A 2-month-old female infant from rural Maharashtra presents with severe cyanosis (SpO₂ 65% on room air), respiratory distress, and a history of poor feeding since birth. On examination, there is a single loud S2 and no murmur. Chest X-ray shows a narrow mediastinum and markedly decreased pulmonary vascularity. Echocardiography reveals the aorta arising from the right ventricle, the pulmonary artery from the left ventricle, and a large VSD. The infant becomes acutely more cyanotic and unresponsive. What is the immediate next step in management?