We will investigate whether levels of blood proteins in children with well-functioning hearts with one ("single") ventricle are similar to levels of these blood proteins in children with two ventricles. For children with hearts with a single ventricle, we will examine blood proteins at various levels of heart function. To assess blood protein levels, we will collect small (6 mL) samples of blood. Heart function will be determined by existing clinical scoring systems. Enrolled patients will receive an echocardiogram, which is a dynamic ultrasound picture of the beating heart.