The relationship between peripheral venous saturation and central hemodynamics (including right and left heart filling pressures and cardiac output), mixed venous oxygen saturation, and contemporary biomarkers of heart failure is not clear. We aim to determine the relationship between mixed venous, central venous, and peripheral venous oxygen saturation in patients with advanced (class III or IV) heart failure. We will determine the relationship between cardiac output measured via thermodilution and the Fick equation calculated using concurrent oxygen saturations from the distal PA, central vein, and peripheral vein. We also aim to examine the relationship of selected cardiac biomarkers with cardiac filling pressures and cardiac output over time in patients with decompensated heart failure.