BACKGROUND: The effects of mechanical left ventricular assist on the nonassisted right ventricle have not been fully elucidated. Current information indicates that the right ventricle benefits from a lower left atrial pressure; however, ventricular septal shifting and increased venous return caused by left ventricular assist impair right ventricular function. Acute intraoperative alterations in mitral and tricuspid valve regurgitation (MR and TR, respectively) may occur as a result of mechanical left ventricular assist but have not yet been documented. METHODS AND RESULTS: Eight patients undergoing implantation of a left ventricular assist device (LVAD) as a bridge to transplantation were studied during surgery by transesophageal echocardiography. MR was present in seven of eight patients, and TR was present in eight of eight patients before LVAD implant (mean MR jet area, 10.6 +/- 2.4 cm2, mean TR jet area, 4.8 +/- 1.0 cm2). Immediately after LVAD placement, MR was still present in seven of eight patients, and TR was present in eight of eight patients (mean MR jet area, 4.2 +/- 0.9 cm2; mean TR jet area, 8.4 +/- 1.9 cm2) (P < .05 preimplant versus postimplant jet area). These changes in MR and TR were associated with a decrease in left ventricular end-systolic dimension (62 +/- 4 versus 48 +/- 3 mm) and an increase in right ventricular end-systolic dimension (31 +/- 4 versus 40 +/- 5 mm) (P < .05 preimplant versus postimplant end-systolic dimension). No patients developed progressive right ventricular failure during 70 to 279 days of LVAD support. CONCLUSIONS: Mechanical left ventricular assist causes an acute decrease in preexisting MR. However, left ventricular assist may acutely worsen TR, presumably by shifting the ventricular septum leftward and increasing venous return to the right ventricle.