Hemodynamic parameters shown to be associated with an increased risk of death include increased mean pulmonary artery pressure, increased mean right atrial pressure, and decreased cardiac index.4 Most guidelines define pulmonary hypertension based on elevated mean pulmonary artery pressure alone. However, with disease progression, mean pulmonary artery pressure may actually fall as the right ventricle fails. For this reason, pulmonary vascular resistance is a more compelling standard for the diagnosis of pulmonary hypertension because it takes into account both pressure and flow. Resistance measurement has not entered guideline care because accurate measurement of pulmonary flow is not possible in the presence of tricuspid regurgitation, typical in these patients, using conventional thermodilution techniques. It is important to consider that the cause of elevated pulmonary pressure is not always pulmonary vascular pathology. For example, in patients with high transpulmonary flow, such as in pregnant women or in patients with anemia, sepsis, thyrotoxicosis, or intracardiac shunt, pulmonary pressure can be elevated in the presence of normal pulmonary vascular resistance. Provocative testing with vasodilators, such as inhaled nitric oxide plus 100% oxygen, is recommended because vasoreactivity predicts responsiveness to prostacyclin analogs, endothelin-receptor antagonists, or phosphodiesterase type 5 inhibitors and also identifies those patients with a better prognosis.