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Decrease preload less venous return. Increase after load, more intrathoracic Pressure.
PEEP actually has a differential afterload effect depending on whether we're referring to the RV or LV. In a spontaneously ventilating person, the LV has to overcome the intrinsic -5 cm H2O of pressure in the thorax before it can begin ejecting (think of this like a suction cup that's applied to the outside wall of the ventricle). When PPV is applied, the LV no longer has to deal with the negative transmural pressure that's resisting ventricular contraction, and thus afterload is reduced. PPV is beneficial for LV failure because as you said, it decreases preload and also because it decreases LV afterload.