Two cases of tricuspid atresia in which spontaneous functional closure of a physiologically advantageous ventricular septal defect (VSD) was documented are presented. We present a third case to show that the obstruction to pulmonary flow in tricuspid atresia is at the level of the VSD. These VSDs are situated in the muscular portion of the ventricular septum, so that the contraction of the septal muscle could close the VSD. The mechanism of onset of functional closure of this type of VSD is unclear; the mechanism may be similar to that in tetralogy of Fallot. Effective palliation by aortopulmonary and cavopulmonary shunts at intervals of time depending upon the clinical status is suggested.
Increasing symptomatology and decreasing intensity of a previously heard murmur in complex congenital heart disease should lead one to suspect functional or anatomic closure of "physiologically advantageous" VSDs.