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Study objectives: We sought to compare catheter studies using a right ventricular ejection fraction (REF) catheter together with echocardiography and MRI in patients with pulmonary hypertension. Patients and methods: We compared hemodynamic findings, echocardiography, and MRI studies in 16 patients with pulmonary hypertension. Six healthy volunteers served as control subjects for the MRI studies. Results: MRI imaging provided accurate assessment of cardiac output in all but two patients. As compared with MRI, the REF catheter constantly underestimated the REF and overestimated right ventricular volumes in patients with pulmonary hypertension. REF, end-systolic and end-diastolic right ventricular volumes, and right ventricular muscle mass, as determined by MRI, were almost identical in patients with preserved cardiac function and those with low-output failure. The only factor that was different in both groups was the severity of tricuspid regurgitation. Conclusion: Right ventricular dimensions and muscle mass do not differ in patients with pulmonary hypertension who have low cardiac output and those who do not. According to our results, the major determinant of cardiac output in these patients appears to be the severity of tricuspid regurgitation. The REF catheter provides invalid data on right ventricular dimensions in patients with pulmonary hypertension. [ABSTRACT FROM AUTHOR]