Electro-impedance tomography used to estimate regional ventilation in patients with acute respiratory distress syndrome during continuous positive airway pressure (CPAP) and airway pressure release ventilation (APRV)/biphasic positive airway pressure (BiPAP) with and without spontaneous breathing. Spontaneous breathing with CPAP is associated with better ventilation in the dependent well perfused lung regions. Spontaneous breathing with APRV/BiPAP is associated with better ventilation in the dependent well perfused lung regions and the anterior lung areas. When spontaneous breathing during APRV/BiPAP is abolished, mechanical ventilation is directed entirely to the less well perfused, nondependent anterior lung areas. PCV, pressure-controlled ventilation.