Given the task definition and input, reply with output. Given an abstract of a paper, generate a title for this paper such that conveys the key focus of the paper.

Abstract: The pressure of pericardial constraint was measured in 20 patients undergoing elective cardiac surgery (10 in Group I with normal cardiac size; 10 in Group II with cardiomegaly) using a catheter with a collapsible latex end balloon. Right atrial pressure and other hemodynamic variables including right ventricular stroke work index were also measured before and after the pericardium was widely opened. The pericardium was grossly normal in all patients and only small physiologic effusions were present. In Group I mean pericardial pressure was 8 +/- 2 mm Hg as was mean right atrial pressure. In Group II mean pericardial pressure was 6 +/- 2 mm Hg versus mean right atrial pressure of 10 +/- 5 mm Hg (p less than 0.05). Excluding 2 of the 20 patients with outlying data, pericardial pressure showed linear correlation with right atrial pressure (r = 0.689). In Group I right ventricular stroke work index rose from 5.0 +/- 2.0 to 6.4 +/- 2.1 g-m/m2 (p less than 0.01) after pericardiotomy with no significant increase in mean right atrial pressure; similar findings in Group II were consistent with removal of external constraint. Thus, even in the absence of an abnormal effusion the normal pericardium exerts a significant pressure on the heart, which is often similar in magnitude to right atrial pressure. In certain notable exceptions, however, right atrial pressure far exceeds pericardial pressure. Such pericardial constraint has important implications for ventricular diastolic mechanics.
Intraoperative measurement of pericardial constraint: role in ventricular diastolic mechanics.