ballon of mitral valve in infants

Went to Boston where they did a CAVC re-repair with Dacron VSD and boine pericardial ASD patches, primary closure of anterior muscular VSD , PA band take down and MPA repair ; uneventful intraoperative and postoperative course; on pre discharge echo, residual mild MS (6-7mm Hg mean), no MR, moderate RV hypertension 1/2-2/3 systemic.  10/29/07 a echo on Cody found increasing mitral stenious and RV hypertension. Showed moderate MS with an 11-13 mm Hg mean inflow gradient(HR 121/min, while awake and moving, No significant MR< mild TR, and RVp estimated at 55 mmHg plus RA  v-wave by doppler of the TR jet, approximately 80 % of the reported systemic blood pressure 79/52.  A heart cath. will be done will careful consideration of balloon dilation of the mitral. Son has Downs. 16mths old. HOW many children has this been done on . HELP Please.  My doctor said only 8 child. with some sucess rate.  going to Boston in a couple of wks.He was on Echmo in n.o. and the repair failed and a Pa band was placed. following that surgery he went to boston.