Relations between mean pulmonary artery pressure (Ppa) minus left atrial pressure (Pla) and pulmonary blood flow in a representative patient with COPD at rest (A) and at exercise (C), redrawn from reference 9. Dobutamine is supposed to induce a passive increase in flow. Pulmonary vascular resistance (PVR) is the slope of the (Ppa-Pla)/Q relation. Passive increases in flow (A to B and C to D) increase pressure less than predicted by the PVR equation. Exercise (A to C and B to D) increases pressure more than predicted by the PVR equation.