although pulmonary arterial pressure (pap) can be estimated on the basis of echocardiography, pressure measurements with a swan-ganz catheter inserted through the right side of the heart provide the most definite assessment.[42] pulmonary hypertension is defined as a mean pap of at least 20 mm hg (3300 pa) at rest, and pah is defined as precapillary pulmonary hypertension (i.e. mean pap ≥ 20 mm hg with pulmonary arterial occlusion pressure [paop] ≤ 15 mm hg and pulmonary vascular resistance [pvr] > 3 wood units). paop and pvr cannot be measured directly with echocardiography. therefore, diagnosis of pah requires right-sided cardiac catheterization. a swan-ganz catheter can also measure the cardiac output; this can be used to calculate the cardiac index, which is far more important in measuring disease severity than the pulmonary arterial pressure.