Fig. 1. Schematic diagram of airway pressure (P^aw)–time course during airway pressure release ventilation (APRV). (Top ) Mechanical ventilation is provided by time-cycled changes between a high and low level of continuous positive airway pressure (CPAP). T^high= time on high CPAP; T^low= time on low CPAP. (Bottom ) With APRV, unrestricted spontaneous breathing is possible during each phase of the ventilatory cycle. The ventilator setting used in this study did not allow the patient to trigger P^awchanges to keep T^highand T^lowconstant. Interfacing of spontaneous breathing included spontaneous breaths on the lower CPAP level, breaths during which the P^awchange to the upper CPAP level occurred, breaths on the upper CPAP level, and breaths during which the P^awchange to the lower CPAP level occurred.