Surgery in VSD after PHT
Once the pulmonary vascular resistance gets too high, closing the VSD becomes dangerous. Yes, part of this is because surgery is a high risk proposition for patients with pulmonary hypertension and/or Eisenmenger physiology. BUT, the biggest reason is because once the pulmonary vascular resistance gets too high, the VSD actually acts as a "pop off valve" for the right ventricle. This maintains the cardiac output at the expense of a lower systemic oxygen saturation.
If you were to close the VSD in someone with Eisenmenger physiology, the right heart would suddenly have to force all of the blood through the lungs before it can get to the rest of the body. This puts increased strain on the right ventricle AND causes a decrease in the patient's cardiac output since the right ventricle can't get enough blood through the lungs. That can lead to low blood pressure, shock and death.
Once pulmonary hypertension develops, your options for surgery are heart-lung transplant, or if the heart is still working well, a lung transplant and repair of the cardiac defect. In many centers, however, the experience is that patients with Eisenmenger's can do reasonably well with therapies to lower their pulmonary vascular resistance (IV prostacyclins, bostentan, sildenafil) and in many cases will actually do better than people who have had a heart-lung transplant.
Your best bet for finding pictures of VSDs is checking a pediatric cardiology text such as the Nadas textbook. Hope that helps!
A stab in the dark: with PHT you have increased pressure in the pulmonary artery, which leads back to the right ventricle. If the pressure becomes too great, the defect can allow unoxygented blood, via the right ventricle to flow into the left ventricle, which should have oxygented blood. Thus you have blood in the left ventricle that is of lower-oxygentation than it should be. This is then pumped to the rest of the body. The danger here is thatin surgery, blood pressure may drop or spike suddenly, which may affect the VSD, with adverse patient outcomes. This is the simple answer, as there are innumerable factors like types of anesthesia, types of surgery, and your bodies own ability to handle stress. Hope this helps a little!