3-DIMENSIONAL LEFT-VENTRICULAR MIDWALL DYNAMICS IN THE TRANSPLANTED HUMAN HEART

To quantify the three-dimensional regional dynamics of the left ventricular (LV) midwall and the centroid and cross-sectional shape of the LV chamber in the transplanted human heart, 12 miniature radiopaque tantalum markers were implanted in the LV midwall of the donor heart at the time of cardiac transplantation in 15 patients. Stereo cineradiography in the late postoperative period (mean, 52 days after surgery) allowed computer-aided measurements of the three-dimensional coordinates of multiple sites in anterior, inferior, lateral, and septal LV regions at 16.7-msec intervals throughout the cardiac cycle. In awake, supine patients, from maximum to minimum LV volume, group mean translations of free wall markers ranged from 0.80 to 1.24 cm, directed toward the LV interior, whereas translations of septal wall markers were significantly less, 0.46 and 0.34 cm (p<0.01), directed away from the LV interior. A component of this translation along the septal-lateral axis was also significantly less (p<0.01) in the septum (0.19 and 0.20 cm) than in the free wall, where it ranged from 0.32 to 0.97 cm. The LV cross section was not circular, and anterior-inferior dimensions (7.18±0.66 and 6.13±0.79 cm, at maximum and minimum volumes, respectively) were significantly greater (p<0.01) than septal-lateral dimensions (5.78±0.65 and 5.12±0.48 cm), yielding an unchanging transverse elliptical LV eccentricity (0.58±0.13). The position of the LV center of volume did not change significantly from maximum to minimum volume in the direction of either the LV long axis or the anterior-inferior axis, but it did change significantly (0.55±0.23 cm, p<0.01) along the septal-lateral axis. We conclude 1) as viewed in a fixed external reference system, midwall sites in the interventricular septum of the transplanted human heart move paradoxically yet are relatively immobile compared with the three-dimensional dynamics of midwall sites in the LV free wall; 2) the transverse cross-sectional shape of the LV in the transplanted human heart is decidedly oval, with significantly greater anterior-posterior than septal-lateral dimensions at the time of maximum and minimum LV volumes; and 3) the center of volume in the transplanted human LV is remarkably stable in the directions of the LV long axis and anterior-posterior axis, suggesting a balance of forces along these axes, yet it moves significantly toward the interventricular septum, presumably counterbalancing the opposite translation during systole of the right ventricular (RV) center of volume. In effect, then, as viewed from an external reference frame, the transplanted heart appears to beat as a "double bellows" (possibly preserving the position of its overall center of mass) in which the free walls of both ventricles contract toward a relatively immobile interventricular septum in systole. We suggest that the altered shape and dynamics of the LV in the transplanted human heart are not due to significantly reduced septal contraction or anterior systolic cardiac translation but are more likely the result of an RV pressure overload.