A 5-year-old boy has a history of repeated pneumonia, frequent acute respiratory viral diseases. Objectively: exertional dyspnea, minor fatigabilty. There is a systolic murmur having its epicenter in the IV intercostal space on the left. Left relative dullness is found along the midclavicular line. According to the findings of instrumental methods of examination (electrocardiography, echocardiography), the patient has been diagnosed with ventricular septal defect, subcompensation stage. What is the main method of treatment?