The condition is a small hole in the wall separating the left and right lower chambers (ventricles). The complications would be shunting (passing) blood from the left chamber to the right chamber. The left ventricle has been oxygenated having passed through the lungs, and that blood would be mixed with deoxygenated blood that is going to be pumped tp the lungs...When serious the addiional blood going into the right ventrical can cause verticular overload and enlarge the ventricle to support the additional blood.
If and when the right ventricle enlarges that may increase the intra chamber pressure and cause right side blood shunted to the left ventricle (termed double shunting). This condition would contaminate the oxygenated blood with unfiltered and deoxygenated blood.
The result can be blood clots getting into circulation and possibly to the heart or brain. Also, the deoxygenated blood into the blood stream can/would reduce the the oxygen to the heart and the condition is medically termed hypoxia. That conditon can weaken the heart.
If the problem is signficant there is transcatheter closure of perimembranous ventricular septal defect. The closure device is threaded through a vessel and doesn't require open heart surgery. The success rate is very good. When or if would depend on size, etc. and 1.8mm is not very large and may close on its own or never progress.
Hope this helps, and if you have any further questions or comments you are welcome to respond. Thanks for your question, take care,
Ken