Acute respiratory failure is accompanied by a severe gas exchange impairment that is signified by a large shunt and no or only little of additional ventilation-perfusion mismatch. The shunt is caused by perfusion of collapsed and consolidated lung tissue that is mainly located in the lower, dependent lung region. The ventilation, on the other hand, is redistributed towards upper, non-dependent regions. By applying external PEEP, or producing an intrinsic PEEP by shortening the expiratory period, not only recruitment of collapsed or consolidated lung tissue may be achieved, but also a redistribution of inspired gas towards more dependent regions. Spontaneous breathing seems to improve gas exchange, and in proportion to its share of total ventilation, when added to mechanical ventilation. A shift from total mechanical ventilation to partial or fully spontaneous breathing may be the road of the future and should be tested further.