Because of the characteristics of the parietal pericardial tissue, the intact pericardial sac has a limited reserve volume, becoming relatively non-compliant when the heart volume is at the high end of the normal physiologic range. Pericardial restraint of the heart is best considered as a contact force. Measured with flattened balloons, the magnitude of pericardial contact pressure is probably several mmHg less than right atrial pressure. The diastolic pressure-volume relationship of the right ventricle, and to a lesser degree of the left ventricle, shifts downward following pericardiectomy. In dogs, the pericardium has been shown to limit maximal cardiac output. Some disease states associated with a rapid increase in total heart volume, including subacute mitral regurgitation and right ventricular infarction, demonstrate an increased pericardial influence on cardiac filling.