Dear Piyush,
A ventricular septal defect (VSD) and an atrial septal defect (ASD) are both congenital cardiac defects, or defects with which your nephew was born with. As mentioned above, a 4 mm VSD is not considered large, unless your nephew was premature or is very small. It likely will not cause problems besides the dilation noted above. The defect may even completely close, or at least become smaller with time. That said, I cannot be totally sure of this without evaluating your nephew. The 6 mm ASD would likely be considered moderate sized, but also has a good chance of spontaneously closing by age 3-5 years. Medications are used to treat the symptoms of congestive heart failure, a syndrome in which the heart is not able to meet the body's metabolic needs. If he has no heart failure, and is eating and growing well, there is no need for medications. As he grows, the chamber dilation should improve. If they remain open but have limited blood flow across the defects, this is not a life threatening problem that requires surgery; there are plenty of patients who have these defects remain open for their life without problems. If, however, the defects remain open and there is evidence of increasing chamber enlargement, it would be appropriate to have the cardiologist re-evaluate the child and consider surgical closure of the defect prior to age 5 years.
A 4 mm VSD in a 9 month old would not be considered large, would actually be small. Has your cardiologist recommended anything? Is your child on medication or displaying any signs of heart failure? Medication will not close the holes, merely help control any symptoms they may exhibit. The holes will either close on their own, or they will require surgery. That said, many holes will grow to be smaller (they will never get bigger), but remain open, and that is fine too. My daughter was born with a large VSD and a small ASD, the ASD closed on it's own, and the VSD is still open but is now small.