The current report describes two patients with severe heart failure due to dilated cardiomyopathy in whom discrepancy between thermodilution cardiac output and clinical status was due to left-to-right shunt. Misdiagnosis of shunting was harmful in the early management of the first case. Secundum type atrial septal defect was confirmed by pathology in both cases. Blood samples for oxymetry should be routinely drawn during right heart catheterization for screening for intracardiac shunts. Using bedside echocardiography pulmonary-to-systemic flow ratio should also be systematically evaluated in the setting of right ventricular dilation.