MAGNETIC-RESONANCE-IMAGING OF CONSTANT TOTAL HEART-VOLUME AND CENTER-OF-MASS IN PATIENTS WITH FUNCTIONAL SINGLE VENTRICLE BEFORE AND AFTER STAGED FONTAN PROCEDURE

It has been previously demonstrated that total heart volume (contents of pericardium) throughout the cardiac cycle varies by <5% and location of center of mass by <3 mm. The hypothesis has thus developed that for maximal efficiency, the heart should expend minimal energy in displacing extracardiac structures by maintaining a constant intracycle total heart volume and center of mass. This is achieved in the normal heart mainly by a piston-like movement of the atrioventricular valve plane toward the ventricular apex. As this has never been studied in the single ventricle heart or at various stages of Fontan reconstruction, it is conceivable that these patients may not exhibit the constancy of total heart volume and location of center of mass, which may lead to a poor outcome in some. The total heart volume and center of mass relationship was therefore evaluated in 25 children (0.4 to 237 months) with functional single ventricles at all stages of Fontan reconstruction using multiphase, multislice spin-echo or cine-magnetic resonance imaging. No significant difference was seen in variation between total heart volume and maximal volume between patients before bilateral cavopulmonary anastomosis (hemiFontan) (5.1 +/- 2.9%), after hemiFontan (3.6 +/- 1.6%) and after Fontan (8.2 7.7%); however, in 4 of 10, 1 of 8 and 5 of 7 patients, respectively, the total heart volume varied by >5%. Across surgical subgroups, significant differences were found in the center of mass displacement (total, anteroposterior and superoinferior planes) but not in directionality of displacement. Significant differences were found in directionality of displacement, however, between patients whose total heart volume varied by <5% and those in whom it did not. Two-factor analysis of variance indicate significant differences in lateral center of mass displacement across surgical subgroups regardless of total heart volume status. Our results suggest that intracycle constancy of total heart volume is not the rule throughout the stages of the Fontan procedure. Center of mass appears to maintain intracycle constancy; however, significant differences exist in directionality and displacement across surgical subgroups and total heart volume status. Long-term pump efficiency may be dependent, among other factors, on the invariance of the total heart volume and the center of mass relationships.