Development of left ventricular muscle in tetralogy of Fallot

Age-related changes of the muscle fiber diameter of the left ventricle were studied in 20 autopsied specimens of tetralogy of Fallot (TOF) and in 34 normal autopsied hearts. According to the amount of pulmonary blood flow, two groups were defined: In Group 1, TOF with a markedly decreased pulmonary blood flow. In Group 2, TOF with a moderately decreased pulmonary blood flow. In Group 1, the muscle fiber diameter was nearly normal at birth, but increased slightly and remained constant after one year, so that it was significantly less than normal. Moreover, histological observation revealed underdevelopment of the left ventricular muscle fibers in this group. In Group 2, the diameter increased with age in a manner similar to normal and, histologically, the muscle fibers were not different from normal ones except for those in one heart with left ventricular hypertrophy. The growth of muscle fibers in the left ventricle seems to depend on the volume of pulmonary blood flow, or the left ventricular inflow volume. If the left ventricular functional capacity correlates with the growth of muscle fibers, left ventricular dysfunction after the corrective operation may originate in the underdevelopment of muscle fibers. Surgical intervention is advisable before the retardation of muscle fiber growth has become apparent.