Evidence for left ventricular constraint during open heart surgery.

BACKGROUND: The degree to which the lungs and other mediastinal structures constrain the heart during cardiac surgery is uncertain.
OBJECTIVES: To assess the degree of constraint to left ventricular (LV) filling that is present during cardiac surgery. PATIENTS AND METHODS: Central venous (CVP) and pulmonary capillary wedge pressures (PCWP), and an index of LV end-diastolic volume (LVEDV) - LV area, transesophageal echocardiography - were measured before and after sternotomy, after volume loading, after pericardiotomy, and before and after sternal closure following the clinically indicated procedure in 12 patients undergoing cardiac surgery. PCWP and estimated transmural LVEDP (PCWP-CVP) were plotted against the LV area.
RESULTS: In all patients, the difference between PCWP and estimated transmural LVEDP-LV area relations over the full range of LV areas was substantial, indicating the presence of important constraint to filling. Even at small LV areas, when transmural LVEDP approached zero, PCWP was almost always greater than 10 mmHg. Because transmural LVEDP approached zero when areas were smallest, transmural LVEDP-LV area relations were judged to be more plausible than the corresponding PCWP-LV area relations.
CONCLUSIONS: Considerable constraint to cardiac filling is effected by the lungs and other mediastinal structures. This constraint must be considered when assessing LV filling pressure - PCWP is not a reliable measure of LV preload in these circumstances.