The term, optimal PEEP, requires redefinition in the light of new clinical data. With the onset of acute respiratory failure heralded by blood gas evidence of decreased oxygenation, PEEP is supplied in quantities sufficient to restore intrapulmonary shunt (Qsp/Qt) to a preselected goal of 15%. This is compatible with published criteria defining adequate blood gas exchange. Now rather than permitting reduction of cardiac output to be the end point of PEEP application, selective cardiovascular interventions to support preload, contractility, or afterload are made as appropriate so that cardiac function may be maintained until the preselected endpoint of shunt reduction to 15% can be made.