Figure 49-2 Mean pulmonary artery pressure (y axis) does not increase until cardiac output (x axis) has increased 2- to 2.5-fold when the pulmonary vascular bed is normal, whereas mean pulmonary artery pressure increases linearly with increasing cardiac output when the pulmonary vascular bed is restricted. (From Robin ED, Gaudio R: Cor pulmonale. Dis Mon May:3–38, 1970.)