, was caused by greater and more even aeration of the lung, whereas the perfusion distribution with PEEP was the same as with IRV. Thus, the strategy of stabilizing the lungs through short expiration times, as in IRV, did not offer any advantages in our lung injury model.

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