Although pulmonary arterial pressure can be estimated on the basis of echocardiography, pressure sampling with a Swan-Ganz catheter provides the most definite measurement. PAOP and PVR can not be measured directly with echocardiography. Therefore, diagnosis of PAH requires a cardiac catheterization. A Swan-Ganz catheter can also measure the cardiac output, which is far more important in measuring disease severity than the pulmonary arterial pressure.