Positive pressure ventilation with positive end-expiratory pressure (PEEP) is used to treat patients with pulmonary edema. PEEP usually raises arterial oxygen tension, but also may reduce cardiac output and stroke volume. The fall in cardiac output could be due to a fall in preload, an increase in afterload, or a decrease in myocardial contractility of either ventricle. Right ventricular afterload increases during PEEP but not enough to cause cardiac output to fall. Right atrial transmural pressure increases; right ventricular diastolic shape changes with an increase in septal to lateral diameter. Left ventricular afterload and end-diastolic volume fall during PEEP even though left atrial transmural pressure is maintained; an exaggerated decrease in left ventricular diastolic septal to lateral diameter suggests that a septal shift impairs left ventricular end-diastolic volume.