Ventilatory support by continuous positive airway pressure breathing improves gas exchange as compared with partial ventilatory support with airway pressure release ventilation

In acute lung injury, airway pressure release ventilation (APRV) with super imposed spontaneous breathing improves gas exchange compared with controlle d mechanical ventilation. However, the release of airway pressure below the continuous positive airway pressure (CPAP) level may provoke lung collapse . Therefore, we compared gas exchange and hemodynamics using a crossover de sign in nine pigs with oleic acid-induced lung injury during CPAP breathing and APRV with a release pressure level of 0 and 5 cm H2O. At an identical minute ventilation ((V) over dot E 8 L/min) spontaneous breathing averaged 55%, 67%, and 100% of (V) over dot E during the two APRV modes and CPAP, re spectively. Because of the concept of APRV, mean airway pressure was highes t during CPAP and lowest during APRV with a release pressure of 0 cm H2O. S hunt was reduced to almost half during CPAP (6.6% of (Q) over dot t) compar ed with both APRV-modes (13.0% of (Q) over dot t). Cardiac output and oxyge n consumption, in contrast, were similar during all three ventilatory setti ngs. Thus, in our lung injury model, CPAP was superior to partial ventilato ry support using APRV with and without positive end-expiratory pressure. Th is may be attributable to beneficial-effects of spontaneous breathing on ga s exchange as well as to rapid lung collapse during the phases of airway pr essure release below the CPAP level. These findings may suggest that the am ount of mechanical ventilatory support using the APRV mode should be kept a t the necessary minimum.