Comparison of positive end-expiratory pressure and inspiratory positive pressure plateau in ventilation of rabbits with experimental pulmonary edema.

We investigated the effects of an inspiratory positive pressure plateau produced by a high (4/1) inspiratory-expiratory (I/E) ratio in rabbits with pulmonary edema induced with oleic acid. With an inspiratory pressure plateau, intrapulmonary shunt was significantly reduced compared to a standard 1/2 I/E ratio. Reduction in shunt was also obtained when positive end-expiratory pressure (PEEP) was added to the 1/2 I/E ratio ventilatory pattern. Since the improvement in shunt could have been caused by either recruitment of alveoli or by maintenance of lung volume at end exhalation we measured tidal volume and thoracic gas volume to distinguish between the two. With both patterns, improvement in shunt was associated with an increase in thoracic gas volume. The increase in thoracic gas volume was greater with PEEP than with 4/1 for an equivalent increase in mean airway pressure. The results suggest that an inspiratory pressure plateau improves gas exchange in pulmonary edema, but that this improvement occurs because of an increase in end-expiratory lung volume. Positive end-expiratory pressure is a preferable method of increasing end expiratory lung volume because a greater increase occurs for the same change in airway pressure.