Effects of inverse ratio ventilation and positive end-expiratory pressure in oleic acid-induced lung injury

Continuous as well as cyclic (with each expiration) lung collapse in acute respiratory failure can be reduced by positive end-expiratory pressure (PEE P) 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 with out external PEEP. During IRV, expiration times of 0.5 or 1.0 s were obtain ed with respiratory rates of 30 breaths/min or 15 breaths/min, respectively . In 15 animals, ventilation-perfusion relationships were studied through t he multiple inert gas elimination technique, and lung morphology was studie d with computed tomography. in another five pigs, blood flow distribution w as studied with perfusion scintigraphy. All three ventilatory modes had sim ilar effects on mean arterial blood pressure, cardiac output, oxygen delive ry, and mean airway pressure. PEEP reduced shunt and improved oxygenation t o a greater extent than the two modes of IRV, although there was a large va riation within each group. The improvement, irrespective of which ventilato ry mode was superior in a particular pig, was caused by greater and more ev en aeration of the lung, whereas the perfusion distribution with PEEP was t he same as with IRV. Thus, the strategy of stabilizing the lungs through sh ort expiration times, as in IRV, did not offer any advantages in our lung i njury model.